Qatar HVAC Design Review Guide

Every project has a Basis of Design (design basis) or Design Basis Memorandum: the document that freezes site conditions, indoor criteria, system philosophy and the code hierarchy before detailed design starts. Read it end to end once, then keep its design-criteria tables open beside every deliverable you review. If your project has no design basis, the first review comment you write is a request for one.

Qatar has no unified national building code. What governs an HVAC design is a stack, and the order of the stack is contractual, not a matter of engineering taste. When two documents disagree, the one higher in the stack wins; when the higher one is silent, you drop one level.

• Budget: three weeks, 20–30 hours. Module hours are shown in the rail. Do the modules in order the first time; afterwards use the rail as a lookup.
• Active recall: each module ends with a self-check and flashcards. Answer before revealing. If you get fewer than four of six, re-read the concept block, not the answer.
• Checklists persist on this device (browser storage). Tick items as you apply them to a real deliverable; the rail shows completion.
• Calculators (Modules 1, 3, 6, 8) run in the page with no external calls. Use them to reproduce a designer's number, not to replace their calculation.
• Badges: Verified public source opened or confirmed by an authoritative snippet; Partial existence confirmed, clause content second-hand; Unverified cite only after you obtain the document; design basis quoted from the project Basis of Design.

Five properties, two of which travel with the air

• Dry-bulb temperature (DB, °C): what a thermometer reads. Drives sensible load.
• Humidity ratio (w, g of water per kg of dry air): the moisture content. It does not change when air is simply heated or cooled above its dew point, so it is the quantity that identifies an air stream through a system.
• Relative humidity (RH, %): moisture content relative to saturation at that temperature. Useful for comfort and corrosion statements; useless for tracking air through a coil, because it changes every time the temperature does.
• Wet-bulb temperature (WB, °C): the temperature reached by evaporative cooling; a stand-in for enthalpy. Coils and cooling towers are rated by it.
• Enthalpy (h, kJ/kg dry air): total heat content, sensible plus latent. Differences in enthalpy give total coil loads directly.
• Dew point (DP, °C): the temperature at which the air starts to condense. Every surface colder than the dew point sweats. This is the number that decides insulation thickness in Qatar.

The three SI air-side equations

With Q in m³/s, the standard-air constants at sea level are:

The ASHRAE Handbook writes the same set with Q in L/s as 1.23 × Q × ΔT (W), 3010 × Q × Δw (W, w in kg/kg) and 1.20 × Q × Δh (W). The psychrometric chart in your library prints them as 1.2, 2.98 and 1.19. All three sets agree within 2 %; the differences come from the density assumed. What you must not do is mix a metric constant with imperial units (the imperial set is 1.08, 4840 and 4.5 with CFM).

Why peak dry-bulb is not peak load

Outdoor air is expensive to condition in proportion to its enthalpy, not its temperature. A very hot afternoon in the desert is dry; a humid morning with sea breeze is cooler but wetter. Compare the two design basis points:

The 34/32 point has 12 % more enthalpy and 50 % more moisture than the 47/30 point. For a treated-fresh-air unit the coil load at 34/32 is about 19 % higher than at 47/30, and the sensible-heat ratio of that load is around 0.2: it is a dehumidifier first and a cooler second. That is why Gulf fresh-air units carry heat pipes or hot-gas reheat: the air must be cooled below its target dew point to wring the moisture out, then reheated so the space does not overcool.

Where the design conditions come from, and what "0.4 %" means

ASHRAE Handbook Fundamentals Chapter 14 tabulates, for Doha International Airport (WMO 411700), the dry-bulb temperature exceeded 0.4 %, 1 % and 2 % of the hours in a year, each with its mean coincident wet-bulb, and separately the wet-bulb, dew-point and enthalpy exceeded for the same fractions with their coincident dry-bulbs. Two consequences:

• The 0.4 % dry-bulb for Doha is in the mid-40s with a coincident wet-bulb in the low-to-mid 20s. A design value of 47 °C DB with 30 °C WB is therefore already a margin above the statistical peak and combines a hot dry-bulb with a wet-bulb that does not statistically occur with it. That is a client choice, not an error; you should know it is conservative.
• The 0.4 % wet-bulb and dew-point for Doha are around 31 °C and 30 °C respectively, with coincident dry-bulbs in the mid-30s. The 34/32 basis sits at the extreme of that band. Fresh-air coils selected on it will be conservative; fresh-air coils selected on 47/30 alone will be undersized on latent duty.

QCS 2014 fixes a third set of numbers that are not load conditions at all but equipment ratings: continuous operation at a maximum daily-average ambient of 50 °C, an operating range of 0–55 °C, and a design humidity of 100 %. These rate the hardware. Never feed "50 °C and 100 % RH" into a coil selection; that state does not exist in Doha's record and would produce absurd loads.

The load components, in the order they appear in a report

1. Conduction through opaque envelope: q = U × A × ΔT_eff. In a manual method ΔT_eff is the CLTD (cooling load temperature difference), which folds in solar absorption and thermal lag; in HAP the same physics is done hour by hour with transfer functions. The U-value is the reviewer's lever.
2. Glazing: conduction (U × A × ΔT) plus solar (A × SC × SHGF, or A × SHGC × incident irradiance in modern form). The shading coefficient is the second big lever; SC 0.3 is a high-performance glass and must appear on the architectural glazing schedule, not only in the HVAC model.
3. People: sensible and latent per person by activity (ASHRAE Fundamentals 2021 Table 1, tabulated at 23.9 °C room; at 26.7 °C the sensible share falls about 20 % and latent rises). Occupancy count comes from the architect or, failing that, ASHRAE 62.1 default densities.
4. Lighting: W/m² × area × usage; add a ballast/driver allowance for fluorescent (1.2) but not for LED with integral drivers if the nameplate is system watts.
5. Equipment and small power: nameplate is not heat gain. ASHRAE gives office load densities of 5.4 / 10.8 / 16 / 21.5 W/m² for light / medium / medium-heavy / heavy workstation densities; kitchens and laundries use appliance tables with radiant and usage factors.
6. Ventilation (outdoor air): computed at the outdoor design state chosen for it (Module 1); counted at the coil, not in the room, when the fresh air is treated centrally.
7. Infiltration: air changes through cracks and door openings; sensible and latent. In a positively pressurised building it is normally zero except at ground floor and entrances; where a building is not pressurised, a stated ACH applies.
8. System gains: fan heat (draw-through adds to the coil, blow-through adds to the supply), duct gains in hot ceilings and roofs, return plenum gains from recessed luminaires.
9. Safety factor: applied last, once, explicitly.

Peak, block and diversity

Each room peaks at a different hour: east rooms at 09:00, west rooms at 16:00, the mess hall at 13:00. The sum of room peaks sizes the terminal units. The block load, the largest simultaneous total, sizes the plant that serves them. For VRF this is exactly why a connection ratio above 100 % is permitted: the outdoor unit is sized to the block, the indoor units to their own peaks. A report that sizes an outdoor unit on the sum of indoor peaks is safe but wasteful; a report that sizes indoor units on a diversified block is wrong.

Method lineage: which engine is in the software

Method differences are small compared with input errors. Do not spend review time arguing TFM versus RTS; spend it on U-values, occupancy, schedules and the outdoor-air basis.

Sanity yardsticks

Use the bands to spot outliers, then trace the outlier to its input. A bedroom at 3 m²/kW is not "wrong"; it is asking you to look at its roof and its infiltration.

ASHRAE 62.1 Ventilation Rate Procedure

R_p is a per-person rate, R_a a per-area rate; both are additive because people and materials each emit. The breathing-zone quantity V_bz is divided by the zone air-distribution effectiveness to give the outdoor air the diffusers must deliver. For a multi-zone recirculating system a further system efficiency applies (Appendix A of the standard); for dedicated treated-fresh-air units serving each space directly, V_oz is what the TFAHU must deliver to that room.

Exhaust rates (62.1-2022 Table 6-2, Annex A Table 3-9)

Toilets private 12.5/25 L/s per fixture, public 25/35 L/s per fixture; shower rooms 10/25 L/s per showerhead; commercial kitchens 3.5 L/s·m² (the hood calculation governs in practice); kitchenettes 1.5; janitor and trash rooms 5.0 (Class 3, no recirculation); soiled laundry storage 5.0 (Class 3); locker rooms 1.25–2.5; parking 3.7 L/s·m². Where a prescriptive ACH and a per-fixture rate both exist, the larger governs and the drawing must say which was used.

Pressure relationships: the map, not the number

Air moves from high pressure to low. Draw the building as a set of boxes with arrows: fresh air in, exhaust out, and net flow at every door. Clean spaces sit at the top of the cascade (positive), corridors below them, toilets, kitchens and laundries at the bottom (negative). The quantities that produce the cascade are small: 5–12 Pa positive across the envelope is enough to stop infiltration in an ordinary building; 25 Pa is the IEC 60079-13 minimum for a pressurised room in a hazardous area; 50 Pa is a smoke-control or process-plant figure that costs door-opening force (about 47 N on a standard door, within the 133 N limit of NFPA 101 but noticeable) and requires airlocks.

The supply needed to hold a pressure is a leakage calculation (the "crack method"): sum the leakage areas of doors, windows and penetrations and apply Q = C × A × ΔPⁿ. BS 5588 Part 4 used to carry the tables; it is withdrawn, and BS EN 12101-6 (pressure differential systems) and ASHRAE's smoke-control handbook chapter are the current sources. A 1 ACH rule for "general buildings" is a convenient stand-in that a designer must still reconcile with the actual leakage; for a leaky warehouse it is optimistic, for a tight office it is generous.

Untreated ventilation and the moisture question

An accommodation room ventilated only by toilet extract receives its outdoor air as infiltration through window and door cracks, at outdoor humidity, straight into the room. At the 34/32 design morning that air carries 30 g/kg. The split unit must remove the difference to 8–10 g/kg while it runs and can do nothing while it cycles off. Whether the room stays below 60 % RH depends on the ratio of latent load to coil latent capacity at part load, which is exactly the analysis a moisture balance provides and a load calculation does not.

Sand, salt and filters

• Sand-trap louvres at 1.0 m/s face velocity on gross area, at every intake, with a washable metal pre-filter behind them; intakes at high level, upwind of pollution sources, and 10 m from any exhaust.
• Filtration is specified by ASHRAE 52.2 MERV ratings (52.1 is withdrawn; a document citing both is out of date on one of them). MERV 7–9 panel or bag filters as pre-filters, MERV 13 on treated-fresh-air units; ISO 16890 is the equivalent European ladder (ePM10, ePM2.5, ePM1) you will see on European datasheets.
• Differential-pressure switches or sensors across every filter bank, reported to the BMS, per QCS Section 22.
• Energy recovery on treated fresh air: plate or wheel; effectiveness stated as sensible or total (enthalpy); an "80 % minimum efficiency" requirement needs to say which. Kitchen exhaust (Class 3) is never recovered through a wheel that can leak into supply.

The system families and what each buys you

How a VRF system works, in the terms a datasheet uses

One inverter-driven outdoor unit modulates compressor speed to hold a target refrigerant pressure; each indoor unit has an electronic expansion valve controlled from its own coil thermistors, so it takes only the refrigerant it needs. Capacity therefore follows the sum of indoor-unit demands up to the outdoor unit's limit. Three consequences matter for review:

• Connection (index) ratio: total indoor capacity ÷ outdoor capacity. Manufacturers permit 50–130 % (some to 150–200 % with derating). Above 100 % the outdoor unit cannot serve all indoor units at full load simultaneously; the design must show the block load (Module 2) is within the outdoor unit's capacity at site conditions.
• Piping geometry: total pipe length, longest actual and equivalent run, length after the first branch, level difference outdoor-to-indoor and indoor-to-indoor. Each limit exists for oil return and pressure drop; each exceedance costs capacity or triggers a manufacturer's refusal to warrant. The schedule must state the values used and the correction factor applied.
• Refrigerant charge: a system's charge is the outdoor unit's factory charge plus a per-metre allowance for liquid line. Charges of 30–100 kg per system are ordinary. ASHRAE 15 / ISO 5149 / EN 378 limit the concentration that a full leak could produce in the smallest occupied room served: for R-410A the refrigerant concentration limit is 0.42 kg per m³ of room volume (ASHRAE 34). A 42 m³ bedroom therefore tolerates about 17 kg. Where the circuit charge exceeds that, the standards require mitigation: smaller circuits, leak detection with alarm, or safety ventilation interlocked to the detector.

Coastal-desert engineering

Salt aerosol, sand loading and ultraviolet are the three degradation drivers. The vocabulary you will see on datasheets: ISO 12944 corrosivity class C5-M (very high, marine) for paint systems, salt-spray endurance in hours per ASTM B117 / ISO 9227 (5,000 h is a heavy-duty specification), pre-coated or post-coated condenser coils (phenolic dip, epoxy electro-coat, or hydrophilic blue-fin as a light option), powder-coated casings with film thickness in micrometres, stainless fasteners, and sand-trap louvres or hoods on outdoor-unit intakes. None of these are free: coatings add condenser air-side pressure drop and reduce heat transfer a few percent, which must be in the capacity claim.

Refrigerant policy in 2026

R-410A (GWP 2,088) has zero ozone-depletion potential, which is the only environmental claim the design basis makes. Qatar is an Article 5 party to the Montreal Protocol, targets complete HCFC (R-22) phase-out by 2030, and had not ratified the Kigali Amendment as of late 2025, so there is no domestic HFC phase-down schedule yet. Lower-GWP alternatives (R-32, GWP 675; R-454B) are class A2L, mildly flammable, with additional charge-limit and ventilation rules; QCS 2014 and the QCDD 2022 guide contain no explicit A2L provisions, so any proposal to use them must be agreed with QCDD early on the basis of ASHRAE 15-2022 / ISO 5149. A reviewer's position: accept R-410A as the basis, require the refrigerant concentration checks, and record the GWP exposure in the risk register for a 25-year asset.

Rating conditions you will see on datasheets

Moving the claim to site conditions

Air-cooled DX capacity falls roughly 1 % per kelvin of condenser entering air above 35 °C and input power rises 2–3 % per kelvin; at 47 °C expect 85–90 % capacity and 125–135 % power, so EER falls by about a third. Indoor entering wet-bulb moves capacity the other way: at 22 °C / 50 % (about 16 °C WB) the coil sees a lower wet-bulb than the 19 °C rating and delivers less total capacity but a higher sensible fraction. Only the manufacturer's selection software or published correction tables give the exact factors; a datasheet without them is incomplete. Add screen or parapet recirculation (Module 4) to the entering-air temperature before reading the curve.

Efficiency metrics and which one Qatar asks for

• EER (Btu/h per W) or COP (W/W; EER = 3.412 × COP) at full load and a stated condition. Kahramaa's tables are EER at T1 and T3 for room and split units, EER at 35 °C for packaged units, COP and IPLV for chillers.
• IEER / IPLV / SEER: part-load weighted metrics. ASHRAE 90.1 Table 6.8.1 uses IEER for unitary and VRF above 19 kW; Kahramaa's 2025 code points to ASHRAE 90.1 Table 6.8.1-3 for chillers.
• Star rating (QS 2663, Kahramaa/Tarsheed): mandatory label for unitary air conditioners up to about 70,000 Btu/h sold in Qatar; minimum 3 stars, tested at T3. A VRF outdoor unit or a packaged rooftop unit above the scope has no star rating; asking for one is a category error, and the applicable instrument is the Kahramaa code table or ASHRAE 90.1.

Noise: from a datasheet number to a bedroom

Datasheets quote sound power (L_w, dB(A) re 1 pW) or sound pressure at a distance (L_p at 1 m). For a unit on a flat roof radiating into a hemisphere, L_p ≈ L_w − 20 log₁₀(r) − 8, so 80 dB(A) sound power is about 72 dB(A) at 1 m and 52 dB(A) at 10 m, before barriers or multiple units (+3 dB per doubling of units). QCS caps outdoor equipment at 65 dB(A) at 1 m. Indoor criteria are in NR (design basis: NR 30 bedrooms and mosques, NR 35 offices and clinic, NR 40 halls, retail, workshops); an NR value is a spectrum, and the datasheet must give octave-band sound power for the acoustic consultant to check it. NC (North American) and NR (ISO) are numerically close but not identical; use one in the specification.

Electrical and controls data the mechanical reviewer still owns

Qatar supply is 415/240 V, three-phase four-wire, 50 Hz; Kahramaa requires the AC load to be declared with the building permit and a power factor of 0.9 or better. A datasheet must give full-load and maximum current, starting current or inrush (inverter units are soft), IP rating of outdoor electrical enclosures (design basis: IP55 inside, IP64 outside; QCS/Shell-based fire-system tables go to IP66 outdoors), motor efficiency class (IE2 minimum per the Kahramaa 2023 code snippet; IE3 is the safer specification), and the control interface (BACnet gateway for VRF, hard-wired points for packaged units).

The vendor-document review cycle

Datasheets arrive with a review code convention: typically Code 1 no comments, Code 2 proceed with comments incorporated, Code 3 revise and resubmit, Code 4 rejected or for information. Your comment must be resolvable: name the design basis or specification clause, the datasheet field, and the acceptable evidence (a curve, a certificate, a corrected value). A technical bid evaluation is the same exercise done line by line in a compliance matrix, with commercial weighting kept separate from technical compliance.

Equal friction: the method and its two dials

Choose a friction rate (Pa per metre of duct) and size every section so that its friction loss equals that rate; the duct gets larger as the air quantity grows. The method is simple and self-balancing enough for low-pressure systems. Its dials are the friction rate and a velocity ceiling: the friction rate sets fan energy and duct size, the velocity ceiling sets noise. A design basis usually fixes both by system type, as this one does in Table 4-7 (AHU supply 0.6 Pa/m and 7.6 m/s; return 0.5 Pa/m and 6.1 m/s; FCU ducts 0.4 Pa/m and 4.5 m/s; extract and fresh-air units 0.8 Pa/m and 9.1 m/s; smoke extract 1.2 Pa/m and 10 m/s). The duct-sizing chart in your library from another Gulf project uses 1.25 Pa/m, which shows how project-specific the number is.

Round ducts carry the chart; rectangular ducts are converted by the Huebscher equivalent diameter, D_e = 1.30 (ab)^0.625 / (a + b)^0.25, which gives the rectangular size with the same friction at the same airflow. Keep aspect ratios at or below 4:1; beyond that the sheet metal cost and leakage climb faster than the space saved.

Acoustic velocity limits are separate from friction limits

ASHRAE HVAC Applications (reproduced in the design basis as Figure 4-2 and Annex A Table 3-10) caps main-duct velocity by the room criterion and the duct location: for RC/NR 25 rooms, 4.8 m/s for rectangular duct within the occupied space, 6.1 m/s above a suspended acoustic ceiling, 8.6 m/s in a shaft; for RC/NR 35 the corresponding figures are 7.4, 8.9 and 12.7 m/s. Branches run at about 80 % and final run-outs at 50 % or less of the main-duct values. Supply outlets: 1.8 / 2.2 / 2.5 / 2.8 m/s "free" opening velocity for RC 25 / 30 / 35 / 40; returns 2.2 / 2.5 / 3.0 / 3.4 m/s. A bedroom at NR 30 with a supply diffuser at 2.5 m/s is over the acoustic limit before any duct noise arrives.

External static pressure is a sum, and the fan is selected on it

ESP = straight-duct friction + fitting losses (loss coefficients, ASHRAE Duct Fitting Database) + terminal devices + volume dampers + filters at their dirty pressure drop + coils + louvres + attenuators, on the supply and the return path, plus system effect at the fan. Fan-coil units are sold in low (30–50 Pa), medium (80–120 Pa) and high (150–250 Pa) static families; a duct calculation that comes out at 120 Pa against a 50 Pa unit has selected the wrong family, and the site symptom is a starved far diffuser.

Construction standard: pressure class, gauge, sealing, leakage

Either standard is acceptable; a specification must name one and its edition, because gauge tables, joint types and leakage classes do not map one to one. QCS 2014 Section 22 Part 6 allows ASHRAE/SMACNA fabrication with DW/144 for BS-familiar contractors; your job is to see that the drawing set, the specification and the test method all say the same thing. A drawing note "DW-142" is a 1980s reference superseded by DW/144 (2013, updated 2016).

Insulation: condensation governs in Doha

ASHRAE 90.1 sets insulation R-values for energy. In Doha the design dew point of about 30 °C (0.4 % annual) means any duct or pipe surface below 30 °C exposed to outdoor or unconditioned air sweats, and roof voids and plant rooms are often at outdoor humidity. The insulation thickness is therefore set by the condition that the outer surface of the insulation stays above the dew point of the surrounding air, with an unbroken vapour barrier on the warm side, and then checked against the energy minimum. Typical Gulf practice: 25 mm glass fibre with foil facing inside conditioned ceiling voids, 50 mm with vapour barrier and aluminium cladding on roofs and in plant rooms, and closed-cell elastomeric on refrigerant and condensate pipes. QCS 2014 Section 22 Part 7 carries the thickness tables (to be confirmed clause by clause); Dubai's 502.11 tables are the published regional reference.

Acoustic lining deserves a caution. Fibrous lining inside ducts carrying humid air is a hygiene and durability risk; ASHRAE 62.1 §5.4 requires liners resistant to erosion and moisture. Prefer external insulation with duct attenuators, or coated/closed-cell liners where lining is unavoidable, and never line ducts downstream of a cooling coil before the air is dry.

Dampers and the drawings that carry them

• Volume control dampers at every branch (the design basis says so), positioned where they can be reached and where their noise does not reach the room (not at the diffuser neck for NR 30 rooms).
• Fire dampers (UL 555 or BS EN 1366-2 / BS EN 15650) wherever ductwork crosses a fire-rated wall or floor, at shaft entries and exits; fusible link 72 °C standard; access door within reach on the drawing, not only in the specification.
• Smoke dampers and combination dampers (UL 555S) where smoke control or HVAC shutdown zoning requires it; motorised, with FACP interlock and BMS status.
• Gas-tight dampers on all intakes and exhausts, normally closed on fire-and-gas emergency, per the client's fire and safety philosophy (a process-plant requirement carried into this project; confirm its applicability rather than delete it).
• Backdraft dampers on exhaust discharges; sand-trap louvres on intakes at 1 m/s; exhaust louvres at 2–2.5 m/s on gross area.

Three lines and what each must do

• Suction (low-pressure gas): carries cold vapour and entrained oil back to the compressor. It must be sized for a pressure drop small enough not to lose capacity (a drop equivalent to about 0.5–1 K of saturation temperature; 2 °F loses roughly 4 % capacity) and a velocity high enough to carry oil up risers. It is always insulated.
• Liquid line: carries subcooled liquid to the expansion device. Its enemy is flash gas: pressure lost to friction and to static lift (a rising liquid line loses about 10 kPa per metre for R-410A) plus heat gained on a sunny roof can bring the liquid to saturation before the valve, and the expansion valve then hunts. The remedy is subcooling margin and insulation on exposed runs. Velocity is kept below about 1.5 m/s to avoid liquid hammer when solenoids close.
• Discharge / hot gas: hot vapour from compressor to condenser (split systems) or the high-pressure gas line of a heat-recovery VRF. Sized like suction but at higher density; insulated for personnel protection above about 66 °C.

Oil return numbers, and why two references disagree

Your library gives two minimum velocities for oil return: McQuay AG 31-011 says 2.5 m/s (500 fpm) horizontal and 5 m/s (1,000 fpm) in risers; the DuPont/Suva handbook says 3.8 m/s (750 fpm) and 7.6 m/s (1,500 fpm). Both are legitimate rules from different eras and oils. A calculation must state which basis it uses; the stricter one is safer for part-load operation, which is where risers fail. Where a compressor unloads to 25–50 %, a double riser keeps velocity up: the small riser carries the minimum load, the large one is sealed by an oil trap at low load and opens at full load. Lines slope toward the compressor (about 1:100 to 1:200), and risers get a trap at the base per the manufacturer's rule.

VRF: the manufacturer's rules replace the generic ones

For VRF the pipe sizes are tabulated by the manufacturer against the downstream capacity index, the branch joints (refnets or headers) are proprietary, and the additional refrigerant charge is a per-metre figure by liquid-line diameter. The generic physics still applies, but the review question changes: show the manufacturer's selection printout for every circuit, with pipe sizes, joint types, lengths, level differences, additional charge and the capacity correction. Installation rules are equally specific: brazing under nitrogen purge, pressure test at the R-410A design pressure (4.15 MPa) for 24 hours, triple evacuation to below 500 microns (−100.7 kPa), and charge by weight. These belong in the method statement and the inspection and test plan, and they are the items that determine whether the warranty is valid.

Insulation on refrigerant lines in Doha

A suction line at 8–12 °C in 30 °C dew-point air sweats through any gap. Closed-cell elastomeric insulation (ASTM C534 / BS 5422 thickness tables), 13–19 mm inside conditioned ceiling voids and 25 mm outdoors, on both lines (the liquid line for solar gain, the gas line for condensation), with UV-stable cladding or jacket on the roof, and sealed joints. Where two insulated lines share a trunking, the trunking must drain. The design basis's condensate-pipe cladding requirement (aluminium sheet outdoors) is the same logic.

Condensate: how much, how big, how steep, where to

Condensate mass equals the latent load divided by the latent heat of vaporisation (about 2,450 kJ/kg): 10 kW of latent cooling produces 14.7 kg/h, i.e. about 15 L/h. At Gulf design conditions a fresh-air unit is 60–80 % latent, so a 150 kW TFAHU makes around 150 L/h at peak, a bath every eight minutes. Sizing then follows a capacity table (the International Mechanical Code table is the common reference: up to 20 tons ¾ in., 21–40 tons 1 in., 41–90 tons 1¼ in., 91–125 tons 1½ in., 126–250 tons 2 in.; verify against the current IMC edition before citing) with a minimum slope of 1 % (1:100) toward the drain, cleanouts at changes of direction, and no reduction in size along the run. A draw-through unit needs a trap whose depth exceeds the negative static at the coil (trap depth ≥ 1.5 × static, plus 25 mm), or the drain pan will not drain while the fan runs; a blow-through unit needs a trap to stop air blowing out. Ceiling-mounted units get a secondary drain pan or a float switch that stops the unit. Condensate pipe is itself cold (12–15 °C) and sweats: insulate it in ceiling voids and clad it outdoors, as the design basis requires. Discharge: the design basis sends condensate to the storm-water network with an air break; a project pursuing GSAS water credits can recover it for irrigation instead.

Chilled water, for completeness

Not used on this project, but you will see it elsewhere: closed-loop chilled-water pipe is sized to about 150–300 Pa/m friction with velocities from 0.9 m/s (small bore, to avoid air-locking and noise) to about 2.5–3 m/s in large mains, per ASHRAE Fundamentals Chapter 22 (older editions Chapter 35). The verification sheet in your library from another project draws exactly that: a 150 Pa/m line up to 300 mm and a velocity-limited line above.

Substations and electrical rooms: cooling first, pressure second, purge last

The load is equipment loss: transformer no-load and load losses (roughly 1 % of rating for dry-type units at full load), switchgear and busbar losses, UPS rectifier and inverter losses, cable losses, plus lights and solar gains through a usually windowless envelope. A design basis figure such as 50 W/m² is a placeholder until the electrical schedule exists; two 1,600 kVA dry-type transformers alone reject about 30 kW, which in a 242 m² room is 125 W/m². Get the loss schedule from electrical before accepting any substation load.

Temperature: switchgear is rated to 40 °C ambient by IEC 62271 and transformers by their insulation class, so 23 °C is a client comfort-and-longevity choice, not a code requirement; it costs capacity because the coil runs closer to the space temperature. Redundancy: an accommodation project substation that trips on HVAC failure in August loses the project, so N+1 packaged units or a ventilation fallback mode with temperature alarms to the BMS are ordinary practice even where the design basis is silent, as this one is.

Now the three quantities that get confused. Supply air at about 8–12 ACH of recirculated, cooled air is normal for a DX-cooled electrical room: 10 ACH of a 968 m³ room is 2,690 L/s, which removes 12 kW at a 3.7 K supply-to-room difference, or 30 kW at 9 K. Pressurisation air is filtered outdoor air sized by leakage to hold the overpressure: at +50 Pa through a reasonably tight room perhaps 0.5–1 ACH (135–270 L/s), which at 47 °C / 30 °C WB costs 9–18 kW of cooling, comparable with the entire internal load, and needs an airlock. Emergency ventilation (purge on smoke or on cooling failure) is a separate mode with its own fan and dampers, and its ACH is not the normal-mode figure. A design that writes "10 ACH" must say which of the three it means; a design that gives all three is reviewable.

Battery rooms: dilution formula, prescriptive rule, and the interlock

Lead-acid batteries evolve hydrogen on charge. EN 50272-2 (now IEC 62485-2) sizes dilution ventilation as Q = 0.05 × n × I_gas m³/h, where n is the number of cells and I_gas the gas-producing current (about 1 A per 100 Ah in float for vented cells), reduced by factors f₁ = 0.5 for low-antimony cells and f₂ = 0.5 for valve-regulated cells; natural ventilation openings then need A ≥ 28 × Q cm². A 110-cell, 400 Ah VRLA bank in float needs about 5.5 m³/h. A prescriptive 12 ACH rule for a 60 m³ room gives 720 m³/h, over a hundred times more; the prescriptive rule governs because it is the client's, and its purpose is safety margin for boost charging and failures. What the formula adds is the logic: extract at high level (hydrogen is light), inlet at low level, at least 2 m between them, no recirculation, fan run-on for an hour after boost charge, and the charger interlock the design basis requires so that a failed extract stops the charging that makes the gas. A hydrogen detector with alarm at 1 % (25 % of the lower explosive limit) is the modern complement. The design basis asks for explosion-proof duty/standby fans; the European standard argues a diluted room need not be Ex-rated, but a Level-1 client requirement overrides that argument. Batteries also want 20–25 °C for life (VRLA life halves for every 10 K above 25 °C), which is a cooling requirement the design basis does not list for battery rooms; ask for it.

Pump rooms and fire pump rooms

Domestic water and fire pump rooms are motor-heat rooms held at 24 °C by fan-coil units in normal mode. A diesel fire pump adds combustion and radiator air when running, per NFPA 20 and the engine manufacturer, and the design basis specifies 10 ACH in emergency mode with supply and exhaust fans interlocked, fed by two-hour fire-rated cables and started by the fire alarm panel. The review point is the louvre and duct sizing for the engine's radiator discharge and combustion air at 47 °C ambient (derate the engine), and the switching logic between fan-coil normal mode and purge mode so that the two do not fight.

Generator rooms and fuel rooms

NFPA 37 governs. The design basis defines three states: 6 ACH with the set off, 10 ACH on standby, and radiator discharge plus combustion make-up air when running, with fuel-tank rooms at 10 ACH continuously. The running case sizes the intake louvre (engine data, at 2–2.5 m/s free-area velocity), the discharge duct or hood, and the attenuators that keep the set within the site noise limit; the standby case keeps the room below the alternator's ambient limit while the set idles; the off case is the summer heat soak. Day tanks inside the room (8 hours in this design) bring fuel vapour rules: diesel's flash point above 55 °C keeps the room unclassified, but leak detection, bunding and a fusible-link fuel shut-off at 75 °C are in the fuel-oil section of the design basis and must appear on the HVAC cause-and-effect matrix.

Fire command centre

Positively ventilated in supply mode at 12 ACH during emergency per QCDD, with its own air conditioning, separated by two-hour construction, started by the fire alarm panel and the sprinkler flow switch, on two-hour cables. Positive pressure keeps smoke out of the one room that must stay occupied.

Commercial kitchens

NFPA 96 and ASHRAE Applications Chapter 34 rule the hood. Exhaust is set by hood type and appliance duty (light, medium, heavy, extra-heavy), broadly 300–600 L/s per metre of wall-canopy hood; make-up air at 80–90 % of exhaust, delivered cooled through a make-up air unit so the kitchen stays slightly negative to the dining room and the dining room stays positive to outside. Grease ducts are welded steel, sloped back to the hood, with cleanouts, enclosed or wrapped for two hours, discharging away from intakes; the fan motor sits outside the airstream. Ecology units clean the discharge in stages (pre-filter, bag filter, electrostatic precipitator, activated carbon) and add pressure drop that the exhaust fan must overcome at dirty condition. The wet-chemical suppression system interlocks with the hood: on discharge, make-up air stops, exhaust continues, and the LPG solenoid closes; the interlock belongs on the HVAC cause-and-effect matrix as much as on the fire one. Kitchen appliance heat gains were deferred in the design basis; the hood removes a large share of them, which is why the load calculation and the hood calculation must be done together.

Central laundry

Tumble dryers exhaust hot, wet, lint-laden air, typically 300–600 L/s each per manufacturer, through lint interceptors into a dedicated extract with anti-static impellers and the motor outside the airstream (the bifurcated fan in the design basis); make-up air must replace it or the laundry runs negative and hot. The design basis names the equipment but gives no temperature, humidity, airflow or lint-load target for the laundry, and assigns its extract rate to the "kitchen specialist"; ask for the laundry equipment schedule, the exhaust per machine, the room design condition and the make-up air, and expect an internal load dominated by equipment (drying, ironing, washing) rather than people.

Clinic

ASHRAE 170 sets total and outdoor air changes and pressure relationships per room type: examination and treatment rooms 6 total / 2 outdoor ACH, waiting areas 12 / 2 and negative to adjacent, toilets 10 exhaust, clean storage positive. "2 ACH" without "outdoor" is incomplete; the schematic submission must tabulate both numbers and the pressure sign per room.

Hazardous areas: context you will meet elsewhere in the site

Not this project's problem, but the origin of several of its design-basis rows. In classified areas (IEC 60079-10-1 zones; the client uses the zone system, API RP 505, not the division system of API RP 500) a building is protected by pressurisation: IEC 60079-13 requires at least 25 Pa overpressure against the adjacent zone, purge before energising, and loss-of-pressure actions; NFPA 496 does the same with Type X/Y/Z purging. Intakes are placed outside the classified zone and above the height where hydrocarbons may collect, with gas detection that closes the gas-tight dampers and shuts the HVAC. That is the logic behind the design basis's normally-closed intake dampers and its 10 m separation and "above the risk zone" intake rules.

Who approves what

The documents behind the approvals

QCDD's governing text is the Civil Defence Technical Requirements Guide 2022 (with a 2023 update), which supersedes the 2015 Fire Safety Guidelines but sits on top of the 2015 annexes: FLS_A1 refuge floors, A2 smoke-stop and firefighting lobbies, A3 external access, A4 fire lifts, A5 fire command centre, A6 fire pumps, A7 electrical rooms, A8 substations, FFS and FAS notes, and ACMV_N1, the air-conditioning and mechanical ventilation notes. Where the guide is silent it adopts NFPA as the principal reference: NFPA 101 (means of egress), 5000 (building construction), 90A (installation of air-conditioning and ventilating systems), 92 (smoke control), 96 (kitchen exhaust), 72 (alarm), 13 (sprinklers), 20 (fire pumps), 37 and 110 (engines and standby power). QCDD does not publish which NFPA edition it enforces; the safe assumption is the current one, and the design basis's fire-fighting code table pins 2022–2024 editions for the client review.

NFPA 90A: the four HVAC rules that recur

• Duct smoke detectors: in the supply downstream of filters for systems above 944 L/s (2,000 cfm), and in the return at each floor or where the return exceeds 7,080 L/s (15,000 cfm) serving more than one floor; detection stops the fan and closes smoke dampers via the fire alarm panel. The design basis puts detectors in AHU return ducts and shuts the unit through the fire annunciation panel.
• Fire dampers at rated walls, floors and shaft penetrations; smoke dampers where ducts cross smoke barriers or where the smoke-control concept requires; combination dampers where both apply (Module 6 carries the drawing checklist).
• Duct materials and clearances: non-combustible ducts, flexible duct limits, clearances from combustibles, plenum rules for return-air ceilings (no combustible cabling or insulation without a plenum rating).
• System shutdown on alarm as the default; systems that must run in a fire (smoke control, pressurisation, fire pump and command-centre ventilation) are the exceptions and get rated cable and emergency power.

Smoke control: what a G+2 project needs and what it does not

Stair pressurisation (NFPA 92: 12.4 Pa minimum across a closed stair door in sprinklered buildings, 24.9 Pa unsprinklered, door force at most 133 N; BS EN 12101-6 class systems at 50 Pa) is a high-rise and enclosed-stair measure that the QCDD fire strategy decides at DC1; low-rise accommodation with open or naturally ventilated stairs may not need it. Mechanical smoke extract applies to large undivided spaces (mess halls, mosques, warehouses) where the strategy calls for it, to enclosed car parks (QCDD: 6 ACH normal, 10 ACH smoke mode, fans rated 300 °C for 2 hours) and to basements. The design basis lists smoke management per the client's fire life safety strategy, names UL-listed equipment, and treats post-fire purge as a non-life-safety convenience. The reviewer's task is to obtain the strategy, list every space where it demands a smoke-control function, and see that the HVAC drawings implement exactly that list, no more and no less.

Fire and gas on a operator-controlled site

a Qatar industrial city buildings follow the client's corporate philosophy for fire and safety: hydrocarbon or H₂S detection at air intakes trips the HVAC and closes gas-tight, normally-closed dampers on intakes and exhausts, so an external release cannot be drawn into an occupied building; intakes sit high and upwind. The project design basis carries these rules over. Deleting them is a client decision; implementing them means a detector at each intake, damper actuators with position feedback, and the shutdown logic on the matrix.

The cause-and-effect matrix

One table, rows are initiating events (smoke detector zone, duct detector, sprinkler flow switch, manual call point, gas detector, kitchen suppression discharge, fan failure, high room temperature), columns are actions (stop AHU, close smoke damper, start purge fan, open relief damper, close LPG solenoid, start FCC ventilation, signal BMS). It is the single document that lets the fire, electrical, controls and mechanical reviewers check each other; ask for it with the first schematic and refuse to sign off ventilation drawings without it.

Architecture: what "DDC per equipment, BMS on top" means on drawings

Each air-handling unit, fresh-air unit, fan set and packaged unit has its own direct digital controller running its sequence stand-alone; a failure of the BMS network leaves every unit running on its last set points. The BMS server supervises: schedules, trends, alarms, set-point changes, energy reports. VRF systems are proprietary networks; they reach the BMS through a gateway (BACnet is the design basis's choice) that exposes a limited set of points per indoor unit (on/off, mode, set point, room temperature, fault). The review artefacts are the system architecture diagram, the points list (inputs and outputs per controller, hard-wired versus network), the sequence of operation for each system, the alarm list with priorities, and the panel schedule with IP ratings and locations.

Sequences to read line by line

• Room units on VRF: wired thermostat per room; set-point range limited (for example 22–26 °C) so occupants cannot drive the system to 16 °C; dead band between cooling and any heating; occupancy or key-card interlock in accommodation if the client wants it.
• Treated fresh-air unit: supply temperature control at 20–23 °C after the heat pipe; the cooling coil controlled on off-coil temperature (dew point), not on supply temperature, so that moisture removal is guaranteed; wheel or plate bypass logic; filter differential alarms; fan on variable speed with a minimum outdoor-air floor per ASHRAE 62.1 when CO₂ control is active.
• Pressurised rooms: a differential-pressure sensor across the envelope modulating the supply fan or a relief damper; alarm on loss of pressure; airlock door interlocks where +50 Pa is confirmed.
• Substations and battery rooms: duty/standby changeover on failure and on run-hours, high-temperature alarm and staged response (second unit, then purge), the battery charger interlock and fan status to the fire-and-gas system.
• Kitchens and laundry: hood exhaust and make-up air interlocked (make-up cannot run without exhaust; exhaust runs on suppression discharge), ecology-unit stage alarms, dryer exhaust interlocked to dryer operation.
• Demand-controlled ventilation: CO₂ set point (typically 800–1,000 ppm, or 700 ppm above outdoor), sensor in the space rather than the return duct for large halls with stratification, and a hard floor at the 62.1 area-based rate.
• Fire interfaces: hard-wired shutdown from the fire alarm panel independent of the BMS network; damper end-switches; status back to BMS (Module 9).

Kahramaa: the permit-side energy check

The Energy & Water Conservation Code (2016, re-issued 2023 as CN-CNT-P2/C1 and 2025 as CN-CNT-P2-C2 Issue 2) is applied at the building permit through Kahramaa's engineer. What it asks for, from the recovered text: envelope U-values and glazing SHGC certified by the manufacturer and calculated for summer per ASHRAE; a cooling-load summary sheet and a cooling-equipment schedule for verification; loads by acceptable handbooks or ASHRAE-certified software; equipment efficiencies against its tables (Module 5), with chillers pointed to ASHRAE 90.1 Table 6.8.1-3 in the 2025 issue and unlisted equipment to ASHRAE 90.1 §6.4.1; polyphase motors IE2 or better (2023 issue); the air-conditioner star label for unitary units; and, through the Wiring Code, the declared AC load with connected and diversified demand per distribution board at power factor 0.9 or better. The summary sheet is the HAP block-load report reformatted; the schedule is the equipment schedule you reviewed in Module 5. Inconsistency between them is the most common reason for a Kahramaa return.

GSAS: the certification-side energy and indoor-environment check

GSAS Design & Build (2019, 4th edition) scores a building from −1 to +3 across categories; certification requires a positive score and stars follow half-point bands (1 star from 0 to 0.5, up to 6 stars above 2.5). The HVAC-relevant criteria: E.1 thermal energy demand performance (envelope and internal gains, the same inputs as Module 2), E.2 energy use performance (modelled energy against a baseline; equipment efficiency and controls from Modules 5 and 10), E.3 primary energy, E.4 CO₂ emissions, E.5 energy sub-metering (the design basis's metering strategy report), IE.1 thermal comfort (ASHRAE 55 predicted mean vote within ±0.5, or operative-temperature compliance), IE.2 natural ventilation, IE.3 mechanical ventilation (ASHRAE 62.1 and the +20 % cap interplay from Module 3), IE.8 acoustics. GSAS is mandatory for government projects and common on the client work; the design basis lists it without a target, which is the first question to put to the client, because a 3-star target changes the envelope, the fresh-air strategy and the equipment class.

ASHRAE 55 in one paragraph

Comfort is operative temperature, humidity, air speed, clothing and activity together; the standard's compliance zone at 0.5 clo and sedentary activity spans roughly 23–26 °C operative at 50 % RH, and it caps the humidity ratio at 12 g/kg for the analytical method. The design basis's 22 ± 1 °C with 40–60 % RH sits at the cool edge of the zone, which is a client preference; the reviewer's check is that the room units can actually hold it at part load without over-cooling (Module 2's cycling problem) and that air speed at the occupant stays below 0.2 m/s for NR 30 bedrooms.

The sequence and its documents

Tolerances: the number that must be written before the test

NEBB and AABC procedures accept terminal airflows within ±10 % of design and totals within −0 % / +10 %; CIBSE Commissioning Code A uses similar bands with tighter values for critical rooms. Outdoor-air quantities are a minimum, never a −10 %. Pressurisation is accepted on measured differential pressure with the doors closed and on door force with the system running. QCS Section 22 Part 1 has its own testing clauses whose tolerances must be read and reconciled with the specification; where the specification is silent, state the NEBB values in your review of the TAB procedure so the report cannot be "within tolerance" of nothing.

The summer problem

Cooling systems commissioned in a Qatar winter cannot demonstrate capacity or dehumidification. Either the contract holds a seasonal performance test (a logged week in July–September with rooms at set point and RH inside 40–60 %) as a condition of final acceptance, or the reviewer accepts that the first real test is the first summer under warranty. Write the seasonal test into the commissioning plan now; it costs nothing at the procedure stage and everything at the punch-list stage.

Construction-phase instruments you will run

• Site queries and RFIs: the design answer must cite the design basis or specification clause it relies on; an RFI that changes a design criterion is a change, not a clarification, and goes through the change-management system.
• Non-conformance reports: categorise by consequence (safety and code compliance; performance; durability; cosmetic); a duct leakage test failure is a performance NCR with a retest, an uninsulated suction line behind a closed ceiling is a durability NCR with an open-up.
• Punch lists: category A (prevents safe operation or occupancy: fire damper missing, cause-and-effect row failed), B (prevents design performance: airflow out of tolerance, missing balancing damper), C (cosmetic or documentation). Mechanical completion is A-clear; substantial completion is A and B clear with C scheduled.
• Mechanical completion report: per system, lists the tests performed with results against the acceptance criteria, the open punch items by category, the deviations accepted, and the documents handed over. It is the document the client's handover procedure will audit.

Read the design basis once with a blank table beside you: reference, issue, severity (H changes a selection or safety function; M changes a quantity or document; L editorial) and a draft comment written to be pasted into a comments resolution sheet. Typical findings on Gulf projects: two outdoor design points without stated use; RH criteria with no humidity control; compounded safety factors; process-plant pressurisation tables copied into non-process buildings; system-per-building lists that disagree with each other; no redundancy philosophy; withdrawn standards (BS 5588, ASHRAE 52.1, DW/142); missing duct, insulation and piping criteria; deferred kitchen and laundry loads; and a green-building rating listed without a target.

Status reflects what could be verified from this workstation on 3 September 2026: government portals (qcs.qs.gov.qa, km.qa, moi.gov.qa, gord.qa, qatarenergy.qa) and ashrae-meteo.info were not reachable, so clause-level facts rest on indexed text and authoritative summaries. Close each open item by opening the document and recording the clause.

• The twenty-three files in the four reference folders were read and catalogued; four are images or legacy formats that could not be parsed (noted in the reference map). Quoted numbers come from the files themselves.
• Qatar regulations were researched from public sources on 3 September 2026. Government portals were not reachable from the build environment, so clause-level statements from QCS, Kahramaa, QCDD and GSAS carry "Partial" badges and a closing action in the ledger; the client standards are not public and carry "Unverified" badges.
• Worked examples use stated assumptions and simplified envelope terms; they teach structure and magnitude, not a design.
• Rules of thumb labelled as such (Gulf W/m² bands, derating percentages, insulation thicknesses) are experience values for screening, not acceptance criteria; the project specification governs.
• Nothing here replaces the project specification, the client's standards, or the judgement of the engineer of record.