HVAC Fundamentals — Engineering Reference

See also (Tier 3 family index): Refrigerants

1. At a glance

HVAC — Heating, Ventilation, and Air-Conditioning — is the building system that maintains the indoor environment within bounds set by human comfort, process requirements, and the physical protection of the building fabric. The four state variables that any HVAC system controls, directly or indirectly, are dry-bulb temperature (T_db), humidity (ω or RH), ventilation rate (outdoor-air L/s per person or per m²), and particulate / contaminant level (filtration class, CO₂ ppm). Pressure (positive vs. negative building pressurization, room-to-corridor in hospitals and cleanrooms) is the fifth, less universal, variable.

The hardware spans five orders of magnitude:

• A 9000 Btu/h (~2.6 kW) ductless mini-split for a 25 m² apartment bedroom — single residential refrigerant loop, ~$1,200 installed.
• A 5-ton (17.6 kW) residential split system serving a 200 m² house — gas furnace + DX cooling coil + ducted air handler, ~$8–15k installed.
• A 100-ton (352 kW) commercial rooftop package (RTU) — McQuay Maverick II / Trane Voyager / Carrier WeatherMaster — serving a strip-mall anchor tenant.
• A 1500-ton (5.3 MW) central plant with one or two centrifugal chillers — Trane CenTraVac CVHF or Carrier 19DV — chilled-water + condenser-water + boiler loops + DOAS + VAV terminals, serving a 30,000 m² office tower.
• A 50,000-ton (176 MW) hyperscale-datacenter mechanical plant — multiple Trane / York / Carrier / Daikin chiller banks + air-handlers + close-coupled CRAHs / liquid-to-chip cooling.

Where HVAC sits in the design stack: it is the application discipline that simultaneously consumes thermodynamics (refrigeration cycles, combustion in boilers), heat-transfer (envelope conduction, coil UA, radiative comfort), fluid-mechanics (duct and pipe pressure drop, fan curves), pumps-turbomachinery (chilled-water pumps, cooling-tower fans, chiller centrifugal impellers), electric-motors (every pump, fan, and compressor), power-electronics (VFDs on every modulating load), and digital-control (BAS PID loops, supervisory sequencing). It is governed by ASHRAE, ASTM, AHRI, NFPA, IAPMO, ICC (IMC/IECC), and dozens of state amendments (notably California Title 24).

2. Why it matters

Buildings consume roughly 40 % of global primary energy and 30 % of CO₂ emissions (IEA 2023). Of that, HVAC is typically the single largest end-use — 40–60 % of commercial-building electricity (ASHRAE 90.1 Performance Rating Method baseline). The economic and environmental stakes are direct: every percentage point of chiller plant efficiency at a hyperscale datacenter is worth single-digit millions of dollars per year.

The other half of why-it-matters is human:

• ASHRAE 62.1-2022 mandates minimum outdoor-air ventilation. Under-ventilated buildings concentrate CO₂, VOCs, and bioaerosols — sick-building syndrome, productivity loss, infection transmission (the COVID-19 era cemented HVAC’s role in public health).
• ASHRAE 55-2023 sets thermal comfort. Mis-tuned setpoints, drafts, or asymmetric radiation drop occupant satisfaction below 80 % — the practical comfort target.
• Condensation control. Any time a surface temperature drops below the room dew point, water condenses. Hidden in wall cavities, that water grows mold within 48 hours and rots the structure over years.
• Legionellosis. Cooling towers and warm-water systems are Legionella breeding sites; ASHRAE 188-2018 mandates a written water-management plan for any cooling tower or building over 10 stories.

Mis-designed HVAC is the rare engineering failure that is simultaneously expensive, unhealthy, comfort-destroying, and legally non-compliant.

3. First principles — psychrometrics

Air conditioning is fundamentally moist-air thermodynamics. Atmospheric air is a binary ideal-gas mixture of dry air (treated as a single species, M ≈ 28.97 g/mol) and water vapor (M = 18.02 g/mol). The bulk pressure P_atm = P_a + P_w, with the partial pressures additive (Dalton).

Key state properties:

• Dry-bulb temperature T_db — the thermometer reading; what comfort and most thermostats measure.
• Wet-bulb temperature T_wb — temperature of an evaporating wet wick under adiabatic saturation; depends on both T_db and humidity. Equals T_db only at saturation.
• Dew-point temperature T_dp — temperature at which the existing vapor partial pressure equals saturation; the surface temperature at which condensation begins.
• Humidity ratio (mixing ratio) ω = m_w / m_a [kg water / kg dry air]; US: grains/lb (7000 gr = 1 lb). Conserved across sensible heating/cooling, changes only when moisture is added or removed.
• Relative humidity RH = P_w / P_sat(T_db). Not a state variable on its own; depends on T_db.
• Specific enthalpy of moist air, per kg of dry air (the ASHRAE convention):

h  ≈  1.006·T_db  +  ω·(2501 + 1.86·T_db)        [kJ/kg dry air, T in °C]

The 1.006 is c_p of dry air; 2501 kJ/kg is the latent heat of vaporization of water at 0 °C; 1.86 kJ/(kg·K) is c_p of water vapor. The reference state is dry air and liquid water both at 0 °C.

• Specific volume v = R_a · T_db · (1 + 1.6078·ω) / P [m³/kg dry air].

3.1 The psychrometric chart

The ASHRAE psychrometric chart (Mollier-Carrier in European tradition) plots ω vs T_db at a fixed atmospheric pressure, with overlaid curves of constant RH, constant h, constant T_wb, and constant v. Five standard processes appear as straight lines:

Process	Description	Path on chart
Sensible heating	T_db ↑, ω const	Horizontal, left → right
Sensible cooling	T_db ↓, ω const, no condensation	Horizontal, right → left
Cooling + dehumidification	Cool below dew point	Down-and-left along coil ADP line
Humidification (steam)	ω ↑, T_db ~const	Vertical up
Adiabatic evap. cooling	T_db ↓, ω ↑, h ~const	Along constant-h line
Mixing	Two streams → one	Straight line, lever rule by mass

The coil apparatus dew point (ADP) is the effective surface temperature of a cooling coil; the chord from the entering air condition to the ADP, intersected at the bypass factor, gives the leaving condition. A typical chilled-water coil has ADP 7–10 °C and bypass factor 0.10–0.20.

3.2 Standard processes

Sensible cooling load (no moisture change):

Q_s  =  ṁ_a · c_p,moist · ΔT_db  ≈  1.20 · V̇_cfm · ΔT_°F       [Btu/h, US]
                                ≈  1.21 · V̇_L/s · ΔT_°C        [W, SI]

Latent load (moisture change only):

Q_L  =  ṁ_a · h_fg · Δω  ≈  4840 · V̇_cfm · Δω_lb/lb           [Btu/h, US]
                          ≈  3010 · V̇_L/s · Δω_kg/kg           [W, SI]

Total load:

Q_tot  =  ṁ_a · Δh  ≈  4.5 · V̇_cfm · Δh_Btu/lb                 [Btu/h, US]
                    ≈  1.20 · V̇_L/s · Δh_kJ/kg_dry             [kW, SI]

The sensible heat ratio SHR = Q_s / Q_tot is a property of the room load and dictates the slope of the supply-air condition line from the room state.

4. Building load calculation

A cooling or heating load is the sum of gains/losses through the envelope, solar transmission, internal gains (people, lights, equipment), ventilation/infiltration, and system losses (duct leakage, motor heat).

4.1 Sensible cooling load components

Source	Expression	Notes
Wall/roof conduction	Q = U·A·CLTD	Cooling-load temperature difference; ASHRAE Handbook tables
Glazing conduction	Q = U·A·ΔT	Frame + glass area-weighted
Solar through glazing	Q = SHGC·A·SHGF·CLF	SHGC from NFRC ratings; SHGF from ASHRAE clear-day tables
Infiltration	Q = 1.21·V̇·ΔT	V̇ from ACH × volume
People (sensible)	Q = N·q_s	70–80 W sedentary, 200+ W active
Lighting	Q = LPD·A·F_use·F_sa	LPD = lighting power density (W/m²)
Equipment	Q = EPD·A	Office plug-load 8–15 W/m² typical
Duct heat gain	1–3 % of supply	If supply ducts in unconditioned space
Fan heat	ηfan inefficiency	Adds to coil load downstream of fan

4.2 Latent load

• People: 35 W (sedentary) to 250 W (active) per person; ASHRAE 2025 Fundamentals Ch. 18 Table 1.
• Infiltration/ventilation: 3010 · V̇_L/s · (ω_OA − ω_room) [W].
• Process moisture: kitchens, laundries, pools, indoor agriculture (greenhouses, cannabis grow rooms — increasingly significant).

4.3 Heating load

Winter design: ASHRAE 99 %, 99.6 %, or 1 % T_db from climate-data tables. Heating load is dominated by conduction + infiltration; solar and internal gains are typically not credited (worst-case morning startup with overnight setback).

Q_heat,design  =  Σ(U·A·ΔT_design)  +  1.21·V̇_infil·ΔT  +  Q_vent,preheat

Outdoor-air preheat is the killer in cold climates: at outdoor design −20 °C and ventilation 17 L/s·person × 200 people = 3400 L/s, the preheat to room temperature (21 °C) is 1.21 × 3400 × 41 = 168 kW of ventilation load alone.

4.4 Methods

• Hand methods: ASHRAE CLTD/CLF (Cooling Load Temperature Difference / Cooling Load Factor) or the more modern RTSM (Radiant Time Series Method).
• Heat balance method (HBM): rigorous, surface-by-surface, hour-by-hour; the engine inside EnergyPlus.
• Hourly simulation: EnergyPlus (DOE/LBNL, open source — the reference engine), eQUEST (DOE-2 derivative), TRACE 3D Plus (Trane), Carrier HAP (Hourly Analysis Program), IES Virtual Environment, DesignBuilder, IDA ICE, TRNSYS.
• Code path: ASHRAE 90.1 Appendix G Performance Rating Method (PRM) for whole-building energy modeling; COMcheck (DOE) for prescriptive compliance.

5p. HVAC system types

The HVAC system family tree branches first by distribution medium — air, water, refrigerant, or hybrid — and second by zoning (single vs. multi-zone), decentralization (packaged vs. central plant), and ventilation strategy (mixed vs. dedicated outdoor air).

System	Distribution	Typical use	Pros	Cons
Single-zone CAV	Air	Small commercial, RTU	Cheap, simple	One zone temp only
Multi-zone CAV w/ reheat	Air	Schools, mid-1980s offices	Multi-zone	Reheat energy waste
VAV (variable air volume)	Air	Modern offices	Efficient at part load	Complex controls; min-flow IAQ
VAV with reheat	Air	Cold-climate offices	Comfort in perimeter	Some reheat penalty
Dual-duct	Air	Legacy (1960s)	Fast response	Largest duct volume; obsolete
Fan-coil units (FCU)	Water + air	Hotels, apartments, hospitals	Per-room control	Condensate drains; maintenance
Chilled beam (active/passive)	Water + induced air	High-end offices, EU	Quiet, low energy	Condensation risk; humidity control
Hydronic radiant	Water	Schools, hospitals, EU resi	Quiet, comfort	Slow response; chilled radiant needs dew-point control
VRF / VRV	Refrigerant	Mid-size commercial, retrofits	Zone control; heat recovery	Refrigerant charge; long line sets
Packaged DX (RTU)	Refrigerant + air	Strip mall, big box	One-vendor; rooftop	Limited turn-down; gasket leaks
DOAS + zone terminal	Mixed	Modern best-practice	Decouples ventilation + sensible	Two systems to design
Mini-split / multi-split	Refrigerant	Residential, light commercial	Ductless, efficient	Aesthetic; limited capacity

DOAS (Dedicated Outdoor Air System) is the dominant modern best-practice: a separate AHU handles 100 % outdoor air with enthalpy wheel or plate ERV, dehumidifies to a low dew point (e.g., 7 °C), and delivers neutral or slightly cool air to each zone. A local terminal (FCU, VRF indoor head, chilled beam, radiant panel) handles the sensible load. This decouples the latent-dominated ventilation task from the sensible-dominated zone task — the simultaneous-heating-and-cooling reheat penalty of legacy VAV-with-reheat largely vanishes.

6p. Central plant — chillers, towers, heat pumps, boilers

6p.1 Vapor-compression chillers

The dominant cooling-plant equipment. AHRI 550/590-2023 rates all water-cooled chillers at standardized conditions and reports both full-load kW/ton and IPLV (integrated part-load value).

Type	Capacity range	Typical kW/ton (full load)	IPLV	Notes
Reciprocating	10–150 tons	0.85–1.00	0.75	Largely obsolete; replaced by scroll
Scroll (multi-circuit)	5–200 tons	0.65–0.85	0.55	Modular; Trane CGAM, Carrier 30RB, Daikin AGZ
Screw	50–1500 tons	0.55–0.65	0.45	Good part-load; York YS, Trane RTHE, Daikin AWS
Centrifugal (single-stage)	200–3000+ tons	0.50–0.60	0.35–0.45	Best at full load; Trane CenTraVac CVHF, Carrier 19XR, York YK
Magnetic-bearing centrifugal	60–1500 tons	0.50–0.55	0.30	Best IPLV; Carrier 19DV/19MV, Trane Sintesis, York YMC², Daikin Magnitude WMC
Absorption (single-effect, hot water)	100–1500 tons	(heat-driven, COP 0.6–0.8)	—	Waste-heat / district cooling
Absorption (double-effect, steam/gas)	100–1500 tons	COP 1.2–1.4	—	Higher firing temp; LiBr-H₂O

Magnetic-bearing centrifugal chillers (oil-free) achieve sub-0.30 IPLV — the most efficient cooling equipment available. They dominate Tier-3+ datacenters and Class-A office plants.

6p.2 Cooling towers

Open evaporative (induced or forced draft), closed-circuit fluid coolers, or hybrid. The condenser water leaves the tower at approach to wet-bulb of 3–5 K; design WB depends on climate (24 °C ASHRAE 0.4 % WB is typical North-American humid).

Manufacturers: BAC (Baltimore Aircoil), Marley/SPX, Evapco, EVAPCO eco-ATWB-E.

ASHRAE 188-2018 Legionellosis Risk Management mandates a written water-management plan; biocide dosing + dispersant + periodic cleaning.

6p.3 Heat pumps

Type	COP_h (rated)	COP_h (cold ambient)	Typical use
Air-source (ASHP) — split	3.5–4.5 @ 7 °C	1.8–2.5 @ −10 °C	Residential, light commercial
Cold-climate ASHP (variable-speed inverter)	3.5–4.0 @ 7 °C	2.0–2.7 @ −18 °C	Northern US/Canada; Mitsubishi Hyper Heat, Fujitsu XLTH, LG LGRED°
Ground-source (GSHP)	4.0–5.5	3.5–4.5 (loop ~0 °C)	Schools, government, premium resi
Water-source (WSHP)	4.5–5.5	(loop 15–35 °C)	Hotels, apartments — boiler/tower loop
VRF heat-recovery	4.0–5.0 cooling, 3.5–4.5 heating	—	Simultaneous heat/cool zones

VRF: Daikin VRV X / VRV A, Mitsubishi Electric City Multi PURY-P / PURY-HP (R2 series for heat recovery), LG Multi V 5, Samsung DVM S2, Toshiba SHRM-e.

6p.4 Boilers

Type	Efficiency	Fuel	Notes
Atmospheric gas	78–82 %	NG, LPG	Largely phased out by code
Forced-draft non-condensing	80–85 %	NG, LPG, oil	Mid-tier commercial
Condensing modulating	92–98 % HHV	NG	Modern best-practice; Lochinvar Crest, Aerco Benchmark, Viessmann Vitocrossal, Cleaver-Brooks ClearFire
Electric resistance	~99 % at meter	Electric	Net-zero buildings, where electric heat-pump-and-boiler split
Biomass	70–88 %	Wood pellet, chip	Niche; rural / institutional

Condensing boilers run with return-water temperature below the flue-gas dew point (~57 °C for natural gas) so that water vapor in flue gas condenses on the secondary heat exchanger, recovering its latent heat. Operating with low-temperature hot-water (LTHW) loops (50/40 °C supply/return) maximizes condensing benefit.

6p.5 Distribution

Loop	Typical supply	Typical return	Δt
Chilled water (CHW)	5–7 °C (42–45 °F)	12–14 °C (54–57 °F)	6–8 K
Condenser water (CW)	27–29 °C (80–85 °F)	35–37 °C (95–98 °F)	5–8 K
LTHW (heating)	60–82 °C (140–180 °F)	50–65 °C (120–150 °F)	10–17 K
Low-temp radiant	35–45 °C	30–40 °C	5–10 K
Refrigerant DX (R-410A)	T_evap 4–7 °C	T_cond 45–55 °C	(phase change)

Variable-primary chilled-water plants with VFDs on every pump are now standard. The legacy primary-secondary “decoupler” arrangement is being phased out in favor of variable-primary with chiller minimum-flow protection.

7p. Ventilation & indoor air quality

7p.1 ASHRAE 62.1-2022 Ventilation Rate Procedure

The breathing-zone outdoor airflow for any space:

V_bz  =  R_p · P_z  +  R_a · A_z          [L/s or cfm]

where R_p [L/s·person] is the people component, P_z is the number of occupants, R_a [L/s·m²] is the area component, A_z is the zone floor area. Selected values from ASHRAE 62.1-2022 Table 6.2.2.1:

Space type	R_p [L/s·person]	R_a [L/s·m²]	Default density [#/100 m²]	Effective L/s·person
Office space	2.5	0.3	5	8.5
Open-plan office	2.5	0.3	5	8.5
Classroom (5–8 yr)	5.0	0.6	25	7.4
Classroom (9+ yr)	5.0	0.6	35	6.7
Lecture hall	3.8	0.3	150	4.0
Hospital patient room	2.5	0.3	10	5.5
Hotel guest room	2.5	0.3	10	5.5
Retail sales	3.8	0.6	15	7.8
Restaurant dining	3.8	0.9	70	5.1
Lab (general)	5.0	0.9	25	8.6
Auditorium	2.5	0.3	150	2.7

After accounting for zone air-distribution effectiveness E_z (0.8 for ceiling-supply heat, 1.0 for cooling) and system ventilation efficiency E_v, the outdoor air intake V_ot at the AHU is computed. Multi-zone VAV systems must apply the more complex Equation 6-8 to avoid under-ventilating the critical zone.

7p.2 Demand-Controlled Ventilation (DCV)

CO₂ sensors at zone or return modulate outdoor-air damper position based on actual occupancy. ASHRAE 90.1 requires DCV in densely-occupied zones (≥ 25 people/100 m²) over 15 m². Typical savings: 10–30 % ventilation energy in offices, 30–50 % in lecture halls and auditoriums.

7p.3 Indoor Air Quality Procedure (IAQP) — 62.1-2022 alternative

Allows reduced ventilation if contaminant concentrations can be demonstrated below cognizant-authority limits, using filtration + sources control + air cleaners. Rarely used outside specialized applications (hospital surgery, semiconductor fab).

7p.4 Energy recovery

Device	Sensible eff.	Latent eff.	Notes
Plate heat exchanger (HRV)	50–75 %	0	Simple, no cross-contamination
Enthalpy wheel (ERV)	65–85 %	60–80 %	Cross-contamination < 5 %; Semco, Greenheck, Munters
Heat pipe	50–70 %	0	Refrigerant filled; no moving parts
Run-around coil	45–65 %	0	Glycol loop; longest separation possible
Membrane (fixed-plate enthalpy)	65–80 %	50–70 %	No cross-contamination; Mitsubishi Lossnay

ASHRAE 90.1-2022 Section 6.5.6.1 requires energy recovery on any system with ≥ 70 % outdoor air and capacity above thresholds varying by climate zone.

7p.5 Filtration

MERV	Particle range	Application
1–4	> 10 μm	Residential pre-filter only
8	3–10 μm	Minimum commercial
11	1–3 μm	Better commercial; “MERV 13” minimum per CDC COVID guidance
13	0.3–1 μm	High-end office; LEED
14	0.3–1 μm	Hospital general areas
16	0.3–1 μm	Hospital surgical
HEPA	≥ 99.97 % at 0.3 μm	Cleanrooms, isolation rooms
ULPA	≥ 99.999 % at 0.12 μm	Class 10 / ISO 4 cleanroom

ASHRAE 52.2-2017 defines MERV. ISO 16890 is the international successor (ePM1, ePM2.5, ePM10).

8p. Thermal comfort — ASHRAE 55-2023 / ISO 7730:2005

The PMV/PPD model (Fanger 1970) predicts thermal sensation from six variables: T_db, mean radiant temperature T_mrt, air velocity V, RH, metabolic rate met (1 met = 58.2 W/m²), and clothing insulation clo (1 clo = 0.155 m²·K/W).

PPD  =  100 − 95 · exp(−0.03353·PMV⁴ − 0.2179·PMV²)        [%]

ASHRAE 55-2023 acceptable range: −0.5 ≤ PMV ≤ +0.5, PPD ≤ 10 %.

Typical comfort envelope (sedentary, summer 0.5 clo, winter 1.0 clo, V < 0.2 m/s):

Season	T_op range	RH range
Summer (0.5 clo, 1.2 met)	23–27 °C (73–80 °F)	30–60 %
Winter (1.0 clo, 1.2 met)	20–24 °C (68–75 °F)	30–60 %

Operative temperature T_op ≈ (T_db + T_mrt)/2 at low air velocity. Cold windows pull T_mrt down → perimeter zones need radiant compensation or higher T_db.

Local discomfort must also be controlled:

• Draft — ASHRAE 55: draft risk < 20 % at occupant ankle (V < 0.20 m/s at 23 °C).
• Vertical air temperature difference (head-to-ankle) < 3 K.
• Radiant asymmetry: warm ceiling < 5 K, cool wall (e.g., window) < 10 K.
• Floor temperature: 19–29 °C (66–84 °F) for bare or stocking feet.

Adaptive model (ASHRAE 55 Section 5.4) — applies to naturally-ventilated buildings where occupants can open windows; comfort range widens with outdoor running-mean temperature.

9p. Distribution — ductwork and piping

9p.1 Duct sizing

Three classical methods (ASHRAE Handbook Fundamentals Ch. 21):

• Equal-friction method: select a friction rate (commonly 0.8–1.0 Pa/m or 0.08–0.10 in WG/100 ft), size every section to that rate. Simple, conservative on noise.
• Static-regain method: size successive downstream sections so that velocity-pressure recovery balances friction loss → equal static along the trunk. Used for large multi-takeoff systems.
• T-method: total-cost (first-cost + life-cycle energy) optimization. Used by Trane Ductulator and ASHRAE Duct Fitting Database (DFDB).

Typical duct velocities (low-pressure systems, ≤ 500 Pa):

Duct	Residential	Commercial	High-velocity
Main supply	3–5 m/s	5–8 m/s	12–18 m/s
Branch	2–4 m/s	4–6 m/s	8–12 m/s
Return	2–3 m/s	3–5 m/s	—
Diffuser face	1–2 m/s	1.5–2.5 m/s	—

SMACNA HVAC Duct Construction Standards (4th ed.) defines gauge thickness, reinforcement, and leakage class (Class 3 = 8 mL/s·m² @ 250 Pa, the modern target). Pressure class typically 500, 750, 1000, 1500, 2500 Pa.

9p.2 Hydronic piping

Closed-loop systems require:

• Expansion tank sized for fluid expansion from min to max temperature; bladder-type prevailing.
• Air separator (often combined with tank fitting) — coalesces dissolved air over time.
• Glycol freeze protection in cold-climate outdoor systems — 30 % propylene glycol drops freeze point to −13 °C but reduces specific heat ~7 % and increases viscosity ~50 %, requiring re-sized pumps.
• Strainers at pump suctions and ahead of control valves.
• Pressure-independent control valves (PICVs) at terminals — eliminate the manual balancing valve and the system re-balance work after any major load change.

Pipe sizing per ASHRAE Handbook tables; velocity 1–3 m/s for water (5 m/s upper for short runs), pressure drop 250–400 Pa/m typical.

10p. Worked examples

Example A — Cooling-load summary, 1000 m² office

Given: 1000 m² single-story office building, occupied by 50 people 9-hr/day, lighting power density 8 W/m², equipment 12 W/m², glazing area 200 m² (south facade), opaque wall U = 0.40 W/(m²·K), roof U = 0.20 W/(m²·K). Design conditions: Atlanta GA, 35 °C / 24 °C WB outside, 24 °C / 50 % RH inside.

Step 1 — Envelope conduction:

• Opaque walls (perimeter wall area, less glazing): 300 m² × 0.40 × (35 − 24) = 1.32 kW
• Roof: 1000 m² × 0.20 × 11 = 2.2 kW
• Glazing conduction (U = 1.6 W/m²·K): 200 × 1.6 × 11 = 3.5 kW

Step 2 — Solar gain (SHGC = 0.30, peak SHGF ≈ 500 W/m² south at solar noon, CLF ≈ 0.7):

• 200 × 0.30 × 500 × 0.7 = 21.0 kW

Step 3 — Internal sensible:

• People sensible (75 W/person sedentary): 50 × 75 = 3.75 kW
• Lighting: 8 W/m² × 1000 × CLF 0.9 = 7.2 kW
• Equipment: 12 × 1000 = 12.0 kW

Step 4 — Ventilation/infiltration sensible:

• Vent = 8.5 L/s·person × 50 = 425 L/s (let’s say 0.5 ACH infiltration adds 250 L/s → total OA 675 L/s for sensible)
• Sensible: 1.21 × 425 × 11 = 5.66 kW (vent only, infiltration usually credited zero in tight buildings)

Step 5 — Latent:

• People latent: 50 × 55 W = 2.75 kW
• Ventilation: ω_OA ≈ 0.0140 kg/kg (35 °C, 24 °C WB), ω_room ≈ 0.0093 (24 °C, 50 % RH); 3010 × 0.425 × (0.0140 − 0.0093) = 6.0 kW

Step 6 — Total:

Component	kW
Envelope conduction	7.0
Solar	21.0
People sensible	3.75
Lighting	7.2
Equipment	12.0
Vent sensible	5.7
Subtotal sensible	56.6
People latent	2.75
Vent latent	6.0
Subtotal latent	8.75
Grand total	65.4 kW (18.6 tons)

SHR = 56.6 / 65.4 = 0.87 (typical office). At 12.5 L/s·m² supply with 11 K supply-to-room ΔT: V̇ = 56.6 / (1.21 × 11) = 4.25 m³/s = 9000 cfm. Coil leaving condition: 13 °C / 95 % RH (cool below room dew point of 12.9 °C → just barely dehumidifies).

Example B — Psychrometric mixing + cooling coil

Given: AHU mixes 30 % outdoor air (35 °C, 60 % RH → ω = 0.0214, h = 90.0 kJ/kg) with 70 % return air (25 °C, 50 % RH → ω = 0.0099, h = 50.4 kJ/kg). Cool the mixed stream to a coil leaving 13 °C and 95 % RH.

Mixed condition (mass-weighted, ω and h are linear with mass fraction):

• ω_mix = 0.30 × 0.0214 + 0.70 × 0.0099 = 0.0133 kg/kg
• h_mix = 0.30 × 90.0 + 0.70 × 50.4 = 62.3 kJ/kg
• T_db,mix = 0.30 × 35 + 0.70 × 25 = 28.0 °C

Coil leaving at 13 °C / 95 % RH:

• ω_LAT ≈ 0.00880 kg/kg, h_LAT ≈ 35.4 kJ/kg

Coil load per kg dry air:

• Total: Δh = 62.3 − 35.4 = 26.9 kJ/kg dry air
• Latent: 2501 × (0.0133 − 0.0088) = 11.3 kJ/kg dry air (note simplified — strictly use h_g·Δω)
• Sensible: 26.9 − 11.3 = 15.6 kJ/kg dry air
• SHR_coil = 15.6 / 26.9 = 0.58

If the room SHR is 0.87 and coil SHR is 0.58, the supply air is over-dehumidified relative to room load. Either accept lower room RH (~40 %) or reheat the supply from 13 °C to ~16 °C to land on the room condition line. Reheat is the canonical example of simultaneous-heating-and-cooling waste — about 11 % of coil sensible in this case — and is the reason DOAS+terminal architectures are now preferred.

Example C — Centrifugal chiller selection, 1000 kW (285 tons)

Given: Office tower peak cooling load Q = 1000 kW = 285 tons. CHW supply/return 6/12 °C, condenser water 29/35 °C, climate zone 4A (Mid-Atlantic). 2000 equivalent full-load hours (EFLH) per year, electricity $0.12/kWh.

Step 1 — Sizing. Apply 1.10 oversize factor (diversity already in load calc): 1000 × 1.10 = 1100 kW. Centrifugal economic break-even is ~300 tons; both screw and centrifugal viable.

Step 2 — Candidate selection:

Option	Chiller	Full-load kW/ton	IPLV (AHRI 550/590)
A	Trane CenTraVac CVHF-330 (oil-bearing, single-stage cent.)	0.55	0.40
B	Carrier 19DV-300 (magnetic-bearing, variable-speed cent.)	0.52	0.30
C	York YS-300 (oil-bearing screw, twin)	0.62	0.45
D	Daikin Magnitude WMC-300 (magnetic-bearing)	0.50	0.30

Step 3 — Annual energy (apply IPLV — most hours are at part load):

• A: 285 tons × 0.40 kW/ton × 2000 h = 228,000 kWh/yr → $27,360/yr
• B: 285 × 0.30 × 2000 = 171,000 kWh/yr → $20,520/yr
• C: 285 × 0.45 × 2000 = 256,500 kWh/yr → $30,780/yr
• D: 285 × 0.30 × 2000 = 171,000 kWh/yr → $20,520/yr

Step 4 — First-cost vs life-cycle. Magnetic-bearing premium typically $80/tonoveroil-bearingcentrifugal=$24,000. Energy savings B vs A: $6,840/yr → simple payback ~3.5 yr. Highly favorable for any 15-yr equipment life.

Step 5 — Auxiliaries. Don’t forget the condenser-water pump (~0.05 kW/ton), CHW pump (~0.04 kW/ton), and cooling-tower fan (~0.04 kW/ton). Total plant kW/ton typically full-load chiller × 1.25–1.35.

11p. Edge cases / gotchas

• Oversized AC short-cycles. Cooling capacity too large for the load → compressor cycles on/off, supply air doesn’t run long enough to dehumidify, room RH rises to 60–70 %, occupants feel sticky and crank thermostat down → mold risk. Size to design load, not to “safe.”
• Coil bypass at high face velocity. Above 2.5 m/s through a cooling coil, the bypass factor rises sharply; not all the air contacts the cold surface and effective dehumidification drops. Standard residential coil face velocity 1.5–2.0 m/s; commercial 2.0–2.5 m/s.
• Stack effect. In tall buildings (> 6 stories), winter buoyancy drives infiltration at lower floors and exfiltration at upper floors. Pressure differential ΔP ≈ ρ_o·g·h·(T_i − T_o)/T_i — a 100 m tower at outdoor −10 °C, indoor 22 °C, has neutral-pressure-plane shift of ~50 Pa across the elevator shaft. Causes condensation in walls, door-pull problems, infiltration loads.
• Building pressurization. Slightly positive (+5 to +12 Pa) building pressure prevents humid outdoor air infiltration. Too positive (>25 Pa) makes doors hard to open and forces exfiltration through joints — driving moisture into wall cavities in humid climates.
• Outdoor-air freezing. Below −5 °C entering coil, water-cooled HX freeze-protection is needed (face-and-bypass dampers + glycol preheat coil + flow switches). DX coils with refrigerant pump-down and ERV with frost-control cycles handle their own.
• Reheat energy waste. A VAV-with-reheat system supplying 13 °C air to a 70 °F (21 °C) zone with a 10 % design load reheats 4 K of every cfm — for the entire shoulder season. ASHRAE 90.1 caps reheat to design min-flow (typically 30 % of max).
• Refrigerant phase-out. R-22 (HCFC) production banned 2020; R-410A (HFC, GWP 2088) phasing under AIM Act 2025 — new equipment ≤ 700 GWP from Jan 2025. Successors: R-32 (GWP 675, A2L mildly flammable), R-454B (GWP 466, A2L), R-1234yf/ze (HFO, GWP < 1, A2L). All are A2L → tighter charge limits, refrigerant-detection sensors.
• VFD harmonics. Every chiller, pump, fan VFD is a non-linear load — typical THD_i 35–50 % without mitigation. IEEE 519-2022 limits THD at the point of common coupling (PCC). Mitigation: 5 % line reactor, 18-pulse front-end, or active harmonic filter.
• Legionella. ASHRAE 188-2018 requires written water-management plans for any cooling tower, plus most healthcare and hospitality occupancies. Hot-water storage minimum 60 °C, distribution return 51 °C; cooling-tower biocide + dispersant.
• Mold from condensation. Any chilled-water pipe colder than the local dew point sweats — insulate to manufacturer-recommended thickness with vapor-tight jacket. Ducts in unconditioned attics carrying 13 °C air through 30 °C / 80 % RH air condense without external insulation.
• Testing, Adjusting & Balancing (TAB). Per AABC, NEBB, or TABB standards — a separate commissioning specialty. Always discovered, never designed-in; budget 1.5–2.5 % of mechanical cost.
• Energy-code compliance. ASHRAE 90.1-2022 prescriptive (per-component minimums) vs. performance path (whole-building simulation, must beat the baseline by mandated percentage). California Title 24 adds even tighter envelope and lighting requirements.

12p. Tools & software

Load calc: Trane TRACE 3D Plus, Carrier HAP, Wrightsoft Right-J/D/N (residential), Elite Software RHVAC/CHVAC.

Whole-building energy simulation: EnergyPlus (DOE/LBNL, open-source — the reference), OpenStudio (NREL GUI / SDK), eQUEST (DOE-2 derivative), IES Virtual Environment, DesignBuilder, IDA ICE (EQUA), TRNSYS, BEopt (residential).

CFD for HVAC: Autodesk CFD, Ansys Fluent, Siemens STAR-CCM+ — cleanrooms, atria, datacenter airflow, smoke modeling per NFPA 92.

Building Automation Systems (BAS): Tridium Niagara (vendor-neutral framework — most installed BAS framework worldwide), Distech Controls EC-Net, Reliable Controls, Siemens Desigo CC, Honeywell WEBs / JACE, Schneider EcoStruxure Building Operation, Johnson Controls Metasys. Protocols: BACnet (IP/MSTP, ASHRAE 135-2020 — the dominant building protocol), Modbus RTU/TCP, LonWorks (legacy), KNX (European resi), MQTT (newer for IoT-style integrations).

Duct sizing: McQuay Duct Sizer (free), Trane Ductulator, ASHRAE Duct Fitting Database (DFDB), Wrightsoft Right-D.

Hydronic piping: Bell & Gossett ESP-PLUS, Taco Plus 30, Pipe-Flo (Engineered Software), AFT Fathom (Applied Flow Technology).

Code compliance: COMcheck and REScheck (DOE, free) for prescriptive ASHRAE 90.1 / IECC; EnergyPlus / OpenStudio for the performance path.

Refrigerant property data: NIST REFPROP 10.0, CoolProp (open-source), Daikin Refrigerant Solution Selector (online).

13. Cross-references

• thermodynamics — vapor-compression cycle, psychrometric enthalpy, combustion in boilers, refrigerant phase-out tables (the parent thermodynamic background).
• heat-transfer — coil UA, envelope conduction, fin-and-tube heat exchangers, radiant comfort analysis, condensation.
• fluid-mechanics — duct pressure drop, fan-system curves, pipe friction, compressible-flow effects in supply duct.
• pumps-turbomachinery — chilled-water and condenser-water pumps, cooling-tower fans, chiller compressor impellers, VFD control.
• electric-motors — induction motors and PMSMs on every fan/pump; premium-efficiency / IE3-IE5 ratings drive plant SEER.
• power-electronics — VFDs on every modulating load; IEEE 519 harmonic limits.
• microcontrollers — BACnet/Modbus field controllers on actuators, sensors, VAV boxes.
• digital-control — PID loops in BAS supervisory and trim control, sequencing logic.
• classical-control — proportional-band tuning, deadband on outdoor-air dampers.
• construction-bim — EnergyPlus input/output formats, IDF, IDD; planned.
• industrial-automation — BACnet object types, Modbus register maps; planned.
• construction-bim — Industry Foundation Classes (BIM) and gbXML (Green Building XML); planned.

14. Citations

1. ASHRAE Handbook — Fundamentals (2025). American Society of Heating, Refrigerating and Air-Conditioning Engineers, Atlanta, 2025. ISBN 978-1947192992. The HVAC engineer’s primary daily reference; psychrometric chart, refrigerant tables, load-calculation methods.
2. ASHRAE Handbook — HVAC Systems and Equipment (2024). ASHRAE, 2024. Equipment-by-equipment design guidance; selection, sizing, integration.
3. ASHRAE Handbook — HVAC Applications (2023). ASHRAE, 2023. Sector-specific (hospitals, datacenters, museums, industrial, residential).
4. ASHRAE Handbook — Refrigeration (2022). ASHRAE, 2022. Industrial refrigeration, commercial refrigeration, food storage.
5. ANSI/ASHRAE Standard 90.1-2022 — Energy Standard for Sites and Buildings Except Low-Rise Residential Buildings. ASHRAE, 2022. The mandatory commercial-building energy code in most US jurisdictions.
6. ANSI/ASHRAE Standard 62.1-2022 — Ventilation and Acceptable Indoor Air Quality. ASHRAE, 2022.
7. ANSI/ASHRAE Standard 55-2023 — Thermal Environmental Conditions for Human Occupancy. ASHRAE, 2023.
8. ANSI/ASHRAE Standard 15-2022 — Safety Standard for Refrigeration Systems. ASHRAE, 2022. Refrigerant safety classification, machinery-room ventilation, charge limits.
9. ANSI/ASHRAE Standard 188-2018 — Legionellosis: Risk Management for Building Water Systems. ASHRAE, 2018.
10. AHRI Standard 550/590-2023 — Performance Rating of Water-Chilling and Heat Pump Water-Heating Packages Using the Vapor Compression Cycle. Air-Conditioning, Heating, and Refrigeration Institute, 2023. Defines IPLV, full-load efficiency rating points.
11. International Mechanical Code (IMC) 2024. International Code Council, 2024. Mechanical-system code adopted (with amendments) by most US states.
12. International Energy Conservation Code (IECC) 2024. ICC, 2024. Envelope, lighting, and HVAC efficiency code; commercial provisions defer to ASHRAE 90.1 in many adoptions.
13. NFPA 90A-2024 — Standard for the Installation of Air-Conditioning and Ventilating Systems. National Fire Protection Association, 2024. Smoke/fire dampers, duct construction for life safety.
14. NFPA 90B-2024 — Standard for the Installation of Warm Air Heating and Air-Conditioning Systems (small commercial/residential). NFPA, 2024.
15. ISO 7730:2005 — Ergonomics of the thermal environment — Analytical determination and interpretation of thermal comfort using calculation of the PMV and PPD indices and local thermal comfort criteria. International Organization for Standardization.
16. SMACNA HVAC Duct Construction Standards — Metal and Flexible, 4th ed. Sheet Metal and Air Conditioning Contractors’ National Association, 2020.
17. McQuiston, F. C.; Parker, J. D.; Spitler, J. D. Heating, Ventilating, and Air Conditioning: Analysis and Design, 7th ed. Wiley, 2023. ISBN 978-1118091807. Long-standing undergraduate text.
18. Mitchell, J. W.; Braun, J. E. Principles of Heating, Ventilation, and Air Conditioning in Buildings. Wiley, 2013. ISBN 978-0470624579. Solid intermediate text with control coverage.
19. Stoecker, W. F.; Jones, J. W. Refrigeration and Air Conditioning, 4th ed. McGraw-Hill, 2018. The classical refrigeration-cycle reference.
20. Wang, S. K. Handbook of Air Conditioning and Refrigeration, 2nd ed. McGraw-Hill, 2000. ISBN 978-0070681675. Comprehensive industry reference.
21. Trott, A. R.; Welch, T. Refrigeration and Air-Conditioning, 3rd ed. Butterworth-Heinemann, 2000. ISBN 978-0750648646. Compact reference with strong commercial-refrigeration coverage.
22. Kreider, J. F.; Curtiss, P. S.; Rabl, A. Heating and Cooling of Buildings: Principles and Practice of Energy Efficient Design, 3rd ed. CRC Press, 2017.
23. Kigali Amendment to the Montreal Protocol (UNEP, 2016). HFC phase-down schedule.
24. US EPA AIM Act Final Rule (2023) — Phasedown of Hydrofluorocarbons. 40 CFR Part 84, technology transitions limiting HFC GWP in new equipment from 2025.