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Room Heating and Cooling Load

Design cooling and heating load of a room or zone by the component method: conduction through walls, roof and glass with the cooling-load temperature difference of each surface, solar gain through glass from its area, solar heat gain coefficient and the peak irradiance of its orientation, internal gains from people, lights and equipment, and the sensible and latent load of the outdoor air brought in by ventilation and infiltration. The cooling load is split into sensible and latent parts for coil and airflow selection; the heating load uses the winter temperature difference with no solar or internal credit. Temperatures are plain numbers in °F (USCS) or °C (SI).

  • Conduction loads per surface from U, area and the cooling temperature difference; heating from the winter ΔT

  • Solar gain through glass by SHGC and orientation irradiance; internal gains from people (sensible and latent), lights and equipment

  • Outdoor-air sensible and latent loads from the airflow and the humidity ratios

  • Sensible, latent and total cooling load in Btu/h and tons, sensible heat ratio, supply airflow at a ΔT, and the heating load

Room Heating and Cooling Load

Unit System: /

Design Conditions (temperatures °F for USCS, °C for SI)







Opaque Surfaces (walls, roof, floor): U in Btu/h·ft²·°F for USCS, W/m²·K for SI; CLTD in °F or °C
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Glass: U as above, irradiance in Btu/h·ft² for USCS, W/m² for SI
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Internal Gains and Outdoor Air








Component Load Method | Method page
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Room Heating and Cooling Load

Component Load Method

The design cooling load of a room is the sum of the heat that enters it at the peak hour: conducted through the envelope, radiated in as sunshine through glass, generated inside by people, lights and equipment, and carried in by the outdoor air. Each is a simple product, and the method's accuracy lies in the inputs.

Conduction through opaque walls and roofs uses the cooling-load temperature difference (CLTD), a tabulated value for the orientation, color, mass and hour that stands in for the sol-air temperature and the wall's thermal lag; 25 to 40 °F for sunlit walls and 40 to 70 °F for roofs at the afternoon peak are typical of the ASHRAE tables. Glass conducts at the plain temperature difference. Solar gain through glass is area × SHGC × the orientation's peak irradiance (about 100 Btu/h·ft² south, 200 to 230 west, 30 north in midsummer at mid-latitudes). Internal gains: a seated person gives about 250 Btu/h sensible and 200 latent; lights and equipment their electrical input (times a usage factor the user applies). Outdoor air brought in for ventilation and leaking in as infiltration adds ṁ cp Δt sensible and ṁ hfg ΔW latent.

The sensible load sets the supply airflow for a chosen supply temperature difference; the latent load and the sensible heat ratio set the coil's leaving condition. The heating load is the winter conduction and outdoor-air load with no solar or internal credit.

This is the simplified component method, suitable for sizing a room's equipment and checking a detailed calculation; it does not model the time lag of radiant gains (the radiant time series or heat-balance methods do) and so tends to be conservative. Temperatures, U-values, CLTDs and irradiances are entered in the units of the selected system.



References

American Society of Heating, Refrigerating and Air-Conditioning Engineers (2021). ASHRAE Handbook: Fundamentals. ASHRAE, Peachtree Corners, GA.

Spitler, J. D. (2014). Load Calculation Applications Manual, 2nd ed. ASHRAE, Atlanta, GA.

McQuiston, F. C., Parker, J. D., and Spitler, J. D. (2005). Heating, Ventilating, and Air Conditioning: Analysis and Design, 6th ed. John Wiley and Sons, Hoboken, NJ.

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