Heat load or heat gain

A building or room gains heat from many sources. Inside occupants, computers, copiers, machinery, and lighting all produce heat. Warm air from outside enters through open doors and windows, or as ‘leakage’ though the structure. However the biggest source of heat is solar radiation from the sun, beating down on the roof and walls, and pouring through the windows, heating internal surfaces.

The sum of all these heat sources is know as the heat gain (or heat load) of the building, and is expressed either in BTU (British Thermal Units) or Kw (Kilowatts).

For an air conditioner to cool a room or building its output must be greater than the heat gain. It is important before purchasing an air conditioner that a heat load calculation is performed to ensure it is big enough for the intended application.

Heat load calculations

There are several different methods of calculating the heat load for a given area:

Quick calculation for offices

For offices with average insulation and lighting, 2/3 occupants and 3/4 personal computers and a photocopier, the following calculations will suffice:

Heat load (BTU) = Length (ft.) x Width (ft.) x Height (ft.) x 4

Heat load (BTU) = Length (m) x Width (m) x Height (m) x 141

For every additional occupant add 500 BTU.

If there are any additional significant sources of heat, for instance floor to ceiling south facing windows, or equipment that produces lots of heat, the above method will underestimate the heat load. In which case the following method should be used instead.

A more accurate heat load calculation for any type of room or building

The heat gain of a room or building depends on:

The size of the area being cooled
The size and position of windows, and whether they have shading
The number of occupants
Heat generated by equipment and machinery
Heat generated by lighting

By calculating the heat gain from each individual item and adding them together, an accurate heat load figure can be determined.

Step One
Calculate the area in square feet of the space to be cooled, and multiply by 31.25

Area BTU = length (ft.) x width (ft.) x 31.25

Step Two
Calculate the heat gain through the windows. If the windows don’t have shading multiply the result by 1.4

North window BTU = Area of North facing windows (m. sq.) x 164

If no shading, North window BTU = North window BTU x 1.4

South window BTU = Area of South facing windows (m. sq.) x 868

If no shading, South window BTU = South window BTU x 1.4

Add the results together.

Total window BTU = North window + South window

Step Three
Calculate the heat generated by occupants, allow 600 BTU per person.

Occupant BTU = number of people x 600

Step Four
Calculate the heat generated by each item of machinery - copiers, computers, ovens etc. Find the power in watts for each item, add them together and multiply by 3.4