Last edited 13 Sep 2019

#  Introduction

Heat load (or heating load) is a term that can be used in several ways when dealing with building physics.

#  Required heating

It can be used to refer to the quantity of heat per unit of time (usually over an hour) that is required to heat a given space in order to maintain it at a given temperature. In poorly insulated buildings, the heat load will be greater than in thermally efficient buildings. In contrast, in a building with a very high level of thermal efficiency, the heating demand can be practically negligible. In Passive houses, this is around 15kWh/(m2a)) which is roughly 10% of the energy used in conventional buildings.

#  Required cooling capacity

The term heat load can also refer to a calculated thermal quantity used to establish the capacity of a cooling system to enable it to maintain the temperature below a required level in a building or space. To do this, it is necessary to account for all potential heat-producing activities (heat sources), including solar radiation, people, machines, lighting, kitchens, computers, and so on within that building or space.

For example, a data centre housing computers and servers will produce a certain heat load that derives from an electrical load. This heat load will have to be absorbed and conveyed to the exterior by the buildingâ€™s cooling system. Once the heat load is quantified, HVAC engineers can design the necessary cooling system to ensure it can effectively keep the space at the desired temperature.

A rough and ready method for calculating heat load in offices containing 2-3 workers and 3-4 computers is given by the following formula:

• Heat load (BTU) = Length (m) x Width (m) x Height (m) x 141
• So, for a room measuring 5m x 4m x 3m = 60 > x 141 = 8,460 BTU.
• (For measurements in feet, the formula becomes:
• Heat load (BTU) = Length (m) x Width (m) x Height (m) x 4)

Where there are more occupants, add 500 BTU for every additional person:

So, if four extra occupants arrive, the heat load will be:

• 8,460 + (500 x 4) = 10,460 BTU.

Heat load (and heat gain) can also be expressed in kilowatts (kW).

• To convert BTU to kW, 1 BTU = 0.00029307107 kW.
• So, from the example above, 10,460 BTU = 3.065 kW.

The method described above can provide an outline idea of the heat load. More detailed methods should be used to achieve greater accuracy.

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