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 building’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.