Unravelling the mystery of the heat load.

We are often told about building heat loads, simulations etc. but what does this mean? It sounds awfully complicated. Let’s get back to basics and see what’s happening.

The basic calculation for a heat load is:

q = U A ΔT

where

q = heat transfer (Watts)

U = overall heat transfer coefficient (U-value is the Conductance, which is the inverse of an R-value), 1/U-value=R-value

A = Area (m2)

ΔT = (Delta T is the temperature difference between inside and outside)

In other words,

Heat Transfer = Conductance x Area x Temperature difference.

When we do this calculation for the floor, walls & ceiling of a conditioned space and add them together we get the amount of energy in Watts needed to heat the space. The smaller the numbers, the less energy is required. (We have more control over the Conductance and Area than we do over the temperature difference).

In the attached image I have written the U A ΔT formula inside a box, and yes, there’s a reason for this. The box represents an airtight envelope. It’s no good if the air that we are trying to heat inside the house doesn’t stay inside the house. The purpose of the insulation is to keep the warm conditions inside the house but this is very difficult to do if the air constantly escapes. Try keeping your coffee warm with the lid off the thermos.

There you go, That’s a basic explanation. Of course there are factors that affect the temperature difference (Delta T) such as surface type, surface colour, absorptance, solar heat gain etc. but all of these end up as figures that are plugged into the basic U A ΔT formula.

Just remember U A ΔT in a box, and work on keeping the controllable figures small.