How much Heat do you Need?

How do you know how much heat your home actually needs?
We are all used to seeing boilers with sizes from anywhere from 24kW way up into the 30s. The reality is though, most homes don’t actually need that
much power to heat them. In fact they hardly need that amount at all.

Combination boilers do benefit from being powerful (on the hot water side) as they are heating the hot water instantaneously but conventional
boilers, which work with cylinders, seem to be way too big for the job. As to why boiler sizes are too big, that’s another story. Arguably, the boiler
industry has lost its way a bit and lots of questions are now being asked of them such as: why are they advising consumers and engineers to have the
flow temp at 75 degrees as this is far too high for the boiler to condense properly.

Despite the large boiler sizes and how homes seem to be constantly described as ‘leaky’ many homes only have a heat loss of around 8kW at design temp. Design temp is the average coldest day of the year (not the average winter temperature) and is around minus 3 degrees Celsius.

What does that actually mean?

Well, it means when there is an external temperature of minus 3, 8kW of power is needed to maintain an internal room temp of 21 degrees Celsius. 8kW (8000 Watts) is another way of saying 8000 Joules of heat energy is needed per second (1 watt = 1 joule per second) to move into the room from the radiators as that is what is moving through the fabric of the home to the outside.

So how do we know what the home needs? Does it need 7.4 kW when it is minus 3 or does it need 12kW? Who decides?

Well, it’s normally the heating engineer who decides. Traditionally it is calculated. An engineer will measure the home’s wall area, window area, thickness of the walls and then make some decisions around the fabric type the home has been built with. And this is where the problem lies.

It is very hard for the engineer to actually know what materials have been used in the construction e.g. were hollow concrete building blocks
used, or high density ones. Another problem is it is sheer guesswork as to how well the cavity insulation has been installed or the windows.

Windows can often have gaps around them which are covered. This all has a massive effect on how quick heat will move through the home.
Although some things have been measured, like the wall and window area, this method does not measure actual speed of heat loss, it is just
trying to calculate what it is expected to be and is often way out.

There are some methods which will actually measure heat loss which we will discuss in detail next month.