28 Jul 2005

Radiator Sizing

Jonathan Ackers asks:

"Is the latest building reg compliant house really that much warmer than, say, a 30s-built house? I was chatting to my plumber mate about how many radiators we'd need and he uses a rule of thumb method acquired years ago, but surely this has changed significantly in our new super-insulated world !?"

Mark reckons:

If you carry out specific heat loss calculations on pre-insulated housing and on housing built to current standards, the theoretical results show that the overall space heating load has been reduced by around two-thirds. A 1930s semi requires around 100 watts per m2 to keep temperature 20°C higher than outside: a house built to 2002 standards should require about 35 watts per m2.

That doesn’t have any bearing on the number of radiators but it should reduce their sizing considerably. In theory, you could reduce radiator sizes by the same factor (ie two thirds), but running counter to this is the uptake in condensing boilers which like to work at lower temperatures. Whereas traditional radiator based systems are were designed to work at a 50°C temperature difference (between the flow temperature and the room temperature), condensers work better at a 40°C difference, which reduces temperature output from radiators by 25%. Taking this into account, an efficiently-designed modern radiator system should still work fine with radiators half the traditional size.

However, plumbers run the risk of being sued if the system doesn’t deliver enough heat and therefore caution is the watchword. Whilst people won’t notice radiators which are oversized, they will complain like stink if they think they are undersized, so many plumbers continue to use traditional rule of thumb calcs for all the homes they work in. After all, how does the plumber know whether a house has really been built to 2002 standards?


  1. I calculated that my 300m2 house requires 96,000 radiator btu.

    Following the logic of reducing radiator sizes, can i also reduce my system boiler size to 46,000 btu output (seems very small to me) considering the size of the house.

    i also will be installing a megaflo 250 litre unvented hot water cylinder.

    Any help on this is gratly appreciated as i do not want to oversize the system and want to make the most of the energy efficiency of the house.

    Many thanks


  2. I am not altogether sure what logic you are employing for your radiator sizing, but these days space heating loads are so small that the limiting factor on boiler sizing tends to be recovery rate of hot water. A 250lt Megalflo will probably have a heat exchanger of 24kW, so ideally you need a boiler this size or slightly larger.