Underfloor Heating. Right or Wrong?

Some of my customers really like underfloor heating but I, as an energy assessor, always have a slight feeling of panic whenever I see it. Why is this? Underfloor heating avoids the need for bulky radiators on the wall and allows an even distribution of heat instead of finding the cat cuddled up under the radiator at night because it is looking for somewhere very warm. From the point of view of creating a house that is comfortable to live in, underfloor heating is wonderful. But from an engineering point of view there are some problems.

The first underfloor heating systems were horribly inefficient. 40 years ago my grandmother had electric underfloor mats in her flat and always worried about the electricity bill. The main problem with an electric heating system is that electricity has to be generated, probably in a coal fired plant which puts loads of carbon dioxide into the air and makes electricity which is very expensive. Nowadays a lot of underfloor heating is gas fired and this is more efficient than electric. The main problem with gas fired underfloor heating is having pipes full of hot water in a concrete floor. A typical concrete floor is 15 inches thick and consists of steel reinforced concrete and hardcore broken brick. This absorbs an enormous amount of heat and this would be wasted. Manufacturers produced underfloor heating systems with pipes in the screed – the last 2 inches of material on top of the concrete floor – and the insulation was put under the screed to separate it from the thicker layers. This has improved the efficiency considerably as the heat is only being lost to a relatively thin layer of screed and not to the whole floor underneath it. There is still the problem of the screed absorbing and wasting some of the heat and in a normal central heating system radiators are slightly better. It is possible to have the hot water pipes over the top of the concrete and under the floor boards. This eliminates the problem and gives you the same efficiency as a radiator.

Another improvement was the use of low temperature underfloor heating. If the water temperature is below 35C it takes less energy to heat it up. Normal radiators run at about 55C. You can have radiators that run at lower temperatures than this, they’re called low temperature radiators, but even they usually run at temperatures of about 45C. If you made a low temperature radiator to run at 35C it would have to be enormous. But the floor is the biggest radiator you could ever get. At low temperatures the screed will still absorb and waste some of the heat but a much greater amount of heat will be saved by having a central heating system that runs at 35C.  Imagine if instead of having to boil your kettle to make a cup of tea you could turn it off at a temperature of 35C – it would take no time at all to heat up. So at low temperatures underfloor heating is more efficient than a radiator.

There are other clever things you can do with underfloor heating. The screed doesn’t actually lose the heat that it absorbs, it stores it. The floor takes a long time to heat up but it takes a long time to cool down again. An underfloor heating system in a concrete floor is a very good storage heater. Using a high efficiency electric heat pump you can turn it on at night and benefit from off peak electricity. Another recent development is the use of electric underfloor heating mats attached to a PV solar panel. This is another way of answering the question of what do I do with all the electricity my solar panels generate while I’m at work? One good answer would be to position heat mats on a concrete floor and make a solar powered electric storage heater.