Underfloor heating

Have you considered underfloor heating? Installation is quite disruptive and it is not especially cheap but there are huge advantages, both for sustainability and for comfort. Here is some advice, including experiences from friends who have taken the plunge: Nicole installed her house with UF heating as part of a comprehensive retrofit 5 years ago. Margaret installed it on the ground floor in their house 2 years ago. Mike installed it 10 years ago in his kitchen.

What makes underfloor heating sustainable?

It can run at a low temperature which is great for running heat pumps efficiently. This means you can run your heating system on low carbon electricity at a reasonable price, while minimising the extra load on the electricity grid.

What is underfloor heating?

There are actually two kinds of underfloor heating: electric and wet systems. The wet system is quite like the radiators we have with a gas or oil boiler except that water flows through pipes under the surface of the floor instead of through tubes inside a radiator on the wall. The pipes loop around the whole floor, providing a gentle, even heat. The pipe layer is protected by a very sturdy layer that can take big loads, and the top finish is usually tiles or sometimes wood. Carpet is also possible.

The floor itself is comfortably warm but not hot. It should be 25-28°C. If you walk with bare feet you will find the floor comfortably warm – even with ceramic tiles.

In an electric system, the heat comes from wires which may or may not be embedded in a mat (see picture). In other respects it is similar, except for running cost – electric systems are much more expensive. This is because electricity is relatively expensive and with direct heating from wires it can be at best 100% efficient - a heat pump can be 300% efficient or more. The rest of this article is about wet systems.

An electric heat map from HeatMat underfloor heating

Both systems are normally controlled with thermostats, as with radiators with TRVs.

There are actually other kinds of underfloor heating for example from warm air ducts, a bit like the Roman hypocaust system.

Underfloor heating tubes run under the whole floor. In this case (a kitchen) they were embedded in concrete screed and the top layer was linoleum.

What is so good about underfloor heating?

There are a variety of benefits to underfloor heating, which probably appeal to different people.

  • The floor is comfortably warm which is nice for your feet – and nice for children to play on. Also in the summer it is cool because of the high thermal mass.
  • There is no need for radiators on the wall – this means you do not have to worry about keeping them clear of furniture and you can put prettier things on the wall.
  • It is an enabling technology for decarbonising your heating with an electric heat pump. It depends on your house and radiator system but with underfloor heating you might expect 400% efficiency from your heat pump and only 300% without. This is because the large surface means that it is possible to run the heating at a much lower temperature than with radiators – 35C is not uncommon.

What are the disadvantages?

Your choices for floor finishes are more limited (see below). Partly this is because they need to conduct heat reasonably well – thick carpet does not!. Also the warmth can damage some things you might put on the floor. For example if you have a leather bean bag, the heat will dry out the leather.

Underfloor heating usually has a high thermal mass and hence takes a while to warm up. This is not necessarily a disadvantage and to some extent it can be adjusted in the design. Nicole says hers takes about 30-45minutes. However, if you come in on a cold day you won’t be able to warm your hands quickly on it, at least not as quickly as you would with a hot radiator.

Will it reduce my energy bills?

The cost to run underfloor heating is more down to the way you heat the water than the underfloor system. You can run it from a conventional boiler or from an electric heat pump, or use water from a thermal store that has been heated some other way. You are unlikely to see much difference in your bill compared to a gas system with radiators, unless you were overheating before and you install better heating controls. Underfloor heating is often installed with a zone per room so you can heat each area individually and with the right controls they can all have different timings too.

How does it work – general principles?

The heat comes from hot water from your boiler or heat pump, just as with conventional radiators. The whole floor is heated evenly (as far as possible). this means you need a conducting layer to spread the heat between the pipes, which are typically 20cm apart. You must have excellent insulation underneath, as you do not want to heat the ground below, or even the ceiling if this is an upper room. You need to direct the heat up into the room. From the top down you need:

  1. floor finish (conductive and heat resistant)
  2. heating pipes embedded in a sturdy conductive material, often a concrete screed
  3. insulation

The middle layer can have more or less thermal mass and you can arrange for the mass to be mainly above or mainly below depending on how responsive you want the system to be.

The warmth from the floor warms the air above and spreads by convection through the room. This is similar to wall radiators too. However, since the whole floor is warm it will tend to heat the room more evenly.

The benefits are greater if the house is well insulated in general – as Nicole says, – there is not much point having warm floors if you have leaky windows/cold walls.

What floor coverings can I have?

You can use anything which is reasonably conductive and tolerant of the heat. Ceramic tiles are common. They will not feel cold, (but they are still good at breaking things you drop on them). Lino tiles are also possible; also thin carpet or rugs can be used. Wood can be used if it is very well seasoned or engineered so that is can resist the heat without warping.

How do I retrofit – solid floors?

If you want to keep the existing floor level you will need to dig down to get more room. Our friend Mike dug down 300mm to accommodate:

  1. Marmoleum (lino) tiles (but other top layers are possible, see above)
  2. 100mm top screed around and above the heating pipes
  3. Uponor flexible heating pipes, clipped through to the EPS to hold them in place.
  4. heavy duty polythene membrane (blue)
  5. 150mm expanded polystyrene insulation (EPS)
  6. 50m screed to provide a level surface

He fitted this system to the whole floor, including under the cupboards etc around the edge. This was more expensive to do and disruptive – but it means in future the kitchen layout can be changed without exposing unheated floor.

Mike finds the new floor comfortably cool in summer as well as warm in winter.

Underfloor heating covered by Marmoleum in a kitchen.

Engineered oak flooring

How do I retrofit – suspended floors?

The principles are the same as with solid floors however, as with any insulation on suspended floors it is important to protect the joists from potential moisture which can lead to rot. The main source of moisture is the room - if warm, moist air from the room gets down to the joists and gets cold, the moisture can condense on the wood and make it wet. Ventilation from under the floor will help to keep the joists dry but it is strongly recommended to add a breathable membrane between the joists and the room. You can also use a wood preservative treatment on the joists.

Margaret describes what she did here: The first step was to clear rubble from the void and put in a lower damp course into the walls, because the new floor joists are deeper than the old ones and now the void under has a concrete base. We covered the floor of the old void with a damp proof membrane. We also checked all the joists and applied applied preservative, to the joists. Then we added 80mm insulation between the joists and an air-tight foil vapour barrier over this and behind the skirting board. The heating coils (20 mm) were laid in trays over this with a fibre cement board planks (22mm) above for thermal mass (Hardie floor) replacing standard timber floorboards. The top layer is different in different rooms: engineered wood or ceramic tiles, with some rugs on top. There are access hatches through to the floor below for access to the wiring.

The main layers are:

  1. Engineered wood/tiles
  2. Fibre cement boards (22mm) for thermal mass
  3. Heating cols in trays (20mm)
  4. Air tight vapour barrier (to protect the wood beneath from moisture in the room)
  5. Joists with 80mm insulation between

Here you can see the foil over the insulation between joists, and the tubes in trays above.

Nicole did it slightly differently with the insulation above the breathable membrane but the membrane is still between the joists and the room above. The layers are:

  1. 14mm engineered wood floor (4mm solid oak and 2 layers of softwood backing) in most rooms, tiles in the kitchen and shower room
  2. 12mm plywood (?)
  3. UFH pipes system (200mm between centres)
  4. insulation between joists – 150mm thermafleece
  5. Breathable membrane:

The insulation is deeper than the joists – the installers created a hammock-like structure within a breathable membrane.

What control systems should I have?

Ideally you need a system that can manage zones so you can heat each room independently. Since it takes a while to warm up timers are essential.

Installation is easier if you use wireless thermostats, otherwise you will need to chase wires through plaster as well as taking up the floors. However, wireless thermstats use batteries and you will need to replace or recharge these regularly.

HeatMiser seems to be a popular control system among our friends.

What temperature does it run at?

The floor should not be warmer than about 28C and the heating water needs to be warmer than that - underfloor heating systems can run with flow down to 35C or even lower. With a lower temperature system the room will take longer to warm up and your choice of floor covering is more critical – more insulating materials will tend to reduce the temperature at the top surface. However the lower temperature flow is better for getting excellent efficiency from a heat pump.

Some gas boilers cannot generate the low temperatures required directly but this is not a problem. Your system will incorporate a tempering/blending valve: this mixes colder water from the underfloor heating system return with hot water from the heat source to supply the correct temperature of water to the underfloor pipework.

The manifold is where the hot water is directed through each loop. The blending valve is the large black object.

What maintenance does it need?

It should not need any maintenance at all, but problems can arise.

Mike had some teething problems with his system: it seems that when the water heating came on in the summer there was a noticeable flow from the main circuit into the underfloor circuit! They had to add an electric shutoff valve to sort this out.

Nicole says she has a problem in one room where the heating was very rarely used and now seems to have stopped altogether. This is after 5 years. She also had a problem with the boiler which was leaking but that is probably an entirely separate issue.

The pipes themselves are hard to access if they are embedded in screed but if the installation has been done carefully they should have no problems. You need to make sure you have access to all wiring and valves. Margaret put in hatches for access to the underfloor void in her suspended floor.

You may have trouble getting a service agreement that covers both your boiler and the underfloor heating system. Margaret had a contract with British Gas but they have refused to include the UF parts.

Viewing with Thermal Imaging

Accessing your underfloor heating directly usually means lifting the floor but you can see what is going on without doing this, using an infrared camera. This is a good way to check that heat is getting to the whole floor. If there are blockages in the pipework this is usually obvious. Here is a bit of Margaret's floor, showing warm stripes. The fibre cement board layer helps to distribute the warmth so the stripes are wider than the pipes and not as clearly delineated as you sometimes see.

The surface temperature is a little below 26°C, as recommended for wood floors. Underfloor heating guide to surface temperatures. (The water in the pipes will be hotter.)

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