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How does underfloor heating work? Your questions answered

Posted by NicoleYouGen on 24 May 2013 at 9:52 am

What is underfloor heating?

Underfloor heating can be either a system that pumps warm water through piping under the floor (a ‘wet’ system) or electric coils placed under the floor (a ‘dry’ system). Taking advantage of the basic principle that heat rises, warmth from under the floor is radiated into the room. 

Underfloor heating is the oldest form of central heating – the Romans used a form of underfloor heating called hypocausts, which heated buildings and baths. Today’s form of underfloor heating is common in Northern Europe, and gaining popularity in the UK. 

Will underfloor heating save energy?

This depends on how well insulated the building is, the efficiency of the boiler, and what kind of heating needs your system will be meeting. 

High occupany commericlal buildings are more likely to see efficiency gains from underfloor heating because it is most effective for periods of continual occpupancy, and because for larger heating needs the capital costs of an underfloor heating system become relatively smaller. 

According to the Energy Saving Trust:

  • In an existing home with average insulation, for an air to water heat pump system, an underfloor heating under insulated timber will save 20% of energy costs over a radiator system (£500/year vs £400 average).

  • In an existing home with average insulation, with a gas or oil-fired boiler, the energy savings are smaller (about £10/year average). 

  • In new and well-insulated homes, underfloor heating with a heat pump will save (£190 vs £230 average) over radiators

  • In a new and well-insulated homes, underfloorfloor heating will save negligably over a radiator system. 

How much does it cost? 

A wet underfloor heating system will cost about a third more than a radiator system, on average. This includes the fact that more insulation is likely to be added below the pipework. If the system is installed at the time of building or renovation, its installation costs will be minimized.

Electric, or ‘dry’ underfloor heating generally costs less to install, but is more expensive to run. Dual storage underfloor heaters involve two sets of pipes and extra thick screed, so they can gain most of their running energy from cheaper nighttime tariffs. This is not a cost-effective form of heating. 

Is underfloor heating suitable for my building? 

A wet underfloor heating system is best suited to buildings that are well insulated. Underfloor systems operate at a lower temperature than most radiator systems (35-45 degrees C). This makes underfloor heating ideal for heating from heat pumps. As a low temperature form of heating, underfloor heating is most effective when kept on continuously. It is slower to respond than traditional radiators, so is not suitable for intermittent heating. Some homes have underfloor heating in a main ground floor room and smaller intermitent heaters in other rooms. 

Commercial buildings might see more efficiency gains than homes, because of having a larger volume of continual occupancy. 

What is it like to live with underfloor heating? 

Underfloor heating is not very responsive, so after you turn it on, you will have to wait longer to feel warm. It’s most effectively used where it can be left on continuously. Electric systems are more responsive, but are more expensive to run.  

Underfloor heating delivers a more consistent temperature, with heat distributed more from the bottom of the room. Whereas radiators will send warm air through the room, but less at the bottom of the room, the underfloor heating will send an even warmth, and will heat more at the bottom and top of the room than in the middle. 

Not having radiators gives you more space. But you will need to consider that furniture items such as cupboards and wardrobes should not sit above the heat coils, as they can be damaged. Bathrooms are harder to heat because the bath takes up a lot of floor space, but if you have a claw-foot tub this will be less of an issue. 

Is underfloor heating suitable for a retrofit?

It is possible to retrofit underfloor heating. You will probably have to raise the floor (and adjust doors accordingly). Concrete floors are generally harder to retrofit, and suspended timber floors tend to be easier to retrofit. Electric systems are usually thinner and easier to install in a retrofit where there isn’t space to raise the floor. 

Will I need a new boiler? 

For a gas powered system, it’s best to have a condensing boiler, which is most effective for low-temperature heating such as underfloor heating. Most new boilers are condensing. Underfloor heating can also be connected to a thermal store, and works well with heat pumps. 

What type of flooring can I install with underfloor heating? 

Underfloor heating is commonly installed with stone, ceramic or terracotta tile floors, but can also be installed with wood, linoleum or carpet.

When installing with wood, you will need to keep in mind that wood will natural expand and contract with changes in heat and moisture more than stone or ceramic tiles will. This is commonly addressed by floating wood boards on a layer of polythene foam that allows subtle movement. Depending on the type of wood floor, it can also be a less effective heat conductor. That said, timber floors are more responsive, so can be more ideal where there are more sporadic heating needs. 

Carpet insulates, which is good for stopping heat loss from the floor, but bad for letting heat from underfloor heating in. But underfloor heating can work effectively with carpet. You need to know the combined tog value of the carpet and underlay, which should be a maximum of 2.5 and ideally lower.  

Do I need a professional to install underfloor heating? 

For a wet underfloor heating system, unless you’re very handy with DIY, you will probably want a professional with experience to do the installation. You can expect to pay more for the installation than you would for new radiators. 

Installation should take a couple of days, depending on the floor space, but screed can take up to three weeks to dry. 

Do I have to install the system with screed? 

Wet underfloor heating systems are commonly installed with a layer of screed on top to create a level surface which the floor is laid onto. Suspended floors can have underfloor heating added without screed. And even without suspended floors, it is possible to install without screed (a ‘dry’ installation, not to be confused with a ‘dry’, or electric, system) using screed replacement tiles, which are fixed together in order to make a level, floating floor. This might be preferable if you want to avoid brininging extra moisture into the building, and also means less wait time before you can walk on the floor. 

More information:

Why heat pumps are ideal for underfloor heating

Heating and hot water information page

Can you retrofit underfloor heating?

Find an installer

Photo credit: Image 1: Irish Typepad

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Comments

9 comments - read them below or add one

roberta98

roberta98Comment left on: 2 March 2016 at 2:37 pm

Great article to read. I have recently had  underfloor heating instaled in my kitchen, livingroom and bathroom. It is expensive to run but i think it would benefit in the future years coming as it is more eficent than radiators.

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paul53

paul53Comment left on: 3 December 2015 at 6:06 pm

if you cant  feel heat  after  a  few days get a  electrician to  check theres  power  at  the  connection point to  the  mat

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mikeyt

mikeytComment left on: 3 December 2015 at 1:00 pm

I recently had an extension built and agreed to install electric mat underfloor heating to the extension and back room. The extension heating works fine, tiles are lovely to walk on.

The heating in the back room however, only worked very briefly. I had the radiator removed. The original laminate boards were replaced with porcelain tiles laid over floor boards with the heating mat in between. The thermostat is currently at max but the tiles are freezing. The builder came back and assured me that there was some heat, not that I felt it, but because it was over a ground floor with a space the heat was dissipating below. I’m no expert but i’m not convinced. Should i feel an appreciable increase in the floor temperature? What should i do next?

Thanks

Mike

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Tasha Kosviner

Tasha KosvinerComment left on: 23 September 2014 at 7:06 pm

Carol Elizabeth

Thanks for your question. It has prompted me to write a blog. I'll post a link back here when it goes live - hopefully next week. 

Tasha

 

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Carol Elizabeth

Carol ElizabethComment left on: 15 September 2014 at 9:06 am

I have just had a wet system installed in my new extension which has a double range of 2x5 bifold doors. I am completely ignorant about how the heating worksand how it is activated from my boiler. My builder has set it and sent me a link for the instructions on how to use it but he did not answer my basic question. With ordinary radiators you have to have your heating switched on and then the radiators come on - for most of us twice a day. Most of  the house works on that basis. However, my builder says the new underfloor heating is better left on using the thermostat. At the moment I don't have any heating on so I am not sure whether the underfloor is on or not - it is showing a consistent temperature but the weather is mild anyway. So question is do I need to switch my heating on all the time to get the underfloor to work and how do I balance that with my radiator situation - I like a cool bedroom at night and frankly don't want heat at night.  Please explain for a complete novice.

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Stuart Young

Stuart YoungComment left on: 20 January 2014 at 2:56 pm

Excellent article. I have electric underfloor heating in my kitchen and conservatory. The running costs were reasonably high at first, partly due to insufficient insulation under my concrete kitchen floor. But once I contacted the manufacturer (Warmup) they helped me optimize the thermostat settings to get maximum heating for minimum cost. It's total luxury - we get to use our conservatory ALL year round now instead of just the summer.

I think they do wet and dry systems as the wet type are ideal for new build and certain renovations. Take a look here: www.warmup.co.uk

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Anthony525

Anthony525Comment left on: 4 June 2013 at 5:27 pm

Underfloor heating saves money because there is less heat to lose from the pipes delivering the heat from the boiler. If you want to venture into underfloor heating then it is probably best to add it to your existing system.

An air to water heat pump will add a thousand UK£ or so to the installation costs - pointless, unless you intend to live there for thirty years or more. A ground to water installation will never pay for itself.

Keep in mind that underfloor heating using a concrete slab will cost more to run, if you intend to turn it off, or down. Concrete is very good at absorbing heat and you will have to heat the concrete slab before any heat enters your room. Concrete is also very slow to cool. If you have large windows facing the sun, the sudden heat build, when the sun comes out can be impossible to live with, a sudden shift of temperature into the 50's centigrade can leave you unable to breath or stay in the room, without opening the windows and doors.

A light weight underfloor heating system, using polystyrene molded sheets that provide grooves for the pex pipe with four inches of insulation below and a fully 

floating wood floor above is easier to live with, quicker to heat and quicker to cool. The wood surface absorbs little heat. 

Any heating system responds better to a well insulated building, underfloor heating is no different - a heat pump just makes it very expensive.

Underfloor heating is a joy to live with, its a turn it on and forget it - thing! As its a low level heat there is no problem in running it under furniture. It is perfect where you have ground to ceiling windows, you can double up on the piping in front of the windows, you can have different zones, making sure the floor is warmer where you want it to be.

In the bathroom, you can make sure the floor of the shower is always warm, diitto the bath, it is good under the toilet pan and cistern keeping them warm and stopping condensation.

Lots of people fit underfloor heating, it needs a reasonable skill level. It can be fitted on top of existing floors, and between joists, keeping existing floor levels.

All new boilers are condensing. With a suitable thermostat, the underfloor heating will continue to push out heat, warming the floor, regardless of covering, until the desired heat level is reached.

If you go for a concrete slab, it is best to have it installed with steel reinforcing mesh, the pex pipe can be tied to this to your desired patten,  you will have to give the slab time to dry before heating the floor.

Concrete dries at 1mm per day at 20 centigrade, a typical 4 inch /100mm slab will take 100 days to dry. 

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Cathy Debenham

Cathy DebenhamComment left on: 24 May 2013 at 1:14 pm

@utility-exchange Pleased to be of use!

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utility-exchange

utility-exchangeComment left on: 24 May 2013 at 10:40 am

Really interesting - thank you! I didn't realise there were wet and dry systems - very comprehensive guide.

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