Stone makes an ideal surface for heating to be under because of its thermal mass and conductivity. The benefits of underfloor heating are clearly winning the argument in many cases because it continues to take market share from radiators. NSS looks at the latest developments.

There are a lot of advantages to using underfloor heating, not least among them that nice warm feeling you get with every sale. And there is hardly a floor surface more suited to underfloor heating than stone. It is no coincidence that the increased use of natural stone for flooring, in both domestic and commercial settings, is mirrored in the growth of underfloor heating.

Both have benefitted from the other. Natural stone in all its various forms creates beautiful floors. But in the British climate they can be cold and, compared with carpets, will add to the cost of heating a room using radiators, fires or stoves.

Put heating underneath the stone, however, and it is transformed. Its thermal mass now becomes a store of heat that radiates up into the room.

Radiators, in spite of their name, radiate very little heat, working instead largely through convection, which sends hot air up to the ceiling. Underfloor heating (UFH), on the other hand, radiates far more heat up to a height of about 2-2.5m, as well as heating the air and creating convection currents.

John Trenell, who set up his own company called Tailored Heating Supplies in 2011 and has been designing UFH systems for 17 years, likens it to a street on a sunny day, half of which is in shadow and half in the sunlight. If you want the warmth you will walk in the sunlight and if you are too hot you will walk in the shadow. The air temperature on both sides of the street will be just about the same. But the sunny side of the street feels warmer because of the heat radiating from the sun.

That’s the difference underfloor heating makes. Because it is like walking in the sun, the air temperature does not need to be as high for you to feel comfortable, which means the water in the pipes of a UFH system does not need to be as hot as it does for a central heating system using radiators, so you burn less fuel.

If you are using an electrical underfloor heating system, the principle remains the same, with radiated heat meaning the floor does not need to be too hot and the electricity bill is not too high.

And John Trenell should know. He chairs the Technical Committee of the trade association BEAMA Underfloor (previously the Underfloor Heating Manufacturers Association).

Another member of the trade association is Antony White of the company Warmup. He says that the water going into a hot water central heating system with radiators is typically 70-80ºC. It is why radiators are hot to the touch. That will heat a room to about 21ºC, which is the temperature at which people generally feel comfortable.

With an efficient, modern hydronic (water) UFH system in a house complying with the latest energy requirements of the Building Regulations, the water only needs to be 5-6º hotter than the floor temperature required, which does not need to be more than 20ºC, although some people take it higher because they like to feel warmer floors. A solid floor should never reach 30ºC and timber floors should not go above 27ºC.

And the lower the temperature, the greater the energy saving. John Trenell says turning down heating by 1ºC from 21ºC to 20ºC will save 6% of your energy use.

Even on older, less well insulated houses the temperature of water in UFH will be lower than it is with radiators. That’s good for energy consumption and good for the longevity of boilers.

Antony also points out that radiators are normally sited under windows to minimise the amount of lost wall space in a room. The convection currents created by the radiator take hot air straight up to the window. And windows lose a lot of heat compared with walls.

It is because windows lose more heat than walls that most new houses do not have conservatories. When conservatories or orangeries are added later, they often incorporate underfloor heating. Even if they are included in new builds, they are not normally heated. That is because if they used the central heating system of a house that would have to be considered in the calculations for energy efficiency, which would make it difficult to reach the conservation of fuel and power requirements of Part L of the Building Regulations.

Regulations state the U-value for walls should not exceed 0.18W/m²K, whereas for windows it is 1.4W/m²K. Before 2000 the window requirement was 4.8W/m²K and it has gradually been reduced since then, so the windows in most existing houses are nothing like as efficient as 1.4W/m²K.

It is also interesting to note that some people like to heat their houses in winter to levels above ambient summer temperatures. The same applies with air conditioning in the summer. Some people cool rooms to a temperature lower than they would accept in winter.

The weather in the UK tends to make air conditioning unnecessary except for the odd day or two, but it is possible to use UFH to cool a space as well as to heat it. Antony White says it is quite common in Spain and Italy to have underfloor cooling systems using a heat pump in reverse, although it is unusual in the UK. He says when he landscapes his garden he will include underground pipes and a heat exchanger to act as a cooler with his UFH. “A 70kW pump is all you need and you get almost free cooling.”

Those cooling systems that there are in the UK have tended to include £10,000 chillers because the people installing the systems like selling £10,000 chillers.

John Gittins, from heating product manufacturer Maincor, says heat pumps are exceptionally efficient. They work on the same principal as a fridge, only in reverse, and give as much as 3-4kW of heat from each kilowatt of electricity they use to pump water around the system.

But they also cost three or four times as much as a boiler to buy and are bulky, so they are more commonly used where heating bills are high (some churches, hotels and offices have installed them with UFH systems) and for properties that are off-grid, where owners buy heating fuel by the tankful. They are most efficient at 35-40ºC, so do not tend to get used with radiators – at that temperature the radiators would have to be enormous – but make an ideal accompaniment to underfloor heating.

But you do not want to make a floor too cold, Antony White warns, because condensation can start forming on your windows at around 19ºC.

The chairman of BEAMA Underfloor is John Arntsen, of Emmeti UK. For more than 30 years Emmeti has manufactured brassware, but its younger UK division has focussed on developing manifolds for plumbing and heating systems.

In recent years it has also begun supplying other key components for underfloor heating and has developed a comprehensive range of simple yet technically competent products driven by UK market needs.

These products include its EWC Wiring Centres for underfloor and other heating systems, including the new EWC-4 for smaller systems. It comes complete with pump / boiler connections. There are also the T3 UFH Controllers and CS-11 Thermostats.

Couple this with its state-of-the-art pipe manufacturing division and Emmeti likes to think it has the perfect range for UFH.

John Arntsen reckons the biggest advances in UFH have come from the control systems used. Apart from that, he says, underfloor heating is much the same as it always has been, with pipes or wires under the floor heating the surface.

There have been some developments in the pipes used, though. In the very early days, some people used rubber water pipes to carry the hot water, which is not a good idea because rubber will perish.

Pipes used these days are Pex, which is cross linked polyethylene; Pert, which is two layers of Pex with aluminium sandwiched between them, so the pipe stays bent when you bend it rather than trying to straighten out; and PB (polybutylene). Copper is not used, largely because it would require joints and they would be areas of potential failure. The ‘plastic’ pipes are sold on rolls of lengths of 50m upwards in various diameters – from 12mm (or even 10mm lately) for thin systems up to 20mm for commercial projects. Pipes in a screed for a domestic system would normally be 16mm.

John Arntsen does not believe it makes any difference to the finished system which kind of pipes are used. They should all last at least 100 years and the only time they are likely to leak is if somebody drills into them, which can be avoided fairly easily using Flir One, an infra red camera that allows you to see where the pipes are under the floor. (Flir One can be downloaded to your mobile at a cost of about £300.)

The new generation of fast-setting screeds have changed things a bit, as well. Especially anhydrite screeds. Anhydrites offer benefits over sand and cement screeds as they are low cost, faster-drying, pumpable, self-levelling and have minimal shrinkage. They are fine for use with UFH as long as pipes are covered by at least 25mm of screed.

Unlike cured sand and cement screeds, calcium sulphate and sand screeds (anhydrites) will go soft again if they get wet after curing. Before a floor surface is laid on them they should contain less than 0.5% moisture. They would typically reach that level in about seven days compared with 21 days for sand and cement.

Accelerated drying of Calcium Sulphate screeds can be used once the screed is at least seven days old. It is at this point that the UFH should be turned on. According to BS 1264 Part 4, the temperature should be taken up to 20-25ºC and left there for three days, then taken up to a maximum of 55ºC at 5ºC a day, where it should be left for at least four days before being gradually reduced to around 15-20°C. John Trennel goes further and says he would turn the heat off for a day or two to make sure any trapped moisture comes out. Capillary action will bring it to the surface. He says he then does another moisture test and if it is damp, he repeats the cycle. The heating should be on when the floor surface is laid.

Before laying stone or any other finish on to anhydrite screeds it is important to remove laitance. This is a weak layer of fine particles left on the surface of the screed as it cures. It is too weak to tile on to and can prevent the screed from drying. It should be removed by light abrasion, such as with a suitable sanding machine and a 60-grit abrasive.

Screeds need sealing before an appropriate adhesive is applied. You have to take account of the need for expansion joints (as a rule of thumb, bays should not be more than 40m²) and you might like to use a decoupling mat to help cope with movement. “If someone wanted a guaranteed trouble free installation it would help to have a decoupling mat,” says John Trenell.

Whichever make of products you favour for fixing tiles (BAL, Weber, Instarmac, Ardex, or any of the others), the manufacturer will have plenty of advice to offer on the use of their products with anhydrite screeds (or any other screed).

Probably the most common cause of failure of a tiled floor is the result of having left insufficient time for the screed to cure, especially if builders and developers are running a fast track programme.

Another innovation is thin floor systems, where pipes are laid into grooved gypsum or plastic board, such as Maincor’s Overboard or Schlüter’s Bekotec-therm. A decoupling layer such as Schlüter’s Ditramat or Dural’s Durabase, or even 6mm ply, is laid over that and covered with 3-4mm of a floor levelling compound. The stone or ceramic surface is laid on top of that.

John Trenell worked for Tarmac’s concrete and screed division before he moved on to underfloor heating, and has a deep understanding of screeds that he makes full use of when he is designing UFH systems for customers, who are usually trade customers rather than consumers or clients.

Nevertheless, he does not disagree with John Arntsen when he says it is the control systems where the biggest changes have taken place, although even John Arntsen admits that some of the mobile phone app controls are a bit gimmicky. “I got stuck into it because I’m a bit of a technophile but after a month I found you don’t really need it,” he says.

John Trenell pulls out his mobile while he is talking to NSS and checks the heating in his home. He finds one of his daughters has turned up the temperature in her room to 22.5ºC. He uses his phone to turn it back down to 19.5º.

Using the GPS on mobiles means the heating system can detect when you are leaving the office and making your way home, so it can turn up the heating at that point. Warmup has such a system in its 4ie thermostats. They detect when you have left the house and turn the heating down, which saves fuel, and turn it up again when you are returning. It is all part of the Internet of Things. Even Google is involved. In 2014 it bought Nest for $3.2billion with its internet connected thermostats. John Gittins points out that Google was not interested in getting into heating but it does like all the information your household appliances feed back to it about you.

Even without the apps, heating systems have become much more controllable, which adds to their energy saving credentials. In any case, most of the manufacturers agree that UFH systems are best set up with timers and low and high settings and left running continuously, even in the summer, without interference – and that can be achieved pretty easily with dials, let alone minute by minute, room by room touch screens.

The manufacturers say to set the temperature controls to, perhaps, 14ºC when nobody will be at home and about 20ºC when the property will be occupied. As with radiators, heat is only supplied when the temperature falls below the required level, so even if the system is on in the summer, most of the time it will not be burning fuel.

The reason they recommend leaving the system running is because it can look after itself. People who are used to turning their heating off during the day while they are away from their home sometimes baulk at the idea of leaving it on all day, albeit at a lower temperature. But underfloor heating has to warm up a bulk of material, which takes time. The colder that mass is before it starts to be heated, the more energy it will take to warm it.

Remote control by apps on your mobile phone seems to some little more than a gimmick, although if you do not operate to a regular routine, it might be an advantage to have a heating system that uses the GPS on your phone to turn the heating down when you leave the house and on again when you are on your way back, so you don’t have to think about it.

And of course, what today’s generation sees as gimmicks, the next might take for granted and be astonished if it is not available to them.

Today’s controls for UFH detect the temperature more accurately than in the past, with sensors measuring the floor temperature in each room rather than the air temperature for more accurate response. This allows bedrooms, for example, to be kept cooler than bathrooms (if that is desirable). This all helps to add to the 10-15% greater efficiency from a modern condensing boiler that UFH generally claims to deliver. That is not just about lower temperatures entering the system, but also about higher temperatures of water returning to the boiler.

New build apartment blocks and self-builders now incorporate UFH as a matter of course. It is also commonly retrofitted into kitchens, bathrooms, bedrooms and living areas. Kitchen and bathrooms will usually have tiles, although in living areas and bedrooms the covering might well be wood or carpet. With a convincing wood effect (as well as the stone effects) now readily available on ceramic tiles, this more heat efficient and stable alternative to timber is proving popular.

Research commissioned by BEAMA from One Poll in 2016 indicates that 40% of people searching for a new home would prefer to have UFH as their first choice and are prepared to pay for it. Projecting the survey to the population as a whole shows a million householders would pay a premium of £5,000 or more for aspirational measures such as UFH and 7.5million would be prepared to pay £1,000.

With that level of recognition and approval it is no wonder the market for UFH products has a long-term growth rate of about 5% a year, with 70% of 45 to 54 year-olds giving UFH a central role in their aspirations of energy efficiency.

And with UFH probably currently accounting for less than 20% of the heating market, everyone involved in it believes there is plenty of growth still to come.

It is difficult to be precise about the level of penetration of underfloor heating because there is no one product that would indicate with any precision how much of it is being installed. Many electrical systems are for DIY projects and are bought from DIY stores and on the internet, which further blurs the picture.

Even the split between hydronic and electrical systems is not clear. Most people say it is 60:40, but some say hydronic systems have 60% and some say electric systems have 60%. It might be a difference in measurement units, depending on whether it is measured in installations or Sterling. Hydronic systems are more expensive than electrical systems, although they are cheaper to run, with gas at about 4p per kWh and electricity about 14p/kWh.

Warmup, which is one of the biggest suppliers in the market, reckons the UFH industry is growing at an astonishing pace and says the market value is forecast to be around £120million this year, which represents an increase of about 25% compared with 2013.

One of the reasons the manufacturers are confident about a continued growth is because they expect the benefits of underfloor heating, in energy consumption and consumer approval, to enable UFH to continue to win market share from radiators, especially as the price of underfloor installations has fallen considerably – the components are a third of the price they were in 2005, says John Arntsen, thanks to economies of scale.

He admits that the first generation of enthusiasts selling underfloor heating in the 1980s tended to be “propeller heads”. He says: “Most of those guys have either retired or passed away. We’re on the third generation now. The second generation had a degree of idealism but now people are in the market just because they can make money out of it.”

And there are enough of them for competition to keep prices keen. The installed cost of UFH is often comparable with the cost of a radiator system in new build. In the right project, especially apartments, UFH can even be the less expensive option as well as maximising usable space.

Warmup believes tightening legislation on energy efficiency is likely to encourage builders and developers to turn to UFH more as a means of meeting more stringent targets.

Most of the volume house builders are currently testing underfloor heating at some of their sites, although it has not yet been adopted by any of them as a preferred system.

One reason for that is probably inertia. They have developed build programmes using a familiar product portfolio that delivers predictable results they are happy with. David Cameron’s ‘zero carbon’ target would have made the builders look at the products they were using, but when he dropped that target the incentive disappeared.

The volume builders also have to consider other aspects of what they do, as John Gittins points out. For example, heat pumps are so efficient it might seem like a good idea to incorporate them into all new buildings as a matter of course. But if you had, say, 100 new houses occupied by people who tend to get up in the morning at the same time, so they all set their heat pumps to start delivering a day-time temperature at 6am, it might require more energy than the mains could deliver.

Hopefully, the comments above demonstrate that underfloor is not quite as straightforward as it might appear but, as John Trenell says, “preparation is king” and if the floor is laid systematically following the guidelines and regulations available, there should not be any problems. He does, however, add: “With the best will in the world, all we can do is attempt to minimise the risk. Problems happen.” He has developed 35 different floor constructions based around four key themes to try to ensure problems do not happen.

But it was because stone companies did occasionally have problems with floors that Stone Federation Great Britain produced a Code of Practice for flooring (see below), which includes a lot of practical advice on avoiding problems. It is already in its fifth edition and is currently being updated. One of the sections receiving attention is underfloor heating.

The Code is available to Federation members in the trade and to designers and specifiers, who can use it to design stone floors that are secure and do not crack.

For housebuilders, the NHBC Foundation in conjunction with BSRIA has a guide to domestic ‘wet’ systems, available as a PDF from bit.ly/NHBC-underfloor. It is specifically written for builders and designers who are less experienced in underfloor heating but are considering using it.

And there is a lot of advice on the BEAMA Underfloor website, as well a list of its members, who manufacture the UFH products for both hydronic and electrical systems. There are pages for homeowners and consumers, installers and contractors and architects and specifiers. The home page is at bit.ly/BEAMA-underfloor.

Stone Federation Great Britain is currently working on the sixth edition of its Natural Stone Flooring Code of Practice. This addresses issues specifically related to the use of stone for flooring, covering all kinds of screeds, even those using lime mortar, underfloor heating and expansion jointing. It references British Standards and CE marking. It is the most widely used of the Federation’s publications among its members – and, in the trade, it is only available to Federation members, although the Federation does also encourage architects and designers to use it when they want a stone floor. Doing so helps avoid some of the problems that usually get blamed on the stone or, if it is used, the underfloor heating. They are almost never the problem. When used properly, both stone and underfloor heating will perform perfectly well for decades. When there are problems, it is nearly always the result of a design or construction miscalculation.

The current edition of the Natural Stone Flooring Code of Practice, pictured here, is 68 pages of A4 size. www.stonefed.org.uk.