Underfloor Heating Explained

How Does UFH Work?

Underfloor heating (UFH) works by turning the entire floor area of each room into a large, low surface temperature radiator. Once the floor’s temperature rises above that of the surrounding air, heat is emitted evenly from across its surface, leaving no cold spots and creating minimal draughts.

Because the surface area of the floor is large, it does not have to be heated to the same high temperature as a radiator in order to emit suffi­cient heat to make the room comfortable.

Heat is delivered into the floor by incorpor­at­ing either pipe coils carrying a flow of warm water, or electric heating elements, within the floor structure. Most people find that UFH is a very comfortable way to heat their home. The floor temperature is pleasant under­foot — only 2-3°C warmer than the desired room temperature and never hot.

Is UFH More Expensive to Install than Radiators?

Few UFH suppliers would deny that UFH is almost always going to be more expensive to install than a standard radiator central heating system. UFH requires more materials and involves more labour — although the latter will be less of a factor for those installing the system themselves.

Estimates for the installed cost of a radiator central heating system (all costs exclude the boiler) are around £12-18/m², compared to £18-28/m² for warm water UFH laid in screed, and £14-28/m² for an electric UFH system in screed.

These comparisons assume standard pressed steel radiators are used — opt for more elegant features, such as column radia­tors, different designs or radiator covers, and the price gap between the two systems narrows substantially.

A popular compromise for those who want the benefits of UFH but are constrained by their budget is to install UFH on the ground floor, whilst opting for a basic radiator system on upper floors, possibly with UFH in the bathrooms. As radiators warm up and cool down faster than UFH, they are arguably a more appropriate form of heat emitter for bedrooms, where heat is usually only required for a 2-3 hour period in the morning and evening.

Is UFH More Energy Efficient?

The operating costs for UFH will depend primarily on which fuel is used, the prevailing price of that fuel and the energy efficiency of the system — particularly the efficiency of the boiler and the level of insulation in the building. These factors are more significant in terms of running cost than any differences in efficiency between UFH and other forms of heating.

People installing UFH typically put additional insulation in the floor to increase its effective­ness – it is essential that the floor is well insulated for UFH to give an effective output – and as a result this will help reduce running costs. The cost of this additional insulation is also one of the reasons why UFH is slightly more expensive to install.

Who Should Design the System?

If UFH is to work effectively, it must be designed to take into account the heat require­ment of each room and match this with the output. Good controls that deliver the desired room temperatures when required, taking into account the time UFH takes to warm up and slow down, are also essential.

The heat requirement of a room will depend on its size and its heat loss characteristics, calculated using the elemental method — individual calculations for heat loss through walls, ceilings, windows and floor. Dividing the total heat loss by the floor area will give you the heat requirement in W/m².

A competent plumber or DIYer can design a UFH system and some of the component manufacturers offer manuals and computer packages to assist them. However, UFH spe­cial­ists would argue that experience is neces­sary to optimise the system for each house design and that mistakes are extremely costly to rectify.

Most specialist suppliers offer a free design service based on floorplans, showing each zone, manifold positions, pipe lengths, spacing, wiring plan etc. To be fully comprehen­sive, the plans must take into account the type of boiler being specified, as this may affect the design of the system and the wiring plan. Some boilers have little more than an on/off switch, whilst others have an integral micro-processor and programmable controls.

Who Should Install the System?

Several of the leading UFH system suppliers offer an installation service and they can quote for this on request. Some suppliers have their own installers but most have a network of approved installers who are self-employed. Check who is responsible for the quality of the work and who, if anyone, is offering a guaran­tee should anything need altering.

An increasing number of plumbers are will­ing to install warm water UFH but it is impor­tant to check that they have experience or are prepared to attend a training course.

Installing the UFH pipes up to the manifold is not a particularly skilled trade and can easily be picked up by a competent DIYer.

Plumbing from the manifold to the boiler requires more advanced plumbing skills, although some UFH suppliers now offer pre-assembled ‘plug and play’ manifolds of various sizes, with mixer valve and pump already attached, making installation faster. Electric UFH is generally considered to be easier to install on a DIY basis. Some small UFH mat systems for use beneath tiles in bathrooms simply plug into the mains.

How Controllable is UFH?

Few suppliers would deny that UFH is less responsive than radiator central heating. It will take an in-screed UFH system 1-2 hours to heat a room from cold, compared to a few minutes for a radiator system. Equally, if heat­ing is no longer required, for instance because the sun has come out, a room heated by radiators will cool down more rapidly, whilst it will take the dense thermal mass of the floor several hours. As a result, more sophisticated controls are required for UFH than for a radia­tor system.

The inherent lack of responsiveness of UFH can be overcome by running the system over a whole 24 hour cycle rather than turning the system on or off to meet heat demand. When less output is required, the system can be programmed to run on a minimum set-back temperature of 16°C, so that the system does not have to start from cold, e.g. in the morning.

The responsiveness, comfort level and energy efficiency of UFH can be further improved by adding weather compensation. This enables the system to anticipate changes in heat requirement according to the weather conditions, as measured by an outdoor sensor, long before the house gets too hot or cold.

Which is Best — Electric UFH or Warm Water?

UFH can be powered either by warm water running through pipes or electric heating cables laid within the floor structure. UFH can be used to heat the whole house, or alterna­tively just to heat certain rooms — for instance only the ground floor or just a conservatory, with the rest of the house heated by radiators. UFH can also be used to provide warming in small areas, for instance to take the chill off tiles in bathrooms. Electric UFH is generally considered to be slightly cheaper and faster to install than warm water UFH, although it is likely to be slightly more expensive to run. Because some forms of electric UFH are very thin, it really comes into its own in some retrofit installations.

Which Type of Pipe Should I Choose?

There are three main types of pipe in use:

Cross-linked polyethylene (PEX, XPE, PE-X) with an oxygen barrier: Known as PEX, this type of pipe is ideal for UFH. It is flexible and pliable, making it easy to lay, although it tends to spring back into its coiled shape and therefore requires fixing down using clips, rail or fasteners. Available in rolls up to 960m, there is no need to bury any joints in the floor. It is corrosion-resistant and able to withstand high temperatures and pressures simultaneously.

The oxygen barrier prevents the build-up of corrosion, created by the reaction of oxygen with metal parts in the boiler and pumps. Corrosion sludge could reduce the system’s effectiveness by blocking up the heating coils.

There is also polyethylene pipe available that is not cross-linked, known as PE. The pipe wall has to be considerably thicker to achieve adequate strength but it is also perfectly acceptable for UFH.

Multi-layer pipe of either PE or PEX incorporating a layer of aluminium: This type of pipe offers most of the benefits of PE or PEX combined with an aluminium layer which acts as an oxygen barrier whilst also allowing the pipe to hold its position once bent. This quality makes it ideal for use in suspended floor systems, laid in diffusion plates or on insulation, where the tendency of ordinary PEX or PE to spring back into its coiled shape can make laying difficult. It is moderately more expensive than PEX or PE.

Polybutylene (PB) with an oxygen barrier: Known as PB, this kind of pipe is also ideal for UFH. Although softer than PEX, it is durable yet flexible and has a high resistance to both pressure and high temperatures. The oxygen barrier virtually eliminates corrosion. Like PEX it has a tendency to return to its coiled state and therefore requires fixing down. Available in long rolls, there is no need to bury any joints in the floor.

Thermoplastic rubber: Some suppliers offer a triple tube system made from thermoplastic rubber. The two ‘outside’ tubes flow in the opposite direction to the centre tube, creating a ‘contra-flow’ which equalizes the tempera­ture across the room, eliminating any cold spots near the end of the heating coil where it is about to return to the manifold. This sort of pipe must be used as part of a proprietory sys­tem with plastic manifolds.

Fittings are the weakest part of any system and so care should be taken to ensure that no fittings are buried — thus allowing easy access if replacement is necessary.

Does UFH Suit All Types of Construction?

UFH is suitable for all forms of house con­struc­tion and can be incorporated into any type of floor structure. The best solution for ground floors is to incorporate the elements within a concrete floor that has a large thermal mass. A boiler can then fire for longer periods at its full and most efficient output, building up heat in the floor which can then be released gradually, rather than repeatedly firing on and off, which is inherently inefficient. Burying the elements in a dense screed is particularly important for electric systems, as it allows the ability to store heat and release it slowly, enabling the system to operate using cheap off-peak electricity.

Those using a suspended concrete first floor can opt to have the elements in a screed upstairs too, although there are also systems for use in floating chipboard floors, or within suspended timber floors.

Can I Retrofit UFH in My Renovation?

Providing the boards can be lifted, or if you have access from below, an existing suspended timber floor can be retro fitted with UFH using any of the systems for new suspended floors. The old flooring can be re-laid without affecting the floor height. Existing concrete floors are harder to retrofit with UFH, as most systems will increase the finished floor level beyond what is practical for existing doorways etc., although electric mat systems are a popular means of heating bathrooms in old houses.

Which UFH System is Best for Screeded Floors?

If the floor structure is a solid concrete slab or suspended concrete slab (e.g. beam and block) the choice is between burying the pipe or cables in a floor screed, or one of several dry fix systems with the cables laid on top of insulation with a floating floor above.

Pipe in Screed: The heating pipes or electric elements are fixed onto a layer of insulation board, with further insulation fitted around the perimeter of the room to allow for expansion and to prevent noise transfer to neighbouring rooms. A floor screed is then cast on top of and around the pipes or elements. The screed, usually at least 65-75mm and some­times containing special additives and/or reinforc­ing mesh, must be allowed to dry out thoroughly before any fixed flooring is laid. The screed makes excellent contact all around the pipe, allowing very efficient heat transfer and therefore good heat output (100W/m²).

Pipe on Insulation: Pipes are laid on extruded polystyrene insulation pre-channelled to house warm water UFH pipes onto which a floating floor is laid. To improve heat transfer from the pipe, screed can be poured into small channels around the pipe, giving good surface contact and a better heat output.

Because of the low thermal mass of the floor, this type of UFH has a faster response than pipe in screed, is quicker to lay, and eliminates the cost of the screed and associated moisture and drying out time. Against these benefits, it is more expensive and has a slightly lower heat output of around 70W/m².

Pipe in Board: The heating pipe is contained within flooring grade chipboard, laid on a layer of insulation. The pipes in the boards simply plug together at the corners and have a foil back to help diffuse the heat. Wood is not a good conductor and so the output of such systems depends on the diffusion via the foil. Also, the pipe has to be of a relatively small diameter to fit in the board, restricting the output of such a system. However, it is a totally dry fix solution. One potential disadvantage, besides the cost of the boards, is that they require at least two connections each – always the weakest part of any system – all of which will be beneath the floor covering.

Electric Mats: For spot heating in smaller areas, electric UFH heating mats can be used directly beneath a tiled or wooden floor. As there will be little or no thermal storage in the floor covering, this system cannot operate on cheap off-peak electricity and is therefore relatively expensive to run.

Which UFH System is Best for Suspended Timber Floors?

If the floor structure is a traditional timber floor with joists suspended between the walls and covered with floorboards or chipboard, one of several warm water UFH solutions is likely to be the most appropriate. The pipe is usually run in long coils running in the direction of the joists, with the joists either drilled, notched or battened at one end for the pipes to cross between the joists and chipboard/boards.

Pipe in Plate: Warm water UFH pipes held in place by aluminium or steel diffusion plates sitting across the joists. The void beneath the diffusion plates is usually filled with mineral wool insulation or polystyrene. Heat transfer relies on contact between the plates and the underside of the chipboard, unless they are painted black, in which case some radiant heat will be emitted even if contact is poor. This system is faster to lay than pipe in ‘pug’ systems and diffuses heat more successfully than pipe on insulation-only systems. The output is around 70W/m². Multi-layer PEX or PE pipe with an aluminium layer is best for this type of installation as it will hold its shape and not coil out of the plate.

Pipe on Insulation: Warm water UFH pipes with an aluminium layer are laid on rigid insulation suspended between the floor joists on battens nailed to the sides of the joists. The insulation is fixed at a height which ensures good contact between the pipes and the underside of the floor. Rigid foil-faced insulation is the best as it reflects radiant heat into the room. The low level of surface con­tact between the floor and pipe means such systems have a relatively low heat output of around 20-30W/m². If the pipe temperature is increased to try and improve heat output, there is a potential risk of ‘striping’ on wood­en floors. Foil faced insula­tion boards are avail­able in standard joist widths, pre-channelled to house UFH pipe. This will improve the output, but not to the same levels achieved by thicker metal diffu­sion plates. UFH pipe can also be fixed to the under­side of the floor to ensure better contact using either a plastic track system, which can be used to fit the UFH from below, or on hangers which sit over the joists.

Pipes Suspended in Void: Warm water UFH pipes with an aluminium layer are fixed to the joists 25-50mm below the underside of the floor and a layer of foil faced insulation fixed beneath to reflect radiant heat back upwards. A pocket of warm air is created which warms the underside of the floor evenly, without any risk of ‘striping’ damage to the floors. The lack of contact with the floor surface and of spreader plates reduces the output of such systems to 20-30W/m².

Pipe in Pug: Warm water UFH pipes are laid on insulation sitting on chipboard suspended between the joists on timber battens. A weak sand and cement screed (a pug) is then laid around the pipe. The pug makes good contact with the pipe and works like a pipe in screed system with a high output of up to 100W/m². This system is the most complicat­ed to install and, like pipe in screed, could have a relatively slow response time. Floors also need to take into account the increased load of the screed.

Pipe in Board: The same system as used for screeded floors can be applied to suspended timber floors taking the place of chipboard flooring.

Panel Modules: Warm water UFH modules comprising a plastic frame of warm water channels with insulation underneath are laid between the floor or ceiling battens. These are quick and simple to lay and provide excellent heat output. They are relatively expensive, however, and do result in joints beneath the floor. Modules are available in 500mm standard width. They can also be installed in walls, ceilings and from below a floor.

Electric Heating Mats: The same mats as used for spot heating for solid floors can be used on top of chipboard for suspended timber floors that are to be tiled.

Questions You Should Ask Suppliers

How long has the company been trading in its present form? Have any of its directors or principals traded under any other similar or dissimilar name in the underfloor heating business?

As with any sector there are so-called ‘phoenix’ companies who install defective pipe, fold and re-emerge under a slightly different name. Approach all companies with caution; sticking with companies that have been trading for a long time is sensible, although it doesn’t always follow that new companies are bad risks.

What is the guarantee on your pipe? It needs to be 50 years and be in writing with at least ten years of it being backed by some external policy. 50 year guarantees are no good if the supplier goes bust. Walk away from anything that says ‘design life of 50 years’ or any other flowery language. Ask for a chart showing life span against pressure and temperature. If they can’t give it there must be something to hide.

Do you have any technically qualified personnel in plumbing or electrics and can I speak to one? Whilst UFH is relatively straightforward, most plumbers and electricians are unfamiliar with it. The vast majority of problems arise because tradesmen do what they think they should do and not what is actually required. Make sure that any tradesmen you employ speaks to the supplier. The vast majority of problems with underfloor heating arise because of incorrect plumbing or wiring.

Will I Need a Special Boiler?

The design of a warm water UFH system needs to take into account which boiler is to be used and the level and sophistication of its built in controls. The more efficient the boiler, the more efficient the system will be. For gas powered systems a condensing boiler is well worth the investment as it will work at its most efficient with a low temperature heating system such as UFH. Thermal stores can also be used. UFH can also be used in conjunc­tion with renewable heat sources such as solar panels and heat pumps.

Does the Floor Covering Make a Difference?

Carpet: It should be borne in mind that carpets act as an insulator so the output from the floor will be reduced according to the thickness of the carpet. The carpet and under­lay should not have a combined tog rating in excess of 1.5. The amount of pipe or cables in the floor will need to be increased to compensate for the reduced heat output.

Wood: There are two types of wooden floor: engineered timber, such as laminates, and solid timber. For use over UFH in screed, most experts agree that the best method is to float the floor on top of a minimal 2-3mm polythene foam, allowing the wood to lay flat, but expand and contract naturally according to its moisture content. For suspended timber floors, solid timber flooring can be fixed to the joists whilst decorative laminate flooring should be floated over a layer of flooring grade chipboard.

Solid wood floors are more prone to move­ment than laminates. Movement can be reduced by properly acclimatising the wood to the room for several days before laying, at the temperature that it is to be maintained, having first let the building thoroughly dry out, in particular the floor screed.

Like carpet, wood is a poor conductor and this will need to be taken into account in the design of the system. The denser the wood grain, the better the output — lighter soft­woods will have a poor output. The worst combination is to have both wood and rugs. This will significantly reduce any heat output.

Flagstones, Marble and Slate: Stone and reconstituted stone flags are ideal for use with UFH. Their high thermal mass makes them excellent conductors achieving a temperature that is very pleasant underfoot and good heat output. Sawn flagstones with a constant bed of 25-50mm are most suitable, laid in a bed­ding containing an additive to allow for slight expansion and contraction of the floor, or an adhesive that allows for this movement. The grout could also be flexible. Thicker, random depth flagstones will work with UFH, but their depth and large thermal mass could slow down the response time.

Ceramic and Terracotta: Ideal for use with UFH. They are excellent conductors and have a relatively high thermal mass. Tiles will need to be laid with a tile adhesive and grout that is compatible with UFH and allows for move­ment. UFH should be switched off during installation.

Linoleum and PVC tiles: These need to be laid directly onto the screed. The tiles are glued with a flexible adhesive that will not degrade with the heat. UFH should be switched off for at least 48 hours before and after installation.