Running your warm water through pipes under the floor is a favoured option of many self builders. The benefits are well-known: a steady, background heat, perceived reductions in energy use and, of course, no radiators — which, in this era of ‘big glass’ (rather than walls to hang them off) solves a tricky design problem.

Underfloor heating (UFH) is now also a serious and cost-effective option for anyone carrying out major work to their home. But it requires a joined-up approach and involves you, as the homeowner, making some key decisions.

Thanks to innovations in this little part of the housing world, the assumed problems about cost, retrofitting and the system failing, are becoming less of a consideration than they were 10 years ago. In short, UFH is about the same price as a ‘designer’ radiator and the only things that go wrong are at the pipe connections, which are above ground. As for concerns about retrofitting, read on to find out more.

Underfloor Heating Types

Wet systems involve standard or small bore pipes full of water powered by a boiler (or heat pump). The heat is generated through the pipes and the floor effectively acts as a large radiator.

Electric systems are popular solutions where underfloor heating is preferred in a single room (such as a conservatory or bathroom). Given the relative expense of electricity it is not usually a sensible option for larger projects.

Costs: Comparing the systems

As an example, a 25m2 room built to current Building Regulations is likely to need 1.25kW to heat it, which would equate to around 1,700kWh per year.

Electric UFH: Assuming electricity at 13p per kWh, electric UFH will cost £219 per year.
Capital cost: £350-400

Wet UFH: The same room with wet UFH, assuming a gas boiler operating at 90 per cent efficiency and mains gas at 6p per kWh, will cost £113 per year to heat.
Capital cost: £500-650.

So, the initial outlay for Wet UFH is higher, but running costs are almost half the price making it more affordable in the long run.

Installation in Existing Homes

The problem with installing UFH to existing houses is that it tends to take up a lot of space. In the good old days, 20mm insulation under UFH was considered enough. In the 1990s that moved to 50mm and now 100mm is considered the minimum. Add 75mm of screed and a floor covering and the floor level rises by close to 200mm — with obvious implications for various fixtures, not least of which will be door head height.

Lo-pro underfloor heating installation over wooden floor

Nu-Heat’s LoProTM10 is an underfloor heating solution for retrofit purposes with a total build-up of just 15mm. The 10mm pipes sit closely together in panel boards

Low-Profile Underfloor Heating

A number of low-profile systems have been developed, like the Nu-Heat LoPro™10 and LoPro™Max that can be laid on top of existing concrete or suspended timber floors. The standard height for low-profile UFH is 15mm, so maybe 25mm to 40mm to include the floor covering. Brent Witherspoon, Managing Director of Chelmer Heating Solutions, says: “The low-profile systems work well, but the best ones are the ones that use high-density insulation.”

Low-profile systems tend to have smaller diameter pipes and run at a higher temperature. The impact of this is that the floor will heat up and cool down far more quickly than conventional UFH. Where UFH is added to an existing radiator system then a different control system will be needed. Luckily wireless thermostats (some of which can be controlled with a phone app) make installation far easier.

Installation in New Builds

Installing UFH to a new build is now commonplace. There are many specialist suppliers and systems for both concrete and timber floors. Insulation is important and on the ground floor this will be no less than 100mm of rigid foam. For upper floors, less insulation (50mm) can be used as any heat escaping through it will serve to warm the floor below.

UFH by its nature is a zonal system as each room will have its own pipe run. UFH therefore lends itself to precise time and temperature control for each room — the most efficient means of running a whole house heating system. 

Screeds

Convention dictates a 60mm to 75mm thick sand and cement screed, usually with glass-fibre reinforcement. The screed acts as the heat sink (or radiator) and at that thickness gives a two to three hour heat up and cool down time.

The alternative is a flow screed. The characteristics of this option means it can be restricted to 40mm thickness, giving a warm-up time of less than one hour. Flow screeds offer better thermal conductivity than sand and cement, and being thinner and lighter means that they can be suitable for renovations and new builds.

Both offer a different approach to timing the operation of the system. 

Underfloor heating pipes in screed

A Build-Up Example

This example, from Anderson Floor Warming shows Anderson’s pipe (it can be 16mm or 20mm depending on the specification) in a typical screed floor construction for a new build. Anderson also provide a range of solutions for the retrofit market based around slotted-in board systems.

  1. Insulation (above floor slab and damp-proof membrane
  2. Clip rail
  3. Pipe
  4. Reinforcement (not always required)
  5. Screed (usually 70mm)

Costs and Tips

How Much Does it Cost to Install and to Run?

Suppliers are, perhaps rightly, reluctant to give a ‘standard’ or ‘average’ price. Situations vary and as UFH is always bespoke the price varies too. According to Roy Pooley from The Underfloor Heating Store, the greater the floor coverage, the lower the marginal costs. “The pump costs the same regardless of room size — so whereas the piping costs on a high output 80m2 single room system are as low as £11/m2, a 10m2 room would cost about £40/m2.”

Average costs

  • As systems are bespoke the best guideline cost we can give is something between £15-£30/m2 for a standard wet system, varying with size and quality.
  • The LoProTMMax from Nu-Heat, which is specifically designed for renovations (and thus more expensive), costs around £1,200 for a 15m2
  • The typical price for UFH for an average 100m2 floor area is around £2,000-£3,000.
thermal imaging of a floor with underfloor heating and a chair

Lower flow temperatures tend to result in modest lower running costs for underfloor heating

Perhaps the biggest benefit is that UFH heats from the floor up, rather than from the ceiling down, as radiators do. This means that the room begins to feel warmer sooner and that the thermostat can be turned down a degree or two. The Energy Saving Trust say that a 10°C reduction in room temperature brings a 10 per cent saving on the energy bill.

Wet systems typically run at 40°C to 50°C, meaning a heat pump can operate at peak efficiency. “An underfloor heating system is more effective in supplying an even heat over the whole room as opposed to a radiator which is sending heat straight up to the ceiling and then convecting it around the room — and would therefore be more cost-effective,” says Roy Pooley.”

Buffer Tanks

There are two main reasons for using a buffer tank; the first is due to the low heat demand in a newly built house. A modern 200m2 house will have a heat demand of 6kW to 8kW and what heat is needed will be largely at 40°C — whereas the gas boiler will be producing 20kW at 70°C. The boiler will then short-cycle (switch on and off a lot) and not reach the temperature needed to operate in condensing mode, reducing boiler efficiency by up to 20 per cent.

A buffer tank allows the UFH to take heat when it is needed and for the boiler to run on longer cycles. A buffer tank also allows the addition of other heat sources — solar thermal, woodburners, etc. A good cylinder with appropriate controls allows multiple heat sources to be properly integrated.

How to Choose UFH and Flooring

Choosing the Right Pipe

UFH pipes come in two forms: single-layer and multi-layer or multi-skin. Single-layer is a simple plastic pipe; multi-skin is usually made up of five layers, the inner and outer layers being PE-RT (a form of polyethylene) sandwiching a layer of aluminium.

  • The latter type of pipe is easier to bend, retains its shape better and is more puncture resistant.
  • The pipe must always be pressure tested before the screed is laid — and full of water while the screed is being laid.
Prowarm pipe build up illustration

Suppliers

“The best advice I can give is to make sure the people you are talking to have in-depth knowledge of the total UFH system,” says Brent Witherspoon, Managing Director of Chelmer Heating Solutions. “There is a significant design requirement with UFH, more than with a radiator system, as it is always possible to fit a bigger or smaller radiator, but once the UFH is installed it is there for good.” Which cannot really be argued with.

There are issues around the quality of the materials; there will always be cheap and cheerful options and better quality, more expensive pipes, fittings and controls. But the efficiency and ultimate success of the system will depend on the quality of the design. Therefore choosing a supplier with good design and backup services is at least as important that the system itself.

For a full list of UFH suppliers visit our Sourcebook

Floor Coverings

Tiles, stone or similar are generally accepted as the optimum covering. They absorb heat rather than insulate and allow that heat to radiate into the room.

Wood will tend to insulate and reduce efficiency, but thinner profile engineered timber has little noticeable impact on heat output. Solid timber is notoriously tricky with underfloor heating — it needs to be acclimatised for around a month. Ask your supplier for advice.

Carpet is generally considered a no-no, but the Carpet Foundation carried out some research in conjunction with the Underfloor Heating Manufacturers Association which seems to show that some carpets can be used with UFH. The research showed that a carpet and underlay with a thermal resistance of less than 2.5 togs does not have a significant impact on efficiently.

Whatever floor covering is to be used, it is a good idea to tell the UFH designer so that the pipe layout can be properly specified.

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Comments
  • Glyn Tutt

    Great article! I would like to add that after having installed 2 systems (one small and one covering a groundfloor over around 180 sqm with 2 manifolds and a total of 7 Zones and 11 pipe circuits) that UFH really work very well when you have the ability to use a thick slab. This means a deep renovation or new build project. Everyone I know that has ‘retrofitted’ UFH of either the wet or electric type find that the floor, despite being very responsive, is hard to maintain at a comfortable level without the use of complicated electronic controls and multiple programmed on/off periods.

    The cost of my larger installation as £39 per sqm – owing to the fact that I used Foil Backed polyurethane insulation (90mm thick) and the pipes were set at about 12 cm apart in a 10-12cm top slab. this ‘over engineered’ installation in a 16th century farmhouse works really well, fed via a thermal store/buffer as it acts as a big heatsink, allowing me to programme the heating controls for 1.5hrs in the morning and 2hrs in the evening in the midst of winter. The total surface area is 180 sqm and my design philosophy was that I wanted to get the pellet boiler to work really hard (it hardly cycles at all at an average cycle of 51 minutes) and very infrequently. The thickness of the floor required specialist contractors who poured a free-flow high density concrete in less than 2 hours with the help of a pump – but I installed the insulation and pipes with my wife over 4 days.

    One other point is that UFH works best if the pipes are laid out in a large spiral – this is easy as long as you keep an eye on the pipe’s printed lengths and work with another person. Our circuits are all 100m long and there are lots of videos on the internet to give you and idea of how to bend the pipes. The advantage is that the whole floor warms up evenly, irrespective of flow through the pipes as opposed to the long zig zag snake that is often shown (warms up one side of the room due to the layout before the other side). I think that most contractors don’t do this because you do need another person to help and it is more complicated it takes more time. I know that some companies doe designs for you (in order to work out the plan) but once you have done one, it all falls into place quickly – and you save yourself money if you do it yourself!

    I’d like to recommend http://www.underfloorheatingsystems.co.uk

  • dave davenport

    Hi Glyn
    Great feedback on your project – many thanks indeed.
    Curious about a couple of things.
    1. spiral design vs interlocking out and return designs for rectangular shaped rooms
    2. given you seemed to have done a lot of your own design and build, why is your recommendation for UFHS so strong

    many thanks in advance

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