Skip to main content
Architect Tips

Underfloor Heating: Top 5 Mistakes And How To Avoid Them

Underfloor Heating - Top 5 Mistakes And How To Avoid Them

We specify Underfloor Heating for 90% of our projects. It is no longer a new technology and yet, we still regularly see mistakes being made: cracked screeds, incompatible flooring, excessive UfH provided and more. Michael Martin, Director and Co-Founder of Formed Architects, takes us through his top 5 considerations to ensure you get it right.

1. Electric or wet system?

The type of system will depend on the following factors:

  • Budget
  • Size and shape of the room
  • Thermal efficiency of the space
  • Use of the room
  • Retrofitted (existing floor) or part of a whole refurbishment or new-build project.

Electric systems are popular for a single room (e.g. bathroom), small space or extension. Electric UfH consists of a woven wire matting system which connects directly into the home’s electrics.

For small areas, that don’t need to be heated for long periods this can be a sensible solution, particularly if you don’t want to add height to the floor as the matting systems can be around 3mm thick. Based on operational costs it is less suitable for larger projects.

Water based systems work with pipes with water flowing through them in the floor. With water systems, you need to consider the thickness and position of the system in terms of the floor build up and how this may impact floor levels especially if you are only doing some areas on a level.

You will also need to allow for a manifold – this is where all the pipes come out of the ground and are fed to the hot water supply – the larger the area that you want to cover, the more zones you will have.

Zones are temperature controllable areas; the size of a zone is roughly limited to 100m of underfloor heating pipe. Different rooms can be separate zones.

The heat emitted from a water based system offers a number of benefits with an even level of heat in a space relative to radiators, whilst also being cheaper to run as typically the temperature for the thermostat on an UfH system will be lower than required for traditional central heating system.

Water based systems are suitable for screed floors and pipes can be set within the screed – this works well in a new build or extension.

If you are opting to lay it over an existing area and do not want to pull up the floor but height is an issue, there are a number of low-profile systems now available which result in an additional height of as little as 16mm.

2. Too much heat or not enough?

Much like radiators, knowing how much heat distribution is required is fundamental when specifying UfH, this will also be impacted by:

  • The heat source (boiler or heat pump)
  • Type and size of UfH pipe
  • The type and location of manifolds (control valves)
  • Screed (type and thickness)
  • Floor finish

You should discuss this with the designer as early as possible so this can be factored into plans when detailed drawings are prepared.

3. Poor zone control

When investing in UfH systems either in a whole-house or within multiple areas of a house, it would be prudent to improve operational efficiency to ensure that you can control the temperature and timing of each zone to suit occupancy needs.

For example, as occupancy patterns change, you might want the kitchen to be warm for a couple of hours in the morning while bedrooms needing to be heated later in the afternoon.

In order to establish the most effective and efficient zone control design layout system for your project, it would be best to liaise and coordinate with a professional to provide a clear and understandable design for your comfort.

Thermostats controlling temperatures can be controlled remotely and via wifi, and there are many options on the market as to how these integrate within smart home systems.

Remember you will have some areas that do not need any UfH and you should make your builder aware of these – you will not need UfH under toilets and showers (you may opt for them if a wet room in which case ensure drainage position is known), in addition there is little point in having UfH under kitchen units and you cannot put them around active fireplaces – there are set distances for this due to the fire risks involved.

4. Wrong screed

A typical standard screed floor associated with extensions and new builds would be between 60mm to 75mm thick, consisting of sand and cement. The UfH system would sit onto the insulation and then be covered by the screed.

This thickness would provide a 2-3-hour (heat up and cool down) thermal mass – heat absorbed and radiated into a space above would be a most efficient setup for a water based UfH system.

Crucially with this type of system is the time it takes for the screed to dry before heating can be tested. I would recommend that the system is tested before any floor finishes are laid so that any issues can be addressed with as minimal disruption as possible.

A typical screed takes about a day to dry for each millimetre of depth – so for 60mm of screed you need 60 days (in typical temperatures) to dry. If you fail to allow for this time before testing the UfH system you can cause cracking in the screed which could then go through the finished floor.

The reason for the drying time is that the screed has a high moisture content – when you heat the screed up if it is not dry enough you force the water out which causes cracking. Specialist devices are available to measure the moisture levels in screed.

There are fast drying screeds available, which can be more expensive and drying times for these should be checked with the manufacturer. You should ensure that the screed is flexible and suitable for UfH.

And when the screed is laid it should be laid under pressure to ensure there is sufficient space for the UfH pipes to expand when water is flowing through them without the risk of cracking the screed.

If you are using UfH in an area where you have an existing suspended timber floor with a new concrete based extension and do not want to lift all the existing timber floors, you can install an overlay type of UfH system throughout.

This will increase the floor height marginally but provide a consistent floor throughout. To the top of this, subject to the system being used and the type of floor finish you can finish this with a self levelling screed to give an even finish. This would typically be about 3-10mm thick.

Again drying times would apply for testing. This type of system has reduced heating times, with all the advantages of water based systems.

Wrong covering

The floor’s substructure will determine the optimum temperature and performance achieved through a given floor finish or diminish it.

As a rule, solid wooden flooring is not suitable for use with UfH, unless specified by the manufacturer. However, engineered wood which consists of a solid wood top layer followed by plywood-like base would be suitable.

Engineered wood differs in grade and be careful to ensure that it is suitable for UfH. It can be robust and reliable; less prone to movement when exposed to changes in temperature and humidity levels, whilst maintaining the look and feel of wood.

Subject to the thickness of the top layer it can be sanded over time if it becomes worn. It is critical that the adhesive for the wood flooring is suitable for UfH, or if you are having a floated floor system that this is factored in and compatible.

Stone, ceramic, slate and terracotta are all good coverings. Although, heat-up time depends on tile thickness – flagstones are prone to take longer to reach optimum temperature, but once reached, it makes no difference in heat quality if it is a thick or thinner floor surface.

If you are using tiled surfaces, then you need to ensure that the adhesive is flexible and compatible with the type of UfH system that you are using – water, within screed and overlay style systems and electric systems use different types of adhesive and the system maybe prone to fail if not compatible.

Large format tiles need to be fully adhered (buttered on both surfaces) as these will be prone to cracking if only dabbed. In addition you should ensure that the grout being used is suitable for UfH.

If you prefer to use vinyl and laminates, then it would be advisable to check both with the flooring manufacturer and the UfH supplier to ensure compatibility.

UfH and carpet is not always considered; although if you are opting for carpet you will need to understand if the carpet is suitable in tog rating. You would not want to exceed 2.5 togs with the carpet and underlay combined so as to not significantly impact on the efficiency of the UfH.

My advice would be to always inform the UfH designer of the intended covering, so that pipe size and layout can be properly specified, and discuss this with the flooring supplier too; make sure at the outset that your builder knows how you are preparing to proceed with the UfH and the height of the overall UfH system and floor finish as this will affect their early stage setting out for the floor slab.

The key to UfH is early coordination and consideration and then you should be able to mitigate any major issues.