Frequently Asked Questions

Electric Underfloor Heating FAQ’s

Your underfloor heating questions answered

Browse through our frequently asked questions about both wet and electric underfloor heating solutions.

If you’re still unsure, our team are here to help. Please get in touch and one of our advisers will respond within one working day.


Are there any restrictions on what furniture or other items I can have on top of my underfloor heating system?

Underfloor heating relies on the heat it produces being unrestricted, if it is restricted it may cause that particular area to overheat and at worst dis-colour the floor. Items such as beanbags and mattresses laid directly on the floor may cause problems. Anything that allows air movement under it is OK.
Please note; that thermally resistive items should not be installed onto underfloor heating in a way where heat can become trapped. This trapped heat could cause failure if left without due care or attention. Possible examples of thermal restrictive items could be heavy rugs, bean bags or a mattress laid in direct contact with the floor. Some items of furniture that do not allow air flow could also cause concern. If you are unsure please contact Gaia.
The manufacturer of the floor finish should always be consulted when determining the suitability of the floor finish with under floor heating systems.  Manufacturer’s guide-lines should always be followed.

Why is the system not heating up?

There are a number of reasons why this could happen:

  • Check that you have a supply (voltage) to the thermostat
  • Check the thermostat is operating
  • Check the settings on the thermostat
  • Check the continuity and resistance of the mat.These checks may have to be carried out by a qualified electrician
What type of tile adhesive/levelling compound/tiling grout do you recommend?

The covering has to be flexible or have a flexible additive.

How thick does the layer of adhesive/levelling compound need to be?

The adhesive bed has to be thick enough to cover the cables; generally 4-5mm is sufficient.

How long do I have to leave the heating system, after tiling, before switching it on?

DEVI would recommend that the adhesive used is allowed to dry/cure naturally and to follow manufacturer’s recommendations.

How long should the heating system take to warm up?

This depends on various factors, such as:

  • What system has been installed?
  • What type of screed has been laid?
  • What depth the floor screed is?

This could be anything from 30 minutes for a Devimat under tiles to 5-7 hours for a storage system.

How long do I have to leave the heating system, after screeding, before switching it on?

It is recommended that any screed be allowed to cure naturally and this would again depend upon its thickness. Generally the length of time from the screed being laid and the system being switched on is sufficient for it to dry out

Do I have to use an RCD?

The use of an RCD is recommended particularly in ‘wet’ areas such as bathrooms & en-suites. An RCD offers greater protection in the event of a fault.

What is the estimated running cost of the underfloor heating system?

The running cost of a cable or mat is dependent upon its rating/output and is based on the units of electricity consumed

A unit of electricity is 1000watts (1KW) for 1hour at the unit cost of electricity i.e 1500w x 1hr x 15p/unit = 22.5p/hr.

What type of supply do I need?

This will depend on the area being heated and the density of cable / mat installed. Your electrician should be able to calculate this once he knows the rating of the underfloor heating installed.

Is my underfloor heating system suitable for use underneath carpets?

Yes, providing the mat / cables are covered with a latex levelling compound first.


What floor finish is suitable for the DEVImat system?

Most floor finishes are suitable for use with underfloor heating providing the floor has been prepared correctly and any manufacturer’s temperature guidelines have been adhered to. With finishes such as wood, laminates and carpets it is essential that the heating mat is completely covered with a latex levelling compound before the specified finish is fitted.

The DEVImat is too big for the room, can I cut it?

The DEVImat cannot be shortened as this will alter the resistance of the mat and may cause it to overheat. It is important that the area that requires heating is measured accurately (free floor area only) and that you always round down the measurements.

My fuse keeps blowing when I use the mat?

Fuses blow or circuit breakers trip when there is either a short circuit or an overload. A short circuit being the more common of the two causes. This is generally due to mechanical damage, for example the heating cable being cut, squashed or possibly drilled through.

Do we require insulation with DEVImat products?

The DEVImat is designed as a retrofit product to be laid directly onto an existing floor surface, so does not require insulation. However, if there is room within the floor to add insulation or a tile-backer board this will enhance performance.

Does the floor need to be primed before laying the DEVImat?

The mat does have an adhesive backing, but if the floor is dusty or greasy it will not stick particularly well. Priming the floor will help the mat to fully adhere to the floor surface.

Which type of mat do I need to order – 100W/m², 150W/m² or 200W/m²?

The 100W/m² mats are for installation on wooden floors. If the wooden floor is ventilated, it is important to have some form of insulation beneath
150W/m² mats are for installation onto concrete based floors
200W/m² mats are for installation onto concrete based floors where a more responsive heat is required.

I want to use DEVImat underneath laminate flooring, what type of underlay do you recommend?

The underlay usually supplied with wood laminate flooring is an acoustic underlay about 2-3mm thick, this is acceptable. If you are installing anything other than this, please call our technical team at DEVI for advice

I have a large floor area that requires two mats, how do I join them together?

The cold tails have to be brought out of the floor and have to be connected in parallel. i.e. core colours connected together, brown with brown, blue with blue etc

Can I put DEVImat underneath a bath or shower tray or kitchen units etc?

DEVI does not recommend that the DEVImat is installed under any such fixtures for a number of reasons:

  • It can inhibit the heat being produced from the system causing it to possibly overheat or damage furniture
  • If ever a fault was to occur, it would be extremely difficult to carry out a repair
I have a floor which is made of both timber and concrete. Which DEVImat should I use?

DEVI would recommend that each section of floor has its own ‘mat’ and thermostat. If one size mat and thermostat were to be used then where do you install the sensor? In this scenario the floors would deliver uneven temperatures.

What width is the DEVImat?

The actual width of the mat is 430mm but it is recommended that it is installed at 500mm intervals to ensure the there is an appropriate space between runs.

What is the difference between twin and single conductor mats?

The difference between the two mats is the ‘cold tail’:

  • The twin conductor mat, there is no second connection tail to return to the thermostat
  • The single conductor mat there is a single cold tail at each end of the mat and they have to be brought back together at the connection point
What is the black thing at the far end of the DEVImat?

This is the connection at the end of the twin conductor mat where the heating element is connected to the neutral return. This is manufactured within the cable.

How do I wire my DEVIclear mist-free mirror mat?

It is recommended that the mirror mats are wired into the lighting circuit, which is switched on and off with the room light. The connection to the lighting circuit can be done using a box set in the wall adjacent to the mirror and sealed with a blank plate or by a joint box within an adjacent void.

What sizes are the mirror mats?

There are 2 sizes available:

  • 800mm x 600mm
  • 500mm x 700mm
My mat/cable has been damaged or broken. Can I fix it?

Yes. In most cases all mats & cables can be repaired. Gaia offers a full fault finding and warranted repair service.

How long is the warranty for DEVI products?

All DEVImats and DEVIflex cables – 20 years for all products installed after 01/09/2013, 10 years for products before this date.

All Thermostats – 2 years (5 year on Devireg Touch)

What does the DEVI warranty cover?

All mats, cables and controls are covered against manufacturing defect

Any failure caused by mechanical damage will be attended to but the cost covered by the customer.

Which cable is the live and which is the neutral from the heating element?

This will vary from product to product.

For twin conductor cables/mats there will be a 3 core cable attached with identifiable core colours.

For single conductor mats there will be a blue & black cold tail, blue being neutral and black being live.

For single conductor cables both cold tails are black and there is no polarity, i.e it doesn’t matter which cable is connected to live and which to the neutral.

What is the maximum floor finish thickness I can have with a DEVImat or DEVIflex heating cables?

This will depend upon the finish specified.

For timber, 20mm for soft wood, 30mm for hard wood. Most wood laminates are OK but may need to be treated on merit.

Most vinyls are suitable as long as manufacturers temperature limits are adhered to.

Carpets and underlays are suitable but the lower the tog rating, the better the system response times.

Can I extend the cold tails or sensor cables?

Both the cold tails and the sensor cables can be extended by up to 50 meters.

Thermostats & Sensors

Where can I put the thermostat?

The thermostat should generally be positioned in the room it is controlling so it can accurately read the air temperature. It should also, if possible, be out of direct sunlight and away from any draughts or any alien heat source that may affect it, i.e. a radiator or TV

Thermostats for bathrooms & en-suites have to be placed remotely either outside the door or in a cupboard and linked to a floor sensor which would be positioned within the bathroom or en-suite floor.

The thermostat display is blank. What does this mean?

Check that you have a supply (voltage) going to the thermostat as this quite often means that there is no supply to the thermostat.

Can the thermostat be positioned in the bathroom?

According to electrical wiring regulations, thermostats should not be fitted in rooms with a fixed bath or shower.

What is a ‘floor sensor’?

A floor sensor is a limiting device that governs the upper limit temperature that the floor has been set to on the thermostat.

What box is the floor sensor in?

The floor sensor is supplied in the base of the thermostat box.

Where is the floor sensor wired to?

The floor sensor is wired back to the appropriate thermostat and connected into the NTC terminal. There is no polarity on the cables therefore it does not matter which cable goes in which terminal.

I haven’t installed a floor sensor. Is it necessary?

The DEVIreg 540 /550 thermostats can be programmed to operate as air sensing, floor sensing or air & floor sensing. A floor sensor must be installed when floor or room & floor sensing is chosen. This is particularly necessary when controlling heating in bathrooms and en-suites where thermostats are placed outside the door.

Technical Hub

Wet Underfloor Heating FAQ’s


What floor finish is suitable for wet underfloor heating?

Most floor finishes are suitable for underfloor heating but to ensure you get the best output we would recommend the flooring has the lowest thermal resistance possible, in the case of carpets we would recommend the combined rating of the carpet and underlay is less than 1.5 TOG and with wood flooring we would recommend the use of engineered wood rather than solid wood if possible. If there are any doubts about maximum floor temperatures, Gaia would recommend checking with the flooring manufacturer to ensure that there are no temperature limits on the flooring.  If there is a maximum floor temperature limit, then the heating can be limited by either adjusting the flow rate or installing a floor sensor.

What heat sources can I use for wet underfloor heating?

Underfloor heating is extremely effective with low temperature heat sources such as Ground Source & Air Source Heat Pumps; although almost any heat source can be used as a mixing valve, which can be used to cool the water down to the required temperature (Typically 35 – 55°C).  The heat source will need to be known before the system is designed, as lower flow temperatures will require the pipework to be installed closer together.

What output can I expect to get from wet underfloor heating?

There are many factors that affect the output of the underfloor heating system (outdoor temperature, required indoor temprature, floor finish, flow temperature). For a screed system, and based on standard requirements, the heat output will usually be around 70W/m² based on flow and return temperatures of 50/40°C, a design room temperature of 20°C, an external temperature of -3°C and a floor thermal resistance of no more than 0.1W/m²K (1 TOG carpet)

Can I put a rug on my floor?

Gaia would recommend rugs are avoided on temperature sensitive floors such as wood and vinyl as the heat will get trapped under the rug and can cause damage to the flooring.  Rugs aren’t such an issue on tiled floors as they typically aren’t temperature sensitive however Gaia would recommend caution with the type of rug as the heat can still get trapped underneath and may damage the rug or the heating.


Why has the pressure dropped on my manifold?

The most common reason for this is air entering the system from the boiler.  However, Gaia would recommend using the services of a plumber to investigate as there may be many reasons for this happening.

The pump on the manifold is running but the manifold is cold?

Gaia would recommend getting an electrician to check the wiring to the wiring centre as it may not be wired correctly to switch the boiler on.

My heating should be on but the pump on the manifold is not running?

Firstly, check that at least one thermostat is turned up and calling for heat, if this is the case but the pump is still not running then it is recommended to get an electrician to check the wiring to the wiring centre as it may not be wired correctly to switch the pump on.

What is the maximum pipe length for a single loop of pipe?

The maximum length of 16mm pipe Gaia would recommend installing as a single loop is 120m due to pressure drop and temperatures.

What pipe diameters are available?

The typical pipe diameter used on Gaia systems is 16mm although other sizes may be available upon request.

What manifold sizes are available?

Gaia Manifold sizes are as follows:

2 port = 164mm

3 port = 214mm

4 port = 264mm

5 port = 314mm

6 port = 364mm

7 port = 414mm

8 port = 464mm

9 port = 514mm

10 port = 564mm

11 port = 614mm

12 port = 664mm

All sizes stated exclude pumpset (130mm) & isolation valves (70mm)

What is the maximum loops available on a single manifold?

All Gaia manifold sizes are between 2 – 12 ports.  If more than 12 loops of pipe are needed then multiple manifolds will be required.

My manifold is noisy, what could be the issue?

If the system is noisy it may be a build up of air within the pipework from the boiler.  In this case, Gaia would recommend a plumber to attend, enabling them to diagnose and resolve the issue.

Thermostats & Sensors

My room isn’t warming up even though my thermostat shows it is on?

As wet underfloor heating is typically installed within a screed, the system can take some time to warm up after the initial install or if it has been off for some time. If this is not the case then you can check that the pump and boiler are running, if they are not running then an electrician or plumber would be required to look into this further.  If the pump and boiler are running but the room is still not getting warm then you should check to see if the thermal actuator is opening up to allow water to flow into the pipework within the room that requires heating.

Do I need a floor sensor?

Floor sensors can be installed with hydronic underfloor heating systems if floor temperature limits are required; however, the floor temperature can typically be limited using the flow rate on the manifold.

I turn on one thermostat in one room but another room in the house heats up instead?

This indicates a possible wiring or control issue with the system.  It is recommended that an electrician checks over the wiring as it is highly likely thermostats are linked to the wrong thermal actuators on the manifold.

Underfloor Heating Systems

Wet Underfloor Heating

We supply wet (hydronic) underfloor heating systems from top brands for a wide range of projects.

Electric Underfloor Heating

We provide a range of electric (dry) underfloor heating systems, including self-adhesive mats, heating cables and decoupling membrane to warm any room.

Frost Protection

We offer a range of frost protection and ice and snow melting systems which protect from harsh winter conditions.

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