Installation Guide

Installing a wet underfloor heating system

Installing underfloor heating is becoming more and more popular in households and people are now turning their hand to the job of installing it themselves. This guide is aimed at providing help with the process of installing a wet underfloor heating system on a solid ground sub floor and on timber joists. While this guide does a good job of explaining the steps involved, if you require help at any stage of your installation call our team who can offer technical advice and further guidance.

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A few points to consider before you begin

– We recommend using 200mm pipe centres as standard, as illustrated in the image below. In conservatories and areas where heat loss is higher (poorly insulated rooms, high ceilings, patio and slide doors), compensate by bringing the pipe closer together to 150mm centres

– If your underfloor heating is combined with a radiator system in the same room, use 300mm centres – If your system uses a ground or air source heat pump, use 150mm centres throughout. As heat pumps run at lower temperatures than oil or gas boilers this is compensated by using tighter pipe centres.

– The pipe should be laid 100mm from the wall and all vertical surfaces in the room

– 100mm is the recommended depth of the sub-floor

– For screed, we recommend a 65mm sand/cement mix. Using less than this can lower the effectiveness of the system as there is less material to store and output the heat

– For optimum efficiency use 65mm foiled back insulation boards by Kingspan or Celotex. Do not use less than 25mm depth board. As a rule, the deeper the insulation the better the system is as more heat is kept in the room.

– When using spreader plates across timber joists, cover with 2-3mm Duralay Heat Flow underlay before laying the floor covering. The underlay removes air gaps caused by uneven floors which reduce the thermal conductivity of the plates and transfer of heat into the floor. The underlay will also eliminate noise and vibration caused by the pumped water flow.

– Pipe clips should be used at a minimum of 1m intervals on straight runs. On bends this can increases to 2 or 3

– Grip rails are an alternative to pipe clips and will reduce installation times and can also be combined with clips. Use 1 grip rail/sqm.

– A run of pipe from the manifold port and back again should not exceed 100m, ideally though up to 90m. If it does exceed this length, start a new run of pipe on a new manifold port.

– Water underfloor heating systems work best and are more efficient when set in screed. The heat output of screeded floors is 100w/sqm. On timber joists using spreader plates it is approx 75w/sqm

– When choosing a floor covering, particularly floor boards, bear in mind do not exceed more than 25mm (1inch) from the screed or spreader plates to the top of the floor covering. More than this will reduce the heat transfer and effectiveness of your system

Installing underfloor heating onto a solid floor

wet underfloor heating in screed

1. If the original floor slab is not level a new sub floor will have to be laid to a depth of 100mm in order to give a smooth level finish.

2. Once this is done it is then possible to insulate the room perimeter with edge insulation strips. This allows the expansion and contraction of the screed and eliminates heat wastage into the walls. The perimeter insulation will have to be bent at 90 degrees so it is tightly against the wall and sub floor and all vertical surfaces where the screed will touch, including columns, steps and inner walls. When done correctly, the insulation strip will be several mm proud above the screed level. Excess is then trimmed off with a blade when the screed has cured.

3. Now that the perimeter insulation is complete, begin laying the Kingspan insulation boards. These should be pushed tightly together and joined by tape. Once in place cover with a waterproof membrane to stop the screed seeping through any gaps causing the boards to rise.

4. With the insulation panels laid it is now time to lay the pipework. Plan your pipe layout in advance and work to the advised layout of your CAD drawing making sure each loop of pipe does not exceed 100m from the flow to the return on the manifold. Underfloor heating ready for screed

5. Begin at the manifold and in line with the pre-designed centres which is usually 200mm. Begin 100mm from the wall you are starting from and run the pipe to the opposite side of the room clipping the pipe to the insulation boards every 1m. At the opposite side of the room and 100mm from the wall form a U bend in the pipe and clip this down and repeat.

6. The pipe should continue to be looped back and forth across the room until the maximum length of 100m is reached. If the room is not covered by a loop of 100m, begin a new loop on a new manifold port and continue until the floor is covered.

7. If using 150mm pipe centres the U bend should be made into a C bend (light bulb shape) so as not to go beyond the bend radius at 200mm centres.

8. With the pipe in place the system needs to be pressure tested in order to ensure that it is all working correctly and there are no leaks. Once this is done it is time to lay the screed. The screed has to completely cover the pipe so that there are no voids or air pockets. The system should sit at a pressure of 6 bars so that the risk of any damage to the walls of the pipe is limited while the screed is being laid. The screed should sit at a thickness of 65mm. The screed should then be covered with a membrane to help the drying process and the floor should be allowed to dry naturally until it is fully cured.

9. Once the screed has fully cured cover with a thin ply using a dense material like MDF. This is to protect the screed but also to help create a secondary level surface for the floor covering to be laid. When laying the ply covering, tape the joints together and ensure that there is space for expansion around the edges.

10. Once the above step has been completed the floor is now ready for the final floor covering. wet underfloor heating in screed

Installing underfloor heating on timber joists

1. The first stage when it comes to laying underfloor heating is to ensure that the space between the joists is tightly compacted with insulation so that the heat emitted from the pipes will not be lost downwards but will be forced upwards into the room. Recommended insulation is rigid polystyrene or foam. It is also important at this stage to work out where heavy furniture will sit in the room so that additional support batons can be added to the joists.

suspended floor install

Underfloor Heating Over Suspended Floor

2. Once this has been done it is time to begin the process of laying the spreader plates. Before laying the plates it is important to ensure that the spreader plates lay flat and are in constant contact with the joist. This means that the insulation is not sitting higher than the joists. The reason for this is so any air gaps or draughts are eliminated between the floor and heating system.

3. Now it is time to notch the joists with 20mm notches so that the pipes can be fitted correctly and it allows the pipe to enter and exit the room. Now the spreader plates can be laid and fixed in even positions to the joists.

4. With the spreader plates in place it is now possible to begin laying the pipe. The pipe will fit into the groves on the plates and it is suggested that the flow pipe from the manifold should be taken to the furthest point in the room as this will allow sufficient heat to penetrate the perimeter of the room.

5. Once the pipe has been correctly put into place it should now be tested to a pressure of 6 bars for one hour before the flooring is laid. The system should also remain under pressure when the floor is laid.

6. OPTIONAL: cover the plates with 3mm Duralay Heatflow underlay. By installing this form of underlay it will provide good thermal conductivity while keeping the heat in the room and eliminate noise from the water flow.

7. With everything in place it is now possible to lay the floor covering.

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