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Underfloor Heating Installation Issues with Heat Pump

Maximumdo · 2026-02-17T21:33:36Z

Hi everyone, I am looking for an experienced underfloor heating specialist who can help us resolve a significant issue. As part of a full house renovation, I spent considerable time researching before appointing an installer to supply and fit our heat pump and UFH system. There was a long gap between design and installation, and the work was completed just before Christmas during what was already a very stressful period for us. We were not living in the property at the time, and I regret not scrutinising the finer details more closely. In short, the UFH has not been installed in line with the design intent, and there is insufficient pipe coverage for the downstairs to reach a comfortable temperature. The current build-up is 16mm pipe clipped to PIR insulation between timber joists, with 22mm chipboard above. There is effectively an air gap between the pipework and the floor deck, which seems to be significantly limiting heat transfer. Upstairs radiators are performing as expected, so the issue is isolated to the UFH. The installer has acknowledged that the system is not right, but we are not being presented with a technically clear resolution. We are now at the stage where we simply need this fixed properly. We have engineered oak flooring ready to be laid and decorating on hold. The uncertainty is taking its toll. Suggestions from a couple of people I have spoken to so far have ranged from adding screed and cement boards, to removing the chipboard and relaying with routed/grooved boards. We are constrained by finished floor heights at the bifold doors and by pipework that runs through a now-tiled utility floor. At this point, we are prepared to incur additional cost if necessary, but we need a competent specialist to assess the situation, review the design versus installation, and provide a definitive solution. If your company has the expertise and capacity to assist, I would be extremely grateful to hear from you. We are based in Farnham, Surrey. Thank you for taking the time to read this.

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MartinHook_UK

(@martinhook_uk)

New Member Member

Joined: 3 weeks ago

Posts: 1

04/03/2026 10:11 am

We suffered a water leak last year in two rooms and had the opportunity to inspect the underfloor heating pipe work. These are the observations that may help you:

- Engineering wood (~20mm thick) is both structurally strong and an excellent conductor of heat. The boards are fitted directly over the joists - no additional chipboard layer is needed. The room with engineered wood now outperforms the solid oak floored rooms.

- The underfloor pipes are carried by steel brackets that run along the joists, overlaid with thin aluminium sheet spreader plates. The wood flooring is in direct contact with these spreader plates.

From your description it sounds like your installer has used the pipe-in-screed construction method, where the pipes are clipped to insulation and then covered with screed up to the desired floor level, instead of a floating floor installation.

Step 1) Remove the chipboard - this is not needed. The engineered wood will be sufficient flooring.

Step 2) Either back fill the gap between the insulation and the top of the joists with screed Or refit the pipes in a suitable spreader plate solution (insulation with grooves or steel brackets) - whichever solution you use the screed or spreader plate must be touching the underside of the flooring.

Your original installer should make good or you can discuss this approach with a new installer.

Martin

👍

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bobflux

(@bobflux)

Trusted Member Member

Joined: 3 weeks ago

Posts: 37

04/03/2026 11:18 am

I'm going to install Thermacome ACOSI+ radiant ceiling. It's a French product, so perhaps not available in the UK, but still some important points:

- The heat spreader plate is 0.5mm aluminium sheet. Some other manufacturers use microns thick aluminium foil, which looks pretty but has no effect on heat transfer.

- Pipe spacing is 125mm and the plates can be easily cut with a box cutter or just snapped every 125mm. For radiant heating, closely spaced pipes provides better heat transfer at lower temperature.

- Total plate thickness 30mm with XPS foam insulation backing.

I'm having trouble visualizing the way your floor is constructed. Can you make a cutaway drawing with dimensions?

From what I understand you have insulation between joists, pipe simply dropped on top, surrounded by air, and chipboard resting on the joists. Final floor height is determined by the chipboard.

In all cases you need a heat spreading material (screed, aluminium plates...) and the floor boards must make good contact on top without air gap.

- Option 1: do it from below. Remove PIR insulation between joists without destroying it, install pipe in heat spreader plates screwed to the bottom side of the floor panels. This ensures the heat spreaders make good contact with the floor panels. Then reinstall PIR insulation from below. If the floor below allows this, this is the simplest solution, it also allows you to finish your floor and do the UFH this summer.

- Option 2: do it from above: Remove chipboard panels. Then you need the heat spreader to make good contact with the floor panels when you put them back, which means either a) it needs to be exactly the same height as what's available between the top of the PIR insulation and the top of the joists, there is no reason for this height to correspond to off the shelf UFH panels or even be the same everywhere, which means it'll have to be a screed or b) you can move the PIR insulation to adjust the height, but in this case it may be simpler to do it from below...

Also please check your PEX has anti oxygen barrier. If it does not it will let oxygen in and corrode your steel radiators. Normally it should have the barrier, but since this is not a normal installation... who knows.

This post was modified 1 week ago by bobflux

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Jonatan

(@jonatan)

New Member Member

Joined: 2 weeks ago

Posts: 6

04/03/2026 5:34 pm

Thanks for sharing your observations, both of you. From what I’ve seen, the key is really ensuring the heat spreader makes good contact with the floor boards, air gaps kill efficiency. I like the breakdown of options from below or above; it’s useful to think in terms of “floor-to-spreader contact” rather than just the pipe position. Also, double-checking the PEX barrier is a good point, something easy to overlook until corrosion shows up.

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Maximumdo

(@maximumdo)

Active Member Member

Joined: 4 weeks ago

Posts: 6

Topic starter 05/03/2026 12:01 pm

Hi @martinhook_uk @bobflux

The installer is making good. This weekend we're having the chipboard removed and a dry biscuit screed filled in between the joists to remove the air gap. On top we are having an 18mm cement board as advised by the company installing the engineered oak flooring, as our joists levels are very mixed!.

The cement board we have chosen has excellent thermal conductivity and is specific for UFH.

👍

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bobflux

(@bobflux)

Trusted Member Member

Joined: 3 weeks ago

Posts: 37

06/03/2026 10:54 pm

This looks like a practical solution 👍

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