Underfloor heating thermostat giving wrong readings

[pr1970ster]

Hi

We have electric underfloor heating in a dining/shower room area with a floor probe connected. Its been installed a few months but now its getting cold have realised its not working well. I`m noticing the probe floor temperature very quickly exceedes the 28c floor limit its set too.
I`ve tried calibrating to a lower temperature but its still rising quickly, so now i`m setting it upto 35c to stop it switching off the heating.

I`ve shared a video here https://photos.app.goo.gl/688zPssB2tC9KU3n7
The floor has been heating on and off for about 30minutes, the thermostat shows the floor temp starts at 31c and a minute later its upto 31.5c . And you can see from my detector its nowhere near that temperature, maximum is about 18c.

Any ideas?

Thanks

 

[SimonH2]

OK, to start with you have a heater underneath a slab of "wood" which is an insulator - so the top surface temperature will be considerably lower than that of the heater underneath it.
So, a few questions about the setup - what sort of heater element is it, is there any thermal mass involved ?
If you've a layer of insulation board, then a heater element, then the wood flooring, you are likely to see the sort of thing you are getting. There's no thermal mass, so when the heater is turned on, the temperature of the element (and things in the immediate vicinity) will rise quickly in temperature. So localised to the position of the heating wires, the underside of the wood will be getting hot - but it'll take time for that heat to conduct through to the surface, and even longer to conduct sideways to the rest of the wood.
Where electric heating is under ceramic tiles, typically it's embedded more or less* in contact with the tile cement, and the ceramics are good conductors of heat. Not to mention the larger thermal mass of the tiles.
* When I helped a mate install a system, the element was taped down to the insulation board, so separated from the tile cement by a layer of sticky tape.

 

[ericmark]

There are two completely different types of electric underfloor heating, one uses a chemical so as it warms up resistance increases so reasonably self regulating, the other has a simple resistance wire and can seriously over heat, so the sensor is very important to stop overheating. It is not the surface temperature but the temperature under the wood floor, the surface your measuring with the unit you show is being cooled by air so will always be lower than sensor is set to.

I have it in a wet room, typically needs to be on an hour before tiles feel warm, and two hours before up to running temperature, and it would never heat the room without the towel rail also being on, even when floor to point getting too hot to walk on as it did when sensor failed, it still would not heat the wet room enough without the towel rail. It was fitted to dry the floor, and that's all it does, had I used black tiles it may have worked better, but never designed to heat room.

If you want 20°C and max permitted temperature is 28°C the transfer rate is very low, with heat recovery units and very good insulation it may maintain the temperature, but takes an age to get there so would need to be on 24/7. Even with radiators which can heat to 80°C warm up time is still an hour, and cool down time very long.

I have wifi TRV heads which can be programmed to only heat room when needed, for the more accurate control part yes works, but as to switching rooms on/off a failure. Now with fan assisted radiators like I had in old house yes, fast heat up and cools down fast as well, as soon as you heard the fan stop, within minutes you felt a chill, as it heated air not fabric of building.

There is only one form of heating worse than under floor and that is storage radiators. The cure is multi types of heating, Plinth installed fan heaters to get room up to temperature fast, and under floor to maintain it. This is what I had to do with my Myson fan assisted radiator, the reverse to your problem, it would heat the room fast, but once it switched off room quickly got cold, so a added a standard radiator which stayed warm for a time after heating had switched off, bridging the gap between the central heating switching off and on.

 

[pr1970ster]

Thanks for the replies. The underfloor heating is the wire type that is laid onto 18mm plywood and then a self leveling compound covers it all. Theres a thin underlay designed for underfloor heating above and then the 15mm engineered wood flooring. I`ve tried adjusting the probe cut off to around 35c which gives a reading of about 24c on the floor surface. On the other side of the room it seems to work fine, i`m wondering if there are also other factors like some of the plywood is laid over existing floorboards and other is over a concrete base.
Anyway The manufacturer has sent me another probe to try but of course the installer has put the original probe straight into the self levelling compound instead of a conduit so its impossible to pull out. So it looks like a matter of lifting some floor boards to put in the new probe.

 

[ericmark]

Your lucky you can lift floor boards, mine is under tiles, did put it in a pocket so it could be replaced but it got stuck. However also a towel rail so not relying on underfloor heating.

The first thing do when we had it installed was to band jack out the original floor, below the wood base the heating wires are on is 6 inches of polystyrene insulating blocks, so today walk on kitchen floor original quarry tiles and wet room floor also tiled and even without heating on the difference is very noticeable. The heating mat should not be on a concrete base.

My son has under floor heating but for a very different reason, he has a solid fuel cooker with a back boiler and water must circulate through the boiler when fire is lit, so the under floor heating is to get rid of the excess heat when cooker is running releasing it slowly throughout the day, I have also heard of it being used with excess power from solar panels, the beauty of under floor heating is the way it stores energy releasing it slowly.

In the main it is used with some other type of heating, storing off peak power in water for use latter needs massive water tanks, having under floor heating as well half's the size of storage tank required. So as part of an integrated system it works well. However that delay works in reverse when used on its own, very slow to heat up, and equally slow to cool down, so has to be used 24/7 where nearly every other form of heating is trying to make it so you only use it when required.

I would look at infrared heating, this has the opposite problem, it is instant both switching on and off, and because it does not heat the air, very hard to control, however as a pair together it may work, PIR's can switch it on just when your using a room, so keeping back ground heat to 17°C and using infrared as a boost as you enter the room could work? Or of course fan heaters, likely a better option fitted in the plinths of furniture again to give that quick boost when you use a room.

We have used under floor heating (hypocaust) since Roman times, but the Roman version normally also heated walls, and they were not worried about fuel costs in the same way as us. So your looking at best way to supplement the heating, you have to accept it's rubbish and make best of it, so use it for background heat which something else to get last 4°C.

 

[SimonH2]

The underfloor heating is the wire type that is laid onto 18mm plywood and then a self leveling compound covers it all.

OK, the levelling compound should act as a spreader plate, but there will be a difference in temperature across the floor - hottest directly over the wires and cooler in between.

Theres a thin underlay designed for underfloor heating above and then the 15mm engineered wood flooring.

A layer of insulation and a layer of thermally insulating wood So there will be a very noticeable difference between surface and substrate (as measured by teh probe) temperatures. Mostly this won't be a problem, however - if there is anything put on part of the floor that prevents heat escaping, then that part of the floor will have a significantly higher surface (and substrate) temperature. SO if, for example, someopne put a nice insulating dog bed in there, the floor surface could get to as high as 35˚ (or even higher).

I`ve tried adjusting the probe cut off to around 35c which gives a reading of about 24c on the floor surface. On the other side of the room it seems to work fine, i`m wondering if there are also other factors like some of the plywood is laid over existing floorboards and other is over a concrete base.

I imagine it's warmer where it's laid over the boards and cooler over the concrete ? Over time, the concrete slab will warm up, and the floor could potentially end up warmer than over the boards (depending on the temperature underneath the boards).

Anyway The manufacturer has sent me another probe to try but of course the installer has put the original probe straight into the self levelling compound instead of a conduit so its impossible to pull out. So it looks like a matter of lifting some floor boards to put in the new probe.

How long's it been down ? Check the installation instructions which usually specify that the probe must be installed in a pocket for this reason. If so, then you call the installer back and tell him to sort it out* at his expense - with the threat of court action if he doesn't.
* And that includes (eg) redecorating if the skirtings have to come off to allow some of the floor to be lifted.

 

[pr1970ster]

Ok thanks. The probe has been glued into the compound , it's impossible to pull the cable and doesn't look like a conduit has been used as suggested in the installation. I spoke to the manufacturer and he said that but also said some installers don't bother. I guess mine is the latter.
There is space for a second probe connection so maybe use that for when they come to redo it.
Thanks

 

[SimonH2]

You don't need the second probe connection - just disconnect the existing probe and connect the replacement in it's place. Then see if anything changes.
And yes, the installer is an ignorant lazy b***ard and you should make him come back and re-do it as he should have done in the first place - including making good afterwards. There's no excuse other than ignorance and laziness for not putting the sensor in a tube so it can be replaced WHEN it fails. But then some people are happy as long as it works long enough for the cheque to clear