Electric Underfloor Heating in Holiday Cottage Ask questions and find answers to many subjects relating to electrics and electrical work

[Barclay102]

I have electric underfloor heating throughout a small holiday cottage I own. During the cool weather last week a guest turned on the underfloor heating and all seemed to be working well except for the living room. For some reason, while some areas of the floor were warm, I noticed that other areas were still cold, as if the heating in these areas had not come on. My question is, is this possible, how and what can be done about it? All the underfloor heating in the living room comes from the same control box on the wall so I am a little puzzled why one area would work and another would not. Thanks for any help and suggestions.

[Mr White]

Electric underfloor heating in its basic form is a cable that gets hot. If parts of a same specific floor are not as hot as other parts of the same floor I would assume the heating cable has failed at those points. I would also guess its wired like the original christmas tree lights, so that if one lamp fails the remainder will continue because of an additional internal link. It may also be that the cable is self-regulating and this is the part which has failed.

[Barclay102]

Thanks for the reply to the electric underfloor heating problem. I had never thought of it before as similar to Christmas tree lights. I was under the assumption that if one part went wrong, the whole thing would stop working. I’m no electrician, but if the fault is as you say, does that mean that the only way to remedy the problem is to remove the floor? It’s a tiled floor, so it would prove to be a big job. Is there a way to check for faults before going down this avenue?
Thanks once again.

[Mr White]

I have to admit electric underfloor heating is not something I have worked with, but as I first said, its just a cable that gets hot, so if part of it is not getting hot, it must be the cable, or the connections to the cable or if it has a local thermostat.

I would suggest you make sure there are no other "local" thermostats controlling the part which are not too hot, you could also check the resistance and while doing that get some one to walk on the floor, and see does the resistance change. (It shouldn't)

[Barclay102]

Thanks for your help. I know a local electrician so I’ll have to give him a call to pop over and do a few checks. The last thing I want to do is take up the floor however. Thanks again.

[ericmark]

Three basic types of heating wire are used.
1) It runs whole length as one cable, any fault and whole cable fails.
2) It has sections often you can see necking at each section, often the cable can be cut just after necking and special shrink sleeve used when laying.
3) Chemical cable there is a chemical between feed and return which increases in resistance as cable warms up, so furniture placed over cable will only have a limited affect on the heat of floor underneath the furniture. (Raychem system)

Test will depend on system used, with system 3 measuring the resistance or current draw tells you nothing. With system 1 there is no point it either all works or all fails. With system 2 normally the installer records the resistance or current used when installed, so latter you can test to see if any faults, I remember testing the one I installed, however finding the results now I think unlikely.

When my wet room was done, it was found the tile cement was out of date and not setting, so all tiles had to be lifted again, when lifted found the builder had damaged cable and twisted together the ends and taped over with insulation tape, this was of course very dangerous with the element being under a wet room floor. I replaced the whole lot before refitting tiles with new cement.

However it does show what people will do, in a way we were lucky builder walked off the job without finishing as that was not only fault found.

The tile cement is special, it allows for expansion as floor heats, however we have just sold house, and we noted the tiles needed grouting again after some 15 years of use. Not really impressed idea was to dry floor but once cooled after having shower they took 30 minutes to warm up again.

Since it is a lift tiles job to repair, getting it tested is not really much help, OK you may know it's faulty, but you can't do anything about it anyway. If you have something like the T2red self regulating underfloor heating then it may be simply not cool enough to work.

The Raychem T2red is safe as self regulating, however with cheap simple resistive types all it needs is for a dog bed to be put on the floor for the floor in that area to over heat. Even a bag of shopping left on the floor can cause local over heating.

[Barclay102]

Thanks very much for the well detailed response. It was very useful and you could possibly have hit the nail on the head with your explanation. The electric underfloor heating at the holiday cottage was installed over 8 years ago and has worked perfectly. However. during a cool snap a few weeks ago, it was turned back on again and it was then when I noticed the cold area of flooring. This cold area was situated underneath and the area real close by a small rug in the floor, so I’m now wondering if that part of the room floor had already hit its set temperature and turned itself off. I was under the impression that if one part of the underfloor heating in a room was on, then all of it should be on, but I may well be wrong with that way of thinking. The heating was turned on when the room temperature was 17 degrees, so what you are saying is that the section that did not turn on probably thought it was at the desired temperature already. I am at the cottage again on Saturday, so I’ll lift the rug up, turn the heating on and see if this solves the problem. Thank you for your help and advice.

[ericmark]

Sorry no, the Raychem T2red is like a modulating boiler, it does not switch off/on but up/down so in theory should have a smaller hysteresis, this is the whole idea of modulating boiler, although many are not fitted with modulating thermostats so in practice they just turn off/on.

The under floor heating if not Raychem T2red needs under floor sensors to limit the floor temperature, seem to remember 28 degrees C is maximum permitted so they use a special thermostat which measures both air temperature and floor temperature.

One of the problems I had was the under floor probe goes in what is called a pocket, it's a long tube so old sensor can be withdrawn and new one inserted, however the old one stuck, so it would have required tile removal to replace, I tested floor and even when on for 6 hours, it never got that warm so I never bothered about it, however my floor was never covered as in a wet room. If used in other rooms it can get covered as can over heat.

In the main electric under floor heating is only used as a back ground heat, because of the 28 deg C limit, although Raychem T2red can work well because of it's self regulation, but other than Raychem T2red then the only really sure way that you don't get hot spots is liquid under floor heating.

All that is needs is for some one to lie a suitcase on the floor for it to over heat, and if temperature fuses are used for safety, these don't auto reset, so to answer the question you need to know what type was fitted, if Raychem T2red then great, but if some cheap and nasty type then what safety features were included?

Sorry to say Raychem T2red is rare, most is the cheap stuff, including what I fitted, so likely it will mean lifting some floor, but if there is a thermal fuse, then possibly just a small section.

[Barclay102]

Thanks again for your reply to the underfloor heating question. It seems like the only way I am going to find out where the problem lies is to first of all take the small rug away and see if that helps, or alternatively start doing some tests to find out what’s gone wrong. The rug has been there for many many years but I can’t ever remember the floor being cold under it when the heating has been turned off. I’ll Let you know how I get on. Cheers again.