Underfloor heating load fluctuating

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Hi,

In my bathroom I have an in screed heating cable kit, 400watts. All has been well for 6 years, until recently.

The thermostat, an Aube TH132F, has started switching off when the heating kicks in, it then switches back on and stays on until the load kicks in again, then repeats the cycle.

Upon inspection, the relay in the backhousing does smell a bit squiffy. Testing the circuits reveals a steady 239v in, probe reads 10.3k ohms, floor load impedance is fluctuating though, which struck me as strange. It dances constantly on the meter, between 39ohms, and 62 ohms. This seems to low to me for a 400w kit.

The thermostat is located in the room next door, as not suitable for in bathroom use. It is a floor sensor only stat, and the floor probe is located under the bathroom floor. The cables are hidden in plastered in conduit, underneath the laminate floor of the room next door, also with underfloor heating, and into the bathroom. So nothing is easy access and I thought best to seek advice before pulling things apart to check.

Do you suspect a problem with the cable? Or could there be another area to investigate between stat and cable kit?

Many thanks

Jamie
 
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My first step would be to carry out an IR (insulation resistance) test on the underfloor heating circuit. But you would need an IR tester.
 
Yes but be aware two completely different types of electric underfloor heating.

1) Chemical
2) Resistance wire

With the chemical type the resistance varies with temperature and it is to an extent self regulating. However when I came to fit underfloor heating with this type a separate earth mat was required when fitting in a wet room.

The resistance type is sub-divided some is laid as a complete loop this was the type I used but others have a points where the cable can be cut to length and you can lose some sections but rest will continue to work.

The latter also tend not to have a built in earth shield.

If there is no built in earth shield it would need a rather large fault to cause a low earth to live resistance but with built in earth it would be very apparent.

Also I certainly used RCD protection with mine and it would be strange to get a low resistance reading without the RCD tripping.

So may be step one is to tell us more about you system. Make, Model, Protection.

The one I fitted failed in the main it was too slow to heat up and dry floor once floor was cooled with shower water but also the thermostat failed a few times then the sensor failed and could not get it out of the pocket. Since rarely used it was abandoned.
 
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Thanks for the reply.

Unfortunately I can't tell you much about the system. It was installed 6 years ago. The invoice only states "digital cable kit, 400w, 37m".

The installation - The only RCD protection is the RCD on the consumer unit. There is a fused outlet on the ring main that feeds the thermostat. From the thermostat there is a flex going down through the wall and under the floor where it connects to the heating element. I know it went in in 1 length. I have only got access to the wires check the load etc at the thermostat end. At the thermostat only the live and neutral are connected, the earth on the feed to the heating element has been clipped, which would indicate there is no earth connection the heating element as I can't recall, nor imagine, that there was a separate earth provided from a different source at the underfloor connection.

Performance has always been good. It provides the heat to an uninsulated fully tiled bathroom, and the floor dries fast when it is on. Will be a cold winter in the bathroom without it!

Hope the above is useful info.

Thanks

Jamie
 
It dances constantly on the meter, between 39ohms, and 62 ohms. This seems to low to me for a 400w kit.

FYI, a 400W mat should be around 132Ω. So there's a problem in there, somewhere.

Where are you taking the measurement?
 
Measurement was taken at the load terminals of the relay. However I have realised I have tested it connected to a potentially faulty relay. Will disconnect and test again later, then post result.

Thanks
 
Well I only have access to the end of the flex that the heating element is connected to. This is where it connects to the stat. The joint between flex and heating cable is under a laminate floor with heating mats underneath, not easy access!!!

This eve I disconnected the load from the stat, tested it and got 150ohms rock steady. Reconnected it to the relay (live feed disconnected) and checked again. 150 solid still. Powered up, let it cycle through two power on/shutdown instantly cycles, checked again. 150.

Kinda weird, but leads me to believe it may be the power base/relay assembly. Which Honeywell have agreed to replace FOC for me despite being 3 years beyond the 3 year warranty period! Awesome customer service (no begging or threatening tactics were employed)!

Will see if it solves the problem.

Thanks

Jamie
 

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