Underfloor heating - fuse and stat are getting hot !

[DIYDaddy]

Hi,

I had electric underfloor heating installed as part of my kitchen refit earlier this year. Now I'm starting to use it I've discovered an overheating problem. The fuse holder on the switched spur is heating up an so is the wall stat itself. This causes it to over-read the air temp and switch off prematurely. Within 30 mins the air readings jump from 20c to 25c despite the fact the floor itself has barely warmed up yet.

Facts are as follows:

- Kit consists of 1 x 10m2 mat + 1 x 5m2 mat @ 200w = 3kw. Both wired back to the stat.
- Resistance of mats checked by me & within guidelines (5m2 = 27ohms & 10m2 = 53ohms)
- Dedicated switch on consumer unit with 16a fuse
- Dedicated wiring from unit to switched spur with 13a fuse
- Current at the spur ~12a according to my clamp meter
- Fuse in spur heats up (plastic around fuse compartment is hot)
- Tried 2 different thermostats & both heat up and over read (back of stat is warm)

I can't understand what the issue is as all the readings seem to check out. I'm thinking of calling in an independent sparky to check the whole install but I thought I'l run it by you guys first in case anyone has seen anything like this before ?

Any pearls of wisdom would be much appreciated. Many thanks !

 

[Berko]

Have you checked the wires from the stat to the matts? They could be shorting. Could disconnect the matts and do some tests.

 

[DIYDaddy]

Have you checked the wires from the stat to the matts? They could be shorting. Could disconnect the matts and do some tests.

Thanks Berko. I've only checked resistances to far. What's the best way to check for that kind of thing with a multimeter ?

 

[GarethH]

Are the cables in the thermostat and FCU screwed down tightly enough?

 

[bhm1712]

edited because i dropped a *******

 

[bhm1712]

are you using the thermostat to switch the 3kw load???? what stat do you have? the only reason i ask is because i have seen some installations not using a dedicated UFH stat and the load was too great for a normal stat

 

[bhm1712]

the resistance is wrong = 80 ohms total is not correct for the other figures stated

12 amps with 80 ohms resistance equals total power of 11.5kw

However 240 volts at 12 amps does indeed give you 2.88 kw

240 volts with 80 ohms = 720watts

with the 3kw figure against 240 volts i would expect 20 ohms total resistance not 80

 

[ban-all-sheds]

Facts are as follows:

These don't make sense - what am I missing?

- Kit consists of 1 x 10m2 mat + 1 x 5m2 mat @ 200w = 3kw.

So far so good - 15m² of mat at 200W/m² = 3kW.

5m2 = 27ohms

P=V²/R, i.e. 2.1kW at 240V, ignoring any resistance temperature coefficient.

2.1/5=533W/m²

10m2 = 53ohms

= 1.1kW = 109W/m²

 

[bhm1712]

i agree B-A-S

I have ignored any resistance temp relationship but it still doesnt add up.

 

[ban-all-sheds]

The total does, (as you realised just in time )

 

[GarethH]

These don't make sense - what am I missing?

Think the OP has the resistance measurements the wrong way round.

Resistance of 10m^2 mat should be approx 1/2 of that of the 5 m^2 mat.

Try with:

5m2 = 53ohms
10m2 = 27ohms

 

[ban-all-sheds]

Doh.

Yes.

And the 10m² mat is actually 2 x 5m² in parallel.

 

[GarethH]

with the 3kw figure against 240 volts i would expect 20 ohms total resistance not 80

the mats will be wired in parallel so resistance won't be R1+R2 but
1 / ( 1/R1 + 1/R2)

1/ (1/53 + 1/27) = 18 ohms

P = V^2 / R
P = 240*240 / 18
P = 3.2 kW

 

[bhm1712]

if its getting too hot, then it can really only be caused by drawing too much current i would say. Is it drawing 12 amps constantly?

 

[DIYDaddy]

Wow - many thanks to everyone for taking the time to reply and go into so much detail !

Apologies on the resistances - I'm quoting all the right numbers but not necessarily in the right order I measured the resistance across the cables for both mats but sounds like I got them the wrong way round - d'oh !

The mats are indeed connected in parallel. The tails from both mats come up into the back of the wall stat and are connected together into the same live and neutral terminals.

Both stats I have tried are definitely UFH specific and rated at 16a / 3.6Kw. The current one is a Warmup WIE.

All terminals are tight, I've checked all of them several times.

Thanks again for your time