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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Electrical Questions >> electric underfloor heating https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1280066459 Message started by harryw on Jul 25^th, 2010, 3:00pm

Title: electric underfloor heating
Post by harryw on Jul 25^th, 2010, 3:00pm

I have a 24m X 0.5m electric underfloor heating mat(ThermoLam) and I need to reduce the length by several meters. How should the ends of the heating element wires be joined. A qualified electrician will be testing and doing the final connection.
Thanks for your help. Harryw

Title: Re: electric underfloor heating
Post by CWatters on Jul 25^th, 2010, 5:57pm

You can't cut the cable. See page 10..

http://www.ambient-elec.co.uk/Files/thermolam-foil-heater-instructions-ambient-elec.pdf

Quote:

ThermoLAM MATS MUST NEVER BE CUT OR TRIMMED TO FIT INTO A SPACE THAT IS TOO SMALL!

I suspect the reason for this is that the length determins the resistance. Cutting it shorter would increase the power dissipated - possibly overheating the mat and the floor.

You can cut the foil (only the foil, not the cable) so that the mat can folded into a different shape. See page 9. If that doesn't help take it back and get one the right size.

Title: Re: electric underfloor heating
Post by CWatters on Jul 25^th, 2010, 6:20pm

I note that they don't list a 24 meter x 0,5 meter mat in the instructions. Are you sure it's not a 20m long mat?

More on the reason why you can't cut it down... Suppose you have a 20m mat athen the resistance is 38 ohms. If you were to (incorrectly) cut say 5 meters off the end the resistance would reduce to 15/20 x 38 = 28.5 Ohms. The power would go up from around 1400W to 1900W. However because the area is also reduced from 10sqm to 7.5sqm the power density goes up from 140W/m^2 to 253W/m^2. That's an 80% increase. The mat isn't designned for that and the might overheat itself or the floor covering.

Title: Re: electric underfloor heating
Post by harryw on Jul 25^th, 2010, 7:58pm

Thanks for your help. I was afraid that would be the answer, but I wasn't sure my logic was right. By the way cwatters we share a surname HW

Title: Re: electric underfloor heating
Post by CWatters on Jul 27^th, 2010, 12:12am

There are a few of us about. We've traced our Watters line back to 1810 ish. They were in India for about 100 years.