Goofed on Underfloor Heating Wattage

[Uheat - Jake]

I don't beleive there is a 300w kit available.
On the subject of spacing, the warm up dcm pro, which is probably the same cable, have a strange recommendation, can anyone enlighten me on how you run that at 190w per m? Do you alternate the spacing ?
View attachment 92196

It's down to the spacings of the cables, if you want 200w/m2 for a heating cable you'd do the following,

5 x 200 = 1000w so you need 1000w cable to heat that area at 200w/m2, you then find a cable around that wattage, so ours would be an Elektra 980w 98m Cable.
You then have to work the spacings out by doing the area divided by the length of the cable. 5 / 98 = 0.051. You then have your cable spacings of 51mm.

If you'd want 160w/m2 or 100w/m2 then you'd just change the equation around to suit them wattages and find a cable to suit.

 

[David Rogers]

Surely dropping the voltage drops the current available drawn by the heating cable hence controlling the heat (current drawn = heat, the max at 240v would be 300w). How can say a 700w rated dimmer be burnt out by a 300w max load? The cable is rated for 240v not dependent on 240v. Applying voltage via a variac would produce the same results.

Using ohms law 240v at 300w would draw 1.25a. The current would not change by varying the voltage but the wattage would. If the voltage dropped to 120v the wattage rated by the heater wires would be 150w. Conversely to drop the wattage to a low 100w would require a voltage reduction to 80v. I think.

John,

Ohms Law is that V=iR where V = voltage, i = current and R = resistance (all applied to DC, but works for many sorts of AC too). The resistance of your UFH cable doesn't change with voltage, but is ultimately responsible for the heat output (watts) defined as i.V. If the cable is rated at 300W with a 240V supply (1.25 amps), then the resistance is 192 ohms (R = V/i = 240/1.25).

If you reduce your voltage to 120V, Ohm's calculation suggests that your new current through that same resistance will be 0.625 amps (i = V/R = 120/192), generating a total heating wattage of only 75W (watts = Vi = 120*0.625) - very much less than you imagine.

I agree with the comment about how modern dimmers on AC supplies actually work (they might either 'chop off' the peak voltage or else 'chop off' the time for which the full voltage is applied). They do not work in the same way as the good old (wire-wound) variable resistances of yesteryear. Whilst I doubt you will fry the dimmer (whatever they're doing, their output will be limited by the 192 ohm resistance of the UFH cable), you should maybe seek advice from the dimmer manufacturer/UFH cable supplier. Good Luck!

 

[john56897452]

Thanks David,

So the maths were about in the right area. Found a 10amp 1000w rated dimmer switch that should be more than up to the job. The theory stands though by reducing the voltage to say even 120v only 75w will be achieved by the heating wire. Still controlled by the floor sensor even if it takes all day to reach 20 c. This has got to be kind to the tiles and more than meets the tile manufacturers recommended 150w spec.

 

[Qwerty]

Do keep us updated with how you get on with this. Do you have a infra red thermometer to read the surface temperature?

 

[john56897452]

Yes I do have a hand held infrared thermometer. Just waiting for the dimmer to arrive before wiring everything up. Floor been laid about three weeks now so should be ready.

 

[john56897452]

Ok, so it seems to work. Took the AC voltage down to about 130v with a 1000w rated dimmer. The floor is probably heating at about 70 - 80w so really gentle for the tiles.

Heating over the past two or three days now, started at 20c now up to 27.1c using an inrared thermometer from work. Takes an age to heat, best part of a day to register changes but gets there albeit very slowly, but that is what I wanted.

Looking at taking the temp up to just under 30c then cooling again. In normal use hoping to leave on all the time. The tiles will be at a tepid heat. If this doesn't negate cracking or warping quartz tiles, nothing will.

Would have been good at the time if the shop had told me the limitaions on these tiles, as at the time I specifically stated I was going to use underfloor heating.

 

[Time's Ran Out]

I think it's a bit like putting diesel in a petrol car - your meant to know.