electric underfloor---working out the running costs

touchdown77

Can anyone help on the following calculations;

My architect has calculated that the heat demand for my new build will be 15w /sq m x 280 = 4.2kw (this is the peak requirement for the coldest period of the year)

If I go for electric underfloor, how do I work out the running costs and the heat that will be achieved?

Lets say the underfloor is on 4 hours per night, thats 16.8kw per day for 180 days, thats 3,024kw per annum, costing about €302.

But how do I know what temperature will be achieved?

2011

touchdown77 wrote: »

My architect has calculated that the heat demand for my new build will be 15w /sq m x 280 = 4.2kw (this is the peak requirement for the coldest period of the year)

Tell your architect that the electric mat will use 4.2kW for as long as temperature is below the set point (desired temperature that is set in to the controller). So if you set the controller to 21 DegC and it measures anything below this value the mat will consume 4.2kW until the set point is achieved. Reaching the set point could take hours or in some cases the set point is never achieved :eek:

A unit of electricity = 1kW for one hour and costs about €0.20 (inc. VAT)
So your 4.2kW mat will cost €0.84 per hour to run.
If the underfloor electric heating is on and below the set point for an average of only 8 hours (conservative figures) a day this will cost more than €400 per bill (bimonthly).


A few thinks for you to think about:

1) Underfloor heating (of any type) only works well when under tiles, not under materials that have a high U Value wood or carpet (no matter what you are told).

2) Anyone that I know that uses electric underfloor heating for heating anything other than a small bathroom has unusually high electricity bills.

3) Ask your architect how he expects you to "fix" the underfloor heating when the element has burnt out (like a filament lamp these elements are not designed to last indefinitely). I had it in my last home, in one room it lasted less than 6 months. Despite promises from the installer that they would be able to find the exact point at which it had failed and repair same this proved not to be possible. Luckily the electric mat in the kitchen did not fail as it was installed under expensive Italian marble tiles and I would have to have lifted all of them to install a new mat :eek::eek: It lasted about 5 years in my friends house.

4) Underfloor heating always works more efficiently when there is insulation under it.

5) This type of heating works very slowly. It takes hours for a concrete floor to heat up, so if you are expecting the room to be warm because the underfloor heating has been on for an hour, think again.

touchdown77

Thanks for that, lots to review here.
I should defend the architect and say that he wasn't encouraging electric over the other options but when I heard 4.2kw, I thought it was very low, therefore easy to go for non-complicated electric but as you point out, it could be on for a long time to achieve the desired temp.

2011

touchdown77 wrote: »

Thanks for that, lots to review here.
I should defend the architect and say that he wasn't encouraging electric over the other options but when I heard 4.2kw

This is a sizeable load (in terms of running cost).
Most/all of the loads that you have that are larger than this will generally be on for shorter periods of time, such as hob, instantaneous shower.
To give you an idea, immersions are typically 3kW.
Turn on your immersion and watch the wheel on the ESB meter increase in speed.

it could be on for a long time to achieve the desired temp.

Not that big a deal once you know about it.
With the timer in the controller you can ensure that the element is switch on in time.

frankmul

Is that 15 w/msq, the energy required to overcome heat losses with an inside temperature of say 18 deg and an outside temp of 0 deg? Do you need to supply 4.2 kW of heat continuously to maintain temperature?