Have any of you estimated and/or measured the running costs of under-floor heating in a way which enables you to suggest a methodology to use?
We are converting a built-in garage into a kitchen, and are considering using UFH - either wet or electric.
On the face of it, electric would be a lot more expensive to run, since the price of on-peak electricity is still three times that of gas. However, you then have to consider efficiency. With electricity, all the consumed energy ends up where you want it. With gas, my boiler is at best 75% efficient and I suspect that it's considerably less efficient than that when running at a low output - like when the *only* thing it's heating is the kitchen floor. [Perceived wisdom seems to suggest that UFH needs to be on either 24 x 7 or, at least, for longer than the radiators in order to allow for the thermal inertia]. Even so, it seems likely that more than 1/3 of the heat generated by burning gas would end up in the floor - so it should *still* be cheaper to run than an electric system.
What effect does UFH have on heatloss calculations? On the face of it, the delta-T between the floor and the soil below is roughly twice as great with UFH as it is with a 'conventional' heating system - so presumably the losses through the floor (everthing else being equal) are roughly doubled, so that the overall room heatloss is a bit higher. I say "a bit" because the loss through the insulated floor is - according to my calculations - only about
6% of the total, so doubling it would add another 6%. Is this a reasonable assumption?Whilst it's easy enough to calculate the steady state heatloss under defined conditions (e.g. ambient temp of -3 degC), estimating the total annual energy requirement is not so straight-forward. I will describe the rationale which I have followed thus far, and would welcome any constructive comments as to its reasonableness.
I use gas for CH in cold weather, and for HW throughout the year, but not for cooking. I know how much gas I use in a year and how much in the summer months when just used for HW. I can thus estimate the annual use for HW and deduct it from the total in order to calculate the annual use for heating. If I assume a figure for boiler efficiency, I can calculate the kWh/annum which actually gets to the radiators. If I divide that by the house heatloss in kW under the defined conditions, I can calculate the *effective* number of hours per year for which the system is running at its rated capacity. [Of course, in reality, it's running for much longer than this - at a lower capacity for most of the time - but this gives a basis for comparison].
Extrapolating this for the 'new' room, if I work out the room heatloss under the same defined conditions and multiply it by the effective hours figure obtained above, I get a figure for the likely annual heat input to the room. For an electric system, this is the number of kWh required and for a wet system the figure needs to be divided by the boiler efficiency to work out the number of kWh of gas consumed. In both cases, the figures are fairly horrific! Are there any fundamental flaws with this approach?