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?
--
Cheers,
Roger
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