To work out the size of any rad you need to know a) The U values of the
walls, floor and windows b) The area of all walls and windows c) The
temperature on the other side of those walls and windows d) The desired room
Then you feed the lot into a Heat Loss Calculator or similar computer
program that works out the heat flowing into and out of all the walls. It
adds the lot up and works out if you will need to heat (or cool) a room to
maintain a particular temperature. It then calculates how much power that
will take and spits out an answer in BTU or KW.....
...or you use your years of experience and guess.
Sorry I think U values (UK) = R values(USA). ...
It's the thermal resistance. In SI units I think its measured in watts per
degree K per square meter. You should be able to find tables for the
particular construction you have somewhere.
I'm not exactly sure what you're trying to size (towel radiator?) or
what you're trying to heat (a shower stall or an entire bathroom?)
In any case here is a link provided to me by "the all knowing Google"
not free but pretty cheap for what it can do
On the other hand you could do an experiment & see how much heat input
you need to heat your space
recently I had the need to keep a shower stall warm over night with the
bathroom window open. I put a 100watt light bulb in the stall & it
kept the stall at about 75F (the outside temp was about 58F over
You could do the same but you'll either have to wait until the weather
ratio the heat demand by the indoor / outdoor temp difference when you
do the experiment to the the indoor / outdoor temp difference for the
coldest day expected.
The size bulb that keeps the shower warm is the constant heat input
that would be required. I would double (or 1.5x) the heat capacity
radiator so that it wouldn't have to run full open all the time plus
it would give you some heating reserve if didn't choose a cold enough
BTW the heat output of the radiator would most likely be expressed in
BTU per hour, watts or kwatts.
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