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Re: Boiler sizing calculations |
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| message from Andy Hall on 16 May 2004 |
Both of the other other replies get you to knowing the heat losses for
the rooms - essentially the Myson program has a set of U values and
does the sums for you.
Once you know the heat loss for a room you can size the radiator.
What you go with depends on the boiler type and operating temperature,
because the actual radiator heat output depends on the mean
temperature difference between the radiator and room.
If you look at radiator manufacturer data sheets, you will find that
there is a main table of ratings. However these come from a test
which is done at a higher temperature than is normal for domestic
installations and has to be scaled with another table.
The two typical designs for UK installations are based around a
non-condensing boiler and a condensing boiler.
For a non-condensing boiler, the flow temperature is taken to be 82
degrees and return is 70, making a mean water temperature of 76
degrees. With an air temperature of 21 degrees, it means a Mean
Water to Air Temperature of 55 degrees. So you can refer to the
table and you will find that the derating factor is 0.89. This means
that the radiator gives only 89% of the test value as an output.
So, if you are starting from your heat loss number, you have to divide
it by 0.89 to determine which radiator to choose because it will need
to be a higher number than the main table suggests. Then you go to
the main table on the data sheet with your number and pick from there.
In other words, if your heat loss is 890W then you would need a
nominal 1000W radiator.
For a condensing boiler, the ideal design temperatures are 70 and 50
degrees. For that, the derating factor is generally about 0.6 which
implies a rather larger radiator.
However, you have a choice here. Condensing boilers will run up to
the traditional 80 degrees if needed and still be somewhat more
efficient than a non-condensing one. So if you already have the
radiators then it doesn't mean you have to change them.
Also, the heat loss calculations assume -3 degrees outside which is
not for much of the time - in effect you are designing for a worst
case. For large parts of the year, less heat is needed and the
boiler will modulate its temperature down into a more efficient lower
temperature range. At this point, the radiators are plenty adequate
anyway. So the sensible thing, is that if you are putting in new
radiators, size them for 70/50 operation, but it isn't mandatory if
not.
For the hot water, if you are using a cylinder and are able to change
it, use a fast recovery one. This is able to accept a lot of heat
from the boiler - perhaps 20kW or more.
Therefore it is not unusual to have a boiler sized at 25-30kW to do
that, when the CH requirement might only be 10-15kW. If the boiler
is a modulating type, this will work properly because it may be able
to drop its output to 4-7kW.
.andy
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Archived message: Re: Boiler sizing calculations (UK DIY Home Renovation)
