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QUESTION:Are there any sites that offer "simple" spreadsheets that can be used to
calculate what size bolier is needed for my CH/HW?
ANSWER: In fact it isn't so much the boiler size that you need to get
absolutely right since many of them now modulate over a range of
output - i.e. adjust their burn rate to match the needs of the
radiators.
You do need to size the radiators reasonably well and there are
programs for doing this.
I have done this exercise numerous times, and they are OKish but you
really have to watch what they are doing.
The basic principle for radiator calculation is to work out the heat
loss from the room for a "worst case" scenario of -3 degrees outside
and then to size the radiator to meet that. The heat loss is
calculated by taking each surface in the room in turn (walls, windows,
doors, floor and ceiling) and working out the loss through it. This
is done by measuring in metres, calculating the area in square metres
and multiplying by the temperature difference through the surface and
a factor called the U value. The larger the temperature difference
the larger the heat loss. Also the larger the U value the larger the
heat loss. Thus the U value for a cavity wall with insulation will
be lower than solid brick for example.
For internal walls the temperature difference will be lower or even
zero in which case there is a small gain or loss from room to room
which could work out to zero. The heat losses (or gains) from each
element are then added together and an amount added for heating the
air assuming a certain rate of change of air per hour. For the
radiators, various factors can be applied such as whether they are in
covers, the operating temperature etc. Thus for each room you
arrive at a heat requirement.
Conventionally, you add all these together, add about a 20% margin and
that gives the maximum likely requirement for the boiler in watts or
kW. Very often, apart from the modulating operation that I
described, the boiler will have a higher output level when heating the
hot water cylinder or in direct heating of cold mains water in the
case of a combi. This is because for a cylinder you want to recover
the hot water temperature quickly and revert the boiler back to the
heating as soon as possible.
The problem with the downloadable software programs that the radiator
manufacturers have on their web sites is that you have to enter the
data very carefully to be sure of counting internal walls correctly
from room to room Otherwise heat losses are not computed correctly.
Moreover, some of them have incorrect U value tables so compute the
wrong answer. I have known them to be out by a factor of three
before now.
The programs on Myson's and Barlo's web sites are reasonably good, but
both have incorrect U values in some cases. Fortunately you can
correct these. A good source of data for that is the Approved
Document to part L1 of the Building Regulations. You can download
this from www.odpm.gov.uk in the construction section.
I did try a web site calculator once - Discounted Heating I believe.
It was a dead loss because U values were clearly wrong and couldn't be
altered.
Do ignore any calculator or magic formula that purports to calculate
what you need as a rule of thumb based on the type of house and your
auntie's telephone number. These are generally wildly innaccurate.
The PC programs aren't bad if you watch what they are doing and sanity
check the results. To be honest, I have found that by the time I've
done that, I might just as well have just sketched a plan of each
floor of the house and marked dimensions on it. Then with a piece of
paper and calculator I can compute each room in a few minutes and the
whole house in a couple of hours or less. Since most of the work is
data entry either way, I tend now to prefer the pencil and paper
approach followed by a check of the results.
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