There are a number of PC based applications for doing this which are
produced by the radiator companies.
Those by Barlo and Myson work OK but do have some issues.
The basic principle they use is that you measure each surface (walls,
floor, ceiling, windows etc. and input the values into the program,
room by room, for each element selecting the material - e.g. double
brick wall with insulation, single glazed window and so on.
For most surfaces, a formula of
Heat loss or gain (in watts) = area x temperature difference x U
Where area is in sq. metres, temperature difference is in degrees C
and the U value comes from a table.
Normally the temperature difference will be from the inside wanted
temperature of the room to the worst case outside temperature.
Conventionally, -3 degrees is used for the worst outside temperature,
but you can put in a lower one if you feel it's appropriate to the
location. For the inside temperatures you can choose different
ones room by room (e.g. 21 for a lounge, 18 dining room, 16 bedrooms,
23 bathroom) or a common temperature for all rooms.
If you use different temperatures, strictly you should calculate heat
losses and gains. For example for the case of a bedroom (at 16)
above a lounge (at 21) there will be a heat loss through the ceiling
of the lounge at a rate determined by a 5 degree drop. For the
bedroom there will be a heat gain, so this ought to be subtracted from
the losses for the bedroom. However, most of the PC programs don't
process a heat gain treating it as zero, only a loss. Of course this
is desirable if you are a radiator manufacturer since the calculations
suggest a larger radiator than you really need. In practice this is
not hugely important since the room to room figures are normally
More importantly, the U values used in some of the programs are wrong.
In one case I found several values that were out by a factor of 3.
You can do a sanity check with other software, or better, use the U
values from the Building Regulations Approved Document to Part L1.
This is downloadable from www.odpm.gov.uk
One thing to watch out for is U values for ground floors. Because the
heat loss is affected by lengths and proximity of outside walls, the U
value to use is adjusted by taking into account the lengths of the
walls and the number that are exterior - the more this is the higher
the U value. There are tables of these to account for an appropriate
The principle of the calculating programs is to do the multiplications
and add the results per room. There are then factors which they may
apply normally by multiplication:-
- Whether the room height is more than a certain amount
- Whether the heating runs intermittently
- Higher than normal exposure of the property
- The boiler operating temperature. This one is important, because
the radiator data sheets assume operation normally at 90 degrees,
whereas the UK convention for a standard boiler is 82 degrees flow and
70 return. If you are designing a new system with a condensing boiler
it is advantageous to operate it at 70 and 50 or possibly even less
since the boiler will operate more efficiently. For any of these
cases the radiator must be derated. Thus for example, if the data
sheet says 1000W for a radiator and you want to run it at 82/70 then
it should be derated to about 0.9 - i.e. you will get 900W not 1kW out
of it. Therefore to make up the heat loss, which is really the
point, you have to select a radiator which matches the heat loss after
all deratings are done.
- Another derating is fo rif you will use radiator covers. This can be
as much as 30%.
The s/w programs are useful to a point but I have always used a sheet
of paper and calculator or a spreadsheet as well so that I can see
that everything has been accounted for properly. You can easily do
the sums for a house in an evening even if you haven't done it before.
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