Re: Heat Loss Calculation



and I

This man is an amateur idiot.
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IMM wrote:

You took the words right out of my mouth!

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How does that compare to a professional idiot ?
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wrote:

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 value
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 small.
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 fiddle factor.
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.
.andy
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