Previous postings on this subject leave me confused.
Good Idea Guide 49 at Wickes gives typical room sizes & recommended BTU output requirenents. But I have a box room to heat and I thought I could calculate the BTU per m3 from the guide (this info is supplied to Wickes by thier supplier - Halstead). Just to check the maths I worked it out at both ends of the scale. Excluding options (north facing / double glazing etc etc) the ratio for 20C is non-linear! i.e. 172 BTU per m3 for a large room on a sliding scale to 279 BTU per m3 for the smallest room. Whereas a reference guide I found at
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works out to within a couple of decimal places to 177 BTU/m3 WHATEVER size the room (21C).
Any advice or thoughts would be gratefully recieved.
It sounds to me as if these are based on 'rules of thumb' - which tend to give extremely approximate - and often conflicting - figures. To do the job properly, you need a calculation based on the U value and area of every surface - and the temperature on the other side of the surface. I tend to use the program which Barlo (now called Quinn?) supply. You can download it from
THe first one is not to mix units from different systems. It is highly likely that a mistake will be made somewhere and you will end up with incorrect results. The trouble is that the conversion factors between certain imperial and metric measures are funny numbers and will add to the confusion.
It is far better to dump the notion of BTUs and to use kW, metres, degrees Celsius and U values in W/m^2.K
Leaflets in the DIY stores are pretty worthless because they usually just use rules of thumb. Depending on the construction of a surface, the heat loss can differ by as much as 10:1
The best way to proceed is either to use a calculator program from one of the radiator manufacturer web sites or to use a spreadsheet. For walls and ceilings, the basic formula is fairly simple
Heat loss = area x temperature difference x U value
For a ground floor, a correction factor is needed related to the amount of outside wall vs. the area. The calculators do this by tweaking the U value as you enter the area, normally.
Once you have the surface losses, you can add them up and then calculate and add the loss related to air changes. Again there is a formula for that based on cubic volume and type of room (assumed air changes).
This method will get you close to what is needed, with some margin in hand. You then have to apply a correction factor from the radiator manufacturers data sheet for the boiler water temperature in use. The rated figures are for a higher temperature than that used in practice in the UK.
If you can't find a calculator program, drop me a mail and I can send you the Myson one (no longer on their web site).
Just point an infra-red thermometer through the window.
Seriously, if you are in a semi or terraced house, you can make *reasonable* assumptions about the temperature on the other side of the party wall based on only a limited knowledge of the heating system employed next door.
One thing to check is that the U values are correct for the material and surface. It's worth cross checking with other sources such as the Approved Documents to the Building Regulations (ODPM web site), or other software or manufacturer's web site.
You could just assume that the temperature is the same as yours, hence zero heat transfer. However, it's probably unwise, since they may not be heating to the same level, may go on holiday etc.
When I've done this calculation in the past, I've assumed 10 degrees on the other side to provide a margin.
You could try plugging in different numbers to see what you get and choose accordingly.
Thanks for the info, I'll have another look at the Barlo calculator this week but if you could email me the Myson one I will be grateful (I can't decipher your email address from this newsgroup since Google made it difficult for spam harvesters). Please add the letter 'etc' after the second underscore.
The only questions I have so far are (1) what are air changes? (2) what temperatures do I use i.e. some calculations assume 20C with an external of -1C, is this the standard method?
I note theres no correction factor for carpets. We're contemplating sanding down the original t&g which by it's nature would create more heat loss than it would otherwise do with a close fitted carpet.
Thanks for the info, I'll have another look at the Barlo calculator this week but if you could email me the Myson one I will be grateful (I can't decipher your email address from this newsgroup since Google made it difficult for spam harvesters). Please add the letter 'etc' after the second underscore.
The only questions I have so far are (1) what are air changes? (2) what temperatures do I use i.e. some calculations assume 20C with an external of -1C, is this the standard method?
I note theres no correction factor for carpets. We're contemplating sanding down the original t&g which by it's nature would create more heat loss than it would otherwise do with a close fitted carpet.
Now I can't decipher.... Please mail me at andyhhallgl
These are as a result of designed ventilation and drafts. The calculator programs have typical recommended/assumed values of airchange per hour of between 1 and 3. To work out the heat loss, the calculator multiplies this by the volume of the room (you will have given it the dimensions) and a constant (like a cubic U value if you like).
This used to be. Nowadays -3 is more typical.
No there isn't really. Is this a suspended timber floor on joists? Quite a bit would depend on whether there is insulation underneath. In any case a ground floor does have the outside wall effect, so U values are sdjusted to account for this using the dimensions to actually modify the U value. The programs *should* do this, but if not, and you need U values, I have a big table of them.
You can combine U values of individual elements if you have things in layers. THe building regs approved document has details, but essentially it is done by adding the reciprocals of each U value and then taking the reciprocal of the result.
If you are planning a complete system revamp, I found that a useful book is the Domestic Heating Design Guide from the HVCA. Costs about £20 and has a lot of helpful information regarding sizing and calculating.
I wrote a program that does most of the calculations (at least the ones I could think off), several years ago. It allows user defined roomchanges/hr,and has built in lists of U for
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