I'm just about to install central heating radiators and the pipework (minus the boiler) and cant find the formula where I can calculate the watts or btu's req'd for each room so that I can size the rads. Could someone pls advise what formula they use or is there even a website to do this?
Also when plumbing in a chrome towel rad in the wc with a concrete floor what is the best advised method of connection is a/ along the wall pipes just above skirting boards or b/ dig a trench in concrete floor?
Finally does anyone have any special pointers when using plastic piping and fittings - I plan to use this up to about 2 m from the boiler where the engineer will fit copper. Also I'll fit copper to below floor boards then plastic.
Do everything in one system of units. If you mix them, there is a high risk of error. BTUs are a deprecated unit, so everything should be done in watts and metres, etc.
There aren't any web sites that provide even close to accurate results that I have found. There are downloadable programs from the radiator manufacturers such as Barlo and Myson which do give sensible results. I can email you the Myson one if you like.
Otherwise it is almost as easy to use a spreadsheet or a calculator and piece of paper.
For surfaces such as walls, ceilings, windows, the heat loss is proportional to the area in square metres and the temprature difference. The two are multiplied together and then by a factor called the U value. There are tables of U values available for different materials.
Floors are slightly different because of the issue of the perimeter length of outside wall and the area. This is handled by having a table of U values factored by the perimeter and area and multiplying by a chosen figure.
There is then the heat required to heat the air in the room. For this, a specific multiplier factor is used and then that is multiplied by the number of air changes per hour and the temperature diference.
Normally, 21 degrees is used for rooms downstairs where one will sit for long periods, 18 for other rooms and 16 or 18 for bedrooms, 23 for bathrooms. -3 is taken as the minimum outside temperature for the purpose.
For each room, the numbers are then added up. Of course, inside wall to inside wall heat transfer will be small and dependent on temperature difference - it will be zero for rooms of the same temperature.
This will give the heat loss for each room from which radiators can be sized. An additional factor of 10-20% is added if the rooms are heated for small periods of time a day.
The radiator manufacturer calculating programs work all of this out for you.
There are then some more things to consider when sizing the radiators. So far you have the heat output required. The next step is to choose operating temperatures and take other conditions into account.
Radiator manufacturer data sheets give nominal heat outputs based on an EN standard which uses 90 degrees as the water flow temperature. In fact, the heat output is proportional to the mean water to air temperature. In a traditional UK CH system, the water flow temperature is 82 degrees and the return is about 70, giving a mean of
76 degrees. For a room at 21 degrees, the MWTA is then 55 degrees. The manufacturer's data sheet gives factors based on MWTA for his radiators. for the example above it's normally about 0.9. This means that a radiator of nominal 1kW output really has only 900W under these conditions. The heat output will be a bit higher if the room is cooler than this.
Since this is a new installation, unless you have particular extenuating circumstances with installation, you will have to choose a condensing boiler. These are capable of running with a flow temperature of 82 degrees to work with existing systems, but operate more efficiently at lower temperatures.
So.... what you can do here is to run the system at 70 degrees flow and
50 return nominal and it will be quite a bit more efficient than at higher temperature. However, of course the MWTA will now only be around 40 degrees for the example above. The radiator fiddle factor is now 0.6 or so, and the output of the 1kW radiator around 600W. Therefore, for the same heat output to compensate for heat loss, you will need larger radiators. Generally, this is not a big problem. You might fit double panel radiators with fins where single panel ones might otherwise have been used.
You can do either. For burial in the floor, copper pipe needs to be sleeved or it is liable to corrode. There is special pipe with plastic sleeve for this or you can use a special gungy tape called Denso.
For plastic pipe, you can get a corrugated sleeving pipe which can be buried and the water pipe run through it.
Use a good quality brand of fittings such as Hepworth, Speedfit or Marley. Use the correct fittings and observe the minimum bend radii and avoid side stressing the pipe. Allow for expansion. Follow the manufacturers instructions on cutting and handling. Invest in a decent ratchet pipe cutter with replacable blade and don't cut pipe with a hacksaw.
I ran their tables for several rooms in my house, just to see what the figures would be as compared with calculating properly. In some cases, the figures came out quite a bit high (>25%) - which is not so important other than it means radiators are unnecessarily large; in others it was a similar amount low which of course is a problem.
They mix the units - millimetres and BTUs (which is a nonsense).
They don't take operating temperatures of radiators into account. Big difference.
They don't take heat required to compensate for heating air due to air changes into account. Another big factor.
On Sun, 27 May 2007 13:59:56 +0100, Andy Hall mused:
That's obviously so they can sell more larger radiators than than people need. Also it makes sure that people feel warm after fitting a Wickes radiator and makes all their other radiators seem poor in comparison, so you buy more.
I used a Myson Java one which was in beta when I downloaded it (probably 5+ years ago). It seemed to vanish shortly afterwards, but I might still have my downloaded copy somewhere. It does seem to have over estimated the rad sizes, but on reflection I'm very pleased as it means I can heat the house with a flow temperature of 45C, which means the condensing boiler operates extremely efficiently.
Being written in Java, I thought I would be able to get it to work on Unix (Solaris), but unfortunately the java program assumes it's running on a case-insensitive filesystem, so it was back to Windows (spit).
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