Radiator Sizing


Postby Treehugger » Wed Jan 16, 2013 5:11 pm

I want to replace two old radiators in my living room. I'm not sure what size to install as most calculation sites only give caluclations for one radiator - my query therefore: is the BTU required the same when fitting two radiators or can this be reduced?
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Postby Atticus Finch » Wed Jan 16, 2013 5:37 pm

Treehugger wrote:I want to replace two old radiators in my living room. I'm not sure what size to install as most calculation sites only give calculations for one radiator - my query therefore: is the BTU required the same when fitting two radiators or can this be reduced?

The figure provided by on-line calculators is just the heat loss for the room; they then assume you only need one rad. If you want to divide this between two, three or even four rads, you can.

So a 2kW loss can be provided by: one rad of 2kw; two rads of 1 kW each; or one rad of 1500W and one of 500W; or even two rads of 800W and one of 400W.

Which calculator have you been using? Many of them are very suspect. This one gives sensible results.
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Postby Treehugger » Wed Jan 16, 2013 8:18 pm

Thanks for that. I have used the calculator you suggest but note that several others come up with diferent figures. It appears that I can replace my old radiators with much smaller modern convection type ones and had been wondering if having two radiators, i.e. one at each side of the room would create some form of improved convection effect thus perhaps reducing the overall size required. I have in fact a fairly small room, only 3.1 x 4.85, it also has a working coal fire so I a am losing a lot of wall space hence why I was seeking to reduce the replacement radiator size's. My walls are also 600mm thick solid stone and the BTU calculator's don't have an option for this - again I am thinking that the thermal mass of the walls would have an effect on the BTU requirement
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Postby Atticus Finch » Thu Jan 17, 2013 11:57 am

Treehugger wrote:I have used the calculator you suggest but note that several others come up with different figures.

You will get different results from online calculators. It all depends on the assumptions they make and the methodology used. Some have glaring errors. The one I suggested has no errors, the methodology is correct and the assumptions are sensible.

My walls are also 600mm thick solid stone and the BTU calculator's don't have an option for this - again I am thinking that the thermal mass of the walls would have an effect on the BTU requirement[/quote]
The U-value of a 600mm stone wall is between 1.4 and 1.68. The on-line calculator uses these U-values: Brick cavity - 1.5; Solid brick 200mm - 2.1. If you entered 'brick cavity' into the calculator, you will get a result which is reasonably close to the correct loss for your room.

The thermal mass does not affect the heat loss; all it does is help iron out the fluctuations in internal temperature. Don't forget that the heat loss calculation gives you the amount of heat required to bring a cold house at -1C up to 21C when it is -1C outside.
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Postby Treehugger » Fri Jan 18, 2013 9:16 am

Many thanks, your advice has been of great assistance.
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