Condensing boiler radiator sizing - CHIC

[D_Hailsham]

The electric fire analogy above is incorrect. USA electricity is 110 volt and ours is 240 volt. Thus their voltage is 46% of ours.

Officially it's 120V in the USA and 230V in the UK.

Reduce the voltage by 46% and you reduce the current by 46% as well. The power output is therefore 46% of 46% which is 21%.

Whilst this is correct in theory, the resistance of the heating element may vary with different operating temperatures, and the actual heat output may be different from the predicted output.

Agreed W= V²/R; though this is only strictly true for DC. If AC is involved you have, in theory, to take into account the inductance of the coil of resistive wire forming the element of the fire.

And finally - the output from a radiator can be calculated by measuring the mass flow rate of the water and the temperature difference. You would need to know both values.

Again, I agree. It was a simplification which assumed that the differential was 10C. Double the differential and, for the same output, you halve the mass flow rate. Which, unless you take into account the change in density of water with temperature, is for most purposes the same as the volume flow rate as one litre weighs one Kg at STP and the error is less than 3% at 80C.

In my defence, I would like to point out was 10:30pm and I was being pestered to finish my supper and have a shower.

The advice not to oversize rads is from the DoE.

Doesn't exist! There are three possible contenders: Defra; the Environment Agency; and the DECC (Dept of Energy and Climate Change).

So which one issued this advice and where do I find it?

 

[ianniann]

I believe the advice, from DECC, is simply quoting a 1989 CIBSE study which recommended weather compensation instead of over-sizing. Of course they were more interested in commercial applications where a few hundred quid for a fancy control system was better value than thousands to upgrade all the pipes and radiators.

Fit weather compensation and save yourself a further 15% over and above what you save merely by having a condensing boiler.

That's right. Spend two hundred quid on a gadget to make your boiler work better some of the time, instead of spending two hundred quid on bigger radiators that make your boiler work better all the time 15% now, is it? Not 30% any more? Where do these numbers come from?

 

[mysteryman]

Boys, boys, you can mock and cackle as much as you like, but let us try to be helpful to the DIYer.

By DoE, I could have included 'or its successor' but there you are.

The 15% figure is as stated, an additional saving over and above what you get merely by changing to a condensing boiler. Both together can easily be 30% more efficient than an old boiler with minimal controls.

You still need something to control the flow temperature after you have oversized the rads. You will then need a different higher flow temp to get satisfactory domestic hot water cylinder heating.

Get a quality boiler with weather comp and this is all done automatically. The timing is built in and you will not need Part P. This can be done at an on-cost well under '200 quid'.

Discussing inductance on an electric fire is not relevant to the issue at hand - and is insignificant anyway.