Sizing a system boiler

[DavidA]

Hi

In a previous thread I asked about alternatives to a Worcester Bosch

30CDi boiler. However, now I am unsure whether 30kW is overspec'd. I used the boiler calculator here:

got a result of 19.0kW. This fits with the current boiler which is

20kW.

However, the house is quite cold and the rads never get very hot. My plumber suggests a 30kW boiler as that will ensure ample power and allow for more rads if I extend the house (which is unlikely).

So I was happy to go with 30kW and accept the =A380 or so extra cost. However, I have just read that condensing boilers don't condense, and therefore are less efficient, if run below their capacity. If this is true, overspec'ing would not be a good idea.

Please will anyone comment on whether overspec'ing a condensing system boiler is a good or bad idea?

Best regards

David

[YAPH]

You mean they never get hot even in moderately cold (rather than brass-monkeys) weather? That suggests that the heat generated by the boiler isn't getting around the system properly. Is the boiler running continuously or cycling on and off during this time? When the boiler is running continuously what power is it consuming?[1]. Is it roughly what the boiler's specified heat input rating suggests?

I'd guess at either a blockage in the heating pipework constricting circulation, or something - such as undersized gas pipework - causing the boiler to run at less than its full capacity.

[1] if the gas meter has a clock dial and measures cu ft measure the time - 't' seconds - it takes for the dial to do a complete revolution and divide 1092 by t to get kiloWatts. If the meter measures cubic metres (m3) take the difference in readings of the digits to the right of the decimal point over a minute and multiply by 0.64 to get kW.

[Andrew Gabriel]

Can't actually make any use of that information without knowing why it's the case, e.g. under-powered boiler, too small/too few radiators, or a fault somewhere.

It's not true. Disregard everything whereever you read that.

Overspec'ing to some degree is necessary in order to heat the house up from cold in a resonable time. The disadvantage is that the lowest modulated down power output tends to increase with increasing boiler size, looks like 8.3kW for that boiler, so when you only need 5kW output, it's going to cycle on and off.

[RobertL]

Aren't combination boilers typically over powered for CH, because they need more power for the HW system than they do for the CH?

Robert

[David]

Yes they are, our boiler has a 37kw hot water output and CH is 28kw but we only need 19kw so the boiler has been range rated, however when it was on 28kw the boiler used to get to the target flow temperature of around 65C very quickly and then modulate down.

[ransley]

Is this an old set up, maybe an old weak pump, is it a condensing unit does it reach set water temp then cycle to running only on pump then refire, what temp is boiler set at, you need to understand the system a bit more before thinking the boiler size is wrong.