I want to get a condensing combination boiler for a medium sized upstairs 2 bedroom flat. I've been through one of the online calculators and it suggests about 10kW rated. I think I was probably erring on the cautious side too. From what I can make out, it seems that most peoples boilers, and most boilers available are massively overrated (most companies minimum size seems to be about 24kW). Is there a reason for this. I haven't been taking water heating into consideration - in order to get a decent shower would i need a boiler rated higher than the heating demand would need?
The yellowhouse say that they calculated they needed only a 3.6kW but had to settle for 8.5kW as the smallest they could get. I'm certainly not as well insulated as them but we are a only an upstairs flat rather than a whole house. Would anyone out there care to predict roughly what they'd expect to be necessary for a flat like ours (double glazing, good loft insulation, but nothing else special). Its just that I don't trust my calculations when I do them!
Thanks
Nigel
posted on 31-10-06
Boiler sizing is not that important if the boiler is a fully modulating one. Most gas boilers have range of heat outputs so that the boiler does not cycle. Note also that if you want a boiler to operate in condensing mode you need to have a low return temperature from the heating circuit.
This is done by reducing the flow temperature and using larger lower temperature emmitters. Therefore if you are replacing an existing boiler you will find that the radiators are sized for higher flow temperatures and to maintain room temperature you will need to either increase the no or size of radiators or increase flow temp(and thereby stopping condensing mode working).
Martian
posted on 31-10-06
Hi David,
As you have already noticed, these boilers are usually sized to cope with the hot water requirements rather than the heating load. Any size of combi on the market should cope with your heating load. If you want a decent shower and are concerned about how long it will take to fill your bath (if you are putting one in) it would probably not hurt to use one of 24Kw.
I have just specified one for a house and opted for one rated at 32Kw as there is more than one bathroom. It is also a "modulating" boiler which means that it adjusts the gas supply according to the requirement for hot water and the temperature of the incoming water.
This particular one is also compatible with a solar pre-heating system which she wants to put in maybe next summer (this requires space to put a hot water cylinder/thermal store).
BTW: I know that combi's are not generally recommended for multi-bathroom applications, but this is a replacement for one which has been in place for over 20 years in a loft conversion. There is no room for cold tanks and the resident does not want a fully pressurised system.
David
posted on 31-10-06
hmmmm. thanks for advice. I didn't really want to replace radiators but perhaps will have to. Reducing the rating of the boiler would presumably be an alternative in that respect but then I wouldn't get a high enough output for the shower.
How much total radiator power would one want to match a 24kW condensing combi then? Is there an easy way of identifying sizes of the radiators I already have?
For each room you calculate the heat losses you then match these to radiators. A radiator will be sized according to its output in Kw at say 80c if you reduce the flow rate to 60C you need to increase the size to compensate for the lower temperature. The above software will do this for you. But it will be somewhere between 30 and 50% greater than you would have previously.
The boiler will then manage the ciscuit temperature to maintain a constant temperature in the system. The ouptut of boiler will vary according to demand but you do not have to concern yourself with that so long as you do not undersize the boiler.
Martian
posted on 31-10-06
David,
Do not change any radiators. A modern modulating boiler with good internal electronics should be able to mostly keep itself in condensing mode by slowing down the pump and dropping the flame size. Even when not condensing it will be more efficient than an old style boiler.
Have a look at boilers by Ravenheat and Atmos.
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