Boiler size

[Hoof Hearted]

I'm about to get central heating installedI in my large 5 bedroomed house. Because of the size of the property, the plumber has recommended the Worstecer Bosch Greenstar 42 cdi. This has a central heating output of 30kw. I have two concerns about this.

1. I did the calculations myself and I reckon that the house requires 20kw to heat it entirely. So maybe 30kw is over the top?

2. Some of the rooms are almost never used. Especially not in winter. I've calculated that the rooms which are in regular use would only require 13kw to heat them.

I dont mind having too big a boiler if there is no trade-off in efficiency, but perhaps someone can tell me.... Is it better to have a small boiler burning for 50% of the time, or a large boiler burning for 25% of the time. With my lack of experience, I would have thought a smaller boiler would be more efficient so long as I took care to ensure that it has sufficient capacity.

Ian

[david williams]

the calculations are done for whole house,whilst you may not use all the rooms all the time should you do so and we have a very cold winter,you would want to blame supplyer of your system when you found boiler having to work beond its capacity and failing

[Hoof Hearted]

I've lived here 20 years and never needed to heat the whole house yet. But that's not what I was asking about.....

According to the calculations, this house needs 20kw to heat it all. If it is fitted with radiators that can output 20kw, then a bigger boiler aint going to heat it any more.

So I am still interested to see an answer to my original question..... Is a boiler that is greatly oversized, going introduce more inefficiencies than a boiler that is moderately oversized? I was thinking in terms of gas wastage during the firing up process perhaps?

Ian

[Bernie2]

A combi which is to have a high hot water flow rate, usually requires a lot more gas than it uses for the central heating.

The 42cdi I think, puts kW into the hot water and you get about 17 litres per minute flow rate.

But you also get 30kW central heating. Although this will modulate down to about 9.4kW.

The thing is, if you want good hot water flow rates, you usually also get a high Kw for the central heating, but they do modulate and only use out of the kW the heat needed.

I suppose you could get smaller but you would probably loose some hot water flow rate if that is important.

[Puddle]

from your post last year on subject,thought penny had dropped,you were given good advise then

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Interesting! I just took a closer look at the technical data for the boiler in question....

The Greenstar CDi Series at a glance - Worcester Homeowner area

Yes it's a 42KW boiler, but the full 42KW is only available to the hot water. The central heating gets from 9.4KW up to 30KW. This is sounding more like it. The 30KW is only 5KW above the radiator capacity.

I couldn't help but notice though, that the 30, 37 and 42CDI models all give 30KW to the central heating. So you pay your extra money just for the better hot water flow.

Last edited by Hoof Hearted; 24-06-2008 at 04:16 PM

Also if boiler fitted undersized to heat the rooms only you want and to heat only the water you want,you will soon moan when you come to sell house and have to deduct price of new boiler to be fitted that is correct size for home
Maybe you could nip to primark and buy some jumpers for any visitors so you can cut back on output more

As you very well know the boiler is very efficient and range rated

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I would have thought a smaller boiler would be more efficient so long as I took care to ensure that it has sufficient capacity.

well if it had sufficient capacity for your property,it would not be a smaller boiler would it

[newbie1]

just what you need a thinks they know it all customer bound to be a real pleasure to work for

[david williams]

perhaps if you contact reactfast they will train you to be a plumber /heating engineer should only take 8 weeks then you wont need any of us

[Plucky]

Thesed boilers modulate anyway do they not?

[badbob]

5 bedroom house here, 10 rads. 32kW boiler fitted. I would think a bigger boiler running lower would be more efficient than a smaller boiler running full chat. Just like a car.

[doitmyself]

5 bedroom house here, 10 rads. 32kW boiler fitted. I would think a bigger boiler running lower would be more efficient than a smaller boiler running full chat. Just like a car.

A boiler is not like a car! A car with a six litre engine which is doing 3000 rpm at 90 mph will certainly last longer than a buzz-box with the engine doing 6000rpm to achive the same speed. But on a boiler the only thing which happens is the gas flow varies with requirements.

If you have a 32kW boiler and the house only needs 20kW to maintain an internal temp of 21°C when the outside temp is -1°C, the boiler will modulate down to provide 20kW. That's fine, but all modulating boilers have a modulating range (9.8-30kW for the 42CDi). Heat requirement is directly proportional to the temperature rise. So if you need 20kW at -1°C, you will need 9.8kw when the outside temp is approx 10°C. If the outside temperature is higher than 10°C, the boiler will stop modulating and go into on/off mode, which is not so efficient