Combi boiler power

[Dave]

Why do most (big) combi boilers have reduced power to the CH? For example the Worcester Greenstar 35CDI is rated 35KW but the CH is limited to 30KW.

[Andy Hall]

It's more the other way round.

Usually the CH requirement for a house is relatively modest (30kW would be the requirement for a fairly large house or a poorly insulated one.).

OTOH, for an instant hot water system, one needs as much power as possible and even 35kW doesn't give exemplary flow of hot water in the winter.

[Dave]

This makes sense for small boilers but if the burner and pump can deliver 35KW to the DHW then I can't see any reason to limit the power to the CH. The heating would heat up quickly then the thermostat(s) would limit the power.

I'm looking for a 35KW heating boiler (to replace two old boilers) and it looks like I'm stuck with a non combi version.

[The Natural Philosopher]

Maybe there are losses associated with the fact that the incoming water for DHW is always cold, but the CH will normally have a hot value in the return.

[Andy Hall]

Typically the burners are modulating so will back of the power anyway as the CH circuit heats up.

There are larger combi boilers around - e.g.

45 model goes up to 47kW on heating and will provide a 20 lpm flow of hot water at 40 degrees.

I have one of this company's previous models and it's an excellent product. Not cheap, however.

[David Hansen]

On 25 Sep 2006 02:32:04 -0700 someone who may be "Dave" wrote this:-

Sounds good to me. A house that is large enough to need that much heating is likely to be too large for one combination boiler to serve adequately.

[Dave]

True but I plan to use the combi section for a single shower room and kitchen tap. The rest of the house will be served from a traditional hot water cylinder heated by the CH part of the boiler.

[Andy Hall]

Then you are looking for something that is the opposite way to most requirements.

All you can really do is to go for the CH requirement and be pleased that you will get a respectabl combi solution into the bargain.

[David Hansen]

On 25 Sep 2006 05:23:34 -0700 someone who may be "Dave" wrote this:-

I trust that the combination boiler will be close to both.

Then you are trading recovery time for input power. I doubt that 5kW will make much difference, unless people are used to bathing in relays. Perhaps what you need is a thermal store rather than a cylinder.

[Christian McArdle]

Normally, it is better to fix the insulation than increase the boiler size. Using reasonable insulation, 35kW is enough to heat a small hotel. No house should require this, unless it is a listed property and fundamentally uninsulatable.

Christian.

[Ed Sirett]

There are a number of factors, one of which would be that circulating more than 30kW of heating is starting to need 28mm pipes and larger than the 'normal' "6m" circulators.

Another is that by limiting the CH power the likelihood of short cycling is reduced and/or the extent of it is softened.

[Ed Sirett]

Have you, formally, calculated/guestimated the heating needs of your mansion yet?

[Dave]

Nice but slightly over budget!

[Dave]

Yes, very.

The combi is just an extra to replace an electric shower. I thought I'd add this because I a 35kW combi is almost as cheap as a 35kW heating boiler. I might still do this if I think I can get away with 30kW of heating.

The existing HW cylinder is adequate and HW use is low so I plan to keep the existing cylinder rather than use a thermal store. If the kitchen also uses the combi the HW cylinder will be left unused much of the time. I appreciate that this is not a normal situation. The house I plan to heat is big, old and very draughty.

[Dave]

Not exactly! Heat calculation is done based on the existing boilers. Each floor has a 60kBTU Potterton Kingfisher. These run almost flat out when it's cold and windy. I would say the house is adequately warm rather than hot!

[Dave]

The house is 300 years old and listed! The draughty windows are the problem. They could be replaced but at huge cost.

[sm_jamieson]

Sounds like you need some tight-fitting secondary glazing panels that can be fitted in the winter. Simon.

[Dave]

Sounds plausible and presents the opportunity for a DIY upgrade!

[Dave]

Sounds plausible and presents the opportunity for a DIY upgrade!

[David Hansen]

On 26 Sep 2006 02:16:10 -0700 someone who may be "Dave" wrote this:-

Far better to insulate it as much as possible than continue trying to heat the garden. How are the draughts caused and what have you done to reduce them. Loft insulation? Presumably cavity wall insulation is not possible.

When you do a heat loss calculation what are the steady state losses?