Boiler sizing

2 Jan 2013
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United Kingdom
I am considering replacing our conventional (very old) gas boiler with a gas condensing combi. I therefore used the following IDHEE site to get an idea of the boiler size required.

I live in quite a small detached house that is well insulated with upvc double glazing. Using the above calculator - it was suggested that 13.2kW was sufficient to run the central heating and hot water. This value includes a 'standard' 2000kw for hot water. However if I was to remove the existing hw cylinder and connect the basins/ bath/ sink direct, the notes on the calculator imply that this 200kw allowance can be set to zero since drawing hot water temporarily disables the central heating. That would make the boiler about 11kW rating!?

This seems very small compared to the recommendations from other DIYnot members who have suggested using as much as 30kW for terraced dwellings. The IDHEE calculator above stresses how inefficient over-sizing can be yet I have read that 24-28kW should be the smallest size of condensing combi considered and that 20kW is probably the smallest available. What am I missing? Any help would be appreciated.
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The smallest combi you can get is 24kW for hot water.

The Intergas HRE 24/18 would be about right for you especially with a little weather compensation, although there are other makes available.

But for simplicity and build... you ain't gonna do better for the money.
Thanks for the VERY prompt replies guys. This is my first post on the forum and I honestly expected to wait a week or so to get a reply (if any). Much appreciated.

Can I assume from this that the IDHEE calculator does not apply to combi's? Also what is the criterion for assessing hot water demand?
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A combi will only ever do one tap at a time, no matter what the claims from the makers/installers.

Requirements for you as an onwer?

Lowest heating output.

24kW will heat 9 litres per minute of bathing water.

28kW.... 11 litres.

In real life... a standard bath takes 15 minutes to fill to overflow (when you get in it) of a 24kW combi.

A 28Kw combi is around 25% faster if the mains supply is there, but the lowest heating output figure is proportionality higher.

Your priorities are the deciding factor. If you shower a lot go for a low modulating, weather compensated 24kW boiler and spend as much as you can on insulation and draft proofing.

If you run two baths a day the extra oomph of 28kW boiler might be worth it. Any more would be a waste.

Heatloss calculators are fine for big houses or those not having combis. For those with anything below 5 bedrooms/2 bathrooms, you only need to worry about room heat losses to size your rads.

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