Choice of boiler capacity

[Tall Question]

Am in the process of replacing my central heating boiler (22KW).
The local gas board suggest using a 18KW replacement but a local contractor suggests 24 KW. This poses the question :- Does an 18kw boiler running at maximum for a lot of the time use more gas than a larger, say 24KW unit running at a lower , more relaxed rate, which is more efficient ?? or more to the point - cheaper.

Thanks - Tall question

 

[stevetheplumber]

Am in the process of replacing my central heating boiler (22KW).
The local gas board suggest using a 18KW replacement but a local contractor suggests 24 KW. This poses the question :- Does an 18kw boiler running at maximum for a lot of the time use more gas than a larger, say 24KW unit running at a lower , more relaxed rate, which is more efficient ?? or more to the point - cheaper.

Thanks - Tall question

most modden boilers modulate to match demand so better to be over unless the original was over sized

 

[Puddle]

As steve says with modulation,moderate oversized not a problem
generaly you do not want an under sized boiler as would be more expensive to run
but also have to be careful as if as said boiler was oversized on last replacement,to oversize again will not be very efficient
however being cynical I would believe and go with anybodies advise over British gas

 

[Beeline]

If you choose 18KW you may find DHW performance inadequate. Both boilers will probably run your heating no probs so choose based on how much DHW flow you want.

 

[doitmyself]

I take it you are not talking about a combi boiler,i.e you have a hot water cylinder.

You need to match your boiler size to the heat loss from your house. The easiest way to do this is to use the [DLMURL="http://www.energysavingtrust.org.uk/business/Business/Building-Professionals/Helpful-Tools/Whole-house-boiler-sizing-wizard"]Boiler Sizing Wizard[/DLMURL]. It's more accurate than the wet finger in the air or "experience" of most plumbers.

 

[migoplumber]

another way to think of it,
if i want to tow a caravan from a to b and know that a mini cooper could do it just (and once), and a fiesta would do it with a bit less hastle, and a jag would do it forever and back, i would rule out the mini straight away and depending on finances choose the fiesta or jag.

 

[doitmyself]

another way to think of it,
if i want to tow a caravan from a to b and know that a mini cooper could do it just (and once), and a fiesta would do it with a bit less hastle, and a jag would do it forever and back, i would rule out the mini straight away and depending on finances choose the fiesta or jag.

That is not an appropriate parallel. Towing a caravan with a mini will put more strain on the engine, brakes, suspension etc than towing with a Jag.

A boiler does not have any comparable components which will suffer the same wear and tear. If you put in too large a boiler all that will happen is the boiler will modulate down to give the correct output. And the problem then is that the boiler may not be able to modulate low enough, so it will run in on/off mode, which is less efficient.

 

[smoggs03]

As steve says with modulation,moderate oversized not a problem
generaly you do not want an under sized boiler as would be more expensive to run
but also have to be careful as if as said boiler was oversized on last replacement,to oversize again will not be very efficient
however being cynical I would believe and go with anybodies advise over British gas

chances are the BG advisor was selling buggies in toy r us or pasties in greggs before they got the BG job!!!

 

[Plucky]

With modern condensing boilers sizing a boiler correctly is very important. Too small and it will struggle to perform but too big and it will barely run in its condensing, most efficient, mode.

New boilers are designed to run at approx 20 degree temp differential. If you oversize the boiler then it will reach its temp alot quicker and will not be as efficient as a condensor that takes a bit longer to reach temp.

Hope that makes sense?

 

[samwilko]

Did either of them calculate your heat loss? If one did and one didn t go with the one who did. Its no good guessing.

 

[doitmyself]

Did either of them calculate your heat loss? If one did and one didn t go with the one who did. Its no good guessing.

That why I suggested using the Boiler Sizing Wizard (click on link in my earlier post).

With modern condensing boilers sizing a boiler correctly is very important.

Most "heating engineers"/plumbers seem to think that you can put in any boiler you like and modulation will take care of everything. This is particularly true of combi boilers, when sizing is dune purely on the Hot Water requirement. So you get the ridiculous situation where a Vaillant 937 is installed because the house has two or three bathrooms, but only needs 10kW for heating because it is very well insulated. The 937 modulates between 12kW and 28kW, so it is unable to modulate low enough. It therefore runs in on/off mode all the time. And don't forget that it only needs 10kW in the depths of winter. When the weather is warmer, the requirement is less than 10kW.

Ideally you should have a boiler which modulates down to half the heating requirement or lower; but very few boilers go low enough. This seems a bit ridiculous when the insulation requirements for new houses are so high. It means most of them are having oversized boilers installed.