What Boiler Output Rating Should I Choose?

[Dan Robinson]

a stretched boiler will be more efficient than one that is too big surely Dan? ie one that takes a reasonable time to get to target flow temperature..

A boiler working hard will have higher flue temps though won't it?

Of course we are dealing with modern condensers here.


Oh the joys of semantics.

 

[neiltheplumber]

An 18kw or 24kw appliance, depends on how many rads you have, loft or cavity insulation etc.

You want a boiler to work hard, the harder it works the more effiencent it'll be.

Sorry mis read post, ignore me!

 

[Agile]

These kinds of 'answers' aren't really helpful, naturally I've done that already and if it was straightforwards I wouldn't bother asking around.

His reply was as follows:

1. It's very difficult to estimate the heat requirements accurately because the wall construction (which depends a lot on which part of the house you are looking at) and local wind conditions can make a big difference to the various calculators and guidelines.

2. It's unlikely that the figures stated in the manual for the (30-plus-year-old) boiler were ever completely accurate and they are likely to be even less accurate now.

We've discussed this pretty thoroughly and I have a quotation for 2 different boiler sizes, but since he's left the final decision up to me I'm taking advice from anywhere I can get it first.

I am sorry that you don't find my reply very helpful! But then I did not find your first post very helpful because it said nothing about what you asked your chosen installer and consequently that wasted my time!

Your installer seems so stupid that he cannot measure the power input of your boiler and knowing the efficiency, work out the power output! You could even do that yourself with the information we give in the FAQ!

Calculating the heat loss is not totally exact, but it can be estimated with a fair degree of accuracy by someone with some experience.

You suggest you need 24 kW or 30kW but tell us nothing about your house! Is it an eight bedroom detached because that the kind of house that needs boilers that powerful.

Adding up the existing radiator power outputs will also give the heat emitter power which needs to be considered.

Tony

 

[Dan Robinson]

We've discussed this pretty thoroughly and I have a quotation for 2 different boiler sizes, but since he's left the final decision up to me I'm taking advice from anywhere I can get it first.

For us the choices left to the customer have VERY little to do with the final performance. Any engineer or company that leaves such a decision to the homeowner should be avoided.

It's very difficult to estimate the heat requirements accurately because the wall construction (which depends a lot on which part of the house you are looking at) and local wind conditions can make a big difference to the various calculators and guidelines

Twaddle. External conditions are factored with a period of historical "worst case" scenarios. Then each room/area is calculated as per its construction and the surrounding environs.

But then of course, to do it properly takes time, and therefore money.

We've done a few recently where the heat loss of a room was negative.

 

[ALEC1]

A boiler working hard will have higher flue temps though won't it?

not if it has a compensated controller on it...

But it is semantics as you say...

I down rate my boilers to 1kw to 1.5kw per radiator, use compensation controllers so that the radiators only get as hot as is needed..but the boiler in a heating period is always burning, albeit at a low temperature if say 45c...flue temp, around 30c...

 

[doitall]

The OP is only having the boiler changed, I think, therefore one assumes the house was warm enough, so as I said just total the rads up, if they add up to 18Kw allow whatever for hotwater and there you have it.