What Boiler Output Rating Should I Choose?

[roberta1000]

I'm at last thinking of replacing my ancient boiler. According to the manual the current one has an output rating of between 24kW and 30kW depending on the manifold pressure. I've no idea what it's actual output is but I suppose it's somewhere between these figures.

Is there any advantage/disadvantage (apart from cost) in choosing a boiler at the top of or above this range? It'll be a system boiler. I don't want to save a few £ and find that the boiler isn't up to the job next winter.

 

[D_Hailsham]

Use Whole House Boiler Size Calculator to find out what size boiler you need.

 

[ALEC1]

top of the range has nothing to do with size...

match the boiler to the heat load of the house and/or radiators to optimise the new boiler

 

[Agile]

Why are you not asking your gas registered installer who can look at your house and give the best advice?

Oversizing boilers reduces efficiency and causes on/off modulation when the heating requirement is low!

Tony

 

[Dan Robinson]

Use Whole House Boiler Size Calculator to find out what size boiler you need.

that thing needs serious updating. It overcooks mine by nearly 50% and thats without Hot Water.

 

[roberta1000]

Why are you not asking your gas registered installer who can look at your house and give the best advice?

Tony

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.

 

[D_Hailsham]

Use Whole House Boiler Size Calculator to find out what size boiler you need.

that thing needs serious updating. It overcooks mine by nearly 50% and that's without Hot Water.

In my limited experience of recommending it to people on this site, whenever it has come up with suspect results it has been due to incorrect data input.

There are other calculators available on the Sedbuk and Baxi websites. But I believe they are using the same U values etc as in my original recommendation.

There is a more up-to-date version but, at the moment, it's paper based. In any case the results from using both methods are very close, which is why I have stuck with recommending the online version.

Domestic heating sizing method (2010 edition)

 

[Dan Robinson]

I fed the data in from scale drawings. Accurate to a few centimetres.

 

[Tom1981]

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.

 

[Dan Robinson]

Not necessarily.

 

[Tom1981]

Not necessarily.

But you do agree they are more effiencent when running on their limits? you dont want a oversized appliance, it's like sticking a V8 in a Ford Fiesta.

 

[smeghead12]

go with the smallest boiler that will do the job , keep it turned up on high , dont go too big , remember any boiler more than 60,000 btu will need 28mm flow and return , so if you go big with 22mm pipework you will have turn down anyway as the heat will not leave boiler as fast as required , 3412 btu is 1kw , a 1000 x 600 double rad is about 1 kw , a cylinder coil is about 1 kw , most houses only require about a 15kw boiler but some people go bigger thinking better it not , hope this helps , tony - qualified gas fitter & qualified plumber .

 

[Dan Robinson]

But you do agree they are more effiencent when running on their limits? you dont want a oversized appliance, it's like sticking a V8 in a Ford Fiesta.

No.

A boiler should be sized so that it is able to cope with the property. A boiler working hard is not efficient. A boiler that can range down to suit the building is better.

Working hard, working on its limits, are not conducive with working efficiently. Oversizing brings in issues of reliability. Efficiency is harmed by oversizing. But not as much as longevity.

Hence the whole house calculator giving an output 50% more than needed receiving my criticism.

 

[ALEC1]

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..

 

[doitall]

If your guy cannot work out close enough plus 10-15% I'd be looking for someone that knows his/her job.

perhaps if you add a location we can recommend someone.

Ps you can get a very good idea by noting the rad sizes and looking on the chart to get the output from each one.