# What size boiler

#### DavidJW

My boiler is nearly 20 years and only 65% efficient so I think its about time it was replaced!

I have received three quotations and all have suggested a larger boiler -18kW; the current boiler is 14.7kW.

I have done a quick calculation, based on the sizes of the radiators and output figures for similar sized radiators from the Stelrad catalogue, and have arrived at 15kW including dhw. This also agrees with the "whole house" calculation on the Sedbuk website. This suggest that the current boiler is the right size.

However, to complicate things. I am having the outside walls cavity insulated (detached house). According to the Sedbuk calculation this mean that I will only need an 11kW boiler.

My question is what size boiler should I install?
18kW (installers suggestion)
11kW (sedbuk calculation)

By the way, its a heating only boiler - I don't want a combi.

Thanks

If you get a "modulating " boiler it will run at lower kW when it senses the return water coming back hot, so no worry over a few kW.

You may have allowed 3kW for the HW??
Modern cylinders can soak up 25kW, so a bigger boiler can be useful for quick reheat.

Depending on the make of boiler you have installed you may be able to range rate it. The Vaillants I fit are told what heat output to run at, at maximum via the display window and set up upon commissioning. I expect and hope other makes allow this as well.

I always advise my customers who have a traditional system fitted as yours, to set the dhw on time in the morning for about 30 mins before ch comes on so dhw heats up very quickly with full power of boiler just for this, then this goes off because cyl stat is satisfied and ch kicks in with full boiler power to it alone again. Usually first thing in the morning is when everything will be calling for heat.

This also helps with the valid point that Chris makes about the h/w coil 'sucking' all the energy to heat it

ChrisR said:
If you get a "modulating " boiler it will run at lower kW when it senses the return water coming back hot, so no worry over a few kW.

OK. But an 18kW boiler costs more than an 11kW and if the 18kW boiler never runs at more than 11kW you have spent money unnecessarily!

ChrisR said:
You may have allowed 3kW for the HW??
Modern cylinders can soak up 25kW, so a bigger boiler can be useful for quick reheat.

Yes, I did allow 3kW. But please explain why modern cylinders are so Watt-hungry. I have only been in the house 18 months, so I don't know when the cylinder was installed. It looks fairly modern as it has the foam insulation attached to the cylinder and has a label: "Choice! Energy and Environment Efficient. ODP Zero Insulation" I can't see a makers label - must be round the back. The cylinder is appprox 1m tall and 50cm diameter.

If I was your installer I would probably go with the 15Kw boiler and range rate it, based on your figures.

Have you any plans for future extensions etc? If so these need to be included in boiler sizing now.

3Kw is the industry standard for a 36 x 18 cylinder.

gas4you said:
Depending on the make of boiler you have installed you may be able to range rate it. The Vaillants I fit are told what heat output to run at, at maximum via the display window and set up upon commissioning. I expect and hope other makes allow this as well.

So the manufacturer can produce a fits-all-houses boiler and leave it to the installer to set it up? I wonder if BMW will be willing to sell me an M5 with the engine limited to 90bhp for pottering around the town?

But how does the installer know what to set the output to? If it's a properly designed new installation with all the U-values know and the radiators etc properly sized, then the installer has a fairly accurate idea as to the requred boiler output setting. But in my case it must be just trial and error.

Which leads me back to my original question: what size boiler, 18, 15 or 11kW?

gas4you said:
I always advise my customers who have a traditional system fitted as yours, to set the dhw on time in the morning for about 30 mins before ch comes on so dhw heats up very quickly with full power of boiler just for this, then this goes off because cyl stat is satisfied and ch kicks in with full boiler power to it alone again. Usually first thing in the morning is when everything will be calling for heat.

Already doing this; but thanks for the suggestion anyway

Don't quite get you You are saying that it is either 18, 15 or 11 that you require, therefore you know what to tell your installer to set the range rating for

As I said, I would go for the 15Kw, range rate it to 11kw and if this was not enough then you can up it

probably only about £50 difference in boiler cost

gas4you said:
Don't quite get you. You are saying that it is either 18, 15 or 11 that you require, therefore you know what to tell your installer to set the range rating for.

I just want to be able to understand how an installer would arrive at the correct size of boiler to specify.

The 18, suggested in the quotes seemed to be based just on the size of the house (How many bedrooms do you have?) without finding out what was already installed.

gas4you said:
As I said, I would go for the 15Kw, range rate it to 11kw and if this was not enough then you can up it

That is a logical suggestion; I like it

gas4you said:
probably only about £50 difference in boiler cost

And it's only the punter's money, so what the ....!!

Websites are not accurate for calculating boiler size. Get £40 quid out and size the boiler properly using a mears calculator. I bet it will come up more than 11kw

x-man said:
Websites are not accurate for calculating boiler size. Get £40 quid out and size the boiler properly using a mears calculator. I bet it will come up more than 11kw

I downloaded the Barlo Rads "Heatloss" program which comes up with very similar figures to the SEDBUK ones. I also downloaded "Superheat 5" from SDA, which calculates the SAP rating and this came up with very similar heatloss figures to all the other programs. So I don't think I am way out in my calculations.

In any case, the existing 15kW boiler is certainly providing adequate heat so a smaller one would seem logical after cavity wall insulation has been installed.

Would someone please explain the difference between a range-rated boiler and a modulating one? Can a boiler be both?

You seem to be unaware that the manufacturers sell boilers at their own choice of maximum output power for each model.

The Worcester 12Ri or 18Ri are good examples for me to give you.

There is little difference in price and the 12Ri would seem to be ideal for you unless you need the hot water to reheat quicker than in about 30 minutes.

Not all boilers have a maximum heat output setting adjustment because the manufacturer's think the modulation will deal with this. Its pretty irrelevant with a heat only boiler and mainly relates to a combi where say 30 kW is required to heat water but only 10 kW for heating the rads.

I am slightly disappointed that the three heating engineers you have met do not seem to have impressed you with their knowledge and ability ( or perhaps you just think that they are all "ignorant plumbers"? )

Tony

You state that your old boiler is 14.7kw. Is thet net or gross?

x-man said:
You state that your old boiler is 14.7kw. Is thet net or gross?

Net output. Why do you ask? Don't all manufacturers quote net output in their sales literature? I have found that you usually have to dig quite deep (in the technical sections) to find the boilers gross input and gas consumption.

Agile said:
You seem to be unaware that the manufacturers sell boilers at their own choice of maximum output power for each model.

The Worcester 12Ri or 18Ri are good examples for me to give you.

So are they actually identical, apart from the fact that the 18Ri has had the wick turned up? (and you pay £50 for the privilege!) I have suspected this was the case.

Agile said:
I am slightly disappointed that the three heating engineers you have met do not seem to have impressed you with their knowledge and ability ( or perhaps you just think that they are all "ignorant plumbers"? )

Ignorant? No way. But I do become suspicious when the person quoting does not bother finding out what size boiler is already installed!

Unfortunately the many good plumbers and heating engineers get discredited by the actions of a very few cowboys, who tend to appear at the beginning of the listings in Yellow Pages and only give a mobile phone number.

DavidJW said:
Would someone please explain the difference between a range-rated boiler and a modulating one? Can a boiler be both?

They can and these days are both. When you range rate a modern boiler it sets the maximum heat output it will give, therefore modulating between the minimum it can go and the maximum you set.

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