Replacement Combi bolier

[gasman26]

Putting in a larger boiler does not give you a better flow rate. All it will give you is the same flow rate, but at a higher temperature.
[/quote]

This was your original statement which is is simply not true.

If you had a incoming supply 20 l/min and you installed a 24kw combi boiler, you would be getting 9-10 l/min at the hot tap at a 35 dergrees rise.

If you fitted a a 35kw combi on the same supply, you would get around 14 l/min at the hot tap at a 35 degrees rise, you would not get 14 l/min out of the 24kw boiler, the water temperature would be too low for use.

So putting in a larger boiler does give you a better flow rate.

 

[mysteryman]

Or, rather, it CAN.

Flowrate may be limited by the capacity of the water main, in which case a more powerful combi is of no benefit.

 

[Fixitrite]

Thanks for the information guys, it is helpful.

Basically my current boiler gives 27KW output and 9.6 litres / min HW.

A 33KW bolier will give me 13.5 litres / min and a 40KW 16 litres per min.

The 33 KW is MORE than capable for CH but the 16 litres / min HW from the 40KW would be nice to have .... plenty of HW for my mixer shower.

BUT ........I don't want to get the 40 and have it running inefficiently because it is too big for the CH demand and constantly short cycles.

what should I do, 33KW and a slightly lesser HW flow to have a more efficient set up ?

Thanks again for your input, I'm going to print this off and have a good read of the advice.

 

[Agile]

Until you have measured the dynamic flow rate then you cannot know the largest size boiler which you could fit.

Tony

 

[D_Hailsham]

Putting in a larger boiler does not give you a better flow rate. All it will give you is the same flow rate, but at a higher temperature.

This was your original statement which is is simply not true.

I accept that my original statement was not sufficiently precise.

So putting in a larger boiler does give you a better flow rate.

No. Putting in a larger boiler may give you a better flow rate. The difference is subtle, but important.

 

[D_Hailsham]

HAHAHAHA, great calculation with a bucket!!! Must try that one with a customer!

I wasn't suggesting that an installer should use this method; he should have the correct tools for the job (weir gauge and pressure gauge).

 

[gasman26]

Thanks for the information guys, it is helpful.

Basically my current boiler gives 27KW output and 9.6 litres / min HW.

A 33KW bolier will give me 13.5 litres / min and a 40KW 16 litres per min.

The 33 KW is MORE than capable for CH but the 16 litres / min HW from the 40KW would be nice to have .... plenty of HW for my mixer shower.

BUT ........I don't want to get the 40 and have it running inefficiently because it is too big for the CH demand and constantly short cycles.

what should I do, 33KW and a slightly lesser HW flow to have a more efficient set up ?

Thanks again for your input, I'm going to print this off and have a good read of the advice.

40kw combi - Without seeing the job it is impossible to say, but unless the boiler is within a few metres of the gas meter then you are going to need a 28mm gas supply minimum.... maybe even 35mm... Your installer will be able to calculate this for you.... With the price of copper this could add a couple of hundred onto the job

 

[gasman26]

Putting in a larger boiler does not give you a better flow rate. All it will give you is the same flow rate, but at a higher temperature.

This was your original statement which is is simply not true.

I accept that my original statement was not sufficiently precise.

So putting in a larger boiler does give you a better flow rate.

No. Putting in a larger boiler may give you a better flow rate. The difference is subtle, but important.

Now you are just being pedantic......
The statement " so putting in a larger boiler does give you a better flow rate" Was in refering to the example i had used above of incoming mains 20l/min dynamic...

It goes without saying, I would assume any gas installer worth his salts will measure the incoming dynamic flow rate.
And pressure being deliverd to the property, before giving you the options of the various boilers available to suit your property.
Pretty bointless fitting a 16l/min boiler to a supply of 14l/min....

 

[gasman26]

I accept that my original statement was not sufficiently precise.

It was incorrect..... Come on say it.....

 

[D_Hailsham]

It goes without saying, I would assume any gas installer worth his salts will measure the incoming dynamic flow rate.

It would appear, from the many topics on this subject, that it's probably the exception.

And pressure being delivered to the property, before giving you the options of the various boilers available to suit your property.

Same goes for this.

Pretty pointless fitting a 16l/min boiler to a supply of 14l/min

At least we agree on something.

 

[gasman26]

The 33 KW is MORE than capable for CH but the 16 litres / min HW from the 40KW would be nice to have .... plenty of HW for my mixer shower.

BUT ........I don't want to get the 40 and have it running inefficiently because it is too big for the CH demand and constantly short cycles.

Is it the duo-tec/platinum 40he?
The output for the central heating side wont be as high as 40kw, but from memory the burners on these are fully modulatin according to the heat demand required.
Your installer should be able to set them maximum heating load within the parameters of the boiler also.

 

[ianniann]

The output for the central heating side wont be as high as 40kw, but from memory the burners on these are fully modulatin according to the heat demand required.

Define "fully"

The 40he has a maximum CH output of 34.4kW, modulating down to 9.6kW (non-condensing output, 12.1kW quoted when condensing). The 24kW model has heating output from 25.9kW down to 6.8kW. Big difference!

Your heating requirements are unlikely to be higher than 24kW even in extreme conditions, and will be lower than 6.8kW most of the time. So how over-sized do you want your boiler to be? Answer is it just doesn't matter. Size it for your hot water needs (if you can see through all the back-biting in this thread) and then deal with the heating from that size of boiler. You may have to range rate the maximum heating output to avoid the boiler switching off prematurely due to being unable to shift the heat fast enough into your system.