Help me choose a new boiler (or tell me I don't need a new one)

Soldato
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18 Oct 2002
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9,436
Background:

I currently have a 24kw Glowworm Combi boiler. Probably about 7 years old.

Up until now this has fed a living room, two bedrooms and one bathroom but an Electric shower (so not feeding any shower).

What Do I need

  • I am extending my flat and making some changes - adding a bigger kitchen and loft extension.
  • Builder has suggested I get a larger boiler as 24kw is very small and may not keep up with the new needs.
  • I will soon have two bathrooms and i'll pretty much doubling the floor space - many more radiators.
  • In addition I will be going for showers that are not electric - I want to get those cool showers that 'shower from above' - you know the square ones everyone is going for!
  • Will integrate WIFI, NEST, etc.

I was thinking of a 30/32w? Will the extra W mean it can keep up with both showers going? IS there any way to get showers that are good with low water pressure? I've seen shower pumps mentioned......

Thanks for any advice
 
Associate
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17 Oct 2010
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1,073
If you dont want elec showers then you have 2 options, combi or regular/system boiler with hot water cylinder

Combi would be simpler straight swap, system would require pipework altering and space for a hot water cylinder


General wisdom seems to be a combi will struggle to supply 2+ points at the same time, may lead to disappointing showers

Large hot water cylinder will supply multiple points no problems but will need time to re-heat and takes up space


Get your plumber to measure your water pressure and flow rate, that is what you have to work with unless you have major work on a new water main, whatever the result will this provide 1 or 2 good showers or not

If you go for a combi get the largest 40+ kw to give the best flow rate, unless your water flow is poor and this would be a waste

If you go for a hot water cylinder you have 2 options, if good pressure and flow you can get an unvented cylinder for mains pressure showers or if low pressure a regular cylinder with a shower pump to boost pressure



Need to get a decent gas man/plumber round really to take flow and pressure readings and give advice
 
Soldato
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WB do some new hiflow combis now that have insane flow rates. we have a bog standard 42w greenstar and it runs two showers really well, it delivers around 17L a minute @ 35degrees.
 
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Soldato
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Thanks both for the input.

Another variable....turns out one reason why the builder is suggesting a new boiler is he removed the existing on to extend the kitchen......and he know plumbers hate plugging in a boiler they have not removed. Seems like a bit of a cop out...why not get the plumber to remove the boiler :/

Also, apparently when the radiators were disconnected the water was pretty much black, never changed in 8+ years. Would this have a negative imact on the boiler?

Loam: Do flow rates depend on existing water pressure?
 
Associate
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Is builder gas safe registered, if not is he allowed to remove the old boiler (check for leaks etc after removal)?
 
Soldato
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Is builder gas safe registered, if not is he allowed to remove the old boiler (check for leaks etc after removal)?

The builder is not but there was a plumber there when it was removed - I did not go in, just saw the plumber van on the drive.
 
Soldato
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Mains pressure wont increase the flow rate of the boiler, its limited by how qquick the boiler can heat the water. I guess it could negatively effect it the mains pressure was utter pants.
 
Associate
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Also, apparently when the radiators were disconnected the water was pretty much black, never changed in 8+ years. Would this have a negative imact on the boiler?

The problem isn't that it hasn't been changed, but that it hadn't been correctly treated.
Leaving that water will have negative effect on the new boiler and any remaining pipework and rads need to be chemically flushed before the new boiler is fitted.
I would also fit a magnetic filter, all of the boiler manufacturers recommend this now.
The new system needs to be treated with an inhibitor on completion.
 
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Soldato
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18 Oct 2002
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9,436
Sorry for the delay, thanks for the replies all.

Looking into combi boilers I see some quite large ones (40kw) are only a little more than the 32KW ones. Any downside to getting one of this size and not really needing it? Faster heating I guess!

Edit: just to add - can all decent boilers do dual-zone? I'm looking at a winchester bosch 40kw or a Baxi 40kw....
 
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Soldato
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21,095
Electricity consumption and efficiency.. if it is switching on/off as it's heating the water too fast with the flow rate of a single shower then that could cause problems for the boiler.

Power flush + mag filter is the norm when updating a boiler.. ours had the same - superb result :D
 
Soldato
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Are combis always rated much higher kw than normal boilers ?

We have an unvented hot water tank boiler system and for our 3 bedroom semi with two bathrooms and 9 radiators we have a 17kw boiler - and it never struggles or has issue regardless how cold it gets or how hot we want things.
 
Associate
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Are combis always rated much higher kw than normal boilers ?

We have an unvented hot water tank boiler system and for our 3 bedroom semi with two bathrooms and 9 radiators we have a 17kw boiler - and it never struggles or has issue regardless how cold it gets or how hot we want things.

Yup, only due to the way it heats the water. Our 30kw combi runs at 30kw on hotwater and 24kw on the rads
 
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