Replace broken standard boiler or install a combi boiler ?
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Replace broken standard boiler or install a combi boiler ?

by JLW » Wed Feb 15, 2023 6:05 pm


My existing 25 year old standard boiler is broken and needs replacing. I have received quotes for a new standard boiler and cyclinder which is similarly old, this requires the relocation of the flue as well. The combi will be more expenisve as I'd like it to be moved to the airing cupboard, currenly housing the water cyclinder, which will also save space in the kitchen.

Are combi boilers any more efficient to run than standard boilers and worth any additional costs, such as gas pipes and adaptor for current power shower which takes hot water from the tank and cold from the mains.

I have one bathroom without a shower and and en suite shower room and don't plan on moving anytime soon.

Any thoughts would be appreciated between the two options.



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Re: Replace broken standard boiler or install a combi boiler ?

by stoneyboy » Thu Feb 16, 2023 10:33 pm

Hi jlw,
If you go down the combi route you will need to check your main supply pipe is at least 25mm with a good supply pressure. If there is more than one person living in the house a combi supply will have issues with the shower because when one person is using the shower and the cold mains is turned on in the kitchen this will affect the shower temperature.
You should also consider the demand on your mains supply - if there are a number of houses with combi’s in your area, at times of high demand the pressure may be too low.
Regards S

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Re: Replace broken standard boiler or install a combi boiler ?

by ericmark » Tue Feb 21, 2023 10:28 am

To day it makes little difference having two boilers, one for CH and one for DHW as using one boiler to do both i.e. combi. Boilers can reduce output to around 6 kW even when they can produce 28 kW for domestic hot water, but moving from storage tanks to a combi makes a huge change.

In the main hot water pipes are larger diameter to cold water due to lower pressure, so they have a lot of water in the pipe, add to this the delay getting the boiler up to temperature, it can mean running a bowl full of water before you get any hot water. If fitted from new, then the smaller pipes mean same time to get hot water.

With a shower there is a limit both minimum and maximum and also if the small reservoir built into the boiler is used, to reduce delay to sink taps, this can cause shower to start cold, go hot, return to cold then go hot a second time so you have to run shower before getting under it for a lot longer.

You clearly can't use the cylinder to use any excess solar energy, so if you ever intend to fit solar panels, you do not want to get rid of the cylinder.

And any direct water heating means it takes an age to fill a bath. OK for a shower, but may as well remove the bath as it takes so long to fill.

Fitted a combi to parents house, as water tanks were leaking, so killed to birds with one stone, but had to also change the piping and shower as the existing power shower would have been illegal with a combi.

My house the cylinder was removed and direct water heating fitted many years ago around 1982, with two separate boilers, this allowed the old airing cupboard to be made into a bedroom, recently the two boilers were changed to a combi, but after I had left the house. My son says he has not noticed the gas bill going down, which one would have expected when he got rid of the 30 year old boilers.

An oil combi is really a cheat, it just has a small tank inside the boiler as oil boilers in the main don't modulate, gas however has been able to modulate for years, my old main 7 DHW boiler was 18 kW, at that time, to get it any bigger caused a problem when you did not want full bore flow, it did modulate a little, but very little compared to modern boilers, my central heating boiler is in this house 19 kW, for a three story house, combi boilers tend to be larger, 28 kW is common, any smaller and it takes too long to fill a bath, my old house would take 1/2 hour or more to fill a bath, and it was so easy to forget and over fill the bath.

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