Replacing 2 boilers with 1
Discussion
Our house has two Potterton Profile 60eL boilers sitting side by side on the wall in the utility room. They were installed by the previous owner’s plumber with the rationale that two smaller boilers are more efficient than 1 large boiler. One boiler heats the upstairs (12 rads and 3 towel rails) and the hot water tank and the other boiler heats the downstairs (4 rads and about 35 sq m of UFH. They each have their own header tank in the loft and there are 4 2 port valves - one each for the downstairs rads, the UFH, the HW and the upstairs rads. They’re controlled by a Wiser 3 channel hub for the 2 sets of rads and the HW and a Polypipe controller for the UFH.
According to the serial numbers the boilers are now 20 years old. They’re rated at 17.5 kWh each. Our annual gas usage is about 16,000 kWh.
The current system works well enough although the boilers do cycle a lot, particularly when we’re only trying to heat one room and a boiler is unable to modulate its output.
The question in my mind is how to do this without a major replumb and therefore redecoration. Is this even possible? The first plumber I spoke with just wants to replace each boiler with a new one.
According to the serial numbers the boilers are now 20 years old. They’re rated at 17.5 kWh each. Our annual gas usage is about 16,000 kWh.
The current system works well enough although the boilers do cycle a lot, particularly when we’re only trying to heat one room and a boiler is unable to modulate its output.
The question in my mind is how to do this without a major replumb and therefore redecoration. Is this even possible? The first plumber I spoke with just wants to replace each boiler with a new one.
What you need to understand is your peak heat requirement.
That depends on how quickly you want to be able to heat your house from cold.
You could probably get away with one boiler not much bigger, if you are happy to run the heating for longish hours in a cold snap.Th most gas I've ever used in a day was about 190 kWh from memory, that was when we had elderly guests in a blizzard, so 22degC indoors and -2 with a gale behind it outdoors.
That was quite expensive enough, but it's well under 10kW average power.
Since then, I've sorted some insulation 'issues'.
It would help if you had smart meter data to understand your worst case use.
I like to be able to warm the house quickly, but my 30kW boiler only does 30kW very briefly. Once the water starts returning warm from the 23 radiators, it modulates progressively. The chap who installed it wanted to 'range rate' it down to 24kW.
Unless you have a monstrous hot water requirement, I'd be looking at one medium sized boiler.
That depends on how quickly you want to be able to heat your house from cold.
You could probably get away with one boiler not much bigger, if you are happy to run the heating for longish hours in a cold snap.Th most gas I've ever used in a day was about 190 kWh from memory, that was when we had elderly guests in a blizzard, so 22degC indoors and -2 with a gale behind it outdoors.
That was quite expensive enough, but it's well under 10kW average power.
Since then, I've sorted some insulation 'issues'.
It would help if you had smart meter data to understand your worst case use.
I like to be able to warm the house quickly, but my 30kW boiler only does 30kW very briefly. Once the water starts returning warm from the 23 radiators, it modulates progressively. The chap who installed it wanted to 'range rate' it down to 24kW.
Unless you have a monstrous hot water requirement, I'd be looking at one medium sized boiler.
16,000kw of gas seems very reasonable for 16 radiators and a load of UFH. We have 19 rads and used to have two boilers which used 30,000kw per year between them. One of them died so I linked the systems together; we now use 30,000kw of gas per year. It’s made zero difference.
For 16,000kw of usage I think I’d leave it as it is until things die.
For 16,000kw of usage I think I’d leave it as it is until things die.
Thank you both for your replies.
I did some analysis - in 2024 the highest daily gas consumption was 175 kW. The highest consumption in any 30 minute slot was 16 kW. I'm surprised at that figure - it seems low. Need to look at 2023 as well.
I get the point of a thermal store but one of my objectives is to simplify things and a thermal store would surely take up space and require extra plumbing and pumps and a control system?
I did some analysis - in 2024 the highest daily gas consumption was 175 kW. The highest consumption in any 30 minute slot was 16 kW. I'm surprised at that figure - it seems low. Need to look at 2023 as well.
I get the point of a thermal store but one of my objectives is to simplify things and a thermal store would surely take up space and require extra plumbing and pumps and a control system?
I’ve looked at the detailed smart meter data I have going back to April 21 and the highest energy use is consistently 1.44-1.48 cubic meters in any 30 minute period. This is roughly 16 kW. The existing boilers are listed as 78% efficient. So in theory a modern condensing boiler rated at 19 kW that’s 92% efficient would be more than sufficient. Or have I missed something?
Edited by AW10 on Wednesday 8th January 23:23
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