Keep gas boiler or install heat pump?

[Walshie]

Hi,

I could use some input on the following please?

We are about to start a fairly major renovation.

We have inherited a new gas boiler - which is a year old, is A rated, has an efficiency rating of 94% and has the capacity we need to support the future plans.

The big question is whether to retain the gas boiler or to take the leap and install a heat pump?

 

If we go the heat pump route - it'd have to be an air source as we have limited space.

 

We will have solar PV and a battery and the house will be very well insulated.

There will be a mix of underfloor heating and radiators.

 

I guess my main challenge is whether to take the financial plunge now or wait another ten years when the boiler is out of warranty and see if there are possibly better incentives and the heat pump tech has moved on some more?

 

What are your thoughts?

Thanks for your help,

 

W

[nod]

From a running cost point of view 

Your one year old boiler will beat the HP hands down 

I’d get the pipe work needed in for a HP and some cables across Which will cost very little Future proofing for when you decide to install a HP 

[JohnMo]

Assuming a system boiler.

 

Use a heat pump cylinder - not a boiler cylinder. This will future proof cylinder, but get very fast reheat time, from the boiler.

 

Set up boiler on X or W plan (priority demand hot water). Definitely not S or Y plan. Design the whole heating to run at one temperature, ufh and radiators, then run weather compensation direct from boiler. Ideally if possible 45 or well below.

 

Do a heat loss calculation, does your boiler modulate to half that figure?

 

If you don't want to, or cannot with the free boiler chuck it or resell.

 

 

[Mike]

  On 26/02/2025 at 17:41, Walshie said:

I guess my main challenge is whether to take the financial plunge now or wait another ten years when the boiler is out of warranty and see if there are possibly better incentives and the heat pump tech has moved on some more?

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Domestically the technology is already mature, though there will no doubt be further incremental gains. Most recent advances have been on high temperature (as in very high temperature) heat pumps for industry. There must still be scope for cost reductions as the market grows, though sales in the rest of Europe are already way ahead of the UK, so it's not a niche market. My guess is that if the price comes down, then so will the subsidies, so I wouldn't expect Government incentives to make a big difference.

 

As for running cost, the latest Climate Change Committee report (just published) calls for the removal of levies from electricity bills (and onto gas) to make electricity cheaper and therefore make the use of electricity more attractive - though whether and when that happens remains an open question. Also with a question mark are the Government plans to fully decarbonise the electricity grid by 2030; if that happens then electricity prices should become more stable compared to gas. From the opposite direction, as people quit the gas grid the standing charges for gas will rise - it's already happening in France -

 

The CCC also say that 'without policy action, the price of running a heat pump will not reach parity with the price of running a boiler until 2035' though the difference is already fairly low, so it wouldn't take much Government action to swing the balance the other way.

 

Personally, I choose the heat pump now; I'd have done so on my current French renovation if I had anywhere to put one.

[marshian]

  On 26/02/2025 at 17:41, Walshie said:

Hi,

I could use some input on the following please?

We are about to start a fairly major renovation.

We have inherited a new gas boiler - which is a year old, is A rated, has an efficiency rating of 94% and has the capacity we need to support the future plans.

The big question is whether to retain the gas boiler or to take the leap and install a heat pump?

 

If we go the heat pump route - it'd have to be an air source as we have limited space.

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Major renovation I assume is to a higher insulated standard than the house is currently - maybe with increased house footprint if boiler capacity has been assessed as good enough to support it and with limited space to site a AHSP unit

 

As @JohnMo said the decision needs some data gathering or decent workings to make that decision.

 

Current property heat loss calcs first - then with that as a baseline look at what the renovations are going to do to the heat loss - it could be that the renovations dramatically reduce your heat loss and as a result the current boiler is then oversized

 

Taking my own example as an illustration of how quickly a suitable boiler can become unsuitable when house is upgraded

 

4 bed detached built in early 1980's - Single glazing with wooden windows, open cavity between inner and outer brick skins, suspended ground floor no insulation and 25mm of loft insulation = 25,000 kWh of gas per year (heat loss 10.5 kWh at -2.4 OAT)

 

Throw in sealed unit double glazed windows and doors, CWI (cavity wall insulation) as well as 70mm of loft insulation and despite a two storey extension annual gas usage is down to 15,000 kWh and already the 24kW boiler is 3 x oversized for the new heat loss 7.5 kWh at -2.4 OAT

 

Add in underfloor insulation, replacing all the rads for lower flow temps into the mix and annual gas usage is down to 8,500 kWh with a heat loss of 4.0 kWh at -2.4 OAT and the boiler is now 6 x oversized and worst still it's minimum modulation 10kW (output is 2.5 x the heat loss)

 

I considered ASHP but it didn't fit well with the silly shaped house so I replaced the now totally unsuitable boiler with one that was only 16 kW but can modulate down to 3.2 kWh meaning when it's -2.4 or lower outside it happily runs 24/7 at close to min modulation.

 

Probably help if we had make and model of the boiler to see what modulation or turndown ratio it has because if you are improving the house having one with a really good modulation would mitigate the improvements.

[joth]

I assume you're on mains gas. Some random thoughts to add to what others have said. 

 

Do you use gas for anything else (hob? Fireplace?). Going to ASHP makes more sense if boiler is the only gas user hence allows you to disconnect gas 

Will you be moving the boiler or meter as part of the renovation? If so then you can save on some of that work by making the transition now, but if not then maybe less incentive to mess with it.

Are you more concerned about financial or environmental savings? Either way increasing insulation and airtightness as part of the build will help, but in particular if you can get your heating demand very low you can do all the heating in overnight cheap rate electric which makes ASHP cheaper to run than gas boiler. With a leaky drafty house you can't avoid running the heating source all day (when building occupied) which makes ASHP more expensive, at least until energy taxation policy is rebalanced away from favouring gas. 

[Walshie]

Thanks for all the helpful comments.

The current boiler is a Worcester GR8701iW - 34KW Class A

 

As we'll be taking the roof off, I plan to fully insulate the roof space and then add Knauf external wall insulation and then clad in timber. We'll also replace all windows and doors with Alu 1.1 system. The extension floor and walls will be insulated as will be new build.

 

So, I think that we'll have insulation covered and be pretty thermally efficient.

I like the idea of keeping the boiler but adding pipework and cabling to accommodate a heat pump in the future.

 

Does this seem like a sensible option?

Thx

W

 

 

[crispy_wafer]

if you want to run a cable for future use, I'd either run a duct so you can pull a cable at a later time, or get a heat loss calc done so you can quickly spec up the right size range HP and what it's cable size requirements are before you start pulling cables.

[marshian]

  On 27/02/2025 at 14:19, Walshie said:

Thanks for all the helpful comments.

The current boiler is a Worcester GR8701iW - 34KW Class A

 

As we'll be taking the roof off, I plan to fully insulate the roof space and then add Knauf external wall insulation and then clad in timber. We'll also replace all windows and doors with Alu 1.1 system. The extension floor and walls will be insulated as will be new build.

 

So, I think that we'll have insulation covered and be pretty thermally efficient.

I like the idea of keeping the boiler but adding pipework and cabling to accommodate a heat pump in the future.

 

Does this seem like a sensible option?

Thx

W

 

 

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Quick search online seems to indicate that that boiler has 11.3 kW minimum (30% of max)

 

It's also not compliant with low temp heating so in my opinion it's going to be in conflict with your rebuild/insulation improvement plans

 

Whilst the boiler might only be 1 year old the install manuals were written in 2013/2016 so it's not a "latest" boiler

[Walshie]

Marshian - Thanks for the comment.

When you say 'in conflict' - is this likely to be a real problem or more that it isn't optimal?

This side of things in not my strength!

[Nickfromwales]

You will likely end up in a scenario where the boiler will be routinely producing more heat than the house can dissipate. This results in boiler “short cycling” and also the return temp may be behind the modulation sweet spot. 

[marshian]

  On 27/02/2025 at 14:38, Walshie said:

Marshian - Thanks for the comment.

When you say 'in conflict' - is this likely to be a real problem or more that it isn't optimal?

This side of things in not my strength!

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It's not going to be optimal and that sometimes is a problem - boiler cycling is a huge hit to efficiency from memory you ideally don't want a boiler to cycle more than 5 times per hours (even that would be a bit shit)

 

My old 24kW with 10kWh min output was cycling more than 10 times per hour in autumn and spring and at best 7 times per hour when house was up to temp in the winter and chewing thro gas

 

My new boiler in winter is doing 22 to 24 cycles per day!!!!

[Walshie]

OK - that's helpful.

Thx - I appreciate it

[marshian]

It helps to describe why cycling is a waste

 

Boiler fires up at between 70 and 80% of max output (so in the case of your boiler 24kWh) it only does that for a couple of mins before modulating down - but in that time it's impossible for all that energy to get into the heat exchanger water so a lot of energy ends up going out of the flue

 

This loss is offset by the time the boiler actually runs and the level of modulation - a little loss at the beginning of the cycle is fine if the boiler then ticks along at minimum modulation

 

Problem comes when those 2 mins of wasted energy is repeated every 5 or 10 mins - the loss is not offset by the time the boiler runs for

 

To use a car related comparison

 

Scenario A

 

You are on a slip road at the bottom of the M1 - you need to be doing 70 mph to safely merge into the motorway traffic and getting the car from 0 to 80 mph is going to result in 7 mpg consumption - however you have 100 miles to cover at 80 mph and your fuel consumption is going to be 35 mpg at those speeds

 

The fuel used in acceleration is offset by the cruising consumption

 

Scenario B

 

You are on a slip road at the bottom of the M1 - you need to be doing 80 mph to safely merge into the motorway traffic and getting the car from 0 to 80 mph is going to result in 7 mpg consumption - however you have 100 miles to cover and it's a bank holiday monday and a 80 mph cruise is impossible - your progress is sporadic and requires frequent acceleration and braking because of the numpties on the road don't understand keep left unless overtaking and don't bother to look in their mirrors before any change of lanes. Your fuel consumption at the end of the journey is 25 mpg even if you average speed and journey time isn't much different.

Edited February 27 by marshian
Car Related comparision

[marshian]

  On 27/02/2025 at 14:39, Nickfromwales said:

You will likely end up in a scenario where the boiler will be routinely producing more heat than the house can dissipate. This results in boiler “short cycling” and also the return temp may be behind the modulation sweet spot. 

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^ this probably said it better than I have

[joth]

  On 27/02/2025 at 15:41, marshian said:

  On 27/02/2025 at 14:39, Nickfromwales said:

You will likely end up in a scenario where the boiler will be routinely producing more heat than the house can dissipate. This results in boiler “short cycling” and also the return temp may be behind the modulation sweet spot. 

Expand  

 

^ this probably said it better than I have

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Nit picking: more heat than the emitters can dissipate. If the boiler is producing more than the house can dissipate it means its summer and time to turn the heating off 🙃

Edited February 27 by joth

[marshian]

  On 27/02/2025 at 16:06, joth said:

 

Nit picking: more heat than the emitters can dissipate. If the boiler is producing more than the house can dissipate it means its summer and time to turn the heating off 🙃

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Nit picking: more heat than the Circuit can accept and dissipate. All the rooms at temp and TRV's have shrunk the circuit does not mean summer

[Nickfromwales]

Nit picking: I’m a busy man so sometimes spout crap. Check out the small print lol.