ASHP vs Gas Boiler - crunching the numbers

[Indy]

Our current plans for the new build have factored in ASHP (at the recommendation of the architect) with UFH throughout (no radiators even on 1st floor). We are aiming for a PassivHaus type building with good thermal efficiency and airtightness with an MVHR system, and ASHP is something that seems to fit the bill as it's a new build and not a retrofit.

 

However, I want to see whether the sums for an ASHP are really justifiable given the rapid increase in cost of electricity. Please bear with me as I work through my train of thought:

 

General usage in our 1930s house (should be lower in the new house hopefully)

6000kWH of Electricity @ 28p/kWH = 1,680

30000kWH of Gas @ 7p/kWH = 2,100

Total cost = 3780 or 315 per month (ouch!)

 

New house - keeping the same numbers for ease

6000kWH of Electricity @ 28p/kWH = 1,680

To get 30,000 kWH of heat output from an ASHP with a COP of say 2 (average across the year) = 15,000 kWH of electricity @ 28p/kWH = 4200

Total cost = 5880 or 490 per month (double ouch!)

 

This is disregarding the capital outlay for an ASHP, which is going to be more even taking into account the BUS grant of 5k.

Gas boiler = 2.5k (example)

ASHP = 10-12k minus the 5k = 5k to 7.5k net

 

I've not factored in all the gubbins like water storage as they'll be common in both scenarios. Does it make sense then to consider an ASHP?

 

Realise both gas and electric prices are up, but the differential between them remains at 4:1 and I can't see gas going away anytime soon. A family member who works in the sector tells me about the introduction of Hydrogen into the gas mix and trials already being run in Scotland, which means there is a long term plan to continue using gas.

 

 

[IanR]

If you plan to build to PassivHaus levels of insulation and air tightness then it's a waste of your time using your current property's space heating figures. You need calculated data, based on real U Values and ACH (air changes per hour).

 

Even with Gas prices going up faster than electricity, and the good reasons that electricity will stabilise and hopefully reduce before Gas does, it's still unlikely that an ASHP will achieve parity with Gas on day-to-day running costs, nor on capital costs now that RHI has been replaced by BUS and grants will be lower in most cases.

 

But, that doesn't mean you shouldn't consider an ASHP, or maybe direct electric if you over-achieve on your PassivHaus aspiration, avoiding the capital costs of the boiller/ASHP installation.

 

Firstly, forget Hydrogen. There is not currently a path to 100% grid/network Hydrogen for UK housing. The Max 20% blend of hydrogen into natural gas, currently being trialled is a red-herring. It's been discussed at length on this site, have a search and read those threads if you wish to know more.

 

Gas boilers in private residence have a finite life. From 2025 they will not be able to be installed on new properties (legislation is not in place yet, but that is the government target with the future homes standard), and replacement fossil fuel boilers will be banned by 2035 (possibly sooner for some housing types).

 

Building regs new for this year now require a low flow temp heating system. You can still specify gas, but it has to be designed around a low flow temp to ensure replacement with non-fossil fuel boilers in the future can be done without too much rework.

 

Your 2.5K gas pricing suggests you are comparing to a combi gas boiler. The Regs are pushing new properties towards a system boiler with a a hot water tank so the Gas v. ASHP installation will now be very similar in all but the boiler itself. If HWC, buffer tank, valves and pumps are all equal, the price difference will just be the difference between a Gas system boiler and the ASHP itself. The 5K BUS grant should get close to closing that gap.

 

If you do go with Gas, to future proof the house you should consider a UVC, maybe a buffer tank for UFH and a route to these from where an ASHP would be located for the pipework. 

Edited April 19, 2022 by IanR

[willbish]

45 minutes ago, Indy said:

A family member who works in the sector tells me about the introduction of Hydrogen into the gas mix and trials already being run in Scotland, which means there is a long term plan to continue using gas.

 

Widespread use of hydrogen for heating looks very unlikely

 

 

[SteamyTea]

How much is the daily had meter rental?

Electrical ones seem to be around 50p/day now.

 

If the government takes the green levies and puts them on gas, that will save 20% on electricty and add about 6% to gas.

[AliG]

You should achieve a COP of just over 3 between heating and hot water on an ASHP, 2 is a very pessimistic estimate.

 

If you use 30,000kWh of heat in your 1930s house you are probably looking at 20-25k in your new house. You might well use ore energy for hot water than heating.

 

So call it 24000kWh with a COP of 3, That is £2250 of electricity versus around £1850 of gas. I am assuming 95% boiler efficiency and 7.37p/kWh.

 

That is a £400 difference, but you will save the gas standing charge of £100 a year, so £300. Get your COP up over 3 and the difference disappears.

 

If you can use cheaper overnight electricity or PV to run the ASHP sometimes then it could well be the same price or cheaper than gas.

 

I don't know who is quoting for your heating but an ASHP should absolutely not cost £8-10k more than a boiler. Maybe £3-4k more installed. Offsetting this is you won't need to install gas to the property which could save a couple of thousand.

 

Net net I don't think there is much difference if you are starting from scratch with a well insulated house. for an older house that already has gas installed it likely remains the better option.

 

As @SteamyTeamentioned if they start to charge for carbon the gap between gas and electricity will get smaller still.

 

 

[Ferdinand]

Yay.

 

Someone doing my homework !

[ProDave]

You don't say what size house but our 150 square metre nearly passive house uses 1400kWh heating the house each year and 1000kWh heating the hot water.  We are in the Highlands so probably a colder climate than you so expect your figures to be lower.

[Indy]

35 minutes ago, ProDave said:

You don't say what size house but our 150 square metre nearly passive house uses 1400kWh heating the house each year and 1000kWh heating the hot water.  We are in the Highlands so probably a colder climate than you so expect your figures to be lower.

 

Overall size of new house is 280 sq m. 6 occupants - 4 adults, and 2 kids under 5.

[Indy]

2 hours ago, IanR said:

If you plan to build to PassivHaus levels of insulation and air tightness then it's a waste of your time using your current property's space heating figures. You need calculated data, based on real U Values and ACH (air changes per hour).

 

Even with Gas prices going up faster than electricity, and the good reasons that electricity will stabilise and hopefully reduce before Gas does, it's still unlikely that an ASHP will achieve parity with Gas on day-to-day running costs, nor on capital costs now that RHI has been replaced by BUS and grants will be lower in most cases.

 

But, that doesn't mean you shouldn't consider an ASHP, or maybe direct electric if you over-achieve on your PassivHaus aspiration, avoiding the capital costs of the boiller/ASHP installation.

 

Firstly, forget Hydrogen. There is not currently a path to 100% grid/network Hydrogen for UK housing. The Max 20% blend of hydrogen into natural gas, currently being trialled is a red-herring. It's been discussed at length on this site, have a search and read those threads if you wish to know more.

 

Gas boilers in private residence have a finite life. From 2025 they will not be able to be installed on new properties (legislation is not in place yet, but that is the government target with the future homes standard), and replacement fossil fuel boilers will be banned by 2035 (possibly sooner for some housing types).

 

Building regs new for this year now require a low flow temp heating system. You can still specify gas, but it has to be designed around a low flow temp to ensure replacement with non-fossil fuel boilers in the future can be done without too much rework.

 

Your 2.5K gas pricing suggests you are comparing to a combi gas boiler. The Regs are pushing new properties towards a system boiler with a a hot water tank so the Gas v. ASHP installation will now be very similar in all but the boiler itself. If HWC, buffer tank, valves and pumps are all equal, the price difference will just be the difference between a Gas system boiler and the ASHP itself. The 5K BUS grant should get close to closing that gap.

 

If you do go with Gas, to future proof the house you should consider a UVC, maybe a buffer tank for UFH and a route to these from where an ASHP would be located for the pipework. 

 

Thanks for the detailed reply. I certainly haven't ruled ASHP out and its useful to read that it does still make sense. One other thing that the architect mentioned when we were in the design stage is that given our build doesn't start until after June this year, new Building Regs kick in. This would make it quite difficult to get approval for a Gas boiler anyway.

 

We are also looking at adding Solar PV in, which means that we could potentially get a few hours (in the summer at least) of 'free usage'.

[George]

Mains gas is cheaper and will be for the foreseeable future. But in a passive house, the heating value will be so low it's not worth worrying about. 

 

Add solar and a desire to minimise fossil fuel usage and ASHP is looks better.

 

However - if it is a choice between a gas boiler but we can afford more insulation OR a ASHP, then choose the gas + insulation. Design the system for low flow temperatures (which you should be doing anyway to get maximum efficiency out the condensing boiler) as this will mean a simple switch to ASHP in the future. 

 

For people without access to mains gas, in my opinion ASHP is already the better choice. Oil and LPG is up and up and very volatile and can't be supplemented with solar PV. 

[Indy]

1 hour ago, AliG said:

You should achieve a COP of just over 3 between heating and hot water on an ASHP, 2 is a very pessimistic estimate.

 

If you use 30,000kWh of heat in your 1930s house you are probably looking at 20-25k in your new house. You might well use ore energy for hot water than heating.

 

So call it 24000kWh with a COP of 3, That is £2250 of electricity versus around £1850 of gas. I am assuming 95% boiler efficiency and 7.37p/kWh.

 

That is a £400 difference, but you will save the gas standing charge of £100 a year, so £300. Get your COP up over 3 and the difference disappears.

 

If you can use cheaper overnight electricity or PV to run the ASHP sometimes then it could well be the same price or cheaper than gas.

 

I don't know who is quoting for your heating but an ASHP should absolutely not cost £8-10k more than a boiler. Maybe £3-4k more installed. Offsetting this is you won't need to install gas to the property which could save a couple of thousand.

 

Net net I don't think there is much difference if you are starting from scratch with a well insulated house. for an older house that already has gas installed it likely remains the better option.

 

As @SteamyTeamentioned if they start to charge for carbon the gap between gas and electricity will get smaller still.

 

 

 

Fair to say that I've used very rough and deliberately pessimistic numbers to make the comparison. Reading a few threads online does make it seem that the certified installers are jacking up prices to take into account the 5k grant, so the higher prices aren't all that unreasonable though I'll certainly be doing a lot of shopping around to get lower quotes.

 

Given that we'll also end up adding Solar PV and if we can get the COP to be closer to 3, I can see that it makes more sense to go down this route and I guess safeguards somewhat from the eventual carbon/green taxes coming our way in the future.

 

 

[IanR]

If you design/engineer to PassivHaus levels your space heating requirement will be ≤ 15kWh/m2. yr

 

Your space heating requirement should therefore be 280 x 15 = 4200kWh per year.

 

This would require 4667 kWh of Gas, assuming 90% efficiency, or 1200kWh of electricity, assuming a SCOP of 3.5

 

In my approx PH level home, I find that my HW energy requirement is roughly twice my space heating requirement, for a family of 5.

 

9 minutes ago, Indy said:

build doesn't start until after June this year, new Building Regs kick in. This would make it quite difficult to get approval for a Gas boiler anyway.

 

Gas can be installed in new builds until 2025 and you'll have no issue getting sign-off, but the system design has to be for a low flow temp.

Edited April 19, 2022 by IanR

[Indy]

2 minutes ago, George said:

Mains gas is cheaper and will be for the foreseeable future. But in a passive house, the heating value will be so low it's not worth worrying about. 

 

Add solar and a desire to minimise fossil fuel usage and ASHP is looks better.

 

I'm going to make a possibly controversial statement here but the 'desire' to reduce fossil fuel usage is not really a factor at all. I am looking at this from a cost/benefit and purely economic perspective (selfishly), and don't mind paying more up front to get longer term reduced bills and a more comfortable house.

 

This responsibility that large organisations have dropped on individuals to 'reduce their personal carbon footprints' while continuing to flout all rules and lobby against real change has made me very cynical in that sense but not something to be discussed on this thread.

 

2 minutes ago, George said:

However - if it is a choice between a gas boiler but we can afford more insulation OR a ASHP, then choose the gas + insulation. Design the system for low flow temperatures (which you should be doing anyway to get maximum efficiency out the condensing boiler) as this will mean a simple switch to ASHP in the future. 

 

For people without access to mains gas, in my opinion ASHP is already the better choice. Oil and LPG is up and up and very volatile and can't be supplemented with solar PV. 

 

We do have the advantage of having mains gas there already luckily, but I am considering having the connection removed/capped if we choose to go down the ASHP route for heating/hot water and induction hob in the kitchen.

[joe90]

Whatever fuel you use you only buy insulation once and lasts (nearly) forever. I fitted my own ASHP (but have no access to gas anyway,) good insulation, solar PV and careful detailing and your energy bill will be minimal. If you design in UFH etc then the “boiler “ whatever it is can be changed out in the future. (Fusion reactor? 🤷‍♂️)

Edited April 19, 2022 by joe90

[JohnMo]

What ever route you go down, with a low energy house, realise things don't work the same with heating as a 'normal' house.  You can operate the whole ground floor as a single zone, without lots of thermostats.

 

I have gas boiler capable of weather compensation, but it just doesn't cope with slow house temperature change and slow change in temperature of the 100mm thick slab.  I now just batch charge overnight for around 6 hours. If I wanted to do it quicker you would just increase the temperature.

 

If you have underfloor heating in a thick slab, you can charge it up over a period of hours, then switch it off for 16 to 18 hours and your house will stay roughly the same temperature.  So if you have an ASHP, you could do the house heating overnight on cheaper electric.  The CoP will not be as good (due lower overnight temperature), but you are paying lots less for the electric.  So you are paying gas prices and need less than a third of it compared to gas boiler.

 

Or do as some use a Willis heater, for £60 each,  always has a CoP of 1, use on cheap rate electric, same cost as gas, use an immersion or heat pump/cylinder combination for DHW.

 

 

 

[JamesP]

We have similar size home of 275sqm but large volume at 750m3.

The ASHP used 1900kWh for heating and 1800kWh for DHW during 2021.

I would recommend some PV and a PV diverter.

Edited April 19, 2022 by JamesP

[George]

2 hours ago, Indy said:

 

I'm going to make a possibly controversial statement here but the 'desire' to reduce fossil fuel usage is not really a factor at all. I am looking at this from a cost/benefit and purely economic perspective (selfishly), and don't mind paying more up front to get longer term reduced bills and a more comfortable house.

 

This responsibility that large organisations have dropped on individuals to 'reduce their personal carbon footprints' while continuing to flout all rules and lobby against real change has made me very cynical in that sense but not something to be discussed on this thread.

 

 

We do have the advantage of having mains gas there already luckily, but I am considering having the connection removed/capped if we choose to go down the ASHP route for heating/hot water and induction hob in the kitchen.

I partly agree - while you're right, corporations and government need to be at the forefront, there's no reason individuals can't also decarbonise when at these decision moments. 

 

Induction is better than gas anyway for almost everything. 

 

It can certainly be financially beneficial to move to full electric household if you also get PV, battery storage and an EV. An ASHP fits nicely into that plan. However, if only considering heating systems and nothing else, kWh for kWh, mains gas wins on price. 

 

But you'll never be able to make your own gas. 

 

 

Edited April 19, 2022 by George

[Iceverge]

ASHP.

 

Fossil fuels aren't sold by nice people. 

 

[SteamyTea]

6 people, domestic hot water is going to be your biggest challenge, whatever type of system you choose.

Consider 3 phase power, if nothing else you can easily fit more than 4 kW/16 Amp PV system.

 

 

[ProDave]

5 hours ago, Indy said:

We are also looking at adding Solar PV in, which means that we could potentially get a few hours (in the summer at least) of 'free usage'.

Solar PV is definitely worth it.  Over a year the PV generates more than it takes to heat the house, but of course not when you need it.  But we self use almost all we generate and it makes a big reduction in the electricity bill.

 

Many of us find for much of the summer. you get much of your hot water for free from surplus PV.

[Indy]

12 hours ago, SteamyTea said:

6 people, domestic hot water is going to be your biggest challenge, whatever type of system you choose.

Consider 3 phase power, if nothing else you can easily fit more than 4 kW/16 Amp PV system.

 

 

Could you expand on that? Is is related to the size of the water tank we get in the property or other issues?

 

Planning to go 3 phase as I'll be moving to an EV when my current lease runs out, and the PV system will give me 'free charging' somewhat. Not that convinced by battery tech as the man maths doesn't seem to justify it just yet.

[SteamyTea]

1 hour ago, Indy said:

Could you expand on that? Is is related to the size of the water tank we get in the property or other issues

Basically down to the time it takes to heat a cylinder up. Gas boilers are usually sized for the water heating needs. Heat Pumps are usually sized for the space heating needs, with a couple of kW added on for the water heating.

Thing about water heating is that usual households want to bathe at similar times i.e. mornings or evenings. With a combi boiler, which is often fitted these days, there is a limit to the flow though the system. This flow rate is the product of the temperature rise required, heat capacity of water and the flow rate, divided by the power of the heater. Why combi boiler are often 40+kW in a large house.

Heat pumps are very really larger than 12 kW, so just under a third of the size. This means they take (roughly) 3 times longer to heat the same amount of water up. 

To get around this, large cylinders are fitted. 500+litres.

This is alright, unless you are only drawing off say 70 litres. Trouble then is, you get the thermal losses of a 500 litre cylinder instead of a 100 litre one.

It is all a numbers game.

[Andehh]

Just to say 'Hello' to the OP, I am in the exact situation you are  . Probably 53% leaning towards gas and 47% towards ASHP.

 

Having mains gas & 250sqm property, SAP of 87 (so less then yours), 3kW PV (due to original FiT limiting expansion).... originally expected ASHP, but with very high ceilings everywhere, builder is walking back on his original ASHP recommendation and is now swinging to recommending Gas. I am still working my way through it all...

[joe90]

Just make sure that the house infrastructure can change to an ASHP in the future if you are going gas but PV,EV, ASHP is the future IMO.