Current gas boiler FUBAR, replace or install Air Source Heat Pump?

Current boiler broken, would be parts darts against a boiler not known for it’s massive lifespan.

Replacement Worcester quoted at £2250, eta 2-3 weeks.

ASHP Valiant 7kw + new tank and buffer tank + couple of new rads and electrical work £8,700 (all relevant grants applied). Eta 2-3 months.

12 year old three bed house, presumed decent insulation over three floors.

What would you do??

Personally, I’d like to see the running cost of a air source pump. Given the cost of electic and where it’s going, a gas boiler should be cheaper. We average about £50 per month to heat and gas hob for a three bed – that’s going up, but not as much as electric. I’d need to see a comparison.

I’d replace the boiler with a new boiler.

@fossy Agreed, but there seem to be a lot of ‘noise’ about the differing taxes on gas and electricity shifting around and the general uncertainty around energy costs that skew the discussion further.

There was a GSHP in a place I rented a few years ago. It also had its own wind turbibe.
.
HPs really need your own electricity generation to be viable. Do you already have solar?

Gas boiler would be a no brainer for me based on the installation cost differential alone. I’d love the answer to ASHP but I don’t think it is for most people, certainly not yet.

HPs really need your own electricity generation to be viable. Do you already have solar?

I agree with this 100%.  Especially if you’re retro-fitting one to an older home that may not be as thermally efficient.

Verbal quote was about 7.5k for a 3kw array, but waiting on a more detailed quote to arrive. £4k for a battery storage of undefined capacity currently.

Absolute rubbish about HP needing their own electricity, and also only working in insulated houses.
If the heat load doesn’t alter because of what fuel deliveries it, what is critical with heat pumps is pipe and emitter sizing.
What is true is it is hard to make savings against natural gas but not impossible with time of use tariffs and buffer storage and again a well designed system all things which can help.
If you are interested in carbon saving then HP is the way forward, if capital cost the a gas boiler, if running cost is the driver for the decision then you need to get some figures from whoever is quoting.
The Vaillant is a very good unit too, there are cheaper ones but I’d stick with Vaillant.

if its a newish house, are the radiators on microbore piping? If so, unless the quote includes re-plumbing each of them I don’t think it works out.

ASHPs generate a significant amount of noise. This has a bigger effect than people realise.

From my investigation into similar, you’ll need a proper heat loss survey to be able to compare apples to apples.

Back in the days of combo boilers and cheap gas, you’ll find they’re typically over specced.

I believe the HP works better at just the right size so it runs for longer but less hard. Too big and it’s stop start all the time.

I’m no expert though, so happy to be corrected. There’s so many variables.

Absolute rubbish about HP needing their own electricity, …………………….if running cost is the driver for the decision then you need to get some figures from whoever is quoting.

I think the issue is that unless the medium you source heat from is capable of transferring a decent amount of heat then over winter the system’s efficiency drops to the point of being almost the same as directly heating with electricity.

So with electricity costing 4x gas. And electricity only being VERY marginally more green (207g Vs 215g CO2/kw) it becomes a stark question of it costing potentially 4x more to heat your house mid winter, for about a 4% reduction in CO2).

That is CO2 per kWh for generation of the energy. The boiler is then 85% efficient, the heat pump 200 to 300%…..

Absolute rubbish about HP needing their own electricity, and also only working in insulated houses.
If the heat load doesn’t alter because of what fuel deliveries it, what is critical with heat pumps is pipe and emitter sizing.

Absolute bollox, commented on this many times before, can’t be arsed yet again.

EDIT : block quote not working

That is CO2 per kWh for generation of the energy. The boiler is then 85% efficient, the heat pump 200 to 300%…..

If it’s got warm air to draw it’s heat from.

If the air is close to 0C then the condensation that forms on the cold plate freezes and the efficiency gains drop to nothing.

They’ll work great when it’s not so cold, but not for the November-Febuary period when it’s hovering just above zero at the times of day when you want to use it early morning and late evening (or as a heat store overnight on cheep rate electricity).

[Edit those figures include the 86% of the boiler comparing it against an electric heater]

I have zero personal experience of heat pumps, only second hand from friends who have installed one. Long and short of it was, unless your house is super insulated, you’ll likely need a second source of heating in winter if you live in the North. High ceilings, stone and any draughts renders them sub optimal. Plus, 2-3months install became 4-6months.

I’d replace with a like for like. Gas will likely be cheaper than electricity based on price forecasts over the coming years (but not months). Like it or not, unless there’s a government subsidised/funded drive, Gas Central heating will be around for the lifespan of your replacement boiler.

I’d try and fix the boiler. What are the symptoms?

I think the issue is that unless the medium you source heat from is capable of transferring a decent amount of heat then over winter the system’s efficiency drops to the point of being almost the same as directly heating with electricity.

So with electricity costing 4x gas. And electricity only being VERY marginally more green (207g Vs 215g CO2/kw) it becomes a stark question of it costing potentially 4x more to heat your house mid winter, for about a 4% reduction in CO2).

^That

That is CO2 per kWh for generation of the energy. The boiler is then 85% efficient, the heat pump 200 to 300%…..

In summer, yes.
On a cold winters night, dipping to frosty, aye, fat chance.
Of course, if you heat your house in the summer and not in the winter, you are quids in.

How does a borehole GSHP cost up? Presumably would perform better in winter as the subsoil temperature is stable year round?

How does a borehole GSHP cost up? Presumably would perform better in winter as the subsoil temperature is stable year round?

aiui, they’re so expensive to install at the moment that the cost never really works out.

How does a borehole GSHP cost up? Presumably would perform better in winter as the subsoil temperature is stable year round?

Boreholes for GSHP are very expensive – £10k+ , and thye are not as good as you think, as they cool the ground around the pipework through the winter, so can be less efficient than ASHP as the winter goes on.
ASHP can be better in the UK, as the air temperature stays above 5 deg. C. for most of the year.
But they just cannot compete with gas CH on price.

interesting, thanks for the replies. I did briefly look at the requirements of a system using a grid of pipes before having a big drive replaced but I don’t have anything like enough land!

Currently involved in decarbonising the NHS’s estate – I’d strongly advise you replace like for like. You’ll pay a fortune for the pump, you’ll probably need to replace all your radiators for ones twice the size, it’ll cost a mega fortune to run, and it won’t feel warm like you’ll be used to as they just work differently.

But they do save a shit tonne of CO2 if that matters enough to you.

And electricity only being VERY marginally more green (207g Vs 215g CO2/kw) it becomes a stark question of it costing potentially 4x more to heat your house mid winter, for about a 4% reduction in CO2).

I assume that doesn’t take into account the coefficient of performance as 1kwh of gas used by a boiler is comparable to 0.3kwh of electricity used for the same heat output (ashp takes most of its energy from the air outside). Obviously depends hugely on the level of insulation in the building, outside air temp, target inside air temp and flow temperatures but you get the idea.

I would look at one of the volume combi gas boiler imstallers.
Heatable or Install gurus and get that quote down to sensible money
. Plus they offer silly monthly payments that arr so low you would barely notice them leaving your account

There are grants of about £5K available for heat-pumps, so should sort of equalise the cost. We have a year old house with ASHP – the key thing being super-insulated and draught-proof. Current electricity bill is £140/month – 110m2 on the West Coast of Scotland.

If someone can explain how 10kw is different when delivered with a gas boiler then please feel free to enlighten me.

Heat pumps work down to minus 20, work in Scandinavia and do work in freezing temperatures. There are SCOP figures that will tell you how they work based on seasonally adjusted figures. Bore holes do not create permafrost unless the system is badly designed.

Borehole costs are approximately £65/m at the moment.

There are grants of about £5K available for heat-pumps, so should sort of equalise the cost.

I read the OPs £8700 as already including the grant (v£2250 for a replacement boiler)

I assume that doesn’t take into account the coefficient of performance as 1kwh of gas used by a boiler is comparable to 0.3kwh of electricity used for the same heat output (ashp takes most of its energy from the air outside). Obviously depends hugely on the level of insulation in the building, outside air temp, target inside air temp and flow temperatures but you get the idea.

No it does (in the worst case).

The cold side exchanger lowers the temperature of the air around it, the relative humidity goes up, and you get condensation. That’s no problem in summer, it actually increases the heat transfer coefficient.

But if the surface temperature drops below 0C then that condensation forms ice, and the system ceases to exchange energy.

There’s the double whammy of it not having the same output temperature as gas so takes longer to heat the building up too, the house is warm all day, which takes more energy than just heating it for a few hours. This is why it’s generally suggested it’s best for very well-insulated buildings (which you could argue would make a similar saving with a gas boiler).

So yes, better on CO2 over the year, but there’s a fairly big but attached to that.

Borehole costs are approximately £65/m at the moment.

Yeah, but when you need 3 at just over 100m depth – that’s almost £20k.

But if the surface temperature drops below 0C then that condensation forms ice, and the system ceases to exchange energy.

They mostly run a quick defrost cycle once every hour and a half or so to avoid that issue.

The figures they quoted for co2 per kWh don’t seem to ring true to me, how can the amount of co2 emitted from a heat pump using 1kw of electricity (generated by a combination of large gas turbines, nuclear, wind, solar and hydro etc), outputting closer to 3kwh once the COP is taken into account, be almost equal to a 90% efficient gas burner using 1kwh of mains gas outputting a theoretical 0.9kwh of actual heat?

£2.2k for a WB fitted is very good, we got 3 quotes, all came in at £4K (no change in pipe work etc)

Even then heat pump wasn’t an realistic option IMO

ASHPs generate a significant amount of noise. This has a bigger effect than people realise.

Not true. Just installed 75 mitsubishi ashp’s in a scheme in Bristol. Kneeling down right next to them I could hardly hear them run. Quieter than a gas boiler running..

200-300% efficient is conservative:

An insulation budget is good sense too. In fact it’s essential to making th emost efficient use of the heat pump.

In the case of some German friends their house is well enough insulated that they heat during the warmest part of the day and the house stays warm enough overnight. When I was there just before Christmas it was between -10°C and 18°C overnight but somewhere around zero in the warmest part of the day – that’s when they ran the heat pump. The house was warm in the evening and perfectly acceptable during the day. Doing the same in the UK you’d have a COP of nearer 3 than 2 in cold periods as it’s rarely below 5°c in the early afternoon 200-300% efficient is pesimistic.

The real problem in the standars of insulation. Even new builds are well below the standards that could be achieved with a small increase in build cost. For an extra £25/m2 of external surface area the heat losses could be halved.

In terms of retrofit an insulated cavity wall is about R=1.5 and an uninsulated cavity wall around half that. Add 100mm of polyurethane backed plasterboard at R=4.5 and you’ve got an R=6 wall – four times the thermal resistance. Before you say you’ll lose interior space consider how how much space your radiators take up because you’ll be able to get rid of most of them. Personally I’m happy with a lving room that’s 2.3% smaller but cosy.

Thanks for the discussion all, looks a replacement boiler will be ordered shortly.

Doesn’t feel quite right somehow but thats a big chunk of cash and might not be here that long. Also the source of the electricity production raises an eyebrow.

It seems daft that modern’ish houses like mine aren’t being bombarded with a kit of PV, batteries and ASHP for a token amount (4-5k) to reduce the national risk on foreign energy sources. But then I really don’t get why the utilities are in private hands anyway.

In an ideal world I’d have a hybrid ASHP but quite frankly these things are so prohibitively expense I can’t see myself ever installing one. I’ve been quoted £26k for a 16kw ASHP system all in…a further £8k for a 2.56kw PV..

It seems daft that modern’ish houses like mine aren’t being bombarded with a kit of PV, batteries and ASHP for a token amount (4-5k) to reduce the national risk on foreign energy sources. But then I really don’t get why the utilities are in private hands anyway.

As alluded to above, decent insulation would go a lot further, heat pumps are nigh on pointless unless you have decent insulation first.

By all means put a bit of microgeneration in as well but do it the right way round.

200-300% efficient is conservative

Doing the same in the UK you’d have a COP of nearer 3 than 2 in cold periods as it’s rarely below 5°c in the early afternoon 200-300% efficient is pesimistic.

Looks about right depending on where you are, that’s certainly the case for 50*C output* based on where I live. That’s the wet west of Scotland so further east and north will be colder.

*I’m quoting 50*C based on keeping radiators rather than fitting under floor heating. Obviously the under floor heating at 35*C is better but we’re talking retrofit here. For a new build that’s properly insulated under floor heating is the way to go.

Boreholes for GSHP are very expensive – £10k+ , and thye are not as good as you think, as they cool the ground around the pipework through the winter

I’ve never heard of this, I know shallow trenches can have that effect but even they are pretty thermally stable. I guess it depends on whatever you’re drilling into.

ASHP can be better in the UK, as the air temperature stays above 5 deg. C. for most of the year.

Again, I’ve never heard of this, can you point me towards any studies that support either of these statements?