What's the case for not getting an ASHP?

[JamesPa]

20 minutes ago, marshian said:

Sorry it wasn’t a criticism of your comment

No offence taken, neither did I take it as a criticism!  If we don't exchange differing interpretations/views (based in all cases on the information we happen to have or our own understanding of the situation) we wont ever learn anything!

 

22 minutes ago, marshian said:

I think you made a valid point that it’s easy to make solving a problem complicated

Thanks you for saying so. 

 

Im a firm fan of both KISS and Occam's razor, particularly when it comes to services which are mission critical to a house.  Its all very well (and fun) doing fancy/unusual  things, but if something goes wrong that needs to be fixed in a hurry and the person who understands it isnt around, then whoever is left needs to get the local tradesman to sort it.  If its complex that's less likely to be easy!  

[JamesPa]

5 hours ago, Roger440 said:

The reality out in the realkworld is that its likely not to be well installed.  

Is that actually true (genuine question)?  About 100k heat pumps were installed in the UK last year.  We hear about only a few, and its surely a heavily distorted sample comprising 

 

1. those who have had a nightmare +

2. those who have an unusually strong interest in heating/renewables (who, as you say, generally experience success with their heat pump)

 

The rest, who have neither had a problem nor have an unusually strong interest in heating, are just getting on with their lives rather than shouting about the success, or otherwise, of their heat pump.  My suspicion is that 'the rest' are the vast majority, because very few people have a strong interest in heating so long as it works.  I don't have and have never seen the stats, but concluding that the relatively few (compared to 100K) who complain are representative of the majority of installs seems a tad presumptuous.

 

Its also worth pointing out that you hear horror stories about all facets of building work.  Yet somehow we seem to have plenty of houses that people are happy to live in.

[sharpener]

17 hours ago, kentar said:

 

Yes to a point, but you can't account for every eventuality. Fitting a 1000l tank in my airing cupboard 'just in case' is not viable.

 

My current tank is 140l. I can drain that in less than 10 mins. Assuming 4 people need a shower, that's 560l. A tank that size is just not going to fit.

 

As others have said if you are already planning to get a bigger one it is perhaps not fair on attributing that requirement to the heat pump!

 

The equation you need is 1 kWh = 860 litre-degrees. So it takes 10 kWh to heat a 215 l tank through 40 degrees. Or a 5kW HP will do it in 2 hrs etc etc.

 

An MCS approved installer (not much of a qualification but you have to use one to get the BUS grant) will use the MCS sizing guide which is 45 litres x (no. of bedrooms + 1) so if yr house has 4 bedrooms that would mean a min of 225l, and with your usage I would go for at least 300. We have a 210 litre OSO cylinder and at 60C it will supply decent showers for two normal ppl plus 2 guests in quick succession. We usually have the HP flow set to 45 which gives just under 40 at the outlet which is enough when there are no guests.

 

There are various claims for fancy tanks such as Mixergy or the new Heat Geek combi boiler substitute but they are mostly spurious and you will do best IMO in spending the money on a bigger conventional tank with a large coil.

 

Have you tried the aerating shower heads which apparently give a full shower experience while using less water?

 

You can download quite good free heat loss s/w from www.heatpunk.co.uk and there is also a quick heat geek cheat sheet which from your description would suggest maybe 65-85W/m^2 is appropriate but you need a more formal method than that.

 

There is a new Heat Geek Zero Disruption process which may be suitable for yr house particularly bc of the existing 15mm pipework.

The planning restrictions @SteamyTea refers to have been relaxed so you can put a HP within 1 m of the boundary and do not need PP for even a big twin fan unit like the Vaillant 12kW (which are slimline and extremely quiet). I would not want to install a HP above ground level, though it can be done subject to the manufacturer's requirements about safe access for servicing.

 

HTH

[kentar]

8 hours ago, MikeSharp01 said:

This depends on a number of factors: The size of ASHP, the area of the heating coil in the tank - the bigger the better, and what you set the flow temperature from the pump for the water heating cycle.

 

Assuming all things being equal - size and configuration of tank, flow temp etc. (why would flow temp be different?)

 

If i'm replacing a 20kw boiler, it would be fair to assume a 20kw ashp would heat the tank in a similar amount of time. However I only see people fitting ashp's with ratings in single digits.

I've read that an oversized heat pump won't be as efficient due to it not being able to modulate down far enough and instead having to cycle on and off. The question is, why? They all run on inverters, in theory they should be able to go to near zero. Meanwhile a mid range Viessmann gas boiler will go down to 3kw, and the top end boiler will go down to 1.3kw.

 

Yes, I will be upgrading the tank from 140l to 250l. However, were it to run out due to say several consecutive showers being required, I'd prefer that it was able to heat up as quickly as a gas boiler could. I know this can be mitigated somewhat by fitting a tank with a longer coil, but the same logic could be applied to a gas boiler.

The central heating side isn't a concern for me. I know I can leave this on 24/7 and let weather comp do its thing.

 

I'm neither for nor against ASHP's, i'm just trying to find a positive argument for installing one over a gas boiler. I know there may be some adjustment to be made, but if it causes more issues than it solves, then what actual reason is there for switching from a gas boiler? If it causes no extra hassle then i'm for it. And if it saves me money over installing a modern boiler that can modulate from 30kw-3kw then that's even better.

 

[kentar]

22 minutes ago, sharpener said:

 

As others have said if you are already planning to get a bigger one it is perhaps not fair on attributing that requirement to the heat pump!

 

The equation you need is 1 kWh = 860 litre-degrees. So it takes 10 kWh to heat a 215 l tank through 40 degrees. Or a 5kW HP will do it in 2 hrs etc etc.

 

An MCS approved installer (not much of a qualification but you have to use one to get the BUS grant) will use the MCS sizing guide which is 45 litres x (no. of bedrooms + 1) so if yr house has 4 bedrooms that would mean a min of 225l, and with your usage I would go for at least 300. We have a 210 litre OSO cylinder and at 60C it will supply decent showers for two normal ppl plus 2 guests in quick succession. We usually have the HP flow set to 45 which gives just under 40 at the outlet which is enough when there are no guests.

 

There are various claims for fancy tanks such as Mixergy or the new Heat Geek combi boiler substitute but they are mostly spurious and you will do best IMO in spending the money on a bigger conventional tank with a large coil.

 

Have you tried the aerating shower heads which apparently give a full shower experience while using less water?

 

You can download quite good free heat loss s/w from www.heatpunk.co.uk and there is also a quick heat geek cheat sheet which from your description would suggest maybe 65-85W/m^2 is appropriate but you need a more formal method than that.

 

There is a new Heat Geek Zero Disruption process which may be suitable for yr house particularly bc of the existing 15mm pipework.

The planning restrictions @SteamyTea refers to have been relaxed so you can put a HP within 1 m of the boundary and do not need PP for even a big twin fan unit like the Vaillant 12kW (which are slimline and extremely quiet). I would not want to install a HP above ground level, though it can be done subject to the manufacturer's requirements about safe access for servicing.

 

HTH

 

That's an informative post, thanks.

[sharpener]

9 minutes ago, kentar said:

 

Assuming all things being equal - size and configuration of tank, flow temp etc. (why would flow temp be different?)

 

If i'm replacing a 20kw boiler, it would be fair to assume a 20kw ashp would heat the tank in a similar amount of time. However I only see people fitting ashp's with ratings in single digits.

I've read that an oversized heat pump won't be as efficient due to it not being able to modulate down far enough and instead having to cycle on and off. The question is, why? They all run on inverters, in theory they should be able to go to near zero. Meanwhile a mid range Viessmann gas boiler will go down to 3kw, and the top end boiler will go down to 1.3kw.

 

 

I think it's more to do with the efficient area of operation of the compressor. It's easier to design a gas burner to work over a wide range of output. The lowest speed the Vaillant 12kW will run at is 27.2% which gives a turndown ratio of over 3:1 and that is typical of better HPs.

  

9 minutes ago, kentar said:

 

Yes, I will be upgrading the tank from 140l to 250l. However, were it to run out due to say several consecutive showers being required, I'd prefer that it was able to heat up as quickly as a gas boiler could. I know this can be mitigated somewhat by fitting a tank with a longer coil, but the same logic could be applied to a gas boiler.

The central heating side isn't a concern for me. I know I can leave this on 24/7 and let weather comp do its thing.

 

 

A preference for matching the behaviour of a (20kW) gas boiler is unrealistic. In most cases it would lead to an extremely oversized HP, with attendant capital cost and inefficiency at low outputs. 

 

A longer coil will not help (much). However you do not need to limit yourself to a single tank, you can cascade them e.g. with a feeder tank in the loft space. If you were planning to scrap the existing one doing this might get you more capacity more or less for free.

  

9 minutes ago, kentar said:

 

I'm neither for nor against ASHP's, i'm just trying to find a positive argument for installing one over a gas boiler. I know there may be some adjustment to be made, but if it causes more issues than it solves, then what actual reason is there for switching from a gas boiler? If it causes no extra hassle then i'm for it. And if it saves me money over installing a modern boiler that can modulate from 30kw-3kw then that's even better.

 

 

It will not save you capital cost now. 

 

It should break even on running cost now, and assuming the govt eventually does something about the cost structure will likely give you worthwhile cost savings in the future.

 

As the cost of solar PV and battery systems continues to fall you will increasingly be able to run an HP off your own power plant.

 

Given the demands on the grid, at some point in the future you may be refused an HP connection by your DNO so best to get in before the rush.

 

As with many incentive schemes the BUS grant may not stay at £7k5 once its initial purpose has been served.

 

You will be able to bask in the glow of the brownie points.

 

 

[JamesPa]

1 hour ago, kentar said:

I'm neither for nor against ASHP's, i'm just trying to find a positive argument for installing one over a gas boiler. I know there may be some adjustment to be made, but if it causes more issues than it solves, then what actual reason is there for switching from a gas boiler? If it causes no extra hassle then i'm for it. And if it saves me money over installing a modern boiler that can modulate from 30kw-3kw then that's even better.

The definite positives are:

 

1 Low temperature heating is much more comfortable.  Of course you can do that with a boiler, but few do and a fair amount of the upgrade cost quoted for ashps is due to the conversion to low temp heating not the ashp per se (and a fair amount of the rest of the upgrade cost is dhw tank replacement which you are planning to do anyway).

 

2 It will warm the planet a lot less (if that matters to you).

 

3 It can cool as well in rooms with fancoils or ufh.

 

In addition

 

4 It may or may not be cheaper to run

 

5  It may or may not be cheaper (or more expensive) to install with the grant

 

That's about it so far as I know.

 

1 hour ago, kentar said:

If i'm replacing a 20kw boiler, it would be fair to assume a 20kw ashp would heat the tank in a similar amount of time. 

 

Not really.  Reheat time is usually limited by the amount of heat you can get through the coil not the capacity of the heat source.

 

 

Edited 8 hours ago by JamesPa

[JamesPa]

1 hour ago, kentar said:

, I will be upgrading the tank from 140l to 250l. However, were it to run out due to say several consecutive showers being required, I'd prefer that it was able to heat up as quickly as a gas boiler could. I know this can be mitigated somewhat by fitting a tank with a longer coil, but the same logic could be applied to a gas boiler.

The central heating side isn't a concern for me. I know I can leave this on 24/7 and let weather comp do its thing.

Ok so judging by this dhw is your main concern.  If you define your requirements it will be possible to do a design and work out what kind of ashp you need or whether it's even possible.  How many l/s for how many minutes for each of the 4 showers?

 

For example a 250l tank heated to 60C will do 4 10min showers at 10l/s or 4 5min showers at 20l/s (assuming incoming water temp 10C, shower water temp 40C).  Is that enough.

 

Many people with ashps heat only to 48C for better efficiency, but you can heat to 60C provided you get the right model (basically an R290 heat pump).  My 7kW ashp heats my 200l tank to 68 C once per week to disinfect.  This is natively on the heat pump, not using the immersion, and prompts the app to issue a warning against scalding!

Edited 8 hours ago by JamesPa

[dpmiller]

ignoring defrosts, with good control and a big coil (or PHE) the heat pump can act almost like a combi. But again, gotta define your flow requirements first...

[JamesPa]

4 hours ago, JamesPa said:

 

 

5 hours ago, kentar said:

If i'm replacing a 20kw boiler, it would be fair to assume a 20kw ashp would heat the tank in a similar amount of time. 

 

Not really.  Reheat time is usually limited by the amount of heat you can get through the coil not the capacity of the heat source.

On second thoughts I should clarify this answer.  To the question you asked (and assuming the same tank) the answer is possibly, it depends on flow temperatures and whether the limitation is the boiler power or rate of heat transfer through the coil.

 

What I should have added is that you cannot assume that you need  a 20kW ASHP to reheat in the same time as a 20kW boiler (because reheat time is usually limited by the amount of heat you can get through the coil not the capacity of the heat source, even with a boiler (which is why there is a market for 'rapid recovery' cylinders.)

 

See my post above for some cylinder size calculations.

Edited 4 hours ago by JamesPa

[Iceverge]

Heat pumps aren't a good fit when you require a very high power heating source. 

 

How do you use your house? 

 

Do you live at home all day or are happy to heat the house when I occupies or do you intermittent heat it? 

 

 

 

 

 

[Roger440]

7 hours ago, JamesPa said:

Is that actually true (genuine question)?  About 100k heat pumps were installed in the UK last year.  We hear about only a few, and its surely a heavily distorted sample comprising 

 

1. those who have had a nightmare +

2. those who have an unusually strong interest in heating/renewables (who, as you say, generally experience success with their heat pump)

 

The rest, who have neither had a problem nor have an unusually strong interest in heating, are just getting on with their lives rather than shouting about the success, or otherwise, of their heat pump.  My suspicion is that 'the rest' are the vast majority, because very few people have a strong interest in heating so long as it works.  I don't have and have never seen the stats, but concluding that the relatively few (compared to 100K) who complain are representative of the majority of installs seems a tad presumptuous.

 

Its also worth pointing out that you hear horror stories about all facets of building work.  Yet somehow we seem to have plenty of houses that people are happy to live in.

 

Im on a few groups of people with old houses. Lots of unhappiness there, but, i know a fair few people with them. None of them are happy with them.

 

Some are because it doesnt respond like their old system, but generally its the running cost. ie, much more than the previous system they had (mostly oil).

 

Theres a reason take up is low. Primarily ludicrous cost, but we know thats just grant harvesters driving the price up, along with the MCS cartel, but people talk to each other still. Mad i know, but it does happen.  But running costs seem to be a close second.

 

High running costs says not installed/designed properly to me. 

 

Accepting that my sample of actual real world experience isnt very big, I would be very surprised if it wasnt the case that the majority of installations are poor to hopeless. 

 

It seems unlikely that the trades doing ASHP's are streets ahead of the rest of the building trade and doing a great job.

 

All that said, ive started to ignore the ASHP thing as i, realistically, will struggle to have one with my weedy incoming supply and still run everything else. I was reasonably confident i could make it work, primarily as i know how much energy the house uses.

 

[marshian]

1 hour ago, JamesPa said:

On second thoughts I should clarify this answer.  To the question you asked (and assuming the same tank) the answer is possibly, it depends on flow temperatures and whether the limitation is the boiler power or rate of heat transfer through the coil.

 

What I should have added is that you cannot assume that you need  a 20kW ASHP to reheat in the same time as a 20kW boiler (because reheat time is usually limited by the amount of heat you can get through the coil not the capacity of the heat source, even with a boiler (which is why there is a market for 'rapid recovery' cylinders.)

 

See my post above for some cylinder size calculations.

 

Very good point - my current cyl would be almost useless with an ASHP - the coil is tiny and incapable of transferring energy quickly

 

In summer I heat HW with a 62 Deg flow temp and it takes anywhere between 35 to 45 mins to get the cyl back to 50 Deg C (I do this to maximise the condensing benefits where the boiler doesn't have a whole house to maintain temp (because summer)

 

In winter I heat HW with a 80 Deg Flow temp and it's done in 20 mins because I want the boiler to get back to CH asap (low temp heating needs to be continuous to replace the heat lost. (at 80 deg flow temp it's condensing for maybe just a few mins out of the 20 mins cycle)

 

An ASHP compatible tank with a 3m2 coil would solve the issue but it's ~£1500 plus install costs.

 

It's one of the reasons I am investigating a PHE if I can improve the HW reheat whilst running lower boiler flow temps I can extract a little more efficiency out of the boiler even in winter.  If that works I only need a direct UVC with no coil and that's a heck of a saving even with the cost of a PHE

[Gus Potter]

On 20/08/2025 at 20:12, kentar said:

My boiler has died and I'm able to get a new ASHP installed via government grant. I have few concerns and i'd like some advice.

My property is early 1900's solid wall construction. The loft has been insulated up to standard 10 years ago. New UPVC double glazing was fitted less than 5 years ago. The heating system is a mixture of new, old and really old. All pipe runs to the rads are 15mm, likely branching off a 22mm main (unverified).

 

Firstly, everyone's telling me not to. Only one person I know has had one. I don't have any details about his system or installation, but he moved into a new house which had one and found it to be unsuitable because it was taking too long to heat the water after the DHW cylinder had emptied and meant having more than one consecutive shower was not possible. It was therefore replaced with a gas boiler.

 

A couple of other concerns of mine are placement and noise. I'd prefer to mount mine high up on the wall against my house, as close as possible to the hot water cylinder on the first floor. This will be around 2m from my neighbours house. Is this possible? How noisy will it be?

If it's not doable, can I have it placed at the end of the garden, around 20m away from the existing boiler? The boiler currently resides in the kitchen, with the hot water cylinder directly above it.

 

Thirdly, heatgeek accredited installations are supposedly regarded as the best. How much of a risk is it using someone not accredited?

 

Are there any other considerations? I'm not interested in saving a few hundred quid a year if it means that an ASHP causes more issues than it solves.

My thoughts as a philistine are. 

 

By all means look to install an ASHP and let it tick over most of the day and night, It makes sense, less wear and tear on the switching and moving valves. Think maintainence and cost of that, this is key to long term performance., often overlooked on BH. 

 

You can have as many fancy controls as you like, but these will all break down at some point and you may want to sell on the house so you might want to make sure your ASHP is an asset rather than a liability.  Let's say you can square that circle, size you tanks and so on. If I was you I would certainly look at ASHP as a solution. But here is what I would do first. 

 

"The loft has been insulated up to standard 10 years ago." This is your low hanging fruit in terms of improving the existing insulation. In Scotland we look for a U value of 0.12 W/m^2K. To give you a flavour for this 400mm of glass wool will get you over the line. But you must make sure the roof void is properly ventilated, loft haches are sealed and you are not pocketing up bathroom down lighters that let moisture into the attic. You can't go flooring the loft unless you ventilate that properly also. Look at the type of roof you have and see how you can ventilate it. 

 

"My property is early 1900's solid wall construction." There is a good chance you will have a timber suspended floor? Now you are going to spend a pile of cash so you want to make sure your floor does not rot. Make sure you get that ventilated properly. Now it may be that you are going to do some work that will let you get extra insulation into parts of the floor. So you trade that off, you have now plenty ventilation under the floor so you need to get the skirtings and so on air tight. 

 

Your solid walls are probably more insulating than you realise! But it's the apertures (windows and door) that will let you down. Spend time getting this detail right. 

 

In summary I think it's doable but unless you are going to live in the place for a very long time then it will take you a long time to get your investment money back. Some folk are happy with the techy stuff as it becomes a hobby and are happy to take the financial risk and potential right down when they sell they house. 

 

In terms of ASHP cost and sizing. There are a few on BH that say.. ok lets design for 11.5  months of the year and if it gets a bit chilly then we will plug in a couple of fan heaters. This make much sense!