Air Source Heat Pump

[dingg]

JuniorD said:

You need to be careful with installing ASHPs and GSHPs in Aberdeenshire because for every 5 that are installed one local will lose their job in the oil & gas industry.

You just made that up didn't you....

[caziques]

There are major issues with air sourced units without thermal mass.

A well designed ground source has a known output, air sourced varies depending on outdoor conditions. If this isn't catered for the end user will have likely experience problems (a combination of being cold and/or high running costs).

Thermal mass, principally concrete, largely overcomes these problems. A 100mm concrete slab is fine for a standard NZ house, 50mm screed is OK for a well insulated property.

Air sourced can work with radiators, but simply fitting a heat pump to existing radiators is recipe for disaster.

[Equus]

caziques said:

There are major issues with air sourced units without thermal mass.

There, at least, UK construction is favourable - the great majority (75%+) of our housing is masonry, and of the small percentage that is timber frame, almost all has concrete floors - it's pretty rare to find the combination of a timber frame house with a timber suspended ground floor.

Personally, I favour more thermal mass than just a ground floor slab, regardless of heating system - if you have a decent level of insulation, you need more thermal mass than that to damp out overheating due to solar gain.

The flip side of the coin is that masonry construction tends to be less airtight than timber frame, so what you gain in thermal mass, you can lose on draughts.

[Matt Dell]

Bit of a bump here but at least this thread was started in 2020.

I'm also looking at installing an ASHP to replace my oil boiler. My house is an 1800's Cotswold stone cottage with 18" thick stone walls and double glazed windows. We had our first winter last year and the house was quite warm.

We are doing work on the house and my architect suggested an ASHP. Also, my sister-in-law had one installed in her house in Kent and absolutely loves it. Her house is of a similar age and is bigger than mine. I know they also installed larger radiators but not underfloor heating.

So I'm surprised to read this thread basically saying you can only install ASHP in a new build.

The system I'm currently looking at is a Stiebel Eltron WPL 25 AS. https://www.stiebel-eltron.co.uk/en/products-solut...

It claims to run up to 65 degrees which is about the same as a boiler isn't it? Installer says I don't need to upsize my radiators because of the better flow temperature.

I would like to install underfloor heating, as well.

Heating experts, what am I missing and why shouldn't I go with this? I was all aboard until I read this thread.

[Evanivitch]

Matt Dell said:

Bit of a bump here but at least this thread was started in 2020.

I'm also looking at installing an ASHP to replace my oil boiler. My house is an 1800's Cotswold stone cottage with 18" thick stone walls and double glazed windows. We had our first winter last year and the house was quite warm.

We are doing work on the house and my architect suggested an ASHP. Also, my sister-in-law had one installed in her house in Kent and absolutely loves it. Her house is of a similar age and is bigger than mine. I know they also installed larger radiators but not underfloor heating.

So I'm surprised to read this thread basically saying you can only install ASHP in a new build.

The system I'm currently looking at is a Stiebel Eltron WPL 25 AS. https://www.stiebel-eltron.co.uk/en/products-solut...

It claims to run up to 65 degrees which is about the same as a boiler isn't it? Installer says I don't need to upsize my radiators because of the better flow temperature.

I would like to install underfloor heating, as well.

Heating experts, what am I missing and why shouldn't I go with this? I was all aboard until I read this thread.

Do it. Piston heads isn't exactly renowned for getting behind change, let alone green change. Will it be cheaper than gas? Probably not. Will it pay for itself? Maybe not. But it will be a lot better than oil. Definitely.

[Matt p]

Matt Dell said:

Bit of a bump here but at least this thread was started in 2020.

I'm also looking at installing an ASHP to replace my oil boiler. My house is an 1800's Cotswold stone cottage with 18" thick stone walls and double glazed windows. We had our first winter last year and the house was quite warm.

We are doing work on the house and my architect suggested an ASHP. Also, my sister-in-law had one installed in her house in Kent and absolutely loves it. Her house is of a similar age and is bigger than mine. I know they also installed larger radiators but not underfloor heating.

So I'm surprised to read this thread basically saying you can only install ASHP in a new build.

The system I'm currently looking at is a Stiebel Eltron WPL 25 AS. https://www.stiebel-eltron.co.uk/en/products-solut...

It claims to run up to 65 degrees which is about the same as a boiler isn't it? Installer says I don't need to upsize my radiators because of the better flow temperature.

I would like to install underfloor heating, as well.

Heating experts, what am I missing and why shouldn't I go with this? I was all aboard until I read this thread.

65 degrees is the absolute limit of this machine. You’ll get close to that with a return of 58 from the tank. Assuming it runs on a leaving water setpoint rather than a return setpoint.

Just had a quick glance over the technical PDF to see what it else is can do and noticed that it has an option to run a booster (immersion heater) are you planning on using it for HWS also or just as heating only system?.

[Evoluzione]

If my boiler was running at only 65'c i'd be ringing a heating engineer. That said i'm interested to hear how this works out given what Equus said and also what i've heard elsewhere about a house needing to be uber insulated to make use of the slight heat these things produce.

[Condi]

I know of 2 houses with ASHPs, both are fairly modern, and electric bills are £170/m each!

If you need hot water as well then go solar water heating, its considerably cheaper and in summer will do most if not all your hot water.

[Matt p]

Evoluzione said:

If my boiler was running at only 65'c i'd be ringing a heating engineer. That said i'm interested to hear how this works out given what Equus said and also what i've heard elsewhere about a house needing to be uber insulated to make use of the slight heat these things produce.

Limited by the head pressure of the refrigerant. Personally I wouldn’t be pushing them past 60 degrees. It’ll be running very close to the HP cutout of 38bar.

I recently commissioned eight ASHP’s for an NHS trust in north London. Five are utilised to supply HWS+Heating. The other three switch between heating an cooling depending on OAT for the Ahu’s. The HWS water is boosted by a pair of boilers when needed. As for the defrost cycle, this is limited to once every 40mins.

The early ASHP’s no matter from which manufacturer imho were pretty garbage and pretty unreliable. Things have moved on a hell of a lot of the past few years. Components have become a lot more reliable. The only thing that I’d be thinking of long term is the R410a replacement which will be coming in the near future.

[uuf361]

I’ve been in my house almost 8 years now (new when I moved in) and it came with an ASHP (NIBE) and only underfloor heating.

Took me a little while to figure out how it worked and to maintain the temperature all the time as nothing is on demand.

It’s been fine, although when it goes wrong it can be painful to get anyone out to fix as there aren’t many companies. Mine has gone wrong 3 times, twice the starter and always in winter, last time was a thermostat (cost to fix £1.24!) that meant the system wouldn’t work or required lots of manual intervention.

Other than that, it’s simple, set up, leave alone as it works. My average usage for all power in the house is around £110/month (2,200 sqft) which is around the same as my old combined gas/electric in my previous 1,250 sqft modern house, plus this one is never cold, as the temp is constant....

[Martin30]

Bumping an old-ish thread, to try and understand if anything has changed over the last 12 months or so.

I am contemplating an ASHP instead of my 25 year old, non-condensing oil system. 70s 4-bed detached house, EPC D but will cavity wall insulate to improve. House is nice and warm currently using 1,500l of oil a year - I assume thus not a poorly insulated environment.

Green Homes grant means capital investment is fairly low. I am not trying to reduce monthly bills, but would like to reduce fossil fuel usage.

With more remote working, keeping the house nicely warm all the time is now normal but I am still a little nervous about the lower heat output from ASHP.

I am wondering if changes usage patterns (at home more) means ASHP make a little more sense.

Any thoughts from the more knowledgable?

Thanks,

Martin.

[Equus]

Martin30 said:

I am wondering if changes usage patterns (at home more) means ASHP make a little more sense

Potentially, yes: ASHP's are certainly better suited to continuous use.

Question is how long the WFH situation will last?

[Siko]

Also interested to hear any thoughts on ASHP. We live in a big(ish) 4 bed Stone cottage. It was renovated a decade ago and UFH was fitted to a thermal store and two log burners. I buy 12 tonnes of cordwood a year go saw, split and burn. It’s a massive pita although it keeps me fit!

I’ve been looking at GSHP (we sit on stone so not really ideal) and it’s come down to ASHP vs oil. Our house is a Cat C EPC and whilst it was highly insulated during the renovation it will always be a not exactly airtight 1800s stone cottage! I like the idea of a RHI paying most of the costs back, but I have also read heaps of horror stories with badly setup ASHPs in older houses like ours giving crazy bills and lukewarm heat.

There’s a fair bit we can do to improve the air-tightness of our home but with heaps of animals and kids in/out all the time it will never be ideally suited to ASHP...as much as I want one.

I’m wondering if oil boiler and log burners might be better for us, any advice appreciated though.

[Europa Jon]

I've had an ASHP for 18 months now. We did have oil, but I didn't want to accidentally run out of oil again. We had space for the extra ASHP gubbins and I fitted massive radiators during major improvements.
Our old oil tank was rusting, and started to leak. Timing was in favour of an ASHP. I wouldn't say the ASHP is any cheaper than oil right now.
So, in conclusion: insulate and draughtproof as much as possible first. Forget about GSHPs: a leak in the collector loop will ost so much to fix, it negates ang efficiency saving over air source. I'd recommend an ASHP if you're needing to spend money upgrading your oil tank or boiler. If not, sit tight while heating oil is cheap.

[caziques]

I exclusively fit air sourced hot water heat pumps to new underfloor heating jobs - and replace oil boilers occasionally (but note this is NZ - winters are not as cold as the UK)

Oil has 10kWhrs of energy in every litre - but even the best boilers aren't 100% efficient - so it can be quite easy to work the figures out.
Heat pumps can be anywhere between 2:1 and 5:1 efficient.

There is an inherent problem with heat pumps and radiators - in order to be efficient, the lower the temperature of the water the better - which means BIG radiators.

Underfloor is no issue - anyone who installs an oil boiler with underfloor should be shot.

The output and efficiency of a hot water heat pumps varies with the outdoor temperature - resolving these issues with radiators may not be straightforward.

The limitations can be the power supply to a house (big heat pumps need three phase) - and balancing the heat output with the load required (a buffer tank usually solves this one).

The considerable extra cost of ground source in NZ means it is never worthwhile. - one big local job with ground source fitted by some competitors, is less efficient than a good air source. (One reason is the 1.1kW pump to circulate water through the ground loops - a waste of energy that reduces the efficiency by about 20%)

[Siko]

Hi Caziques, thanks for your reply. Can I just pick up on a couple of points? Are you saying that an oil boiler wouldn’t work well with UFH and do ASHP need 3 phase electric supply?

[Equus]

caziques said:

Oil has 10kWhrs of energy in every litre - but even the best boilers aren't 100% efficient - so it can be quite easy to work the figures out.
Heat pumps can be anywhere between 2:1 and 5:1 efficient.

This is a quite a misleading statement if taken at face value.

• Firstly, it is unheard of to achieve average 5:1 efficiencies in the UK, or anywhere close. You're lucky to get over 3:1, and 3.5:1 would be considered very good.
• Secondly, the efficiency drops as outside temperature reduces, hence an ASHP will be at its least efficient when you place it under highest demand; and under such circumstances efficiency can drop to parity or below. If we get a cold winter, running costs will spike.
• Thirdly, and perhaps most importantly, it takes no account of the cost of the input energy. Currently, Google tells me that average electricity price in the UK is about 14.4 pence per kW/h. Heating oil costs about 4.8 pence per kW/h and a good, modern condensing boiler is about 92% efficient.

... so if we're generous and assume an average COP of 3.5:1 for an ASHP, the actual costs for a kW of energy as it comes out of your radiators are:

ASHP = 14.4/3.5 = 4.1 pence per kW/h

Oil = 4.8/0.92 = 5.21 pence per kW/h

You then have to factor in the unresponsiveness of heat pumps, which really means that they need to be left running the whole time, whereas oil can be throttled back or switched off when not needed.

I'm probably a bit odd, but even at this time of year I sleep with the bedroom window open and my oil-fired heating system switched off at night (basically 1/3rd of the time) - it goes back on in the morning. Lots of slightly more normal people are out at work all day, under normal circumstances. In either case, you can fire up an oil fired boiler and the house is warm 10 minutes later. Try that with a heat pump...

TL:DR version: the actual difference in running costs is negligible, and will probably even favour oil, depending on your usage pattern, whilst the flexibility of oil is much better.

[Equus]

dhutch said:

Equus said:

The basic shell of an early 1990's property will be very poorly insulated compared to modern standards; if it's masonry, it may still have an uninsulated cavity, which will be at best 50mm (twice that would now fail to meet current regs). It will almost certainly have been built with an uninsulated ground floor, and no attempt at airtightness.

There is a reasonable chance you are right, however that is a broad and sweeping statement.
My parents house built in the late 80's has a 75 cavity with 50mm insulation, insulated flooring, mhrv and due to using conventional plaster probably as good or better air tightness as building half its age. A lot of this was uncommon, but certainly not unavailable.

Just to pick up on this, which I missed first time around: your parents house, as you acknowledge, would have been exceptional for its age.

... but to put things in perspective, current construction standards require approximately twice that thickness of an insulation that is twice as efficient, in the walls (ie. about 400% better than a house that was already exceptional in its day). And the regs are about to change (June 2022) to require another 31% overall uplift in energy efficiency.

The Government has a major push that will drive us toward heat pumps on new houses; a further upgrade to the Regs, expected in 2026, will effectively outlaw fossil fuel heating systems on new houses. But don't confuse that with their suitability for existing housing stock - the way we're building new houses makes them dramatically better suited to heat pumps than most existing buildings.

[Evanivitch]

Equus said:

I'm probably a bit odd, but even at this time of year I sleep with the bedroom window open and my oil-fired heating system switched off at night (basically 1/3rd of the time) - it goes back on in the morning. Lots of slightly more normal people are out at work all day, under normal circumstances. In either case, you can fire up an oil fired boiler and the house is warm 10 minutes later. Try that with a heat pump...

You'd just use a thermostat in your room to kill the heating in that room overnight.

[Equus]

Evanivitch said:

You'd just use a thermostat in your room to kill the heating in that room overnight.

With a heat pump, yes, you would.

But the point being you'd still be running the heat pump overnight, heating (and losing heat from) empty rooms, just to compensate for the fact that the system can't cope with rapidly ramping up the temperature again in the morning.

'Normal' people will be doing the same thing while they are out at work for 8-12 hours a day, just so that they can come back to a house that isn't freezing for the first couple of hours when they get back in the evening.

This will usually negate any cost advantage in terms of price per kW/h that heat pumps can theoretically achieve over oil boilers, on older housing stock.