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High Temperature Air Source Heat Pumps for Domestic Heating

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Steve Skelton 108/10/2020 18:38:09
130 forum posts
3 photos

We have a modern well-insulated house in South West UK with underfloor heating. When building the house, I looked at ASHP’s and even went on a Daikin training course to learn more about them. I decided not to go ahead at the time (2013) as most of the “real person” reports (as opposed to the salesmen) were saying that they did not yield the efficiencies (COP – coefficient of performance) that were claimed and that most people were finding them more expensive to run than the oil or LPG systems that they replaced. Based on this I installed a modern condensing LPG boiler.

When I investigated why they did not seem to work as advertised I came to the conclusion that they were not performing due to ice build-up on the condenser heat exchangers such that the ice acts as an insulant stopping the transfer of energy from the ambient air into the condenser thus reducing the efficiency of the unit.

It is well known that ASHP’s work fine in Scandinavia so why would the same unit not work in the UK? I think the answer is the difference in air humidity. Scandinavia tends top have cold dry air, whereas we have less cold but much more humid air, and thus in this country units ice up much more quickly.

Now that I have a bit more time I am looking again at whether the technology has moved on and the units can actually work at COP’s greater than 2.5 and therefore be cost-effective against LPG.

Does anyone here have any experience of ASHP’s and specifically the high-temperature air to water units (which are I believe are all split units with a separate box in the house to get a second stage reverse refrigeration process)?

Ideally, I would be looking to install a unit in parallel with the LPG boiler (for when the external temperature is very low) to run the hot water and underfloor heating.

Cheers, Steve

Martin Connelly08/10/2020 18:56:53
2020 forum posts
214 photos

I have just had a Daikin air to air heat pump split unit fitted (wired up 1 week ago). I had an old gas fired warm air heating unit that I thought would be condemned if a gas safe engineer ever saw it. The options were have a wet radiator based system or make use of the under floor ducting that is already in the ground with a new system for producing the warm air. The heat pump system was chosen because the outside unit could be mounted on a suitable (south facing - may have some good effect against ice ) wall and the indoor unit in the loft. Minimal disruption compared to having to pick inside walls to mount radiators on and then have all the piping routed round the place. It is basically similar to the ceiling mounted air conditioners that are found in many shops and offices but ducted to the existing system and had a very similar cost to having a wet radiator system fitted. It's too soon to tell what it has done to my energy usage and overall costs but seems to be working fine in the current climate.

Martin C

Oily Rag08/10/2020 19:02:12
531 forum posts
172 photos

Good business to be investigating, there will be much investment in alternative forms of heating in the near future. Whilst not specifically experienced in ASHP's I was involved in a ground source heat system which a friend of mine got involved in (due to the fact he had a rather nice JCB back hoe). In a neighbouring village the customer wanted his sizeable back lawn dug up to a depth of 4.5 M and laid with radiators and pipework to extract the heat from the ground. The pump was run from solar panels and the system seemed to work well in spring, summer (hot water heating) and autumn, but in winter the ground became 'permafrost' even when ambients were above freezing! This led eventually to the lawn dying and the excavation site sinking, filled with water and he finished up with a skating rink well into April!!!

I agree with your point about Scandinavia having dry air - from memory very cold air (sub -20C) is considered absolutely dry air. Dew point in this country is a curse we all face!

I'll ask about the system he used - I think it had a house based heat exchanger system of some kind.

john halfpenny08/10/2020 19:24:36
198 forum posts
27 photos

Steve, we have had a Mitsubishi ashp ( made in Scotland) for over 10 years, in a new build 350sqm house, with concrete floors up and down; 30 deg for heating and 55 deg for hot water. We are very happy with it, no maintenance to speak of, and current electric bills (that's everything) of around £1400. We got all the certificates, and the government pays our bill for 7 years under the rhi scheme. The system is slightly under specified because I didn't want 3 phase costs, so we use a wood burner to give direct heat when temp is below 0deg. The system works well above an air temp of 5deg, but below 0deg, it's just an immersion heater - cop of 1. We find it most economical to run it on economy 7 tariff at night only, except on the very coldest days when we may run for a few hours in the day as well; internal temp is typically 21deg. The air at night is of course colder than in the day, but night electricity is cheap, so the arrangement works for us. Most of the time we are only 2 occupants, so we use the zoning to turn off active heating in unused bedrooms. Noise from the fans is unobtrusive, but we are on a big plot. The system occasionally reverses in cold weather to melt ice away, but ice does accumulate around the base of the unit - this could have been dealt with on installation if the problem had been appreciated. Plumbers are generally useless, as are electricians; they don't understand the technology, - our one problem was quickly addressed and solved by a local air conditioning business. A good choice if you already have underfloor heating. I'm sure tech is better now.

Stuart Bridger08/10/2020 19:33:38
536 forum posts
29 photos

The GSHP example seems a bit extreme. We installed a an IVT unit 13 years ago with panels a metre deep in the back garden. We have never experienced the permafrost effect and it has no effect on the lawn. I do keep an eye on the ground loop temperature and only one year has it been sustained below zero on the incoming side. We do supplement with a wood burner in the winter though.

Robert Atkinson 208/10/2020 19:54:06
1152 forum posts
20 photos

We had a Dakin multi-split Air-Air system installed about 18 months ago. Two outdoor units feeding 5 indoor (capacity for one more). It wors really well and heating costs have gone way down. Even using it for cooling in hot weather our bills have not gone up.
Like most modern systems this uses a variable speed compressor and fans so it is very quiet and does not switch on and off with a big change in noise like the old fixed seed systems. I installed a similar Mitsibushi system in my last place about 15 years ago and it was great as well. When I sold it I dad to explain to the energy assessor that it wasn't just air con. He initially put it as electric heaing but after taking advice back at the office our energy rating improved a number of points.

The odd thing was that all the installers on the new system wanted to put single split systems (5 outdoor units) in. No idea why and unlike the system I put in the old place the two multi split outdoor units were slightly dearer than 5 smaller singles.

Robert G8RPI.

Martin Rock-Evans08/10/2020 20:17:43
24 forum posts
14 photos

We had an air source head pump installed last year that works well with our underfloor heating system. Here in the South West (Mid Devon), the ambient air is sufficient that it rarely showed any ice last winter, so COP has been good. Much of the power is from our Solar PV panels, so free electricity leads to free heating, but we did need to be careful to set the thermostats sufficiently low out of daylight hours to prevent excessive bills from nighttime heating.

One thing we considered was keeping our old oil fired boiler alongside the new ASHP, to provide heat in freezing weather, however if you do, the RHI will have to be metered. However, the RHI is still capped with the same maximum annual RHI being based on your ECP rating if you have a meter or not, so in terms of maximising the RHI, we opted to completely replace the heating system and have the RHI unmetered.

Another thing to consider is the actual design of the system and location of ancillary equipment. Although our ASHP is almost silent, the heating circulation pump is not (same in any large central heating system) and with our new heating layout system, the circulation pump was installed in our airing cupboard, just outside our bedroom. This is sufficiently noisy to wake me earlier than I'd otherwise want...

Andrew Binning08/10/2020 21:54:53
39 forum posts
9 photos

Just come across this thread on ASHPs . I have a Daikin unit and my annual bills are up on what they were with the condensing oil fired boiler and in summer I get a good slice of power from my 3.4Kw solar array. This is of course no use for savings against mains supplied energy for the ASHP in the heating season. I have a wet radiator system controlled by an industrial PLC which allows individual room heating to be selected for heating periods and temperature. This means that the ASHP has to run at a higher temperature than would be ideal for an ASHP. If I could, I would go for a split system with underfloor heating and a higher temperature circuit for upstairs rads ( heating not needed in bedrooms except first thing in morning and bedtime ) and the hot water. Hot water with my present setup is largely free from solar from March to September so the ASHP is shutdown during the summer. The house is a 1938 detached construction with solid brick walls. Roof is insulated to modern standards and extensions front and back are cavity wall construction insulated to 1990 building standards. I don't regard the house as being well insulated . My current electricity bills are about £140 a month, before the ASHP was installed I was using about 1500 litres a year when the oil price was about 45p a litre. But of course when I installed the Heat Pump the bottom fell out of the oil market ( 2015 ) and the price of oil halved! Electricity consumption prior to ASHP installation in 2015 was about 14,000 units a year. The electricity consumption for the year ending April 2020 was 11,250 units at 14p per unit. The solar panels generated 2750 units in the last year to March 2020 and my FIT payments this year will be well over £1200. My hot water is also heated by a solar water panel array which has run for 37,500 hours since its installation in 1984 but I cannot tell you how much energy they have actually contributed , suffice it to say that even before the solar panels were installed I was getting hot water from April to September with hot water cylinder temperatures sometimes exceeding 80°C. Now surplus PV power is diverted to the immersion heaters in the hot water cylinder as well. Hope you can extract what you need to know from this essay! If you need to know more I will try and assist. Best Wishes.

Post Script:  Have posted and re read what I wrote,  I realised I have contradicted myself somewhat!  My electricity consumption is less post ASHP installation and I do not have fuel oil bills so I have benefited more than I had previously thought.  PV panels make a significant energy contribution to overall consumption but not of course during the winter when their contribution would be really useful.  I need to see if I could make any savings by running the ASHP during the night on economy 7 rates, perhaps setting back the temperatures a bit from daytime levels.  By the way I have seen no evidence of ice formation on my outdoor heat pump radiator ( absorber)  and the fans are very quiet and are load dependent variable speed running.

Edited By Andrew Binning on 08/10/2020 22:04:38

Edited By Andrew Binning on 08/10/2020 22:09:40

not done it yet08/10/2020 22:49:52
6519 forum posts
20 photos

Not much mention of COP yet. Looking forward to some real world figures.

Neil Lickfold09/10/2020 08:17:05
756 forum posts
129 photos

We placed our Bedroom outside unit so that it is in the sun, and the warmer side of the house, as we use it mainly for winter heating the cold end of the house. The main living room unit is placed in the cold side of the house, as we want it for cooling in summer, more than heating in winter.

So far it has worked very well for us. We can't see the power consumption for the heat pumps, but the power company we were with, had a home comparison for electricity used based on comparable house specs. Our power bill was consistently 25% to 30% lower than the comparable homes. We do have a well insulated home, but the main difference is we have 3 individual units complete with their outdoor units. The system they wanted to put in was one large outside unit feeding 3 inside units, out here is called a multi split system. The smaller 2 2.5kw units are more energy efficient than one 5kw unit. The lounge has a single 5 kw unit for cooling the space and is the correct size for the area to cool and more than enough to heat in winter. Running all 3 still uses less power than the one big unit.

The 2.5 kw units have a rating of 4.3 and the 5kw is 4.0 the 10 kw that they wanted to put in is only 2.9

Now 18 months later , the newer 2.5kw units are down to 6.5 and the 5kw units are 5, so producing more heat or more cooling for even less energy.

Roger Best09/10/2020 12:05:16
333 forum posts
43 photos

Hi Steve

A recent innovation is R32 refrigerant, which is better suited to to our climate and makes the LG system highly attractive. I don't know if the competition are catching up yet.


My Mrs is completely sold on electric heating. I put in a wood burner with back boiler about 12 years ago, always intending to hook up thermal tubes to the bespoke multi-coil boiler, but never got round to that, or to having a gas boiler to warm the house when we were at work. After all those winters messing about with logs and probably getting lung cancer she is ready to rip it all out. I am crying inside after all that work, but our work and health have changed with age, we need a warmer house, and the technology has moved on.

We got a load of quotes last year - most where 14k - obscene! One of the issues was the government green skies subsidy, which doesn't like alternative heat sources, even when they are a no-brainer like solar thermal. So if you want the government to chip in then you will have to play a bit of a game.

I would be very interested in anyone's experiences of Air-source and solar thermal too. The posts above are most appreciated, ta.

Brian Sweeting09/10/2020 13:44:32
453 forum posts
1 photos

Steve, I was going make a suggestion regarding the Scandinavian options as it was raised many years ago when I was involved in the trade.

Although dry air obviously has an impact you would also find that they used a wider fin spacing on the coils. This reduced the don't problem because the ice could not combine with the ice on adjacent fins and allowed much quicker defrosts.

Robert Atkinson: Recommending single splits over multis indicates lack of skill/knowledge regarding the installation.

Steve Skelton 109/10/2020 14:04:41
130 forum posts
3 photos

Firstly, many thanks to everyone who has contributed.

Martin, I think you have a good option with air to air. I thought of ducting warm air to rooms when I built – perhaps this would have been the best way?

Oily Rag, I have a friend who installed a GSHP, although he has had a few problems with it. 4.5m below ground, wow that’s deep! This is the first time I have heard of the ground freezing due to a GSHP, but I suppose logic would suggest it is possible if the energy coming up from the earth is not sufficient.

John, we also use a wood burner to heat up our thermal store which the UFH can draw from and we find this significantly reduces the amount of LPG we consume. I take your point about the COP when the outside temperature is below 0 deg. C - this is why I want to have an ASHP in parallel with the existing boiler. We are fully zoned by room using the UFH. Do you have any feel for what it does to the COP when the ASHP is in reverse flow due to ice build-up?

Robert, your system looks interesting – unfortunately, due to my heating design, I have to have an air to water system.

Martin. We are close to you (near Wellington) so, like you, we should not get long period of freezing weather. Our UFH is buried in 12T of screed which acts as a good thermal buffer so along with our high levels of thermal insulation (100mm in floor, 140mm in walls and 250mm in ceiling) should mean we only need to use an ASHP during the day when the outside temperatures tend to be higher.

Do you have a split system and do you heat your hot water as well from the ASHP? What outlet temperature do you run the ASHP at?

Andrew, bad luck with the oil pricing! I take it you have a mono system? What output temperature do you use? I do not have the facility to link the ASHP to the UFH without using the flow and return pipework from our LPG boiler via the thermal store – hence why I need a high-temperature ASHP which I believe has to then be a split system. I do not have any confidence in the sales pitches by the mono system manufacturers.

Neil, sounds interesting, are you using air to air split systems and whereabouts, are you?

Roger, have you installed the LG Mono system? I see it quotes 65 deg. C as a maximum outlet temperature. I have tried to get technical info from the LG site without success – I may not be looking hard enough. Generally, with mono systems the COP at this temperature is very poor – do you have any knowledge of the LG performance?

So once again thank you all for your comments, please keep them coming.


Andrew Binning09/10/2020 14:54:07
39 forum posts
9 photos

Steve, I have what I think you would call a Mono system. The oil boiler was removed and the ASHP unit connected to the pipe system supplying the radiators. The discharge temperature from the ASHP is set at 70°C. If you read my previous post you will know that in the summer all my hot water comes from solar hot water and PV systems. In the winter if there is occasionally a bit of winter sunshine to boost the DHW temperature otherwise the DHW is heated by the ASHP. Sales pitches from the manufacturers did misslead me as well. They are more interested in getting their money and not too concerned if you have an optimum design for your situation. They should have told me to get rid of the downstairs radiators and install UFH running it from a split system with low temperature for the space heating from UFH and a high temperature circuit to heat DHW and the odd upstairs radiator. Benefit of hindsight! However having checked the figures again I am gaining from the COP of the ASHP but couldn't put a figure on what that is. The figures speak for themselves. Pre ASHP with a condensing oil boiler

Electricity :- 14,000 units at 15p £2100 Plus oil :1500 litres at 0.45p/litre £675

Post ASHP installation

Electricity :- 11,250 units at 15p £1687 and no oil cost

The PV contribution to energy consumption has been basically the same so can be ignored for comparison purposes. Hope this helps.


Cyril Bonnett09/10/2020 15:41:49
244 forum posts
1 photos

We've had a Nibe f2015 for 9 years, it was installed in two days, outside unit, piping, eight radiators and controls, 250lter tank, warms the house, costs around a grand a year, we use calor for cooking, one large bottle a year.

The COP was stated as 2.5 but they reduced that and I think they now claim its around 1.5/2.

Heats the house, but we have never used the two immersion heaters, 5 degrees and below they kick in and turn the heating system into a very expensive hot water system.

Had no problems at all, it's a bit noisy and huge, produces hot water at around 55 degrees, the new ones are smaller, quieter and some produce water at around 80 degrees some it's claimed down to -6 degrees.

The house has exterior insulation which helps a lot in the winter but does have a down side, it keeps the house cool in the summer, sometimes too cool.

Recommend it yes, the annual service/insurance is quick and cheap as well.

Neil Lickfold09/10/2020 19:28:10
756 forum posts
129 photos

Andrew Binning, We live in Hamilton NZ, in winter it gets to a frost, so no colder than -6 C and summer will get to 30c but with a very high humidity. Out here, most brands are using R32 refrigerant. The system of having multiple split sytems instead of 1 larger unit and the same multi split system is not what the installers reccomend. But there have been some distinct advantages to what we did. Our running costs are lower per unit per room, even with all 3 running. The workshop 2.5kw is set for 22 c heat most year round, except peak of summer when I change it to cool 22c. Having the outside unit so that it gets the least amount of weather on it also helps in my view. We have not had the outside units freeze on us yet. But the combined fin area for the 3 outside units is greater than that of the 1 10kw unit had we gone that way. The neighbours were complaining this winter that there outside unit kept freezing on them. The main cause of the freezing I think was the airflow around the outside unit had some restrictions, ie bins placed beside the unit. Not infront though. Our outside unit is placed on a concrete plinth that sits on compacted sand. We do not hear the outside units running. The units for the workshop are placed at the back of the house where there is a predominant wind blowing through. At the end of that area, it opens to a courtyard and we have the washing line in line with what I call the tunnel. It dries very quickly most days. This breeze that flows though is very helpful to the outside units it seems . The house has large eves, and so most of the time, the outside units are dry. They seldom get wet, and seldom get dew on them either. Our system has a dehumidify mode, which can be set automatically in the program. It does blow the air around and can feel cool as it is dryer air being moved. We changed the direction to blow to the ceiling , and that seems to also work well for us.

Neil Lickfold09/10/2020 19:36:38
756 forum posts
129 photos

If we replaced our 3 units with the latest Daiken high efficiency units, our max power to run all 3 will be 2.35kw total. Our current system is 2.9kw to run all 3 according to the specs of power consumption. The largest of the high efficiency units is a 5 kw heat pump.

Edited By Neil Lickfold on 09/10/2020 19:40:52

john halfpenny09/10/2020 21:32:40
198 forum posts
27 photos

In our damp uk climate, water condenses on the fins as heat is extracted. In cold weather, that water freezes, and to combat this our ashp reverses occasionally for about a minute. Warm water at 30 deg is sufficient to melt the ice which runs down to ground level. In our case, no provision for draining that water away was provided, so it freezes again, and over a few days the ice can build up. Of course this requires sustained air temp of below 0deg, which we seldom get for long. No harm caused, but I recommend some deliberate means of directing the melted water elsewhere. COP is of course dependent on the temperature difference across the heat exchanger, and accordingly varies significantly with ambient air temp. We run at night, so COP is low, but it doesn't matter because electricity is cheap - my point being that COP is not the sole governing factor. We also have a huge mass of concrete as heat store (not very green I know) which keeps the internal temp very stable.

BTW, anyone reading this thread must distinguish between air/air ashp which provides direct warm air, and air/water ashp which provides warm water for underfloor heating.

Edited By john halfpenny on 09/10/2020 21:35:11

Chris Jones 310/10/2020 04:03:49
28 forum posts
75 photos

This is slightly off topic, does anyone have experience of PVT (combined photovoltaic solar thermal) systems?

Roger Best10/10/2020 23:00:07
333 forum posts
43 photos

Hi Steve, no we didn't go forward with the system, we didn't get a warm feeling from the people who contacted us, I got some quotes via a company advertising on Facebook called Greenmatch and I was sure they where high. Its on the cards for next year though, probably with a local company.

My house wasn't designed to have services other than a gas fire and cooker, so it has to have pipes and radiators, I am jealous of those who can use nice, simple, air.

I recall that the LG unit gives a good COP at cold temperatures, when other gasses stop working. I am sure they all have poor COP at high temperatures too, gas boilers simply do a better job.

I don't know where I have the brochure or in what form, memory playing tricks again, it doesn't help that the heating paperwork has been tidied away. I thought they sent me a pdf but its missing.

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