Air source heat pumps

We have one, and it is a nice heat as with stone floors having the underfloor heating is a great pleasure. It’s in our gîte, so a renovation project with loads of insulation, new insulated roof, new windows etc etc so not far off a new build. We keep at low temperature, and then have a small woodstove to bump temperature up if need be. But we’ve found that if people’s feet are warm they are ok at a lower temperature.

However the thing that really bugs us, and is our fault for not asking the right questions, is that we have to keep hot water flowing through it even when gîte is empty or bloody thing freezes. So we have to have radiator circuit at 20 - with radiators turned off of course - to make sure that it’s ok and there’s warm water circulating with the pump. The hors gel setting is useless as if we leave the whole thing on the pump itself freezes because it is making cold air even colder. Admittedly we are in a cold area so if you wee somewhere less cold it may not have the same problem.

As far as costs go we monitor it carefully, and it’s not too bad. Certainly no more than ordinary electric heating. What is hugely inefficient is one year we had longer term renters over winter (never again) who liked to live in T-shirts so wanted it set at 24 degrees. Luckily they were paying the charges!

In one of our UK homes… OH fitted fan-assisted radiators under the work units (within the plinth)… one on each of the four sides… so warm air was wafted right across the floor. It was all part of the (wet) CH system and worked like a dream…


Jane - ours has/had antifreeze mixed into the water.

So they’re ok on UK plugs with adapter?
If so I’ll bring some over in September as we are going to UK for a wedding.

They supply them with EU / French plugs.

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Hi Stella, my system in the uk was gas fired, not via a heat pump. We did have a cold winter at -8 and the underfloor heating still worked well in the relatively new house. It really is down to the correct insulation under the heating system in the floor and the house was also super insulated in 1985 terms - probably now about the standard in new UK houses.

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That’s a question I haven’t asked! Thanks!

Sounds like you got things just right then… well done…

I’ve posted quite a few times on this subject.

It doesn’t matter if the outside temperature is below zero; drop the temperature of a cubic metre of air by 10° and you get almost the same heat out whether it starts at 20°C or -20°C, and there are systems which work down to -20 and below but there are some considerations.

One is that efficiency does drop off as the temperature of the intake air drops - the worst is probably with intake air just above freezing when ice build up on the heat exchanger is a real problem. Systems usually have a way to deal with this such as an auxiliary heater, ours reverses the heat exchanger to draw heat from the heat store but whatever approach is taken it reduces the heat output from the system - and as Simon observes, just when you most need the system to produce heat not waste it defrosting itself.

I suspect that many are under-specified, ours has a nominal output of 16kW which is already low for the size of property but drops quite quickly if the intake air is below 5-10°C. Had the system been sized so that enough heat was generated at an intake of -10 or so it would have worked better but, I suspect, have been even more expensive (it was installed by the previous owner in about 2008 at a cost of about 15k€).

Me, for one.

Hmmmm, well, yes but even though mine is underpowered there is still a COP of 2-2.5 even with intake air at -15°C so it is better than electricity alone.

It is worth spending a little time to understand how they work to better understand the limitations.

The “Hors Gel” system on yours probably does something similar to the “Hors Gel” setting on mine - almost nothing! Basically it amounts to little more than keeping the pump running to circulate water through the heat exchanger - keeping it moving will help stop it freezing but if it is cold enough for long enough it will eventually freeze. In fact on mine if the thermal store temperature drops low enough it will simply stop the pump thus removing even the minimal protection afforded by keeping the water in the system moving.

So, essentially, you are right - you need to leave the system on to ensure there is enough heat to allow frost protection. I now leave ours with the room temp set at 11-13°C when the house is shut up for Jan & Feb.

A further important point - flow temperatures are generally less than 60°C and the higher you go the less efficient the heat pump, which means that older radiators are much less suited, underfloor systems work better with low flow temperatures and are much better for heat pump systems.

All that said while I was very antagonistic to our system at the beginning I have grown to realise that it is probably the least worst option - oil is on the way out, gas means inconvenient storage tanks, and wood pellet systems can be distinctly temperamental and all require regular deliveries of fuel. Our house is not well suited to a log burner (we have an inset burner but a lot of the heat goes straight up the chimney).

A ground source system would probably have been better but not everyone has the room to install the piping.

What I would say is that if you are considering an air source system make sure that you see, and understand, the data on heat output vs ambient & flow temperatures in conjunction with weather data for an average and worst case winter to make sure you will have enough heat output - not just looking at the headline figure which is almost certainly at an ambient temp above 10°C and will not represent the performance which you will get when you need it the most.


Very interesting Paul…

Incidentally… there is no need for the heat from an insert to simply go up the chimney… the heat can be distributed to any other rooms which “touch” the chimney… a relatively simple procedure at the time of installation…but, of course, not always possible, depends on the property layout… :thinking:

Thank you, that is helpful. And I’m glad to know that it’s not just us who has a problem with hors gel. The first winter I set it at 6 degrees as a normal hors gel sort of temperature, and the pump promptly froze as soon as the temperature dropped. Since I don’t want to heat a whole 3 bed house to 13 degrees all winter that’s why we tried just heating a few metres of the radiator circuit. So far, so good…no frozen pump. Do you think that’s a workable solution or have we just been lucky?

The installer did sort of explain the graphs, but a) very technical for me and b) not sure he was much of an expert since he set it up to go into summertime mode on 1st April - when we normally still have snow! I despair of him, so did contact manufacturer to try to get sense over the flow settings for our weather conditions with no great success. So, for the service this coming autumn we have asked for a senior technician to come from about 100km away so hopefully that will help.

I’m slightly surprised at that - I had kittens when I arrived to find the system showing a fault (and thus completely shut down) knowing that the previous few nights had been between -10° and -8°C but by that time I had figured out how to get into the maintenance settings so cleared the fault got the system running and prayed that there wasn’t a split pipe in the heat exchanger - which was actually fine, the pipes are fairly well lagged.

I believe these systems are often setup with a large thermal store to act as a buffer between the heat exchanger and circulation through the radiators - ours is 200l I think, having 200l of water sitting at 50-60°C is easily enough heat to keep the system frost free.

For us leaving the system powered up for 2-3 months is expensive but much less stress inducing; the long term plan is to get some remote control over the 'net - sadly the system is old enough that the manufacturer cannot supply the necessary bits so it is a DIY electronics project and I haven’t had the time to sort it out yet.

Yes, we’ve rented a property which had a fan & vents in the upper rooms, quite effective but I don’t think it would be easy to retro-fit; in any case the log burner is only a backup for the coldest weather.

Nearly 100% sure ours doesn’t - no way to get it in as far as I can see but it would seem to be an eminently sensible thing to do. I know that the ground sourced systems normally have antifreeze in the ground loop.

There are some quite complex graphs to look at during installation but when sizing a system you want to find a graph like this

Which shows heat output vs flow (line 1, red = 35°C flow temp and line 4=60°C) temperature and intake air temp, the model in this case is a Stiebel-Eltron WPL13S (now quite an old design) which is nominally 13kW output (I have the 16kW model).

From this we can see that the rated output is only actually available if the outside air is above about 22°, below that decent output is possible down to 10° but only at the lower flow temperature. If you have conventional radiators you need the flow temp as high as possible (a gas or oil system would have flow temps of 70° or so) but the air sourced systems do not work as well - in fact if you set the flow to 60° the system gives up the ghost a couple of degrees above freezing.

Turning the flow down to 55° (line 3) mitigates the performance at low air temps quite a bit but, while the system will produce some heat down to -10, it’s only kicking out half its rated output at that point though. Also 55° is a bit low for the hot water temp, so if you are using the heat pump for domestic hot water you need to make sure that there is an immersion heater and that the system is setup to do a pasteurisation cycle once a day - I’m a bit stuck as there is an immersion but not controlled by the system controller so I have the hot water set at 56° - just hot enough to kill legionella, with the flow a couple of degrees above that, not brilliant for efficiency but as low as I dare go.

The other graphs to look at are power in vs heat output - in the case of the WPL13S these look like this

So, you can see the system is most efficient (COP of 5) for a flow temp of 35° and intake air of above 15° - basically when you don’t need it :slight_smile: However there is still a COP of 2 at -10 with a flow of 55° Not great but better than just using electricity for heating.

phew… after several glasses of wine, with friends, to celebrate Easter… I haven’t got a hope of understanding your graphs etc…

Every home is so different, that care and thought needs to be put in… before deciding to spend money on anything… IMO the only thing one can never have too much of is… Insulation… hic… I’ll leave the conversation to others until I have calmed down… :wink::crazy_face::woozy_face::woozy_face::woozy_face:


Is this video just tosh from your experience?

First thought…

“running appliances in our homes”

Why, oh why… do the French “norms” insist that an electrical ventilation system be include in new-builds… has no-one ever heard of “opening a window”… :thinking:

It is because opening a window is not a quantifiable design regarding forced ventilation. a given occupancy requires a given amount of ventilation in these new build, high sealed, insulated homes, compared to it doesn’t matter drafty old ones :slightly_smiling_face:
If the wind changes direction the window may waste heat or not ventilate at all.

So much nicer to have a proper explanation than some superstitious meaningless poor installation condemnation of a valuable system to combat excess electrical usage.

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@anon88169868 Ours is also a Steibel, so v relevant. I can’t find the box where I put all the tech info but should be able to later on. However I think I’ve worked out the graphs, and I need to find out whether ours is on factory settings or whether they adjusted it. Thanks - all v helpful.

What’s rather reassuring is that it also seems to show that our set up is sensible, as we have wet underfloor heating downstairs - with loads of insulation underneath and then stone floor on top. It’s one large room, with a central open stairwell so heat rises nicely to upper floors, and the wood stove also positioned centrally. So when we have people in the gîte you hardly need the radiator circuit at all.

I should be clear my experience is limited to running the system in the French house, plus what I have gleaned online. The claims can be checked with a bit of analysis though.

Most of the video was marketing speak but I did spot a couple of claims.

The first was the rebate stuff which was UK specific and, to be honest, I have no idea how accurate that was.

Then there was (I think) 40% less CO2 emissions than gas - this page gives some comparison figures for CO2 emissions per kWh for various fuels.

Electricity (UK) - 0.527kg CO2 per kWh compared with 0.185 kg / kWh for natural gas and 0.245 kg per kWh for heating oil.

So, to be 40% better than natural gas you need to get down to 0.185 x 0.6 - 0.111kg per kWh. That needs a COP of 4.74 or better - uncannily almost exactly the figure they claimed in the video so that, on the face of it, stacks up.

But wait a moment gimoozaabi - the installation manual for their 8, 12 and 16kW units tells a slightly different story.

The performance data is on page 44, in the form of a table with vastly fewer data points than the Stiebel unit but what is there is sufficient

A7W356.3 / 4.4212.3 / 4.7212.3 / 4.72
A7W556.2 / 3.112.5 / 3.212.5 / 3.2
A7W656.3 / 2.712.4 / 2.612.4 / 2.6
A-2W356.7 / 2.9 12.0 / 3.315.5 / 3.2
A-2W557.1 / 2.112.0 / 2.515.6 / 2.4
A-7W357.0 / 2.912.0 / 3.013.2 / 3.0
A-7W557.1 / 2.112.0 / 2.215.3 / 2.2

A7W35 means intake air 7° with a water flow at 35° and the figures are heat output/COP.

The claim that heat output is maintained down to an air temp of -7°C seems to hold, more-or-less but the COP drops way down to just 2.2 worst case for the 16kW unit so you aren’t going to be saving the planet 40% CO2 compared with gas. They give a “seasonally adjusted” COP of 3.6 which is hard to check but is a bit more realistic than 4.7 - at that level you would still be ahead on CO2 but only by a shade over 20%.

I note they are fairly candid that the system works best with lower flow temperatures and is not well suited to conventional radiators.

So, in conclusion I think their CO2 claim is “optimistic” rather than actually mendacious.

However while I’m happy to feel all warm and cuddly saving the planet I would like to know whether I’m saving myself money - the UK average electricity price is, apparently, 16.3p per kWh and that of gas 4.2p - a factor of 3.88 - given that seasonally adjusted COP of 3.6 it suggests that a heat pump would actually be more  expensive than gas heating in the UK - I guess that’s why they are pushing the environmental aspect :slight_smile:

Obviously the economics will differ slightly in France and, overall, their 16kW unit looks to have slightly better performance than my WPL16S - but not radically better.


Thanks for the detailed reply.

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