Pompe a chaleur

Hi following the advice of a friend we are looking at the merits of a eau/air system , the brand recommended is Dimplex. But having been on one or two sites what seemed a simple solution has become more complicated. Our new home is in a zone where temperatures can reach -10 (Drome 26) but rarely do due to climate change. We would like to replace our existing chaudiere fioul with the pompe à chaleur system. The house is 140 m2 built in 1940’s. There is insolation in the roof and double glazing everywhere plus a poêle bois in one room. We have existing radiateurs. We need to know peoples experience of this brand or any others and their opinion /experience of this type of system especially how it copes with cold periods. Also where to find the most reputable installer and also where to turn for advice about grants. Thankyou so much Fiona

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The radiators will be undersized for a heat pump, you’ll need 50% larger as a guide. What size is the plumbing pipework? Having insulation is no guide to how much and how well its fitted. Badly fitted it does very little.

There are six sizable radiators in cast iron. On what basis do you consider that undersized radiators cause a significant problem, given that we are considering the thermal transfer downline from the heat pump? Put more simply if the room temperature reaches its set point quickly then surely the heat pump will have to work less hard. I find it hard to believe that the heat pump cannot shed the thermal energy sufficiently quickly given that its output will need to serve sizable radiators plus domestic hot water. The plumbing pipe work is standard 14mm diameter with short runs of approx one and a quarter inch.

The problem with hooking traditional radiators to a pompe à chaleur is that they were designed with a boiler output of 70-80°C; in mind. Heat pumps generally top out at 60° - and it isn’t a great idea to run them that hot as efficiency drops with increased flow temperatures. In fact the best match is not a traditional radiator at all but under floor or in wall systems with flows of about 30-35°

For domestic hot water you also have to consider whether your temperatures will be high enough to kill legionella - mostly they won’t be as you need 60° for that and the system should be set up to cycle an immersion element once a day to get the water above 60.

You are correct to question performance with low temperatures outside. Heat pumps start to work noticeably less well once the air temperature is less than 5-10°C with lower heat output and less efficiency but most do produce some heat over and above the electrical power that they consume, even down to -10.

We have a Stiebel Eltron system - the system has a number of compromises, including feeding traditional cast radiators. It works but it would be a stretch to say flawlessly.

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Exactly as Paul describes except 60 deg from a heat pump is pushing it, some do but 40 is more common and sometimes less than that which is why they are better suited to underfloor heating.
Putting in radiator fans would help warm the room as radiators dont really do that well. Controversial but thats the truth as far as comfort goes.

Have you considered replacing the existing chaudiere with a new and more efficient model which is capable of burning fuel that has 30% ethanol added. That way there would be no need to change any pipework or radiators.

We too have a Steibel heat pump, and live in the mountains where it does get cold. So yes the efficiency plummets as the temperature drops and for a short period every year it is essentially electric central heating. With the linky I can track this to the centime!

As other have said the heat flow is the issue. Ours goes to underfloor heating of stone floor, and although we have two large radiators upstairs the open well staircase means that heat gentle rises so low output radiators are enough.

The things I don’t like about it are that you can’t really fluctuate temperatures much between night and day/weekend and week etc. With the stone floor there is really no point in having more than a degree or so difference between night and day as it is so slow to respond. I also don’t like that you can’t really turn it off when you go away and the hors gel setting is pointless where we are as too cold.

Agree, even the pump is officially rated at 60° it’s probably using a supplemental heater to get there.

Our is a WPL 16S, as previously discussed in depth :slight_smile:

It will be useful for Fiona to get as much performance data as possible before choosing a pump (ideally full graphs as below but the manufacturer’s data should include at least a few data points), and to go through heat loss calculations to get the right size. Ours is, I think, undersized in practice (nominally it is 16kW output which would be OK, it’s just that it can only generate that 16kW when you don’t really need it).

Heat output is as below:

Sorry the quality isn’t great but - the first thing to note is that the rated output is only achieved with an air input temperature of 25° - you certainly wouldn’t need central heating at that point!! This is quite an old model and newer ones might be a bit better.

It is, at least, above 12kW provided the incoming air is at 7° which leads me to the first point about air source heat pumps - ideally you probably need 25% more rated power than you think.

Below 7° power output falls off fairly rapidly, that maks sense as it’s about the temperature that the system needs to periodically waste some heat defrosting the heat exchanger (remembering that the outflow air should be about 10° colder than the inflow air).

The 2nd general point is that you need a lot more power for high flow temperatures - at 2° inflow (the lowest inflow temp at which the graph includes 60° outflow water temp) you need 4.5kW to maintain a water output of 60° compared with 3kW for a water temp of 35° - in fact that remains true pretty much across the range of input temperature.

Going down to -10° (the lowest we’ve had in Brittany while I’ve owned the house is about -8) you can still get 7kW out, for about 3.6kW in (so there’s still a bit of a gain) but if you sized the system as needing a 16kW conventional boiler you will be shivering with only 7kW of heat output.

Fiona - we’ve discussed all this at length (probably too much length :slight_smile: ) on the forum previously, have you searched through the old posts?

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We have tried to find someone to give an estimate for a chaudiere biofuel but have not found anyone that recommends it because of shortages in the future (apparently climate change affects the crops - Colza needed for this.). We thought that a fioul chaudiere no matter how efficient would not be accepted after 2022-23 and that it may not be possible to refill your tan. If you know more about this please could you give us more information. We are quite concerned about the mixed reviews on pompe a chaleur and the chaudiere granular seems to be more expensive for a friend who bought a chaudiere granular than they imagined. We have a very limited budget so the choice of system is pretty important. Thank You for putting this idea forwards.


With 140m2 you’ll need something in the 16kW range and will be looking at 10-15k€

Yes - biofuels don’t seem to be the answer from an environmental point of view - since they can be more profitable than growing food - or of course preserving wild forests - they seem to be incentivising destructive land use, and therefore doing more environmental harm than good.

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That’s about right! 16000 is our budget and we would hope to have a grant with that. Great site!

Indeed, sadly unintended consequences have a habit of biting you in the bum.

OK, that is, at least, realistic.

Going back to radiators - 6 seems low, we’re about 150m2 and there are (from memory, haven’t been able to get over to France since last summer) 10. With 6 I’d be slightly worried you just won’t have enough surface area & you might have to change them for modern ones more suited to your heat source.

As to Dimplex - no idea, I guess it will depend on what your chosen installer is used to, they seem to have an extensive range. The wesite seems to suggest 9kW up to 180m2 which seems very low to me - but it depends on how well your home is insulated.

Totally agree! Marketing over selling, cost accountants under specing the heat exchanger side. Only people to believe is the poor engineers called out to try and correct the problem later and they are the ones paid the least. They are always the ones to shoulder the complaints so they know better than anyone the real world sizing and design. Great in a passive house with tripple glazing. I am a big fan of the tech but it has to be in the right environment.

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I can see that. We have time to keep researching as there is still a lot of fioul left in the tank. If we were younger we would have log fires as we have access to wood and although there are chaudieres with automatic log fillers they are expensive. We will get there !

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If your finances are tight I wonder whether you need to really think through your whole strategy of where you spend your money to get the best long term result? As you know it’s not just the initial investment that hurts but long term running costs too.

So have you investigated costs of improving all your insulation? This can be cost effective as even small reductions in need for heat can pay back many times over in future years.

And then look at your house and how you live in it and see whether a mix and match approach might be the best? We have oil, wood and air source heat. The downside is maintaining the equipment, but it has allowed us to make improvements little by little rather than ripping out a complete system and starting from scratch. For most of the house our elderly oil boiler does the work, but in one section it’s a wood stove (and for the gite it’s the air source heat pump) And now we have installed air source hot water, so elderly boiler can have a rest for 6 months of the year. I reckon this will pay itself back in 4 years as we are saving quite a bit on fuel already. And hopefully will also extend the life of the fuel boiler…

And do you have gas? As that’s also worht considering.

They are listed on the grants as well.

Hi Fiona,

Unfortunately there is a good deal of misinformation circulating on this subject as folks struggle to interpret the technicalities of the regulations coming into force during 2021/2.
The government has NOT banned the installation of new chaudieres running on liquid fuel, but what they have done is banned the installation of such a chaudiere that emits more than 250gCO2eqKwh in CO2 emissions from 1st July 2021 for new build properties, and from 1st January 2022 for existing buildings. The consequence of this is that as there are no chaudieres which run on 100% fossil fuel (fioul) that can meet this criteria, that particular type of chaudiere has effectively been banned. There is a good explanation (in French) to be found here : Décret sur l'interdiction du fioul ce qui est prévu et ce qu'en pense le secteur - FioulReduc
Please do remember that there are many sources of biased advice who would love to sell you a very expensive alternative heating system.

The good news is that a chaudiere which runs on biofuel F30 (70% fossil fuel oil and 30% bio-ethanol) has an emissions level of only 219,8 gCO2eqkWh and so comes in under the limit of 250g and is therefore acceptable.
Most existing chaudieres can easily run on biofuel F10 (10% ethanol) with only minor adjustment, or possibly a change of Gicleur (fuel spray nozzle), and it is envisaged that this fuel will become available in the market place during the first or second quarter of this current year (2021). The sale of 100% fossil fuel (fioul) will be stopped as from 2024 and everyone will then be using F10 as a minimum.
Changing to biofuel F30 (30% ethanol) will be possible towards the end of this year (2021) when the fuel becomes available in the market place once the thorny issue of how to tax it has been thrashed out. For most existing chaudieres it will be necessary to change the burner unit (Bruleur) to accept F30 fuel but this will be much cheaper than changing the whole boiler of course.
Further information about the timeline of fuel composition changes is available in French here: Le biofioul à base de colza : l'avenir du chauffage au fioul - FioulReduc

The government recognises that for many people living in rural areas there is really no choice other than to use a liquid fuel for heating purposes, and it is therefore envisaged that a switch from fossil fuel to bio-fuel will take place over time. In 2030 the F10 fuel will cease to be available and F50 (50% ethanol) will come on stream. In 2040 F30 will cease and F100 will become available. In 2050 the F50 will cease and from then on it will only be the 100% ethanol liquid fuel that is available.

I would strongly suggest that you talk to one of the larger fuel suppliers in your area to find out more precise details of when they will have F10 and F30 available. If you use one of the more competant chaudiere servicing companies that use exhaust gas analysis to fine tune the chaudiere, then they will be able to advise you as to how they can adjust your existing boiler to use firstly F10 and subsequently F30.

Overall this process will be much cheaper than changing the entire heating system, and the money you save from that can be put to improving the insulation of your home. Obviously it goes without saying really that the most eco-friendly energy is that which is never used, so the watchwords have to be insulate, insulate, and insulate again.


Hi Robert,
We appreciate very much your time and knowledge on this subject. It is this kind of information we need before experts come out to see the house and how it is configured. Because our french is ok but limited, although my husband is an engineer and already familiar with many of the technical terms and processes, we are sometimes caught out by the less honest people.

Have a great day!