What is the real story around heat pumps?

[nctnico]

But keeping on the wood burner for the 22 expensive days is not an option for me. I'm struggling with it as is and it is not going to get better unfortunately. That is the whole reason for the change.

Hence my suggestion to go for the diesel burner. You can install a high-tech system right now but who is going to fix it in 10 years from now? You might not be able to do this yourself. If most of the neighbours have a diesel burner then there will always be somebody nearby who can at least take a look at it when something is wrong with it.

Also, are there ways to improve the insulation of the house? Replacing regular double glazing with HR++ glazing makes a massive difference. I'm in the process of doing that. Another system high on my list is a balanced ventilation system which recovers the heat which otherwise just blows out of the house. I expect this to reduce heating costs and improve comfort by a large margin.

[Zero999]

It's true that air below freezing still contains some water, but going from what I've read, it's much less of a problem, when it's below freezing, especially once the ground is frozen, so no water can evaporate into the air. I've noticed that in winter, when the wind is blowing from the east, off the cold continent, the air is do dry, ESD is a problem and I feel my sinuses drying up.

A practical example: my ES/Amitime 9kW unit runs defrost cycle around 20-25 times a day with temperatures around 0degC. This is the worst case. At -15 .. -20 degC, the heatpump easily makes 2 - 3 hour stretches so just around 10 defrosts per day, even when the RH is the same or even higher.

If the heatpump implements detection of frost properly, defrosting is not a dealbreaker even in very cold environment, because the situation indeed gets better as it gets colder. There are stories of bad products which overdetect the need of defrosting, e.g. doing it every 40 minutes in -20degC, which then totally kills the COP.

Doesn't that completely kill the performance?

I would have thought icing up will be worst around 5°C, which is very close to the average temperature, in the coldest month, in the OP's location. At higher temperatures, then the heat can be removed from the air without freezing.

A ground source hear pump is probably more optimal in the OP's climate but it depends on the geology.

[pcprogrammer]

But keeping on the wood burner for the 22 expensive days is not an option for me. I'm struggling with it as is and it is not going to get better unfortunately. That is the whole reason for the change.

Hence my suggestion to go for the diesel burner. You can install a high-tech system right now but who is going to fix it in 10 years from now? You might not be able to do this yourself. If most of the neighbours have a diesel burner then there will always be somebody nearby who can at least take a look at it when something is wrong with it.

The diesel burner is not a very good option due to the smell and it is also expensive. Could probably get better quotes for it then the one we got, but still.

Upcoming trend is heat pumps. We see them getting installed more and more around here. Mostly used is fire wood. For as far as I know already 4 air to water heat pumps and maybe two houses with a diesel burner. This is a very small village with ~100 inhabitants.

Edit: The house has HR double glazing for the new build and probably mostly standard double glazing in the old part. We build the biggest part of the house ourselves. Walls and roofs are insulated with 75mm fiberglass insulation.

The question here is which of the two heat pump solutions would suit us best. Air to water or ground to water.

So far the disadvantage of an air to water system seems to be the freezing of the evaporator and a lower seasonal COP.

How about maintenance of an air to water unit. Our house is surrounded by trees so a lot of leaves to deal with. Will this be a problem for the ventilator in the unit.

[pcprogrammer]

A ground source hear pump is probably more optimal in the OP's climate but it depends on the geology.

That also is the opinion of one of the three companies we asked for a quote. But how objective is his opinion, since the company specializes in geothermal. The first was resolute diesel or pellets is the way to go, because that is what he does most. The third opted the air to water solution, because again what they do most.

Hard to find good objective information on the subject. We are leaning towards the geothermal solution. We know it is expensive and will never see a return on investment, but living costs money and comfort is worth it, to us at least.

[coppice]

A ground source hear pump is probably more optimal in the OP's climate but it depends on the geology.

That also is the opinion of one of the three companies we asked for a quote. But how objective is his opinion, since the company specializes in geothermal. The first was resolute diesel or pellets is the way to go, because that is what he does most. The third opted the air to water solution, because again what they do most.

Hard to find good objective information on the subject. We are leaning towards the geothermal solution. We know it is expensive and will never see a return on investment, but living costs money and comfort is worth it, to us at least.

Just beware of people selling low cost ground source heating. If you take the heat from a big enough underground reservoir it can work well. However, to keep the price and installation disruption low a lot of people will offer solutions that get too much heat from a small area. That may work great for a year or two, but you need to worry about the long term, and whether the heat you draw from the ground can be replenished. If you have hot summers and cold winters things even out from pumping heat in both directions, but it doesn't sound like you want to pump any heat down the hole.

[RoGeorge]

but my central heating system as is also needs electricity to work

Not really.  Heat pumps require significant amounts of electricity for the compressors (to pump the heat) and defroster, or else they can not pump at all the heat from a lower temperature to a higher one.

Classical stove OTOH, does not require electricity, except maybe for a small recirculation pump, but that's for comfort only, it is not essential.  The recirculation pump only needs insignificant amounts of electricity, it only moves water.  Heat comes entirely from the combustible itself (wood), and the flow from heated water to room happens naturally.  The small recirculation pump can be eventually removed by a clever design, or can be actionated ocasionally by other means (backup batteries, or a small oil-based generator, depending on how long the power surge lasts).

Heat pumps do not work at all without significant amounts of electricity, unless you live nearby a hot water geyser (AFAIK there are none in France).  Otherwise, a heat pump requires way too much electricity to be supplied by a battery, or by a local backup generator.

Another thing to consider if you live in France, you should estimate the future cost of electricity (for heat pumps) at much higher price then it is now. 

So far France has had plenty of cheap electricity coming from nuclear power plants that were fueled by relatively cheap Uranium from Africa.  The situation in Africa has changed recently, and nuclear fuel will become expensive.  Many will have to shut down.  This will impact not only France price of electricity, but the entire EU.  Do not assume nuclear power plants can be replaced by wind or solar.  When compared with nuclear, wind and solar are an absolute joke.

[nctnico]

A ground source hear pump is probably more optimal in the OP's climate but it depends on the geology.

That also is the opinion of one of the three companies we asked for a quote. But how objective is his opinion, since the company specializes in geothermal. The first was resolute diesel or pellets is the way to go, because that is what he does most. The third opted the air to water solution, because again what they do most.

Hard to find good objective information on the subject. We are leaning towards the geothermal solution. We know it is expensive and will never see a return on investment, but living costs money and comfort is worth it, to us at least.

AFAIK geothermal wells need to be re-drilled every 15 years or so. I might be wrong, but make sure to check how long a drill hole can be used.

[pcprogrammer]

A ground source hear pump is probably more optimal in the OP's climate but it depends on the geology.

That also is the opinion of one of the three companies we asked for a quote. But how objective is his opinion, since the company specializes in geothermal. The first was resolute diesel or pellets is the way to go, because that is what he does most. The third opted the air to water solution, because again what they do most.

Hard to find good objective information on the subject. We are leaning towards the geothermal solution. We know it is expensive and will never see a return on investment, but living costs money and comfort is worth it, to us at least.

Just beware of people selling low cost ground source heating. If you take the heat from a big enough underground reservoir it can work well. However, to keep the price and installation disruption low a lot of people will offer solutions that get too much heat from a small area. That may work great for a year or two, but you need to worry about the long term, and whether the heat you draw from the ground can be replenished. If you have hot summers and cold winters things even out from pumping heat in both directions, but it doesn't sound like you want to pump any heat down the hole.

Yes, also something that is on our minds. I asked the sales person of one of the companies what the expected life time of the source would be, and if pumping heat back in the summer is needed. He said no. He himself has a system for 20 years now and still going strong he said.

On the net searching about this says "these sources are not unlimited" but they don't mention what the average time is before it is depleted.

With the system he offered it is possible though to cool the house in the summer and pump back heat into the hole, but it is active cooling, which means more electricity consumption. And it only works with the underfloor heating, and these rooms don't need that much cooling. The upstairs bedrooms can run up to ~29 degrees Celsius which is hot, but these only have the radiators. 

Attached are some pictures of the current wood burning system and the control.

I attached servos to the burner to control the air inlet valves. Ended up replacing them with brush less high torque servos. The smaller ones did work, but the motors wore out quickly.

The 5000 liters is divided across two tanks. Had to modify them because at first I used PVC sewage pipes to prop up the secondary heat ex-changer. It collapsed due to the long term exposure to >80 degrees water. Now it is held up with copper pipes and wire.

Every September we throw down ~22m3 of wood and stack it like shown on one of the pictures. In January another 3m3 follows. Hard work.

Also had to redo the controller after lightning damage. Now fully opto isolated between the controller and the sub stations.

[Zero999]

A ground source hear pump is probably more optimal in the OP's climate but it depends on the geology.

That also is the opinion of one of the three companies we asked for a quote. But how objective is his opinion, since the company specializes in geothermal. The first was resolute diesel or pellets is the way to go, because that is what he does most. The third opted the air to water solution, because again what they do most.

Hard to find good objective information on the subject. We are leaning towards the geothermal solution. We know it is expensive and will never see a return on investment, but living costs money and comfort is worth it, to us at least.

Just beware of people selling low cost ground source heating. If you take the heat from a big enough underground reservoir it can work well. However, to keep the price and installation disruption low a lot of people will offer solutions that get too much heat from a small area. That may work great for a year or two, but you need to worry about the long term, and whether the heat you draw from the ground can be replenished. If you have hot summers and cold winters things even out from pumping heat in both directions, but it doesn't sound like you want to pump any heat down the hole.

Yes, also something that is on our minds. I asked the sales person of one of the companies what the expected life time of the source would be, and if pumping heat back in the summer is needed. He said no. He himself has a system for 20 years now and still going strong he said.

On the net searching about this says "these sources are not unlimited" but they don't mention what the average time is before it is depleted.

With the system he offered it is possible though to cool the house in the summer and pump back heat into the hole, but it is active cooling, which means more electricity consumption. And it only works with the underfloor heating, and these rooms don't need that much cooling. The upstairs bedrooms can run up to ~29 degrees Celsius which is hot, but these only have the radiators. 

Attached are some pictures of the current wood burning system and the control.

I attached servos to the burner to control the air inlet valves. Ended up replacing them with brush less high torque servos. The smaller ones did work, but the motors wore out quickly.

The 5000 liters is divided across two tanks. Had to modify them because at first I used PVC sewage pipes to prop up the secondary heat ex-changer. It collapsed due to the long term exposure to >80 degrees water. Now it is held up with copper pipes and wire.

Every September we throw down ~22m3 of wood and stack it like shown on one of the pictures. In January another 3m3 follows. Hard work.

Also had to redo the controller after lightning damage. Now fully opto isolated between the controller and the sub stations.

Simple: use a solar thermal system in summer, to replenish the heat. The pumps can be solar powered.

A ground source hear pump is probably more optimal in the OP's climate but it depends on the geology.

That also is the opinion of one of the three companies we asked for a quote. But how objective is his opinion, since the company specializes in geothermal. The first was resolute diesel or pellets is the way to go, because that is what he does most. The third opted the air to water solution, because again what they do most.

Hard to find good objective information on the subject. We are leaning towards the geothermal solution. We know it is expensive and will never see a return on investment, but living costs money and comfort is worth it, to us at least.

AFAIK geothermal wells need to be re-drilled every 15 years or so. I might be wrong, but make sure to check how long a drill hole can be used.

I wasn't thinking of a well, but a large coil of pipe buried in the land next to the house i.e garden/yard. It obviously needs to be large enough to collect sufficient heat. The average annual temperature, including summer and whether it's also going to be used for air conditioning might make a difference. I suppose, if you live in an area with cool summers and don't need air conditioning, you could use solar for hot water in summer, which will allow the ground to warm up before winter.

[coppice]

A ground source hear pump is probably more optimal in the OP's climate but it depends on the geology.

That also is the opinion of one of the three companies we asked for a quote. But how objective is his opinion, since the company specializes in geothermal. The first was resolute diesel or pellets is the way to go, because that is what he does most. The third opted the air to water solution, because again what they do most.

Hard to find good objective information on the subject. We are leaning towards the geothermal solution. We know it is expensive and will never see a return on investment, but living costs money and comfort is worth it, to us at least.

AFAIK geothermal wells need to be re-drilled every 15 years or so. I might be wrong, but make sure to check how long a drill hole can be used.

That's problem problem I described above. Too much heat being drawn from too small an area, faster than the replenishment rate, eventually results in a useless source for the heat you need. Its avoidable, but it may require considerable extra up front costs and disruption to put an adequately sized system in place. I'm sceptical of the calculations people use. They try to make it a simple one size fits all approach. The ground varies a lot from place to place, and clearly heat flow rates are going to vary a lot too.

[coppice]

I wasn't thinking of a well, but a large coil of pipe buried in the land next to the house i.e garden/yard. It obviously needs to be large enough to collect sufficient heat. The average annual temperature, including summer and whether it's also going to be used for air conditioning might make a difference. I suppose, if you live in an area with cool summers and don't need air conditioning, you could use solar for hot water in summer, which will allow the ground to warm up before winter.

Few people have enough area available for such a system, so most people have to go deep rather than wide. They still use pipework, though.

Would it be better to replenish the underground heat with solar thermal, or solar panels driving the heat pump?

[coppice]

With the system he offered it is possible though to cool the house in the summer and pump back heat into the hole, but it is active cooling, which means more electricity consumption. And it only works with the underfloor heating, and these rooms don't need that much cooling. The upstairs bedrooms can run up to ~29 degrees Celsius which is hot, but these only have the radiators. 

I would avoid using these systems for cooling, especially if you have humid summers. The condensation on the radiators can be a nightmare for mould formation, even if they are basically wipe clean surfaces. Its hard to deal with mould on the rear. A cool floor can be very uncomfortable, an increasing condensation on a carpet is a really bad idea.

[pcprogrammer]

That's problem problem I described above. Too much heat being drawn from too small an area, faster than the replenishment rate, eventually results in a useless source for the heat you need. Its avoidable, but it may require considerable extra up front costs and disruption to put an adequately sized system in place. I'm sceptical of the calculations people use. They try to make it a simple one size fits all approach. The ground varies a lot from place to place, and clearly heat flow rates are going to vary a lot too.

Our terrain, even though it is ~1Ha, does not have enough grass land to put a horizontal system in. So we need vertical drill holes. We are waiting for the second quote on this, but the first specified that two holes of 85m deep would do. They have to be spaced out at least 7m if I understood him correctly. (French and me does not go together that well. I speak it a bit but understanding is more difficult. The wife is better at it but gone when it becomes technical  )

[pcprogrammer]

With the system he offered it is possible though to cool the house in the summer and pump back heat into the hole, but it is active cooling, which means more electricity consumption. And it only works with the underfloor heating, and these rooms don't need that much cooling. The upstairs bedrooms can run up to ~29 degrees Celsius which is hot, but these only have the radiators. 

I would avoid using these systems for cooling, especially if you have humid summers. The condensation on the radiators can be a nightmare for mould formation, even if they are basically wipe clean surfaces. Its hard to deal with mould on the rear. A cool floor can be very uncomfortable, an increasing condensation on a carpet is a really bad idea.

The summers are indeed humid. The radiators are of the kind sold by this company https://jaga.co.uk/lst-heating-radiators/tempo When cooling they should remain cutoff of the system. The floors with heating in them are covered with wood. 14mm of chestnut. Temperature should not be set below 18 degrees to avoid condensation, but I'm not to keen on it anyway.

[Zero999]

I wasn't thinking of a well, but a large coil of pipe buried in the land next to the house i.e garden/yard. It obviously needs to be large enough to collect sufficient heat. The average annual temperature, including summer and whether it's also going to be used for air conditioning might make a difference. I suppose, if you live in an area with cool summers and don't need air conditioning, you could use solar for hot water in summer, which will allow the ground to warm up before winter.

Few people have enough area available for such a system, so most people have to go deep rather than wide. They still use pipework, though.

Would it be better to replenish the underground heat with solar thermal, or solar panels driving the heat pump?

I honestly don't know. Solar thermal doesn't involve running the heat pump and the associated wear and tear, but unlike photovoltaic, it can't be used for other purposes.

[pcprogrammer]

I wasn't thinking of a well, but a large coil of pipe buried in the land next to the house i.e garden/yard. It obviously needs to be large enough to collect sufficient heat. The average annual temperature, including summer and whether it's also going to be used for air conditioning might make a difference. I suppose, if you live in an area with cool summers and don't need air conditioning, you could use solar for hot water in summer, which will allow the ground to warm up before winter.

Few people have enough area available for such a system, so most people have to go deep rather than wide. They still use pipework, though.

Would it be better to replenish the underground heat with solar thermal, or solar panels driving the heat pump?

I honestly don't know. Solar thermal doesn't involve running the heat pump and the associated wear and tear, but unlike photovoltaic, it can't be used for other purposes.

It would also require special pluming on the side of the source. At least a three way valve to switch over to the solar panels and a separate pump to circulate the water. The question is if it is worth it.

The wife found on the net that a source like that can be used for up to 30 years, so good for us 

[pcprogrammer]

but my central heating system as is also needs electricity to work

Not really.  Heat pumps require significant amounts of electricity for the compressors (to pump the heat) and defroster, or else they can not pump at all the heat from a lower temperature to a higher one.

Classical stove OTOH, does not require electricity, except maybe for a small recirculation pump, but that's for comfort only, it is not essential.  The recirculation pump only needs insignificant amounts of electricity, it only moves water.  Heat comes entirely from the combustible itself (wood), and the flow from heated water to room happens naturally.  The small recirculation pump can be eventually removed by a clever design, or can be actionated ocasionally by other means (backup batteries, or a small oil-based generator, depending on how long the power surge lasts).

Heat pumps do not work at all without significant amounts of electricity, unless you live nearby a hot water geyser (AFAIK there are none in France).  Otherwise, a heat pump requires way too much electricity to be supplied by a battery, or by a local backup generator.

Another thing to consider if you live in France, you should estimate the future cost of electricity (for heat pumps) at much higher price then it is now. 

So far France has had plenty of cheap electricity coming from nuclear power plants that were fueled by relatively cheap Uranium from Africa.  The situation in Africa has changed recently, and nuclear fuel will become expensive.  Many will have to shut down.  This will impact not only France price of electricity, but the entire EU.  Do not assume nuclear power plants can be replaced by wind or solar.  When compared with nuclear, wind and solar are an absolute joke.

Everything can change in the future for sure. Fossil fuels also get more expensive. Fire wood also gets more expensive.

If global temperatures keep going up we might not need heating anymore, and so on. Not realistic and makes living harder if for every decision you start to think about the future and what it will bring.

[Siwastaja]

Heating 5000 liters with my 38 KWh wood burner takes between 10 to 12 hours and on the cold days it only lasts for 36 hours coming from ~85 down to ~30 degrees.

5 m^3 * dt=55degC is whopping 320kWh of energy, so 213kWh per 24 hours or 9kW average thermal power. Sounds like you have a pretty large house, or poor insulation. To compare, I need 5kW thermal to maintain +21 indoors at -25 outdoors, but then again I'm only heating some 120 square meters, maybe your home is bigger.

With a heat pump you probably won't reach much higher then 55 to 60 degrees and it will have to work the whole night to get there.

You are correct - with a heatpump, storing energy by increasing dt means immediate compromise in COP, so you get the best efficiency and least energy used by not storing but running continuously. Well with large enough storage, you could run the heatpump during higher air temperature then use the produced heat during lower air temperature thus increasing COP. But realistically if the outdoor temperature varies by 10degC during 24hrs, and if you then have to increase storage temperature by 10degC to get the energy stored during 24hrs period, you have already lost the gain. If you have to increase by 20degC, you are losing on COP. For that to make sense, you would need a pricing scheme where hourly prices have large enough differences. The problem is, lower prices at night coincide with lower temperatures at night, so in the end it might be best to just let the heatpump run on its own curve algorithm without any storage; during night when it's colder the thing ramps up, and that coincides with cheap hours.

Here we usually have a price peak at 7-8 am and avoiding that one brings significant savings without need for any large storage reservoir.

But keeping on the wood burner for the 22 expensive days is not an option for me. I'm struggling with it as is and it is not going to get better unfortunately. That is the whole reason for the change.

I hear you. Personally I have found it's much easier to struggle with something for a few weeks than all the time, but 22 days is quite a lot actually. It's still a good idea to keep the existing system because it will help you when that record cold which only happens only every 5 years hits you and the heatpump is approaching COP=1. It you then keep burning for a week or two you have dodged the bullet, without having to sacrifice your whole life to burning wood - a hobby for some.

Realistically, the heatpump would be running resistive aux heating in most extreme conditions, which is OK.

[Siwastaja]

I would have thought icing up will be worst around 5°C, which is very close to the average temperature, in the coldest month, in the OP's location.

On my unit, I have never seen freezing at +5degC (well on low power level, an occasional small stripe on the evaporator, but nothing that grows enough to require a defrost cycle), need for defrosting starts at around +3 degC and gets worst at around 0 degC then gradually better at colder temperatures. (Of course "better" is quite relative word, more frequent defrosting at coldest temperatures would be catastrophic for both average output power and COP.)

Even at 0-ish (0, +1) degC, in most humid conditions ("British weather") the need for defrosting actually decreases or even completely disappears, because so much energy is released by condensing the water it keeps the evaporator clean of ice. It basically rains water under the machine in such conditions and COP is great.

A ground source hear pump is probably more optimal in the OP's climate but it depends on the geology.

OP's climate is, I think, pretty good for air source heatpumping. Even here in much colder climate, many choose air source, but sure, here ground source wins hands down. As I said earlier, given OP's specifications, I believe air-to-water unit would come close to SCOP of 4. 100% sure it will significantly exceed 3.0. With ground source you would get to maybe 5.0. In such Middle European conditions, days of +5degC or so dominate in heating bill. Optimal for air source. Ground source would give only slight advantage.

AFAIK geothermal wells need to be re-drilled every 15 years or so. I might be wrong, but make sure to check how long a drill hole can be used.

Here we have a lot of experience of ground source drilled in many different soil types over 30-40 years or so and this kind of problem is rare. Maybe if you have some seismic activity then quite obviously it can damage the plastic pipe that's put into the ground, but usually people assume the lifetime for the hole is at least 40-50 years.

[Siwastaja]

How about maintenance of an air to water unit. Our house is surrounded by trees so a lot of leaves to deal with. Will this be a problem for the ventilator in the unit.

I have a cherry tree close to my outdoor unit. Expect to have to remove a leaf or two every now and then stuck on the evaporator coil, or in the bottom tray. Not a big deal IMHO. Otherwise it's been maintenance-free. Given your climate plus relatively low distribution temperature I would definitely bet on the air-to-air unit. Some people have bad experience but it's based on some specific brands/models being bad, so do your googling before committing to purchase.

On English-language websites like this, people sometimes make defrosting sound like a big issue. But it's normal operation, not a problem. The only problem might be finding real-world data for a certain unit so you have to rely on specifics you hear from others. In the end, ignoring some random problems like a flawed defrost logic on some Mitsubishi Electric LN series air-to-air units, most heatpumps perform quite similarly, more expensive brand doesn't necessarily give you any better COP. You get the best COP by minimizing distribution temperature, regardless of brand, and this you have already done, so  . Just ignore totally unsuitable lowest-tier units designed for the warmest climates (e.g. basically swimming pool heaters) and you should be fine even with a cheap unit. Presence of bottom tray heater and compressor heater already communicates the intended usage in cold environment.

[baldurn]

I propose air to air units for the upper floors. It will be 1500 euro per room. Keep the radiator for redundancy. However with this you no longer need 55 degree hot water and can choose a more reasonable temperature for the floor heating downstairs. You will get optimum COP because the air to air units only need to deliver air at the temperature needed in the room.

In the summer you can use the units for air conditioning.

My other advice is to oversize somewhat. The endless cycles of defrosting can be attributed to undersized systems struggling to get the job done. I have one air to water and two air to air. We have had no trouble this rather cold winter.

Regarding the noise, you will not notice it indoors. But you will outdoor. Even the more silent systems are not 100% silent. This is of course worse on the days you do not want to be outside however. One advantage of the geo thermal system is no noise at all.

Indoor you often have another type of noise. Radiators are not completely noise free if you are sensitive to that. The air to air units also have indoor fan noise. I am not too bothered about it, but your milage may vary.

[Siwastaja]

I propose air to air units for the upper floors. It will be 1500 euro per room. Keep the radiator for redundancy. However with this you no longer need 55 degree hot water and can choose a more reasonable temperature for the floor heating downstairs. You will get optimum COP because the air to air units only need to deliver air at the temperature needed in the room.

In the summer you can use the units for air conditioning.

If the underfloor heating is downstairs and radiators upstairs (I missed that but did not read too well, sorry for that), then this is pretty good advice: even if the radiators are of low temperature type, they still need higher temperature than the underfloor loop. E.g., in worst cold conditions, maybe 30degC for the floors, 40 for the radiators.

Then doing a hybrid solution and getting smaller air-to-water unit for the floors, and air-to-air units for upstairs would make sense. The air-to-water unit must deal with the higher of the two temperatures, so if you only use it for the lower temperature circuit you are getting better COP all the time. You also get more power output from the same unit with lower temperature, since the power limitation is actually input power limitation! And then, given that you are only heating part of the house, smaller unit will do, and the air-to-air units would then take care of the upstairs separately.

For summertime cooling, upstairs is usually where the heat is the worst anyway so most cooling needed there, especially if bedrooms are upstairs. Additionally cold air easily goes down so you need to just to open a door to the stairway and some cool air goes down so you might manage without cooling solution downstairs. Air-to-water heatpump can provide a little bit of cooling through the underfloor circuit but it must be non-condensing meaning it will increase RH% by reducing temperature without reducing absolute humidity. Therefore having air-to-air units is a good idea as they offer real dehumidification, in humid-hot summer conditions (I don't know if that's a problem for you, though).

[pcprogrammer]

5 m^3 * dt=55degC is whopping 320kWh of energy, so 213kWh per 24 hours or 9kW average thermal power. Sounds like you have a pretty large house, or poor insulation. To compare, I need 5kW thermal to maintain +21 indoors at -25 outdoors, but then again I'm only heating some 120 square meters, maybe your home is bigger.

See for your self. Excluding the basement it is ~240m2. Lots of exterior wall and roof space. Not good for energy saving, but I wanted a lot of space for my hobbies. The last section on the right is a two story part with 50m2 per story. All for me. The rest of the house is for the wife. 

[pcprogrammer]

I propose air to air units for the upper floors. It will be 1500 euro per room. Keep the radiator for redundancy. However with this you no longer need 55 degree hot water and can choose a more reasonable temperature for the floor heating downstairs. You will get optimum COP because the air to air units only need to deliver air at the temperature needed in the room.

In the summer you can use the units for air conditioning.

My other advice is to oversize somewhat. The endless cycles of defrosting can be attributed to undersized systems struggling to get the job done. I have one air to water and two air to air. We have had no trouble this rather cold winter.

Regarding the noise, you will not notice it indoors. But you will outdoor. Even the more silent systems are not 100% silent. This is of course worse on the days you do not want to be outside however. One advantage of the geo thermal system is no noise at all.

Indoor you often have another type of noise. Radiators are not completely noise free if you are sensitive to that. The air to air units also have indoor fan noise. I am not too bothered about it, but your milage may vary.

I'm very sensitive to noise. I can hear the rumble of the underfloor heating in my room, but it is not loud enough to be hindered by it. A blowing fan is a different story. We have an air purifier bought for relieving the wife her allergy that turned out to be something else, and I could not stand the noise of it.

So no air to air units for me.

After we get the quotes from the last company we had around we will make a weighed decision. I do appreciate the specific input on the core of the question. 

[pcprogrammer]

I propose air to air units for the upper floors. It will be 1500 euro per room. Keep the radiator for redundancy. However with this you no longer need 55 degree hot water and can choose a more reasonable temperature for the floor heating downstairs. You will get optimum COP because the air to air units only need to deliver air at the temperature needed in the room.

In the summer you can use the units for air conditioning.

If the underfloor heating is downstairs and radiators upstairs (I missed that but did not read too well, sorry for that), then this is pretty good advice: even if the radiators are of low temperature type, they still need higher temperature than the underfloor loop. E.g., in worst cold conditions, maybe 30degC for the floors, 40 for the radiators.

Then doing a hybrid solution and getting smaller air-to-water unit for the floors, and air-to-air units for upstairs would make sense. The air-to-water unit must deal with the higher of the two temperatures, so if you only use it for the lower temperature circuit you are getting better COP all the time. You also get more power output from the same unit with lower temperature, since the power limitation is actually input power limitation! And then, given that you are only heating part of the house, smaller unit will do, and the air-to-air units would then take care of the upstairs separately.

For summertime cooling, upstairs is usually where the heat is the worst anyway so most cooling needed there, especially if bedrooms are upstairs. Additionally cold air easily goes down so you need to just to open a door to the stairway and some cool air goes down so you might manage without cooling solution downstairs. Air-to-water heatpump can provide a little bit of cooling through the underfloor circuit but it must be non-condensing meaning it will increase RH% by reducing temperature without reducing absolute humidity. Therefore having air-to-air units is a good idea as they offer real dehumidification, in humid-hot summer conditions (I don't know if that's a problem for you, though).

For us the least invasive solution is one with a heat pump based on water on the house side. Be it air to water or ground to water. All that is needed is to bring a couple of pipes into the basement and connect it to the existing system.

The wife will kill me if I suggest to have work done in the upstairs bedrooms to add an air conditioning unit above her bed. Se is not to trilled about either the mess of the work being done and the noise of the finished system once in place.