Exchange oil heating for heat pump with underfloor heating and wood heating

Hello, we plan to renovate the basement of our house, current condition:

[*]two-story house (living area approx. 243m², built in 1977) with flat roof (roof renewed and insulated a few years ago)
[*]underfloor heating on the ground floor (from the beginning), radiators in the basement
[*]the basement is half embedded in a slope, so far only one room is actively used as an office, which is heated during the week, the rest is unheated
[*]heating and hot water generation by oil (annual consumption approx. 2000l/year; the energy consultant calculated an independent final energy consumption of 39,500 kWh/a and a primary energy demand of 191 kWh/(m²a))
[*]almost all windows on the ground floor have been gradually replaced over the past years

The energy consultant's renovation roadmap foresees the following measures in the first step:

[*]insulation of the exterior wall including new windows in the basement (possibly internal insulation of the wall embedded in the slope)
[*]insulation of the basement floor, replacement of radiators with underfloor heating
[*]replacement of the oil heating system by an air-to-water heat pump
[*]according to the energy consultant, this leads to: area-related primary energy demand of 49 kWh/(m²a), expected final energy consumption of 9,700 kWh/a

Further insulation measures are planned in the renovation roadmap for later steps, but will not be implemented for now because they are too complex and therefore currently too expensive.
In addition, I am considering installing a controlled residential ventilation system myself (indoor climate, humidity) and a wood heating system for the winter (possibly a wood gasifier or replacing the old inefficient fireplace in the living room with something efficient); we practically get firewood for free (we only have to "do it" and bring it over).

I was able to learn quite a bit about this topic, among other things here in the forum, but I also have new questions:

[*]I often read here that a heat pump combined with underfloor heating should ideally be operated without a storage tank. The reasons are plausible to me, but the question arises about the concrete implementation. In my heating offers, storage tanks are included (probably specified by the respective heat pump manufacturer). Since I am not an expert and cannot/will not enforce installation against manufacturers' instructions (in case something happens, it could fall back on me), I wonder, are there heat pump manufacturers who do not require a storage tank? Or generally, how do the "no-storage advocates" manage it at home?
[*]One heating installer generously offered an 18 kW heat pump, another 12 kW. With the previous consumption of 2000l heating oil, something around 8 kW should be sufficient, especially in combination with the wood heating, right?
[*]The "old" underfloor heating on the ground floor has relatively large installation distances (you can feel warm and cold spots). The newly to be installed underfloor heating in the basement should of course be installed with closer spacing (better heat transfer -> lower flow temperatures). The heated floor areas are about the same size. Can problems arise from this, for example with pressures, flow rates, or that the old underfloor heating unnecessarily raises the flow temperature? If so, what would be appropriate measures?
[*]Until the children are old enough to move into the basement, only the office will have to be heated there for the next few years. From an energy-saving point of view, is it sensible/possible to hydraulically/thermally balance the heating so that the basement (possibly except for the office) is somewhat cooler at first and then do a new balancing when the children move downstairs (which could coincide with future energy measures on the ground floor)?
[*]Is there a risk for the heat pump if the old underfloor heating is connected directly (corrosion, materials that are no longer used today, etc.)?
[*]The bathroom on the ground floor has underfloor heating but does not get warm enough when the children bathe. So far, we have managed this by turning on the towel radiator in time, which is connected to the high-temperature heating circuit in the basement. But this will then be eliminated. Can the towel radiator be connected to the hot water boiler for this purpose (appropriately sized, possibly as a combi boiler, we want a fresh water station or similar for hot water anyway)? Is this reasonable or even permissible? It would have the advantage that the heat pump could run more efficiently for "normal" heating and only has to provide a higher temperature level briefly for the special case of children bathing. If not, any other ideas?
[*]Which (piece wood) heating system is recommended in this case? Wood gasifier in the basement or a new stove in the living room? As I see it:


[*]Advantages of wood gasifier: heat can be used throughout the house for heating or hot water and possibly grants are available
[*]Advantages of a stove in the living room: cheaper to buy, works also during power outages, has the "coziness factor"
[*]Disadvantage of living room stove: heat is only available locally and not for (less efficient with heat pump) hot water generation -> is a noticeable (electricity) savings effect to be expected in winter here at all, or does it dissipate because I still have to heat the rest of the house and the hot water with the heat pump and only the living room is a bit warmer?

Many thanks in advance for your patient reading and answers!

Does no one want to say anything about it? Not even to a single question?

That's also quite a lot of information and a complex project. But I'll throw in a little opinion:

1: If you can't (or don't want to) enforce the installation without storage, you should go for the lesser evil, a small ~50-liter inline buffer tank (in the return line). Manufacturers usually allow that and it doesn't cause much damage. I simply did it and signed that I would retrofit one if there were any disturbances. Nothing can really break. And while you're at it: you also don't want individual room controllers, no second pump group, no hydraulic separator. Read a bit on the topic, balance for a year, done.
2: You need the correct heating load of the house. Once the heat pump is installed, you will heat differently. Namely around the clock and all rooms. It will no longer be possible to heat only parts of the house and only on demand. That is inefficient on the one hand, and on the other hand, a much too large heat pump would have to be purchased for that. Once you have the heating load, preferably calculated room by room, choose a suitable heat pump WITHOUT any fear or domestic hot water surcharges. 18 is definitely too much, 8 might be tight if the figure of 39500 kWh/a is correct. Oh, and: if the energy amount is correct, you should energetically improve the house beforehand. You'll have the money for that when I get to the last point.
3: Yes, it can be problematic. Solutions: additional radiators in the rooms. Also equip one or two walls or a ceiling with surface heating. Or install one or two air conditioners that can assist. The worst solution would be to run a much higher supply temperature just for these rooms.
4: Always heat everything. Two, maximum three degrees colder is not a problem; more than that is avoided for two reasons: the supply temperature then has to be chosen higher because energy is withdrawn from occupied rooms. That must be compensated. That is less efficient than heating everywhere. Reason two is possible mold with strongly differing temperatures and existing air exchange.
5: Yes. Metals can lead to deposits. If you know what was installed (or take water samples) you can counteract this by adding certain additives.
6: My suggestion for the bathroom would be an infrared heater that is only switched on when needed. Otherwise, you could also add an additional ceiling heating.
7: This is the wildest point, but you asked. My opinion then: leave wood out. For one, it costs money and space, makes dirt, is strenuous, and then it doubly harms the environment. First through smell and fine dust. And then, just like oil, it releases CO2 that CANNOT be reabsorbed in time. By the time the trees you burned are standing again, the issue of climate change (with us) is over. Sustainability is a myth. Sustainable is leaving wood in the forest. Then alone the chimney sweep costs as much as two months of heating with the heat pump. That means financially you will never get that back. Economically also nonsense. Then there's an annoying chimney on the roof taking up space for important photovoltaic modules while partially shading five others. Since it will never be cold for you anymore, as described above, because you heat continuously, there's no reason to light a fireplace. If you don't care about all that, then I would tend towards a wood stove in the living room. It can actually serve as backup during a power outage and is much, much cheaper. Just integrating and controlling a second heat source into your heat pump system not only messes up the hydraulics but also your wallet. It is unnecessarily complex and thereby again much, much more expensive than you could ever save even with gifted wood. By the way, for the same reason, solar thermal is also being phased out in favor of photovoltaics with heat pumps. LOTS of photovoltaics! Keep it simple! And now have fun with this exciting project! :)

Thank you very much for the detailed response, it helped me a lot. I have been thinking about it for the past few days, especially about the last point.

1: If you can’t (or don’t want to) insist on the tankless installation, you should go for the lesser evil, a small ~50-liter buffer tank (in the return). Manufacturers usually allow that and it doesn’t cause much damage. I just did it and signed that I would retrofit one if there were any disturbances. Nothing can really break.
And while you’re at it: You also don’t want individual room controllers, a second pump group, or a hydraulic separator. Read into the topic a bit, balance it for a year, done.

Would it also be possible to use the heat pump for cooling in summer with this setup?

18 is definitely too much, but 8 could be tight if the figure of 39,500 kWh/a is correct.
Ah, and: If the energy amount is correct, you should improve the house energetically beforehand. You have the money for that if I get to the last point.

The 39,500 kWh/a is the calculated value for the current state. With the currently planned measures, according to the energy consultant’s calculations, we are expecting a final energy consumption of about 9,700 kWh/a. Although the first value seems a bit high to me (we currently need just under 2,000 liters of heating oil per year (but only the ground floor is "properly" heated) and the second almost a bit low (the insulation measures in the first step only involve the basement.
Insulation of the ground floor will come later because that would make the whole project much more complex (the terrace needs work, two grapevines at the house, a covered balcony/winter garden across almost the entire width of the house, plus it is a boundary building and the insulation must extend onto the neighbor’s property and the roof must also be accordingly adapted).

Skip wood. For one, it costs money and space, makes dirt, is exhausting, and then it harms the environment twice as much. First through smell and fine dust. And then, just like oil, it releases CO2 that CANNOT be re-bound in time. By the time the trees you burned are standing again, the climate change issue (including us) will be over. Sustainability is a myth. Sustainable is leaving the wood in the forest.
Then the chimney sweep alone costs as much as two months of heating with the heat pump. That means financially you will never recoup that. Economically also nonsense.
Then there's an ugly chimney on the roof taking up space for important photovoltaic modules while partially shading five others.

Since for you, as described above, it will never be cold again because you keep heating through, there is no reason at all to light a fireplace.

If you don’t care about any of this, I would tend towards a Swedish stove in the living room. It can actually serve as backup in case of a power outage and is much, much cheaper. Just incorporating and controlling a second heat generator into your heat pump system will screw not only the hydraulics but also your wallet. That is unnecessarily complex and therefore again much, much more expensive than you could ever save with gifted wood. By the way, for the same reason, solar thermal systems are being dismantled in favor of photovoltaic with heat pumps. LOTS of photovoltaic!

Keep it simple! And now have fun with the exciting project! :)

Photovoltaics (currently just under 10 kWp) are already installed, but chimneys as well ;)
I understand that a water-based wood heating system doesn’t make sense (even though it would have been great for hot water, where the heat pump is not so efficient). But my motivation for using a wood heating system additionally is not to heat permanently. Rather, it is more about being able to add heat when it is really cold and the (air) heat pump is least efficient (which will be a few days a year) and having a backup system (which hopefully will never be needed):

[*]in case of a prolonged power outage (not necessarily a full blackout, then we would probably have quite different problems, but even a local storm can cause a multi-day power outage)
[*]I feel better having the heating energy needed for winter stored locally and not dependent on the whim of a supplier (what happened with gas prices last winter could also happen with electricity prices)

Of course, one could argue that this is quite unlikely, but three years ago very few expected a pandemic and a year ago a war in Europe.
Moreover, the heat pump only runs purely on (own) green electricity in summer and partly in the transitional period. In winter, it uses the available electricity mix which is currently becoming less green (the expansion of green electricity currently barely keeps up with the growth of electrical consumers). ;)

Would it also be possible to use the heat pump for cooling in the summer with this setup?

Yes. But since water-based heat pumps cannot dehumidify, which is the most important thing in our summers, it doesn’t really help much. If you want it to be comfortable, you won’t get around air conditioning.

... an expected final energy consumption of 9,700 kWh/a.

That would really be great. Make sure the heat pump doesn’t become too large. If the value really drops that low, even the small 5 kW units are already at the upper limit (I’m estimating, better calculate yourself)! Especially if you still have a fireplace, the heating element in the heat pump, and maybe even air conditioning as backup.

[*]in case of a longer power outage
[*]I feel better if I have the required heating energy for the winter stored on site u

There’s nothing against a little caution. And if you only burn a bit of wood in emergencies or occasionally for the romance, that’s not an ecological problem either. But then everything speaks in favor of a comfort fireplace. Water-based systems need flow, so you can’t use them as backup in a power outage. Also, financially that would really be nonsense. That bit of hot water (at 50°!), where the heat pump is not quite as efficient, you make from your own electricity at noon. So that doesn’t really matter.

It will be a cool project. Read up a bit more about heat pump heating, that saves a lot of money and trouble. And then good luck!

Slowly everything is becoming more concrete and it is about to design the heat pump, or rather to decide on which renovation status it is best to size it. With the installation of the heat pump, the basement will be energetically renovated; the ground floor will remain as is for the time being, but it is planned to be done later as soon as there is more financial and time flexibility.

If the heat pump is sized for the first expansion stage, it will be oversized after the renovation of the upper floor. If it is sized for the complete renovation, it will initially be undersized.

Here is the consideration to take up the proposal with the air conditioning:

Yes. But because water-based heat pumps cannot dehumidify, which is the most important thing in our summers, it doesn’t really help that much. If you want it to be comfortable, you can’t avoid an air conditioner.
[...] maybe even have an air conditioner as a backup.

and then to use this in winter for heating as a supplement to the heat pump. Of course, the sizing must be correct here as well, and that raises the questions:

[*] Not only must the heating capacity match the house, but also the cooling capacity. If these two deviate too much, it might possibly make sense to use two or three small units instead of one large one and use them only as needed, right?
[*] How can I size an air conditioner based on heating load? For the heat pump, this works via a corresponding design diagram which shows the heat pump's capacity depending on outdoor temperature at a certain flow temperature. For air conditioners, I have so far not been able to find such a diagram; at best, a fixed value is given for heating capacity at a certain temperature combination (for example, 4 kW at 7°C outdoor temperature and 21°C desired temperature). But that doesn’t help me for the design case in deep winter. So how to proceed here?