Is a multisplit air conditioner suitable for heating?

I can hardly imagine that. In 2020, when we moved in here, we had problems with the new heating system which didn’t work for a week. We heated our KFW55(!) house with our climate split system just to the bare minimum. We really only ran it a little when the rooms got too cold, and we only have 3 split units, and that was anything but cheap / economical. I can’t imagine an economical scenario for an uninsulated old building. But I’ll read along with interest.

Explain to me where you had problems with it. On the other hand, I can hardly imagine that you can’t get a KFW55 house wonderfully warm with a split climate system. Split climate sets are available in almost all power classes and also with a good COP when heating. They are completely independent of the existing radiators and so on.

I can't imagine an economical scenario for an uninsulated old building. But I'll follow with interest.

That certainly depends on the starting situation, for example if the building is currently heated with night storage heaters. Then it's also about what is feasible without completely tearing the place apart. Split air conditioning is practically minimally invasive and is likely to be three times cheaper in consumption. In addition, air-to-air heat pumps are eligible for a 35% Bafa subsidy in existing buildings.

Explain where you had problems with it. On the other hand, I really can't imagine that you can't get a KFW55 house wonderfully warm with a split air conditioner.

Sure, you could have gotten it wonderfully warm with that, but you could also just as well sit in front of the electricity meter with a bowl of popcorn. It was simply really expensive. Accordingly, we only turned the units on as long as we had to. Our air conditioner is of course not designed to heat the house, I don't know how much more you could optimize it to make it more economical. But it doesn't make sense to me in the uninsulated old building, how that should work. Warm air rises, you're basically blowing the expensively warmed air straight up to the roof?

if heating with an air-to-air heat pump in your house is much more expensive than with an air-to-water heat pump, then something is wrong. My first tip would be that you did not operate the devices 24/7. Just like the heating heat pump, an "air conditioner" likes to run steadily at low power.

Efficiency differences do not exist in theory at all - in practice, you just have to be a bit more careful when ventilating and it is even more important not to let the apartment get cold, because it takes longer for the building fabric to warm up. And often air conditioners are much cheaper than heating heat pumps, which of course can sometimes also be a disadvantage in efficiency.

And naturally, an old building is not ideal for heat pumps. But neither is it for other heating systems. And if grandma stays in the house for another 10 years before it is demolished and rebuilt, air conditioners are certainly an alternative. And in passive houses, for emergencies, too. Heating with them is definitely good for the climate.

My contribution was serious, the old building is not a cave but rather a castle inhabited by a 65-year-old castle maiden.

So a building whose insulation has been correspondingly neglected. Windows from the 80s. Masonry or basic structure from 1930 with 40-50 cm brick and plaster, the ceilings consist of lattice slats and ash filling or whatever it is called. Here and there, more or less poorly insulated by own effort (attic), so I call the castle an uninsulated old building.

The whole thing is currently heated with a 14 kW pellet boiler and radiators. About 4-5 tons of bulk material per year over 3 floors of 60m2 each.

With [Klima-Split], which after all is also a heat pump, it was meant to support the existing pellet heating during transitional periods and/or at correspondingly warmer daytime temperatures, and possibly reduce pellet consumption.

I wouldn’t do anything else with an air-to-water heat pump as a hybrid heating system except that I either have to enlarge the radiators to lower the flow temperature or have to throw in underfloor heating to manage 35-40°C, in order to integrate the whole thing into the existing building stock and I have to see if the pipe network can circulate enough water at all.

If I am correctly informed, for example, a multi-split outdoor unit can supply up to 5 indoor units.

The affected ground floor consists of an open living-dining room with kitchen, hallway, bedroom and bathroom in the ground floor. The cooling split lines plus possibly necessary condensate lines (but I think I only need them in cooling mode) could be installed under the basement ceiling. The other two floors only have to be tempered and kept frost-free because they are unoccupied. Renting is not an option without turning the whole place upside down, which I don’t want to do to the castle maiden anymore.

A 12.5 kW multi-split air conditioning system with 5 indoor units at 2 kW each I find online for €6,500, if I do everything through a refrigeration company it might cost around €15,000 minus subsidies.

If I go to a heating contractor, they ask for around €20,000 to €30,000 for a 12 kW air-to-water heat pump with new radiators or underfloor heating and integration into the existing heating system. Considering the ceiling and floor structure, underfloor heating is probably out of the question anyway, it would probably have to be a wall heating system.

With the split system, there would be the added value of cooling. Hence the idea, but I am probably really crazy.

With a climate split system, which after all is also a heat pump, it was meant to support the existing pellet heating during the transition period and/or at correspondingly warmer daytime temperatures, and possibly reduce pellet consumption.

For saved pellets, electricity then goes on the bill. Have you compared that? What does 1 kWh of heat from pellets and from electricity cost (assuming a COP of 3.5)?

The biggest disadvantage and perhaps also the main reason why an air-to-air heat pump is not particularly appreciated even in new buildings is the heat transfer via air. That is simply quite uncomfortable. To bring the necessary thermal energy into an old building, a gentle breeze is not enough; you need a bit more throughput. The mentioned air is also quite dry.