Which heating system for a multi-family house, heat pump not eligible for subsidies?

There is no thermal insulation certificate yet because I have done everything myself so far and without subsidy applications I didn’t need one. I only compared the U-values of KfW55 and KfW70 and am somewhere in between. Here are my (approximate) values: U-value 36.5cm Ytong wall: 0.24 U-value floor slab insulation with 12cm insulation: 0.26 Roof with 8cm Pavatex and 20cm insulation between rafters: 0.15 Windows: 0.89 Roof windows: 1.10 A friend has taken a bit more for his single-family house, also extra for hot water because it is often problematic he says. He already took a 12kW heat pump. My building materials dealer has already entered it into his program and even said 30kW. Are 14kW really enough? And which one would you recommend then? In addition, my photovoltaic feed-in tariff ends in 2027, as it stands now selling electricity would no longer be worthwhile and then the 23kW system would be available for hot water or buffer storage.

Heating demand according to Google on several platforms: KfW-55 house 35 kWh/m², KfW-70 house 45 kWh/m²

Means for me about 42 kWh/m² living space. With my living space of 435m2 then about 18.27kW. For domestic hot water an additional charge of about 20% results in about 22kW. So I would need a heat pump with at least 22kW, right? How do you come to only 14kW for my living space?

My known heating engineer has roughly calculated with a 24kW boiler for an oil or gas heating system based on my data.

I would like to put forward the thesis that a 12kW heat pump will already be sufficient for heating the apartment building.

Heating installers, unfortunately, based on my own painful experience, mostly recommend rather oversized heat pumps. The reason is simple: it gets warm quickly and reliably, design flaws are surely concealed, and the customer only realizes much later that this results in a poorer overall efficiency with frequently cycling heat pumps.

An empirical value from an apartment building with these key figures: apartment building 475m² with 4 residential units and 8 residents:

some component values: wall 0.15 W/m²K window 0.75 W/m²K roof 0.11 W/m²K

The final energy consumption of the apartment building is 15.8 kWh/(m²*a). The house has comparatively large window areas with about 170m².

The heat supply is provided by a 12kW ground source heat pump with 3 x 80m geothermal boreholes and "central" controlled residential ventilation systems for each residential unit. Actual consumption: ~8200 kWh / year including central hot water preparation.

At the time, I had prepared a very detailed heating demand calculation (9,200 kWh/year). Taking into account factors such as building time constant and solar and internal (= people/devices) heat gains, the actual heating demand is further reduced compared to the calculation. These effects are usually not considered in superficial calculations. According to the calculation, a 9kW heat pump would have been sufficient, but the heating installer refused and only installed a 12kW unit.

You are planning for 14 people in full occupancy. At peak times, for example, when all working residents want to shower simultaneously in the morning, the 12kW heat pump might possibly reach its limits. This depends on the chosen system for hot water preparation. In the above-mentioned apartment building, a 1000L hygiene buffer tank with stainless steel corrugated pipe is operated, and with 8 residents there have never been complaints so far.

With 14 residents, it would be worth considering making the buffer tank and/or the heat pump somewhat larger if necessary (1,200-1,500L, 16kW). Another approach would be hot water via instantaneous water heaters (possibly photovoltaic-supported electricity usage) in the residential units – this also avoids the legionella issue.

I myself rent out 10 apartments with a total area of 500 sqm. Politically, private small landlords are dead. And the fewer apartments you rent out, the greater your risk becomes.

As a private landlord, you are legally fair game without any rights. There are associations like the Tenants’ Association with free lawyers who dissect every utility bill for you. Even if you have these done by a company. But even these billing costs cannot be fully passed on.

That is why I can only advise you to pass the cost risk on to the tenants. For 4 apartments, I would install a small heat pump with a hot water heat storage tank in each apartment. These already exist similar to a gas unit heating system. This way, it is connected to the tenant’s electricity meter and they have to pay. You save yourself the heating cost statement, which has become very complicated and always contestable when heating is central.

A judge at the local court once told me that nowadays it has become impossible to create a legally secure utility and heating cost statement. And as a landlord, you are even supposed to bear part of your tenants’ CO² charges. You avoid this this way.

And the electricity provider can cut off the tenant’s electricity for the heat pump. Of course, you are not allowed to do that. And if you heat centrally, it is technically difficult to separate a single tenant.

In the end, the question always remains whether the tenants can and will continue to pay the ever-increasing energy prices. Ultimately, the landlord is left sitting with the costs.

I have already paid off my rental house, so I no longer have to make any debt service payments. But if I still had loan obligations on it, I would probably sleep very badly. I am at least calculating that you will no longer be able to pass on property tax to the rent, and that it will rise sharply. Then they will cap the pass-through heating costs or come up with other tricks to penalize the bad landlords. So I just want to warn you a little.

Renting out property in Germany has become a tough business. The money doesn’t come by mail. That’s why I admire people like you.
I wonder how anyone is supposed to rent out reasonably with construction costs beyond 3000 euros per sqm.

For 500 sqm of rentable area, it will cost 2 million euros including land. Probably even more. At 4 percent interest and 2 percent repayment, it would take me 28 years to pay off the property. The monthly rate would be 10,000 euros. Even under optimal conditions, I would have to take in 20 euros per sqm rent. Utopian. And now the fact-checkers please explain to me why I should take this risk?

As we can currently see, the outlook for 400,000 newly built apartments every year looks bad. That will probably not happen.

Heat pump, Google and oil-based heating installers are an explosive mix, where in the end a lot of crap can hit the fan.

There is no heat protection certificate yet because I have done everything myself so far and without funding applications I didn’t need one.

According to the Building Energy Act 2020, the heat protection certificate is part of the building application. Someone must have calculated something. Whether it corresponds to reality is another matter.

An acquaintance took a bit more for his single-family house, also extra for hot water, because it is often problematic, he says

More helps more has always been the saying with oil and gas boilers. 20% more power is no problem, just costs about 10% more, but during operation you hardly notice it. Heat pumps, on the other hand, that are too big, always deliver enough heat – but above 0°C quickly go into short cycling mode because they cannot modulate down that far. It’s not the bare kW number that matters here, but also the lower performance limit at moderate outside temperatures (heat pumps do not have their lowest possible output at higher (0°C-10°C) outside temperatures). It is not necessarily harmful to choose one size larger if the lower modulation limit stays the same. In general, no one needs more than 8 kW in a new single-family home – if yes, then the hot water tank is too small, or the rain shower is supposed to run at the same time as the bathtub, after the 3 kids have come back from sports and showered everything empty. That has to be taken into account in the planning.

My building materials dealer has already entered it into his program once and said even 30 kW.

I didn’t know they also do HVAC planning now.
Input mask – field1: "Area", field2: "Number of residential units", field3: "Year built"?
Output: 30 kW

Is 14 kW really enough there? And which one would you recommend then?

I don’t know. Keyword: room-by-room heat load calculation and domestic hot water demand calculation. Software: Hottgentroth, BKI Energieplaner or others.

Heating demand according to Google on several platforms:

Maybe Chat GPT knows more.
There is no fixed heating demand for KfW standards or their energy classes, nor for GEG2020.
The allowable maximum heating demand per year for new buildings must not exceed that of the respective reference building.
The reference building is a virtual house of the same volume with the same component areas (windows, roof, wall, floor) built from the respective valid standard components and with a standard heating system according to the Building Energy Act 2020 with correspondingly improved values according to KfWxx.
So you can build a 3m wide 20m long tube-shaped building with one floor and a complete glass facade on the north side with very high heating demand and still fulfill GEG2020. Or you build a cube with minimal windows, all facing south, and also have the building standard according to the Building Energy Act 2020 – but significantly less heating demand.
The larger the house is, the lower the heat demand per m² usually is, unless it is a tube.

For domestic hot water, a surcharge of about 20% then results in about 22 kW

Where does that come from? Flat rate? Why? Makes no sense. If the heat load is greater than the peak load required for the used buffer/ hot water tank and the heat load calculation already includes standby times for heat pump tariffs and hot water preparation, you don’t need a surcharge for hot water. If yes, -> change tank size.

My acquaintance heating installer roughly calculated a 24 kW boiler for oil or gas heating with my data.

Oil/gas heating → totally different story in sizing, because it doesn’t matter at all if it is 100% too large.

For 4 apartments I would install a small heat pump with hot water thermal storage in each apartment.

I have little to say about the rant, but the suggestion is not bad at all, especially if no separate HVAC planning is to be done. The problem here is that the smallest heat pumps on the market usually have 5 kW nominal output and about 2 kW minimum output and then all are too large because the heat load of the respective individual apartment is too small.

has already written how he solved it. Just as I suggested – calculate the heat load, size heat pump and suitable buffer storage with fresh water station accordingly. A flat rate won’t work.

The building application along with the planning was already done and submitted by myself in 2019, at that time a thermal insulation certificate (nonsensical rubbish) was not yet required, at least not in our area. These were very detailed explanations now, but what I would be mainly interested in is which manufacturer/type of heat pump could be used for the multi-family house in terms of price/performance/guarantee without any concerns (without fear and stomach aches compared to the well-known manufacturers failing in their service), so that my costs would not be (much) higher than for an oil or gas heating system (for which I would also need a chimney in comparison to the heat pump, which would add another approx. 2500-3000 euros).