ᐅ Heat Pump for Renovating Older Buildings: System Setup, Domestic Hot Water, Buffer Tanks, and Ventilation?

Hello everyone,
I have been researching heat pumps for some time now in order to make the right decisions for an upcoming renovation project. However, I am not sure if I have completely understood all the technical aspects and would appreciate corrections if I have any misunderstandings or errors.

To start, the renovation property was built in 1978 and has about 150m² (1,615 sq ft) of living space across two floors, with the second floor located in a collar beam roof. The basement is unheated and not intended for residential use, but possibly for a fitness room with occasional occupancy.
Maximum number of occupants is 4, currently 2.
The plan includes adding cavity wall insulation to the two-skin masonry construction as well as inspecting/updating the insulation of the top floor ceiling and roof. A full surface heating system is planned. A photovoltaic system with battery storage is also part of the project. The ventilation solution is not yet finalized.

Now regarding heating:
A heat load calculation and similar assessments are still pending but will definitely be done. I would not rely on past energy consumption figures since oil heating was used and previous owners were different.
I am considering an air-to-water heat pump with only a small buffer tank for the heating system. Instead of an Electronic Expansion Valve (EEV), I might prefer zone control to reduce heating in the second floor, which is currently only used occasionally. Therefore, the heat pump circuit would be equipped with an overflow valve. Why are EEVs so expensive? I would need a distribution manifold for balancing anyway. Or are the thermostats and control valves accounted for at such a high cost?

Domestic hot water is planned to be provided via a fresh water station. The domestic hot water storage tank will likely have a volume of about 300 liters (80 gallons) to ensure sufficient energy at low temperatures. Additionally, heat storage from excess photovoltaic energy is being considered for the summer, although I am still unsure whether it might be more sensible to store the energy in batteries instead.
One area where I am particularly uncertain is the heating of the domestic hot water tank:

• On one hand, the tank must be heated by the heat pump. The heat pump would switch repeatedly between space heating and the domestic hot water tank, correct? How does this work when, for example, the heating circuit is at 35°C (95°F) and the hot water tank is at 45°C (113°F)?
• On the other hand, I considered a second small heat pump in the basement using ambient air to also help dry the basement. Alternatively, I could use exhaust air from the bathroom and toilet for this second heat pump, which would provide a small ventilation system, though without heat recovery. From this second heat pump, I expect an optimized size that matches performance and temperature demands, and in summer the main heating heat pump could be completely off.

Does anyone have any insights or additions to help complete my understanding? Once this is settled, the next big topic will be which installer delivers quality work and which manufacturer to choose. But one step at a time 😉

007sascha schrieb:

I am considering an air-to-water heat pump for heating with only a small buffer tank. Instead of an EER, I might prefer using zoning control to reduce the temperature in the second floor, which is currently used less frequently at times.

ERR (Energy Efficiency Ratio) is legally required—you can apply for an exemption with a well-founded request. Controlling by floor is not practical, nor is neglecting proper heating of the upper floor.

007sascha schrieb:

A heating load calculation and similar assessments are still pending but will definitely be done. I wouldn’t rely on previous consumption data since the house was heated with oil and owned by different previous owners.

Since you are renovating, relying on anything else would not be effective. I would definitely get an external consulting firm or a heating designer who cooperates with an external office.

007sascha schrieb:

…with only a small buffer tank in front

If possible, use no buffer tank and no bypass valve. Just run the system steadily.

007sascha schrieb:

Domestic hot water should be supplied via a fresh water station. The domestic hot water storage tank will probably be around 300 liters.

Fresh water station or domestic hot water storage? Choose one but not both (personally, I find the storage tank sufficient). Fresh water stations are considered more hygienic (although Legionella bacteria practically do not occur in single-family homes with normal use).

007sascha schrieb:

On one hand, this would need to be heated by the heat pump. The heat pump then switches back and forth between heating and the domestic hot water storage tank, right? How does this work when, for example, the heating circuit is at 35°C (95°F) and the domestic hot water tank is at 45°C (113°F)?

A three-way valve is used. If a temperature sensor in the tank indicates heating is needed (connected to the heat pump), the valve switches and directs the heating water to the tank. The water is heated to the required temperature—up to 60°C (140°F). This continues until the tank water reaches the target temperature. During this time, the heating circuit is not running. With heating once a day for 15-20 minutes, this is acceptable (frequency depends on water use and tank size).

007sascha schrieb:

On the other hand, I was also thinking of a second small heat pump that uses surrounding air in the basement to dry it out simultaneously. Alternatively, I could use the exhaust air from the bathroom and toilet for this second heat pump, which would also give me a small ventilation system, though without heat recovery. I expect the second heat pump to be optimally sized for its performance and temperature, and in summer, the heating heat pump could be switched off completely.

Keep it simple. A second heat pump costs a lot of money and also consumes electricity, generating ongoing costs. You won’t need that much hot water to justify a second domestic hot water heat pump. That is why it is not common practice. It would be like installing a second engine in a car as a backup: extra costs, potential for failure, and added complexity (analogy: plug-in hybrid electric vehicle). Save yourself the expense and skip the second heat pump.

Fresh water station or domestic water storage tank? One or the other, but not both

However, I still need to supply the fresh water station with hot water, and for this, a buffer tank is necessary, right? Or is the floor heating used as a buffer for the fresh water station as well? The low floor temperature alone is not sufficient. Most likely, this is just a terminology misunderstanding—I understand the fresh water station as the heat exchanger for the fresh water. If the fresh water station refers to a complete unit including the buffer tank, then clearly no additional buffer tank is needed.

Three-way valve.

Okay, so the heat pump simply raises the temperature during the period of domestic water heating and then returns to the lower temperature used for the heating system afterward, correct?

007sascha schrieb:

But I still have to supply the fresh water station with hot water, and for that a buffer tank is necessary, right?

A buffer tank makes sense (and therefore also the entire floor heating circuit, since everything is connected). However, a domestic hot water storage tank is something different (this stores hot water for showers, etc.).
The fresh water station (the storage tank is naturally heated by the heat pump) – the heat exchanger in the station transfers the temperature to the flowing domestic water, similar to a tankless water heater:

Domestic hot water storage tank:

The heat pump circulates hot water (on the left side) through a heat exchanger inside the storage tank and heats the domestic water stored inside. On the right side, you draw off hot water (at the top) while cold water enters from the main supply at the bottom.

Is that clearer now?

007sascha schrieb:

Okay, so the heat pump just raises the temperature during the domestic hot water supply period and then returns to the lower temperature for heating afterwards, right?

Exactly.

Now clear?

Absolutely, we meant the same thing, I just called it by the wrong name. Thank you.

In your diagram, there is another optional three-way valve on the return line of the fresh water heat exchanger. Is this used to prevent turbulence in the cold water area to achieve the lowest possible inlet temperature for the heat pump?

007sascha schrieb:

In your diagram, there is another optional three-way valve on the return line of the domestic hot water heat exchanger. Is this to prevent turbulence in the cold water area in order to achieve the lowest possible inlet temperature for the heat pump?

This is just a random diagram. In this case, it shows a stratified charging tank, which can have different inlets depending on the temperature.

For ME, a domestic hot water station is just a "nice to have." It does not change the domestic hot water, offers no real hygienic advantages, and is SIGNIFICANTLY more expensive.