ᐅ Heat Pump for Renovating Older Buildings: System Setup, Domestic Hot Water, Buffer Tanks, and Ventilation?
Created on: 20 Nov 2025 15:26
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007sascha
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:
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 😉
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 😉
Please do not use a bypass valve or buffer tank. Since you are fully switching to underfloor heating, there is enough energy available for defrosting.
You will heat the upper floor anyway. Reducing the heating there will only disrupt your hydraulic system.
You will need a room-by-room heating load calculation anyway for designing the underfloor heating system.
Use a standard domestic hot water tank compatible with heat pumps. These have a large heat exchanger surface.
Also, make sure to eliminate thermal bridges and insulate the basement ceiling. Then you will have done practically everything correctly.
As mentioned before: keep it simple. Unless you are applying for subsidies, you can also omit the room thermostats. You will control the system during balancing using the flow meters.
You will heat the upper floor anyway. Reducing the heating there will only disrupt your hydraulic system.
You will need a room-by-room heating load calculation anyway for designing the underfloor heating system.
Use a standard domestic hot water tank compatible with heat pumps. These have a large heat exchanger surface.
Also, make sure to eliminate thermal bridges and insulate the basement ceiling. Then you will have done practically everything correctly.
As mentioned before: keep it simple. Unless you are applying for subsidies, you can also omit the room thermostats. You will control the system during balancing using the flow meters.
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007sascha22 Nov 2025 09:26Where did you have your heat load calculation done?
Did you do it yourself or use an online tool that complies with the required DIN standard, a design office, or directly through the heating installer? I generally have reservations about tradespeople because nowadays it's really hard to find a good one, or I’ve often had bad luck. There are also plenty of design offices to choose from. How can I perform a reliable quality check in advance? Experiences from acquaintances as well as in forums often range from one extreme to the other for the same company.
Did you do it yourself or use an online tool that complies with the required DIN standard, a design office, or directly through the heating installer? I generally have reservations about tradespeople because nowadays it's really hard to find a good one, or I’ve often had bad luck. There are also plenty of design offices to choose from. How can I perform a reliable quality check in advance? Experiences from acquaintances as well as in forums often range from one extreme to the other for the same company.
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nordanney22 Nov 2025 09:40007sascha schrieb:
Where did you have your heat load calculation done?Heckmann. He’s considered an expert online, especially for DIY projects. Links aren’t allowed here, but almost everyone installing, for example, a Panasonic Monoblock Sunset comes across this engineering office.You’ll also get a layout plan for the underfloor heating. Everything is customized to your preferences (room temperature, desired supply/return temperatures, etc.). I have used their service myself—once for underfloor heating and once for radiator sizing.
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