ᐅ Recommendation: Air-to-Water Heat Pump versus Local District Heating Network for KFW40 New Construction
Created on: 29 Oct 2023 08:16
M
Mark_xx
Hello everyone,
We are about to start construction on our single-family house.
Key facts:
2 full floors
200 sqm (2,150 sq ft)
No basement
KfW40 standard
3 people
Photovoltaic system 7-9 kWp (depending on heat pump or district heating), battery storage optional
Underfloor heating throughout the house
There are two specific energy supply options we have been offered:
Option 1: District heating
Technology: Transfer station, meter, and downstream combined storage tank for heating and hot water (CS Solar KSR 1000), plus two 3 kW heating rods with control for feeding in photovoltaic electricity. Supply temperature from the district heating network: 65-80 degrees Celsius (149-176 degrees Fahrenheit)
Flat price per kWh independent of consumption: 300 euros + €0.0995/kWh
One-time connection fee including transfer station: 14,000 euros
Option 2: Air-to-water heat pump
Technology: iDM ALM 4-12 with integrated storage tanks (100 l (26 gallons) heating, 295 l (78 gallons) hot water)
Comparable price: 18,000 euros
Overall costs over the service life, assuming a heat pump lifespan of 15 years, favor district heating.
However, I have two questions:
I am not sure whether a combined storage tank is really practical with district heating, especially in summer. For example, do you have to heat the entire buffer tank to temperature for showering even if the heating water isn’t needed? Could this possibly lead to issues with legionella?
Also, there is a claim that a photovoltaic system works more efficiently with an air-to-water heat pump than with a heating element, because the PV system likely does not produce 3-6 kW at the time of the heat demand.
Does anyone have experience with one of these options or answers to my two questions?
I would also appreciate any suggestions for improvement.
Thank you!
We are about to start construction on our single-family house.
Key facts:
2 full floors
200 sqm (2,150 sq ft)
No basement
KfW40 standard
3 people
Photovoltaic system 7-9 kWp (depending on heat pump or district heating), battery storage optional
Underfloor heating throughout the house
There are two specific energy supply options we have been offered:
Option 1: District heating
Technology: Transfer station, meter, and downstream combined storage tank for heating and hot water (CS Solar KSR 1000), plus two 3 kW heating rods with control for feeding in photovoltaic electricity. Supply temperature from the district heating network: 65-80 degrees Celsius (149-176 degrees Fahrenheit)
Flat price per kWh independent of consumption: 300 euros + €0.0995/kWh
One-time connection fee including transfer station: 14,000 euros
Option 2: Air-to-water heat pump
Technology: iDM ALM 4-12 with integrated storage tanks (100 l (26 gallons) heating, 295 l (78 gallons) hot water)
Comparable price: 18,000 euros
Overall costs over the service life, assuming a heat pump lifespan of 15 years, favor district heating.
However, I have two questions:
I am not sure whether a combined storage tank is really practical with district heating, especially in summer. For example, do you have to heat the entire buffer tank to temperature for showering even if the heating water isn’t needed? Could this possibly lead to issues with legionella?
Also, there is a claim that a photovoltaic system works more efficiently with an air-to-water heat pump than with a heating element, because the PV system likely does not produce 3-6 kW at the time of the heat demand.
Does anyone have experience with one of these options or answers to my two questions?
I would also appreciate any suggestions for improvement.
Thank you!
Daniel-Sp schrieb:
Hello,
From what I can see on IDM’s website, it’s a combined storage tank installed as a hydraulic separator. This separates the heat pump circuit from the heating circuit. That is not ideal!
Such an installation makes sense when you have a high-temperature heat source and a low-temperature heat sink, like underfloor heating.
With a heat pump, the goal is to produce only the required amount of heat, adjusting the supply temperature to the actual demand. With a setup using a separation tank/combined storage, you would need to closely match the flow rate of the heating circuit (underfloor heating) to the flow rate of the heat pump—only then will energy losses from the separation tank be minimal. However, the heat pump offered is a modulating heat pump, which is actually correct. The problem with separation tanks is the adjustment of flow rates. The heat pump controls not only the compressor output but also the flow rate to maintain an optimal temperature difference within the heat pump loop. Meanwhile, the circulation pump for the heating circuit runs at a fixed flow rate. Therefore, the flow rates can’t be optimally matched, resulting in an efficiency loss.
The solution is to connect the heat pump directly to the underfloor heating, ensuring that enough heating circuits are open at all times to maintain the minimum flow rate (disable ERR). For domestic hot water production, a three-way valve switches from the underfloor heating to the domestic hot water storage tank. It is also possible to plan a small buffer tank (30–70 liters (8–18 gallons)) in the heat pump return to support defrosting cycles. While this is not strictly necessary since the screed provides more heat for defrosting than a buffer tank would, it helps reduce efficiency losses to a negligible level.
Summary:
1. Have a heating load calculation carried out.
2. Then specify a SMALL heat pump accordingly, without accepting a safety margin in heating capacity; this margin should already be included in the heating load calculation. Avoid separation tanks/combined storage or similar constructions.
3. It is also important to calculate the room-by-room heating load and design the underfloor heating accordingly, considering temperature requirements for each room. If needed (usually in bathrooms), consider additional wall heating. If towel radiators are planned, do not connect them to the heating circuit; operate them electrically instead.
After moving in:
4. Perform the thermal balancing of rooms in the first winters (not via the ERR!). This is something to be done by the homeowner, not the heating installer.
5. Regularly check operating behavior and cycling times.
Best regards from Hamburg Thank you very much for your detailed response!
I have already requested the heating load calculations.
If I want to stay with IDM, would your recommendation be to choose the Aero ALM (2–8) model and combine it with a domestic hot water storage tank?
The so-called hygienic storage tanks offered by IDM as alternatives are also basically combined storage tanks with similar issues to those you described, correct?
In principle, I liked the idea of a heat pump with an integrated combined storage tank, especially since space is quite limited. But maybe the AERO ALM together with a separate domestic hot water tank will also work space-wise.
Best regards from the south!
Mark_xx schrieb:
There are two specific alternatives for energy supply that were offered to us: Congratulations. Local heating as a real option is quite rare – usually, it is offered without any choice, more like an imposed benefit.
https://www.instagram.com/11antgmxde/
https://www.linkedin.com/company/bauen-jetzt/
D
Daniel-Sp30 Oct 2023 16:05Mark_xx schrieb:
The alternative hygienic tanks offered by iDM are essentially combination tanks with similar issues as you described, correct?
Correct, you should reject that one.
I’m not familiar with IDM in detail, but I believe they offer this unit both as a combined storage tank and as a domestic hot water storage tank (without the stratification plate). However, I think it’s at least 500 liters (130 gallons) in size, so it’s definitely oversized for use as just a hot water tank. But you don’t necessarily have to use IDM – external storage tanks can usually be integrated without any issues.
Thank you very much for all the feedback!
We have now received the first response regarding the heating load calculation. It showed a peak of 5 kW, which supports the statement that the 2–8 kW option is more than sufficient.
My conclusion regarding the heat pump is as follows: I plan to switch from the iPump ALM 4–12 including the combined storage tank to the Aero ALM 2–8 and connect a domestic hot water tank to it. I will skip a heating buffer and instead direct excess energy into the screed rather than into the buffer.
Does anyone have an opinion on district heating? If I decide to go that route, would you recommend adding a combined storage tank after the transfer station, or possibly just using a domestic hot water tank here as well? The district heating water enters the transfer station at about 70–80 degrees Celsius (158–176 degrees Fahrenheit).
We have now received the first response regarding the heating load calculation. It showed a peak of 5 kW, which supports the statement that the 2–8 kW option is more than sufficient.
My conclusion regarding the heat pump is as follows: I plan to switch from the iPump ALM 4–12 including the combined storage tank to the Aero ALM 2–8 and connect a domestic hot water tank to it. I will skip a heating buffer and instead direct excess energy into the screed rather than into the buffer.
Does anyone have an opinion on district heating? If I decide to go that route, would you recommend adding a combined storage tank after the transfer station, or possibly just using a domestic hot water tank here as well? The district heating water enters the transfer station at about 70–80 degrees Celsius (158–176 degrees Fahrenheit).
R
RotorMotor31 Oct 2023 07:33For a maximum demand of 5 kW, 2-8 still seems quite a lot to me. Isn’t there a smaller option?
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