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,
According to the website, the ALM 4-12 has a minimum output of 4 kW, which seems oversized to me.
What does the heat load calculation say? The smaller one should already be large enough. Is the storage tank a combined buffer, or are the domestic hot water and heating circuits separate?
Have them provide a quote for one/the smaller ALM and recalculate. Apparently, you are not obliged to connect. How long are the prices for district heating guaranteed?
Greetings from Hamburg Hello,
Thank you for your reply. To be honest, I’m not sure if the heat load calculation has already been done. As part of the KfW40 application, my energy consultant calculated some key figures, but the heat load calculation is probably done by the plumbing company, right?
What I have:
Annual heating demand = 8,559 kWh/year
Annual primary energy demand = [I]7.31 kWh/m² (7.31 kWh/sq ft)
I will contact our energy consultant and plumbing partner regarding this.
As far as I understand, the ALM has a combined storage tank integrated inside the unit, but I can’t clearly find that on the manufacturer’s website.
There is no obligation to connect. The supplier guarantees the prices for 3 years. After that, the price is €9.95 plus possible market fluctuations according to the following shares (45% wood chips + 15% electricity + 15% inflation + 25% wheat price)[/I]
Harakiri schrieb:
Which expert designed your 12 kW heat pump? "KfW40" houses, even with 200 m² (2,150 sq ft), have "difficulty" fully utilizing 5 kW heat pumps, and that only if they have to heat continuously at the design heating load (which practically never happens). To make a meaningful comparison, you should try to get an offer for a significantly smaller heat pump – ideally one that can modulate down to 1.5 kW. And please, without a buffer tank – you don’t need it, it only adds cost and reduces efficiency.
Do you also want to cool in summer? That option is obvious, especially if you have a photovoltaic system.Cooling would be a nice bonus, but it’s not a must right now.You were probably offered the iPump ALM series – there is a version with a 100 l (26 gallon) buffer tank and a 295 l (78 gallon) domestic hot water tank integrated. I would definitely not choose that in your case – ask for at least the iPump A 2-7 model as a comparison: it is much better sized, has no buffer tank, and only a 200 l (53 gallon) domestic hot water tank, which in my opinion is sufficient for a household of Hamburg's size.
If your priority is to have R290 as the refrigerant and a nominal COP above 4.5 (which the iPump A 2-7 does not meet), then as an alternative consider the Aero ALM 2-8 plus an external domestic hot water tank (200 to 300 l / 53 to 79 gallons), provided you have enough space.
If your priority is to have R290 as the refrigerant and a nominal COP above 4.5 (which the iPump A 2-7 does not meet), then as an alternative consider the Aero ALM 2-8 plus an external domestic hot water tank (200 to 300 l / 53 to 79 gallons), provided you have enough space.
Harakiri schrieb:
You were probably offered the iPump ALM series – they have a version with a 100 L (26.4 gal) buffer tank and a 295 L (77.9 gal) integrated domestic hot water tank. Personally, I would definitely not choose that option – at least ask for a quote on the iPump A 2-7 model for comparison: it is much better sized, has no buffer tank, only a 200 L (52.8 gal) domestic hot water tank, which in my opinion is sufficient for a home the size of those in Hamburg.
If your priority is to have R290 refrigerant and a nominal COP above 4.5 (which the iPump A 2-7 does not meet), then as an alternative consider the Aero ALM 2-8 plus an external domestic hot water tank (200 to 300 L / 52.8 to 79.3 gal), if space allows. Yes, exactly, the iPump ALM series was offered – what is the reason you would prefer an external domestic hot water tank over the integrated combination tank (100 L / 26.4 gal buffer plus 295 L / 77.9 gal domestic hot water)?
Space could indeed be a concern, but I guess the Aero ALM series is actually smaller than the iPump ALM series.
As mentioned, there is initially no objection to an integrated hot water storage tank. However, I would not spend extra money on a buffer tank on the heating side. It basically only exists to compensate for design errors. Therefore, the recommendation is the iPump A 2-7 as an alternative – with only an integrated hot water storage tank, no buffer tank, and much better suited to the heating load.
D
Daniel-Sp30 Oct 2023 12:31Hello,
As far as I can tell from IDM’s website, this is a combination storage tank installed as a hydraulic separator. It separates the heat pump circuit from the heating circuit. This is not ideal!
Such an installation makes sense with 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, adapting the supply temperature to the actual demand. With a setup involving a buffer tank/combination storage, the flow rate of the heating circuit (underfloor heating) would need to be closely matched to the flow rate of the heat pump. Only then is the energy loss through the buffer tank minimized. The heat pump offered is a modulating heat pump, which is actually correct. The problem with buffer tanks, however, is adjusting the flow rates. The heat pump controls not only the compressor output but also the flow rate to maintain the optimal temperature difference in the heat pump circuit. The circulation pump for the heating circuit, however, operates at a fixed flow rate. So the flow rates cannot be optimally matched, resulting in a constant loss of efficiency!
The solution is to connect the heat pump directly to the underfloor heating. To ensure the minimum flow rate, enough heating circuits must always be open (disable ERR). For domestic hot water, a three-way valve switches from the underfloor heating to the domestic hot water tank. It’s also recommended to include a small buffer tank (30–70 liters (8–18 gallons)) in the heat pump return line to stabilize defrosting cycles. Although this is not absolutely necessary—because the screed provides more heat for defrosting than a buffer tank—this reduces the efficiency loss to a negligible level.
Summary:
1. Have a heating load calculation performed.
2. Based on this, select a SMALL heat pump with no safety margin in the heating capacity, as this is already included in the heating load calculation. Avoid buffer tanks/combination storage or similar setups.
3. It is also important to calculate room-by-room heating loads and then design the underfloor heating accordingly, specifying temperature requirements for each room. Where needed (usually in bathrooms), consider additional wall heating. If towel radiators are planned, do not integrate them into the heating circuit; operate them electrically.
After moving in:
4. Perform hydraulic balancing of the rooms during the first winters (not via ERR!). This is the homeowner’s responsibility and is not provided by the heating installer.
5. Regularly monitor operating behavior and cycling times.
Best regards from Hamburg
As far as I can tell from IDM’s website, this is a combination storage tank installed as a hydraulic separator. It separates the heat pump circuit from the heating circuit. This is not ideal!
Such an installation makes sense with 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, adapting the supply temperature to the actual demand. With a setup involving a buffer tank/combination storage, the flow rate of the heating circuit (underfloor heating) would need to be closely matched to the flow rate of the heat pump. Only then is the energy loss through the buffer tank minimized. The heat pump offered is a modulating heat pump, which is actually correct. The problem with buffer tanks, however, is adjusting the flow rates. The heat pump controls not only the compressor output but also the flow rate to maintain the optimal temperature difference in the heat pump circuit. The circulation pump for the heating circuit, however, operates at a fixed flow rate. So the flow rates cannot be optimally matched, resulting in a constant loss of efficiency!
The solution is to connect the heat pump directly to the underfloor heating. To ensure the minimum flow rate, enough heating circuits must always be open (disable ERR). For domestic hot water, a three-way valve switches from the underfloor heating to the domestic hot water tank. It’s also recommended to include a small buffer tank (30–70 liters (8–18 gallons)) in the heat pump return line to stabilize defrosting cycles. Although this is not absolutely necessary—because the screed provides more heat for defrosting than a buffer tank—this reduces the efficiency loss to a negligible level.
Summary:
1. Have a heating load calculation performed.
2. Based on this, select a SMALL heat pump with no safety margin in the heating capacity, as this is already included in the heating load calculation. Avoid buffer tanks/combination storage or similar setups.
3. It is also important to calculate room-by-room heating loads and then design the underfloor heating accordingly, specifying temperature requirements for each room. Where needed (usually in bathrooms), consider additional wall heating. If towel radiators are planned, do not integrate them into the heating circuit; operate them electrically.
After moving in:
4. Perform hydraulic balancing of the rooms during the first winters (not via ERR!). This is the homeowner’s responsibility and is not provided by the heating installer.
5. Regularly monitor operating behavior and cycling times.
Best regards from Hamburg
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