ᐅ Heat load calculation is 10.3 kW; is an air-to-water heat pump with 9.5 kW capacity sufficient?
Created on: 5 Mar 2020 22:09
H
hippjohaHello everyone,
I just had a brief phone call with our heating engineer. The heating load calculation resulted in a value of 10.3 kW. He would recommend a 9.5 kW air-to-water heat pump (nothing else is possible since drilling, etc., is not allowed), among other reasons because it is eligible for funding through BAFA.
My question now is whether this air-to-water heat pump is adequately sized. In the open-plan living room, we also have a wood stove.
I hope someone here can clarify this for me.
Thank you very much!
Best regards,
Hannes
I just had a brief phone call with our heating engineer. The heating load calculation resulted in a value of 10.3 kW. He would recommend a 9.5 kW air-to-water heat pump (nothing else is possible since drilling, etc., is not allowed), among other reasons because it is eligible for funding through BAFA.
My question now is whether this air-to-water heat pump is adequately sized. In the open-plan living room, we also have a wood stove.
I hope someone here can clarify this for me.
Thank you very much!
Best regards,
Hannes
D
Daniel-Sp5 Mar 2020 22:34Hello,
10.3 kW heating load—what for? New construction? How was it calculated?
Generally, the heating load is calculated based on the design outdoor temperature. How often have these low temperatures actually occurred in recent years?
At a glance, without detailed information, it is likely that a lower heating load will suffice most of the time because outdoor temperatures are higher than the design temperature, and internal and solar gains are usually not considered.
Check the average minimum winter temperatures, the heat pump’s output at those temperatures, and the heating demand of the house at those temperatures.
You should also insist on a modulating heat pump. Do not install a water jacket in the wood stove; it only causes hydraulic problems. The heat pump should be connected directly to the underfloor heating system, with no buffer tank or mixing storage, and no additional solar thermal system.
Avoid oversizing the heat pump.
Regards, Daniel
10.3 kW heating load—what for? New construction? How was it calculated?
Generally, the heating load is calculated based on the design outdoor temperature. How often have these low temperatures actually occurred in recent years?
At a glance, without detailed information, it is likely that a lower heating load will suffice most of the time because outdoor temperatures are higher than the design temperature, and internal and solar gains are usually not considered.
Check the average minimum winter temperatures, the heat pump’s output at those temperatures, and the heating demand of the house at those temperatures.
You should also insist on a modulating heat pump. Do not install a water jacket in the wood stove; it only causes hydraulic problems. The heat pump should be connected directly to the underfloor heating system, with no buffer tank or mixing storage, and no additional solar thermal system.
Avoid oversizing the heat pump.
Regards, Daniel
Hi Daniel,
I haven’t received the exact data yet. It’s a new build. If I understand correctly, this value applies at -14°C (7°F)?
The wood-burning stove will not be hydronic / connected to the heating system.
The air-to-water heat pump would be a Viessmann monoblock (split unit).
If you say it shouldn’t be too large, then should it be somewhat smaller? Do I understand the calculation correctly that at -14°C (7°F) we would need 10.3 kW, and the system would then be too small (assuming the heating is turned fully on in every room)?
Best regards and thanks,
Hannes
PS: The average standard outdoor temperature is -11.5°C (11°F).
I haven’t received the exact data yet. It’s a new build. If I understand correctly, this value applies at -14°C (7°F)?
The wood-burning stove will not be hydronic / connected to the heating system.
The air-to-water heat pump would be a Viessmann monoblock (split unit).
If you say it shouldn’t be too large, then should it be somewhat smaller? Do I understand the calculation correctly that at -14°C (7°F) we would need 10.3 kW, and the system would then be too small (assuming the heating is turned fully on in every room)?
Best regards and thanks,
Hannes
PS: The average standard outdoor temperature is -11.5°C (11°F).
D
Daniel-Sp5 Mar 2020 23:20With a heat pump, EVERY room must always be heated to ensure sufficient airflow. Otherwise, the heat pump cannot remove the heat properly and will trigger a fault. Alternatively, a large portion of the heat flows past the underfloor heating via a bypass valve directly into the return line, causing the heat pump to shut down early and cycle frequently. You should read up a bit on heat pumps, especially on how to operate them efficiently and in a way that protects the equipment.
The heat pump should not be selected based on the lowest ambient temperature (NAT) or the average minimum temperature. It should be able to provide the required heat mostly through the compressor alone. The few days when this is not enough and the electric heater activates should not be significant. That’s why it’s important to know how cold it really gets on average.
We have an NAT of -12°C (10°F), recently adjusted to -10°C (14°F), and I can count on one hand the number of days in recent years when it dropped below -5°C (23°F).
It is important to find a good bivalence point—the temperature at which the electric heater must assist. This point is higher than the NAT and varies depending on the model.
Is the offered heat pump modulating? This is also very important. You should avoid fixed-speed heat pumps.
Yes, 10.3 kW heating load at NAT.
That sounds quite high for a new build. How large is the house? Is it just standard according to the energy saving regulations?
The heat pump should not be selected based on the lowest ambient temperature (NAT) or the average minimum temperature. It should be able to provide the required heat mostly through the compressor alone. The few days when this is not enough and the electric heater activates should not be significant. That’s why it’s important to know how cold it really gets on average.
We have an NAT of -12°C (10°F), recently adjusted to -10°C (14°F), and I can count on one hand the number of days in recent years when it dropped below -5°C (23°F).
It is important to find a good bivalence point—the temperature at which the electric heater must assist. This point is higher than the NAT and varies depending on the model.
Is the offered heat pump modulating? This is also very important. You should avoid fixed-speed heat pumps.
Yes, 10.3 kW heating load at NAT.
That sounds quite high for a new build. How large is the house? Is it just standard according to the energy saving regulations?
Thank you for your response. Okay, I will take a closer look at it again or wait for the documents from the heating installer.
We have a total of about 220 sqm (2,368 sq ft) of living space (170 sqm (1,830 sq ft) + 50 sqm (540 sq ft) granny flat). The building complies with the energy saving regulation. According to the energy certificate, our primary energy demand falls between KfW 40 and KfW 55 (36.5 kWh/m²a).
We have a total of about 220 sqm (2,368 sq ft) of living space (170 sqm (1,830 sq ft) + 50 sqm (540 sq ft) granny flat). The building complies with the energy saving regulation. According to the energy certificate, our primary energy demand falls between KfW 40 and KfW 55 (36.5 kWh/m²a).
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