Hello! We are currently building a single-family house to KfW40 standard and are supposed to have a Bosch air-to-water heat pump installed. Our heat pump was ordered back in December 2022, but it has not been delivered yet, and there is no clear indication of when it will arrive. Our construction company now says they can install the same model (Compress CS7400iAW ORMB-S) within two weeks, but with lower capacity, specifically 5 kW instead of 7 kW. According to our heating load calculation, our house requires 2.8 kW without hot water and a total of 3.4 kW including hot water. So, it seems that 5 kW should be sufficient. The question is – is this really the case, or am I misunderstanding something? Or should we wait for the 7 kW unit, even though delivery is completely uncertain? We are a family of five living in Schleswig-Holstein. I also asked about alternatives, for example, an air-to-water heat pump from Vaillant. I was told that this is an option, but it would have to be ordered new, with a delivery time of about three months, which should be more certain. Could someone recommend: take the 5 kW unit, wait for the 7 kW, or go with the Vaillant?
N
nordanney2 Mar 2025 14:06derdietmar schrieb:
A heat pump that is properly or slightly undersized can run into issues here and may not produce enough heat even at its highest modulation level to compensate for defrost losses.However, this can only happen at the No-Action-Temperature (NAT). Because on 99.5% of days, it is significantly warmer and the heat pump is only lightly loaded.If I think about it… In recent years, I can only recall 1-2 occasions when the NAT was actually reached.
D
derdietmar2 Mar 2025 14:09Hello,
unfortunately not, this especially happens during the transitional seasons. Icing is less critical and less common at low temperatures because the humidity is low. In autumn and spring, when the outside temperature is around 1-2°C (34-36°F) and the humidity is above 95%, the machines often ice up.
Best regards
unfortunately not, this especially happens during the transitional seasons. Icing is less critical and less common at low temperatures because the humidity is low. In autumn and spring, when the outside temperature is around 1-2°C (34-36°F) and the humidity is above 95%, the machines often ice up.
Best regards
D
derdietmar2 Mar 2025 14:16Hello,
The editing time has unfortunately expired, so here is a new post:
This image illustrates the behavior quite well. In the early morning hours, the unit is operating at full capacity.
I don’t want to go into too much detail, just wanted to point out that defrost cycles should be taken into account when selecting the performance level of the air-to-water heat pump.
Best regards
The editing time has unfortunately expired, so here is a new post:
This image illustrates the behavior quite well. In the early morning hours, the unit is operating at full capacity.
I don’t want to go into too much detail, just wanted to point out that defrost cycles should be taken into account when selecting the performance level of the air-to-water heat pump.
Best regards
H
Häuschenbauer42 Mar 2025 14:32First of all, thank you to everyone for the many responses!
@derdietmar
You seem to be very knowledgeable, so thank you again for the detailed explanation.
My question is this: my heating capacity of about 5.6 kW refers to a design temperature of -12°C (10°F). Since the defrost cycles mostly occur at high humidity around freezing point, at 0°C (32°F) the 55/6 unit provides approximately 7 kW and the 75/6 around 10 kW.
That should presumably be sufficient. I don’t know the heating load demand for the house at 0°C (32°F), but if it is 5.6 kW at a design temperature of -12°C (10°F), then the available capacity of 7 kW from the 55/6 at 0°C (32°F) should more than cover the need, right?
I would appreciate any feedback.
Best regards
@derdietmar
You seem to be very knowledgeable, so thank you again for the detailed explanation.
My question is this: my heating capacity of about 5.6 kW refers to a design temperature of -12°C (10°F). Since the defrost cycles mostly occur at high humidity around freezing point, at 0°C (32°F) the 55/6 unit provides approximately 7 kW and the 75/6 around 10 kW.
That should presumably be sufficient. I don’t know the heating load demand for the house at 0°C (32°F), but if it is 5.6 kW at a design temperature of -12°C (10°F), then the available capacity of 7 kW from the 55/6 at 0°C (32°F) should more than cover the need, right?
I would appreciate any feedback.
Best regards
D
derdietmar2 Mar 2025 14:50Hello,
I would actually go for the larger unit. It not only has the capacity to compensate for the withdrawn energy, but it will also need to defrost much less often and operate more frequently at a low modulation range.
A possible downside during transitional periods could be that it shuts off on some days earlier (lowest modulation stage reached, yet a lot of heat remains), compared to the smaller unit. However, this can be partly counteracted through appropriate settings, which transform cycling into longer pauses or run times.
I read a user report elsewhere that matches your conditions:
Almost 150 m2 (1600 sq ft)
Heating load at NAT: 5 kW (17,000 BTU/h)
Underfloor heating
In this case, the larger unit 75/6 is in use, and the described issue does not occur, unlike with the smaller 55/6 unit used by other users. In particular, the number of defrost cycles is much lower than with the smaller device.
Best regards
I would actually go for the larger unit. It not only has the capacity to compensate for the withdrawn energy, but it will also need to defrost much less often and operate more frequently at a low modulation range.
A possible downside during transitional periods could be that it shuts off on some days earlier (lowest modulation stage reached, yet a lot of heat remains), compared to the smaller unit. However, this can be partly counteracted through appropriate settings, which transform cycling into longer pauses or run times.
I read a user report elsewhere that matches your conditions:
Almost 150 m2 (1600 sq ft)
Heating load at NAT: 5 kW (17,000 BTU/h)
Underfloor heating
In this case, the larger unit 75/6 is in use, and the described issue does not occur, unlike with the smaller 55/6 unit used by other users. In particular, the number of defrost cycles is much lower than with the smaller device.
Best regards
N
nordanney2 Mar 2025 14:50derdietmar schrieb:
Icing is less critical and less common at low temperatures because the air humidity is low. In autumn and spring, at 1-2°C (34-36°F) outside temperature and 95+% air humidity, the machines ice up very frequently. Yes, but neither the house nor the heat pump requires much power then. That’s what I mean. The system briefly defrosts and then continues heating. During this time, a new building does not noticeably lose heat—even with defrost cycles every 30 minutes.
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