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?
D
derdietmar2 Mar 2025 14:58Hello,
Yes, cooling down probably won’t be much of an issue in most cases. However, energy is still lost during defrosting, and the coefficient of performance (COP) drops significantly.
Since it depends heavily on the environment, it’s hard to label any general approach as right or wrong.
Best regards
nordanney schrieb:
Yes, neither the house nor the heat pump require much power. That’s what I mean. The system briefly defrosts and then continues heating. During this time, no new building cools down noticeably – not even with defrost cycles every 30 minutes.
Yes, cooling down probably won’t be much of an issue in most cases. However, energy is still lost during defrosting, and the coefficient of performance (COP) drops significantly.
Since it depends heavily on the environment, it’s hard to label any general approach as right or wrong.
Best regards
N
nordanney2 Mar 2025 14:59Häuschenbauer4 schrieb:
I would really appreciate some feedback. I stand by my point: definitely do not use the larger unit. You will never need that capacity, and it is completely oversized.
derdietmar schrieb:
It not only has the reserve to compensate for the lost energy, You don’t need that reserve because the 55/6 already has sufficient capacity.
derdietmar schrieb:
it will also need to defrost much less often and run more frequently at low modulation levels. Let’s put it another way. In normal operation, I see the 55/6 running at low modulation levels 99.9% of the time. The 75/6 will not be able to run at low modulation levels any more often.
derdietmar schrieb:
One potential disadvantage during the transition period could be that it switches off on some days (lowest modulation level reached, but the excess heat still does not dissipate), more often than the smaller unit. That’s roughly 60-70% of the heating season.
derdietmar schrieb:
In this case, the larger 75/6 unit is in operation and the described problem does not occur, unlike other users with the smaller 55/6 unit. Has the user actually tested both units to compare? Maybe they have a well-sized house and system where the 55/6 works just fine.
For me, the heat pump is not the weakest link to focus on. It’s the design of the underfloor heating system. If you have that planned properly, you greatly reduce the load on any heat pump. So, consider reducing the pipe spacing so you can achieve better flow and allow the heat pump to run more easily. In my case, 50% of the rooms have pipe spacing less than 10cm (5 inches). This way, the heat pump never has to struggle or work hard.
N
nordanney2 Mar 2025 15:01derdietmar schrieb:
However, energy is still lost during the defrosting process, causing the coefficient of performance to drop significantly."Sinking significantly" sounds quite dramatic. Can you specify exactly—numerically—how much the coefficient of performance drops? How much energy is actually lost? Do you have any experience data or calculations on this?H
Häuschenbauer42 Mar 2025 15:08nordanney schrieb:
Yes, neither the house nor the heat pump requires much power for that. That’s what I mean. It will just defrost briefly and then continue heating. During that time, a new building does not cool down noticeably – not even with defrosting every 30 minutes. I also think that no significant cooling occurs, especially since the 55/6, as mentioned, still delivers 7 kW at 0°C (32°F). That is already more than what is needed by NAT.
Besides, there would still be the electric heater available, right?
D
derdietmar2 Mar 2025 15:09Hello,
I am only familiar with experience reports; in one example, the daily performance factor dropped to 2.5–2.8, and energy consumption tripled. Defrosting occurred 1–2 times per hour.
Best regards
I am only familiar with experience reports; in one example, the daily performance factor dropped to 2.5–2.8, and energy consumption tripled. Defrosting occurred 1–2 times per hour.
Best regards
H
Häuschenbauer42 Mar 2025 15:15I will take another look at the topic as well.
Do the two models actually differ in the output available for hot water production?
Or should the same values as for the heating system be used?
Do the two models actually differ in the output available for hot water production?
Or should the same values as for the heating system be used?
Similar topics