ᐅ Assessment of the Heating Concept for a KfW 70 Energy-Efficient House
Created on: 6 Jan 2016 17:22
O
oerlein
Hello everyone,
I have already looked closely at the current offer we have for a KfW70 house and am not entirely sure about the air-to-water heat pump being used.
The following is currently included in the offer:
I have researched this heat pump online but cannot judge whether these are just some isolated negative experiences and satisfied users don’t speak up, or if the unit is actually that “poor.”
One significant concern for me was that the electric heating element activates already at around 0°C (32°F).
At a house viewing appointment, I was able to experience the heat pump, and despite it not being a very cold day, the outdoor unit seemed quite noisy to me.
What do you think about this combination/heat pump?
Are there alternative units you would recommend?
I am generally comfortable with the idea of an air-to-water heat pump, but those many negative impressions of the above Rotex unit have made me cautious.
Thank you very much for your support!
oerlein
I have already looked closely at the current offer we have for a KfW70 house and am not entirely sure about the air-to-water heat pump being used.
The following is currently included in the offer:
- KfW70 house
- 160 sqm (1,722 sq ft) of living space heated with underfloor heating
- central controlled ventilation system (manufacturer unknown)
- Rotex HPSU 6 compact heating and cooling with 500-liter (132-gallon) stratified storage tank up to 7 kW
I have researched this heat pump online but cannot judge whether these are just some isolated negative experiences and satisfied users don’t speak up, or if the unit is actually that “poor.”
One significant concern for me was that the electric heating element activates already at around 0°C (32°F).
At a house viewing appointment, I was able to experience the heat pump, and despite it not being a very cold day, the outdoor unit seemed quite noisy to me.
What do you think about this combination/heat pump?
Are there alternative units you would recommend?
I am generally comfortable with the idea of an air-to-water heat pump, but those many negative impressions of the above Rotex unit have made me cautious.
Thank you very much for your support!
oerlein
B
Bieber08157 Jan 2016 10:51I could have also mentioned that the storage tank and domestic hot water are closed (not drain-down) systems. The brochure actually refers to it as a hygiene storage tank, and Grym’s description is, as far as I know, completely accurate.
But the question of whether this storage tank is unfavorable in combination with a heat pump or not is still open. (At first, I don’t see any significant disadvantages, but that doesn’t necessarily mean there aren’t any.)
It’s also unclear to me whether the Rotex HPSU Compact uses a (this?) storage tank for the space heating function.
But the question of whether this storage tank is unfavorable in combination with a heat pump or not is still open. (At first, I don’t see any significant disadvantages, but that doesn’t necessarily mean there aren’t any.)
It’s also unclear to me whether the Rotex HPSU Compact uses a (this?) storage tank for the space heating function.
Large storage tanks naturally have greater heat energy losses. If you don't need a lot of hot water every day, 500 liters (130 gallons) is not the most cost-effective option (even with showers, a 200-liter (53 gallons) tank with one recharge per day is sufficient for us; when the children are older, a second may be added). Even if heat stays within the building envelope, oversizing due to higher charging temperatures is not efficient.
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Bieber08157 Jan 2016 20:15Saruss schrieb:
Large storage tanks naturally have greater heat energy losses. Compared to smaller tanks? The larger the tank, the better the surface-to-volume ratio, so losses are generally lower. Or am I missing something?
What kind of system do you have? How do you know how often the tank is “recharged”? When is the tank recharged?
Bieber0815 schrieb:
as small storage tanks? The larger the tank, the better the surface-to-volume ratio, so fewer losses occur. Or am I thinking about this incorrectly?
What kind of device do you have? How do you know how often the tank is "charged"? When is the tank charged? The ratio improves, but of course, you have much more surface area! That increases faster than the ratio improves; otherwise, at a certain size, no energy could be lost at all.
The absolute losses grow with size. Therefore, the tank should be as small as possible without sacrificing comfort. This naturally varies depending on the number of people and showering/bathing habits.
I have a brine heat pump with the mentioned tank size. I log the heat pump’s operation on a Raspberry Pi and therefore know when and how often it is charged. I do not have a fixed charging time set; it depends on consumption and hysteresis in my system.
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Bieber08157 Jan 2016 22:48Saruss schrieb:
The ratio improves, but naturally you have a lot more surface area! That increases faster than the ratio improves, otherwise above a certain size you wouldn’t lose any energy at all.V ~ r³, A ~ r². V/A grows faster than A. Heat losses relative to the volume become infinitely small for an infinitely large tank (this is the theoretical limit).
Saruss schrieb:
Absolute losses increase with size. Yes, that’s true, but be careful; in my opinion, the interesting aspect is the losses relative to the stored heat. The source term in the balance (the heat supplied electrically by the heat pump should be independent of the surface-to-volume ratio) is, in my view, irrelevant here.
Background: I’ll try to explain in words: dQ/dt = k·A·dT, where dQ/dt = c_p·ρ·V·dT/dt; after integration this results in (T_end - T_start)/(T_outside - T_start) = exp(-k·A·t / (ρ·c_p·V)). This means that the time t after which the contents reach ambient temperature depends on V/A. And V/A increases the larger the tank is.
Saruss schrieb:
I log the heat pump operation on a Rasp.pi Does your heat pump have a suitable interface for that, or did you have to build one yourself? Is there something like that also available for engineers (for example, in Excel)?
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