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
I found standby losses for a 500-liter (132-gallon) storage tank of between 2.1 and 2.3 kWh per day. Calculating with 2.3 kWh over 180 non-heating days results in 414 kWh. At 0.05 euros per kWh, this amounts to annual costs of 20.70 euros. Reduced by a factor of 1.8, the 200-liter (53-gallon) storage tank therefore has costs of 11.50 euros. However, the 500-liter storage tank is also a hygiene tank, meaning the temperature can be lowered, thereby reducing standby losses. Lowering the temperature from 60°C to 50°C (140°F to 122°F) reduces the temperature difference to room temperature from 40 to 30 degrees. May I assume that this decreases standby losses by 25 percent? That would result in costs of 15.53 euros. The temperature difference compared to the 200-liter tank at 60°C (140°F) corresponds to annual costs of 4.03 euros...
My 200-liter (53-gallon) tank is set to a maximum standby temperature of 48°C (118°F). The storage costs will remain the same even on heating days, unless you take cold showers in winter. This is because charging the tank is significantly less efficient than operating the heating system. For an air-to-water heat pump, a cost of 0.05 euros per kWh is especially optimistic during winter.
For a gas boiler, in my opinion, the cost per kWh of heat produced should not be significantly different. These losses then become internal gains for heating, so it is basically a zero-sum game.
48 degrees Celsius (118°F) combined with a hygiene storage tank, right? For standard storage tanks, I find 60 degrees Celsius (140°F) absolutely appropriate and 48 degrees Celsius (118°F) hygienically questionable.
48 degrees Celsius (118°F) combined with a hygiene storage tank, right? For standard storage tanks, I find 60 degrees Celsius (140°F) absolutely appropriate and 48 degrees Celsius (118°F) hygienically questionable.
Ok sorry, that was just quickly typed on my smartphone while on the go, without having the whole thread in view. But I had the impression anyway that the discussion was slowly turning into an academic debate...
Theoretically, you can argue with the exchange rate, yes. However, for houses with more than two dwelling units, something like that would be prohibited, and I generally believe such regulations are not there without reason, even if this rule about the 60-degree outlet temperature at the boiler does not apply to single-family homes, but only from three dwelling units upwards.
Everyone has to decide for themselves, but for me, a traditional boiler requires a minimum temperature of 60°C (140°F) (and not much higher to avoid limescale), and 43-50°C (109-122°F) for a hygiene storage tank. Hygiene tanks are usually considerably larger, but they also have lower thermal losses due to the lower temperature compared to a conventional boiler running at 60°C (140°F).
On the topic: The Rotex HPSU Compact 508 with 6 kW achieves an annual performance factor of 3.72 in heating mode and 3.77 in hot water mode (edit: calculated using an example with -14°C (7°F) standard outdoor temperature, 35°C (95°F) flow temperature, and other 'realistic' assumptions). With a heat pump tariff of 19 cents, you reach the 0.05 EUR I mentioned; at 20 cents, slightly above. Of course, there are differences between summer and winter; on the other hand, there are significantly more than 180 heating days and fewer than 180 non-heating days. Standby losses are somewhere between 15 and 30 EUR per year and could not even be halved with a 200-liter (53-gallon) tank, but they would be somewhat lower. The potential savings might be around 10 EUR per year with a considerably smaller tank. Let’s say 50% more — 15 EUR.
For 15 EUR, there is no need to recommend a smaller tank. The tank is perfectly fine, has no noticeably higher operating costs than a 160-liter (42-gallon) or 200-liter (53-gallon) tank, and is absolutely top level in hygiene. Additionally, the tank can be controlled at 50°C (122°F), which might be questionable with traditional tanks (meaning, I say it *could* be questionable, and you have a different opinion).
Theoretically, you can argue with the exchange rate, yes. However, for houses with more than two dwelling units, something like that would be prohibited, and I generally believe such regulations are not there without reason, even if this rule about the 60-degree outlet temperature at the boiler does not apply to single-family homes, but only from three dwelling units upwards.
Everyone has to decide for themselves, but for me, a traditional boiler requires a minimum temperature of 60°C (140°F) (and not much higher to avoid limescale), and 43-50°C (109-122°F) for a hygiene storage tank. Hygiene tanks are usually considerably larger, but they also have lower thermal losses due to the lower temperature compared to a conventional boiler running at 60°C (140°F).
On the topic: The Rotex HPSU Compact 508 with 6 kW achieves an annual performance factor of 3.72 in heating mode and 3.77 in hot water mode (edit: calculated using an example with -14°C (7°F) standard outdoor temperature, 35°C (95°F) flow temperature, and other 'realistic' assumptions). With a heat pump tariff of 19 cents, you reach the 0.05 EUR I mentioned; at 20 cents, slightly above. Of course, there are differences between summer and winter; on the other hand, there are significantly more than 180 heating days and fewer than 180 non-heating days. Standby losses are somewhere between 15 and 30 EUR per year and could not even be halved with a 200-liter (53-gallon) tank, but they would be somewhat lower. The potential savings might be around 10 EUR per year with a considerably smaller tank. Let’s say 50% more — 15 EUR.
For 15 EUR, there is no need to recommend a smaller tank. The tank is perfectly fine, has no noticeably higher operating costs than a 160-liter (42-gallon) or 200-liter (53-gallon) tank, and is absolutely top level in hygiene. Additionally, the tank can be controlled at 50°C (122°F), which might be questionable with traditional tanks (meaning, I say it *could* be questionable, and you have a different opinion).
Your calculation is still fundamentally incorrect.
You cannot calculate using the annual performance factor (seasonal COP) like that. Besides the fact that these are theoretical values, they depend heavily on user behavior, weather, and other factors. In summer, the heat pump achieves a very good COP for domestic hot water, but the waste heat from the storage tank is not needed. In winter, the COP for domestic hot water might be around 2 if it’s not too cold; in this case, the waste heat can be used, but heating it this way is significantly more expensive than heating for the space heating operation. So, your calculation is definitely too low.
Of course, it’s not about large sums, even if you double the numbers.
I also know people who prefer hot water for dishwashing, for example, and therefore charge the storage tank to around 60°C (140°F) instead of 45°C (113°F).
What I have been saying the whole time applies here: for maximum cost efficiency, you should choose a storage tank that fits your individual usage pattern in size—no bigger than necessary. This saves some on purchase and operating costs, though not a lot.
However, if you save around 30 euros or more at several points within the house, it adds up in total.
You cannot calculate using the annual performance factor (seasonal COP) like that. Besides the fact that these are theoretical values, they depend heavily on user behavior, weather, and other factors. In summer, the heat pump achieves a very good COP for domestic hot water, but the waste heat from the storage tank is not needed. In winter, the COP for domestic hot water might be around 2 if it’s not too cold; in this case, the waste heat can be used, but heating it this way is significantly more expensive than heating for the space heating operation. So, your calculation is definitely too low.
Of course, it’s not about large sums, even if you double the numbers.
I also know people who prefer hot water for dishwashing, for example, and therefore charge the storage tank to around 60°C (140°F) instead of 45°C (113°F).
What I have been saying the whole time applies here: for maximum cost efficiency, you should choose a storage tank that fits your individual usage pattern in size—no bigger than necessary. This saves some on purchase and operating costs, though not a lot.
However, if you save around 30 euros or more at several points within the house, it adds up in total.
Similar topics