Assessment of heating concept KfW70

  • Erstellt am 2016-01-06 17:22:52

Grym

2016-01-08 15:51:53
  • #1
I have found standby losses for a 500-liter tank of 2.1 to 2.3 kWh per day. Calculated over 180 non-heating days at 2.3, this 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 tank thus has costs of 11.50. However, the 500-liter tank is also a hygiene tank, meaning the temperature can be lowered and thus the standby losses reduced. A reduction from 60 to 50 lowers the delta to room temperature from 40 to 30. May I now assume that this reduces the standby losses by 25 percent? Then the costs are still 15.53 euros. The delta to the 200-liter tank at 60 degrees is then 4.03 euros per year...
 

Saruss

2016-01-08 15:59:48
  • #2
My 200l tank is set to 48 degrees standby maximum. The tank will incur the same costs on heating days, unless you shower cold in winter. Because tank charging is much less efficient than heating operation. For an air-to-water heat pump, 0.05 euros per kWh is particularly optimistic in winter.
 

Grym

2016-01-08 16:05:15
  • #3
For a gas boiler, the costs per kWh of produced heat should in my opinion not be _significantly_ different. These losses then turn into internal gains for heating, so basically a zero-sum game here.

48 degrees and then a hygiene tank as well, right? For standard tanks, I find 60 degrees absolutely appropriate and 48 degrees hygienically questionable.
 

Saruss

2016-01-08 17:23:50
  • #4
With a heat pump, it makes a significant difference. See the start post of this thread. Do not go off-topic or give wrong advice!! 48 degrees are not concerning at all if the [exchange rate] is high enough.
 

Grym

2016-01-08 19:02:07
  • #5
Ok sorry, I just quickly typed this on my smartphone while on the go, without having the whole thread in view. But I already had the impression that the thread was slowly turning into an academic discussion...

Theoretically, you can argue with the exchange rate, yes. However, for houses with more than 2 residential units, something like that would be prohibited, for example, and I am generally of the opinion that such regulations are not there for fun, even if this regulation with the 60-degree outlet temperature at the boiler applies not to single-family homes [EFHs], but only from 3 residential units onwards.

Everyone has to decide for themselves, but for me, a traditional boiler requires a minimum temperature of 60 degrees (and not significantly more because of lime scale) and 43-50 degrees for a hygiene storage tank. Hygiene storage tanks are usually somewhat larger, but they also have fewer thermal losses due to the lower temperature compared to a conventional boiler with 60 degrees.

On the topic then: 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 with an example at -14 degrees standard outside temperature, 35 degrees flow temperature, and other 'realistic' assumptions). At a heat pump tariff of 19 cents, you arrive at the 0.05 EUR I mentioned, slightly above at 20 cents. Of course, there are differences between summer and winter; on the other hand, there are also significantly more than 180 heating days and significantly 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 tank, but they would be somewhat lower. Savings potential is maybe 10 EUR per year through a significantly smaller tank. Let it be 50 percent more – 15 EUR.

Because of 15 EUR, there is no need now to recommend a smaller storage tank. The tank is perfectly fine, has no noticeably higher operating costs than a 160-liter or 200-liter tank, and is absolutely hygienically top. In addition, the tank can be controlled at 50 degrees, which might be questionable with traditional tanks (i.e., I say it could be questionable and you have a different opinion).
 

Saruss

2016-01-08 19:22:54
  • #6
Your calculation is still fundamentally wrong. You cannot calculate with the annual performance factor like that, apart from the fact that these are fictional figures; it depends too much on the user, weather, etc. In summer, the heat pump has an extremely good hot water COP, but you do not need the waste heat from the storage; in winter, the COP for hot water may be around 2 if it is not too cold; here you can use the waste heat, but it is significantly more expensive than heating for the heating operation. So your calculation is definitely too low. Of course, it is not about large sums, even if you double the numbers. Otherwise, I also know people who, for example, like to have hot water for washing dishes and therefore prefer to charge a storage tank to 60 rather than 45. So what I have been saying all along applies: for the greatest cost efficiency, you take the storage tank that fits your own usage behavior exactly, but not much bigger. That way you save a bit on purchase and operation, even if it is not much. However, if you save 30 Euros or more here and there at 10 places in the house, it adds up in total.
 

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