ᐅ BAFA Innovation Grant for Air Source Heat Pumps

I would replace "In general" with "Always." In which combination would an ST be worthwhile? Self-construction not considered.

Lumpi_LE schrieb:
I would replace "In general" with "Always." In which combination would an ST be worthwhile? Not considering self-construction.

I didn’t want to be so categorical. That only provokes unnecessary disagreement. Now I have done exactly the opposite. However, there are indeed subsidies that can make an ST available at zero cost. Even then, though, a photovoltaic system is probably more economical...

Best regards, Nika

To calculate the seasonal performance factor... Unfortunately, this forum is very strict regarding links, even if they do not contain advertising or similar content. :-( Well, rules are rules, even if this limits the usefulness of the forum significantly.

In any case, the calculated seasonal performance factor according to VDI for most heat pumps can be easily calculated online by yourself. This calculation can be submitted directly to BAFA, provided it is signed by an authorized person. Simply enter "seasonal performance factor calculator" in your preferred search engine.

Best regards
Nika

gnika77 schrieb:

P.P.S Under no circumstances install a heating buffer tank with a heat pump.

Why not use a buffer tank? I’m currently considering retrofitting one to my Stiebel Eltron LWZ 504 to better utilize the SG Ready function and store heat.
Are there any disadvantages besides heat loss (which anyway stays inside the house) that I might be overlooking?

United98 schrieb:
Why not use a buffer tank? [...]
Are there any disadvantages besides the heat loss (which stays inside the house anyway) that I might be missing?

Yes, there are disadvantages. These have also been demonstrated in practice by the Fraunhofer study. Look it up – it’s even highlighted in bold there.
Basically, a buffer tank causes a higher supply temperature for the heat pump. This can be 5°C (9°F) or more. For every 1°C (1.8°F) increase, you can expect about a 2.5% drop in efficiency. That means with 5°C (9°F) higher, efficiency would be about 12.5% lower.

Also, why do you need a buffer tank?
The screed flooring is already a much better thermal store. You can easily absorb the occasional excess photovoltaic energy during the heating season by increasing the supply temperature by 1 to 2°C (2 to 4°F). You usually won’t notice this change in room temperature, if at all.

Regarding efficient heat pump operation, a forum specializing in building services engineering would be a better place to get detailed advice. I would assume that instead of a buffer tank, a mixing valve (primary-secondary circuit or hydraulic separator) might be installed in your system, which also lowers efficiency. Also, there are other measures to improve efficiency. Optimizing your heat pump’s operation mode will give you better results than adding a buffer tank.

Oh, and independently from efficiency: since a buffer tank costs money, the storage costs per unit of energy in a buffer are higher than the costs of feeding in and buying back photovoltaic electricity. That is also one of the reasons why the specific heat generation costs of a solar thermal system are so high.

Best regards, Nika

gnika77 schrieb:
For every 1K, you have to expect about a 2.5% efficiency loss. That means at 5K, the efficiency would be 12.5% lower.

No, that would be (if the assumptions are correct, which I cannot assess or confirm) approximately 11.9%. Percentages should not be added directly.