Is a buffer tank useful in a heat pump?

So you have 170m² of heated area with screed.
If you assume 100kg/m², that is 17t of "buffer mass".
If you now add 500l, that is about 3% more.

I am familiar with the considerations of operating heat pumps in single-family houses without a buffer tank. The buffer tank costs some efficiency. Nonetheless, a buffer tank makes the overall heating system more "robust".

The calculation in the quote can be done that way, but the consideration and conclusion are not 100% correct. For one, it does not take into account that water mixes with water inside the buffer tank, i.e., heat transfer inside the buffer tank occurs significantly faster than heat release from the water through the outer walls of the heating coils into the screed.

Especially in autumn and spring, a buffer tank can help reduce frequent cycling of the heat pump.

As an example: roughly, the heat pump can "charge" about 11.5kWh into a 1000L buffer tank when it heats the storage from 35 to 45 degrees. By comparison: on a day in February, many rooms in the house may hardly "consume" heat due to solar gains — in that case without a buffer, the heat pump will run for much shorter times and tend to cycle more frequently.

If the house has photovoltaics or a variable electricity provider like Tibber is used, it can be interesting overall to charge the buffer tank during the day or at low-price times.

Since apparently very frequent cycling occurs in the current system, a buffer tank could at least help with cycling, possibly as a combined tank with a stainless steel coil in the buffer for hot water preparation in through-flow mode. But someone with expertise on site needs to examine this; remote diagnoses can be quite off.

...a buffer tank could at least help with cycling, possibly as a combination tank with a stainless steel coil in the buffer for domestic hot water preparation in flow-through mode.

Thanks for the answer!

I also found combination tanks interesting. However, according to the German distributor, the heat pump is apparently not "compatible" with combination tanks.

I actually thought that with modulating heat pumps with inverter technology, a buffer tank is not necessary, but if it helps in our case, that's fine with me. The heat pump is simply oversized at 3-12 kW for a heating demand of 6.3 kW.

It is undisputed, however, that a buffer tank makes the entire heating system "more robust."

"Robust" against poor planning?
Certainly not robust against high consumption.

The calculation in the quote can certainly be done this way, but the consideration and conclusion are not 100% correct. For one thing, it does not take into account that water mixes with water in the buffer tank, i.e. the heat transfer within the buffer tank occurs much faster than the heat release of the water through the outer walls of the heating coils into the screed.

"For one thing," but what about the "for another"?

And the "for one thing" is really pretty irrelevant.

As an example: roughly, the heat pump can "load" about 11.5 kWh into a 1000L buffer tank when it heats the tank from 35 to 45 degrees.

1000L is of course already extreme. Added to that is that many heat pumps at 45 degrees operate at only half the efficiency they have at 35 degrees.
Now you could say that this better reduces the cycling of the system.
Whether you want to do that with double the electricity use, however, I find very questionable.

If the house has photovoltaics or a variable electricity provider like Tibber is used, it can be interesting in the overall consideration to charge the buffer tank during the day or at low price times.

Who actually does that? Such a complicated control system.
The OP apparently can’t or doesn’t want to understand even the simple control now.
And what good does it then do at half efficiency?

Since frequent cycling apparently takes place in the previous system, a buffer tank could at least help with cycling, possibly as a combination tank with a stainless steel coil in the buffer for on-demand domestic hot water production. But someone with expertise on site has to look at this; remote diagnoses can be pretty off.

Very bad idea. Halving efficiency permanently in the hope of saving a few cycles during heating.
Also, you would first have to clarify what the heat pump can handle, i.e., whether it can even do these features you advertise like overloading and co.

: What have you already tried in terms of optimizing settings?
Working with time windows?
Tried "time to on next"?
ALL valves open?

The OP doesn’t even seem able or willing to understand the simple regulation now. : What have you already tried to optimize in terms of settings? Working with time windows? Tried "Time to on next"? ALL valves open?

I find the first sentence somewhat inappropriate. I studied mathematics and physics and enjoy working my way into the processes. At the moment, I can’t try anything because the heating is broken. Would the following be a good strategy? Since January, we have connected a pool to the heating. For the pool, I have to specify how many minutes per hour the pool should be served. It was last set to 15 minutes. Of course, I could also increase that to 30 minutes and correspondingly lower the flow temperature for the pool (e.g., reduce it from 35° to 33°). Then the heating would have a forced break of 30 minutes every hour without the compressor needing to be stopped, so compared to operation without the pool, it would have to perform the heating work in half the time. That could compensate for the oversizing. Right?

I find the first sentence a bit inappropriate.

Yes? I don’t think so. You created the first thread a year ago saying that the compressor of your heating system is broken. So you’ve had a year now to improve the settings. Also to get reasonable alternative offers. But the repair is coming tomorrow? Another problem is that you fragment your threads a lot. You drop bits of information here and there and then omit them again in other threads. A more systematic approach would certainly be sensible here.

You created the first thread a year ago stating that the compressor of your heating system is broken. So you’ve had a year to improve the settings. But the repair is coming tomorrow?

That’s the dilemma. For a year, I have tried to get
- the installing company
- the German distribution of Ecoforest
- heating engineers who don’t work with Ecoforest,
to readjust the heating system so that it cycles less frequently, but no one wanted to do it.

Ultimately, the heating engineer who connected the pool promised to go through the settings, possibly with telephone support from Enrgi (the German distributor of Ecoforest). This was commissioned in June and started in September. However, the work was only completed in January. The pool has only been connected since January. The last step, reconfiguring the heat pump, did not happen because the heating system is now defective.

The repair appointment was initially canceled because Enrgi insisted that we pay in advance and that they would only come if the money had already been received. Since the transfer could not be made that quickly, the appointment was canceled again.

Since the costs are very high:
- Compressor replacement €4,800
- possibly purchase of a 200-liter buffer tank for €2,400 plus mixer kit €600 plus 3 temperature sensors €90
- installation of buffer tank ??? €
Total approximately €8,000

we are currently obtaining quotes for a new heat pump:
https://www.hausbau-forum.de/threads/waermepumpe-fuer-5-000-7-500-eur-reparieren-oder-durch-neue-waermepumpe-ersetzen.46862/

It will be even more expensive, but we could switch to a larger manufacturer with a better service network and have a new heat pump, this time correctly dimensioned, e.g. 2-6 kW instead of 3-12 kW, with a new 5-year warranty.

With a repair, there would still be the risk that even with a buffer tank the heating system cycles that often and the compressor breaks again in 7-8 years. (Or would that be very reliably excluded with a buffer tank?)