Is a buffer tank useful in a heat pump?

"Robust" against misplanning?
Robust against high consumption certainly not.

Correct, robust against misplanning. I mentioned that a buffer tank does not increase efficiency.

"On the one hand", but where is the "on the other hand"?
And the "on the one hand" is really quite irrelevant.

That is indeed an important aspect: the heat pump must also be able to get rid of its heat during operation. At low flow temperature and a small flow/return temperature difference, the system must be designed for a volume flow adjusted to the heat pump model. All of this is feasible, but if the commissioned heating installer does not put in the effort, the bufferless system may not function as reliably in the worst case. This is the only point I wanted to make.

1000l is of course already extreme. Additionally, many heat pumps are only half as efficient at 45 degrees as at 35 degrees.
Now one can say that this naturally reduces cycling even better.
However, whether one wants to do this with double the electricity consumption I consider very questionable.

My example was obviously formulated in a misleading way. It was not about the specific temperatures in the example, but rather a somewhat illustrative example of the thermal energy needed to raise the temperature of 1000L of water by 10 degrees. Perhaps there was too little context. I think a further discussion on this point does not help here.

For a bufferless system, it would be important to me that the heat pump is modulating and under no circumstances also oversized at the same time. From my point of view, it can even be a size smaller than in the calculation, as long as it is still sufficient for hot water preparation (depends on the system).

In the usual heating demand calculation based on component values, no solar gains are accounted for, no heat emission from people and appliances in the house is considered, and the building time constant is not taken into account—that is, the heat storage capacity of the materials used in the house (solid construction, etc.).

And one must not forget that the calculation considers the stress-test case. In the worst case of noticeable undersizing, the house is not cold all year round but perhaps just a bit cooler in the living room for a few days every five years.

I took the trouble to calculate this as precisely as possible for a house with 475m²—I arrived at 9kW = sufficient sizing. 12kW were installed because the heating installer refused to install the smaller model. The actual annual values are very close to the values calculated back then and suggest that 9kW would have sufficed.

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

What kind of clowns are these? This is the factory customer service of a heat pump manufacturer and not a small independent heating engineer, right?

What kind of clowns are these. This is the factory service of a heat pump manufacturer and not a small independent heating engineer, right?

Germany distributor for the Spanish heat pump brand "Ecoforest"

And you are really still seriously considering repairing the can with all this drama?

And are you seriously still considering repairing the unit after all this drama?

Option 1: €4,800 service appointment by Energie: - Compressor replacement - Checking the heating for sources of errors (check dirt trap heating circuit/brine circuit, temperature sensor, bypass valve) - Optimizing hydraulic balancing (noticeably less heat reaches the upper floor than the ground floor when all individual room controllers are open) - Optimize configuration (limit maximum speed, possibly increase hysteresis?, possibly night setback?) - Increase pool heating time from 15 minutes per hour to 30 minutes per hour If the repaired and optimized heating system still cycles, then installation of a buffer tank for X,XXX EUR (€2,000?)

Option 2: Installation of a new brine heat pump from a major brand - 30% subsidy - Price XX,XXX EUR (€20,000?)

Currently, I do not yet have an offer for retrofitting a buffer tank.

Currently, the cheapest offer for installing a new brine heat pump is €34,000 (which would be €23,800 with subsidy).

If that remains the case, we will decide to repair.

If we receive an offer of €18,000 for a new heat pump (which would be €12,600 with subsidy), we will opt for a new heat pump.

We do not yet know exactly where the price threshold lies that separates the two options.

How would you decide? Considering the €4,800 repair (plus possibly costs for installing a buffer tank) and the frustrating circumstances with Ecoforest, how much would you be willing to spend on a new heat pump?

(1) After the appointment with Enrgi, the problem with the cycles is resolved. In the last 24 hours, there were only two cycles with a duration of 3:21 h and 2:07 h respectively

The following measures were taken:
- ERR all on (previously this was not sufficient to solve the problem)
- Inverter replaced
- Software update
- Power changed from 3-12 kW to 2.5-7.5 kW

(By the way, the bypass valve was completely closed.)

(2) After the software update, the options for time programs are greatly limited. I can only activate/deactivate the entire heat pump, but not specific operating modes (heating, cooling, hot water, pool).

The only type of time program results in spring/autumn through the heating limit. Contrary to my assumption, this does not refer to a 24-hour average, but to the current temperature. This means that in spring/autumn, heating only takes place at night when the temperature is <12°. If the temperature is >12° during the day, heating does not take place. I consider the limitation of heating times during the transitional period to be advantageous, but during the day the heat pump could be operated with self-generated solar power, at night I have to buy electricity.