Is a buffer tank useful in a heat pump?

Why a stratified storage tank? Why not directly into the underfloor heating. For hot water production, a 3-way valve then switches towards the hot water storage tank, preferably also with a fresh water station. Keep it simple! Such a stratified storage tank often causes problems. The high volume flows of a heat pump too often disturb the nice thermal layers in the tank in practice. I’m afraid you were poorly advised there.

To the TO. How was the heat pump operated then? Were ERR active? Were all rooms heated? Hydraulic short circuits? Before investing money in a heating buffer, a cause investigation should be carried out first. 8Kw heating load also seems a bit high to me.

I roughly calculated it: Heating load for 170 sqm 5 people hot water Mainz (-12) 5.1 kW without ventilation, with ventilation it is even less. You probably have a combo tower where everything is inside, i.e. pump-hot water-storage. Hmm. Heat pump oversized Hot water storage: tight, to put it mildly. How did the 8 kW come about? From the Energy Saving Regulation certificate? Is the child that far down the well or can it still be saved?

Thank you for the answers!

Here is more information:

- Data
Energy standard: almost KfW-55
Annual primary energy demand Qp according to energy saving ordinance: 34.5 kWh/(m2a)
Specific transmission heat loss H'T: 0.305 W/(m2 K)
Heated building volume 920 m^3
Living area: 220 m^2
Heated living area: 170 m^2
Controlled residential ventilation with heat recovery is present.

- The individual room thermostats are fully opened on the entire upper floor and in the guest WC, so that the heating does not run at full power, but the valves slow everything down. On the ground floor, the valves are set to 22 ° because otherwise it gets too warm downstairs and too cold upstairs.

- 7-8 kW was the statement of an uninvolved expert based on the energy certificate and the calculations.

- In neighbors with a similar house, the identical heating system works with 5 times fewer starts (12,000 starts vs. 60,000 starts in 7 years).

Can the installation of a buffer tank save the situation? Which one would be recommended then? Would a buffer tank have been recommended anyway? Or is a buffer tank not useful at all with a modulating heat pump and underfloor heating?

Or are there other things that could be responsible for the too many starts, which could be checked? Control valves in the manifold? Bypass valve?

Or instead of a €5,000 compressor replacement, would it be better to get a new, better dimensioned heat pump? What would that cost (9,000–13,000 euros plus installation)? What delivery time would have to be expected? (So how long would we not be able to shower?)

The heating system is now defective. Presumably, the compressor has given up due to far too many starts.

Unfortunately, the installing company Brunnenbau Heth from Lorsch refuses to carry out the repair, citing a shortage of skilled workers. Two other Ecoforest partners in the area have also declined because they are unable to replace the compressor. Ultimately, someone from the German branch of Ecoforest in Wangen (400 km away) will come on Monday to replace the compressor for around €5,000.

In the meantime, I have received a list of possible causes of error from a heat pump specialist. I have written to the company coming on Monday asking them to please check the following points:

- Check and, if necessary, clean the dirt trap in the heating circuit
- Check and, if necessary, clean the dirt trap in the brine circuit (hot gas monitoring)
- Check temperature sensors (flow temperature and return temperature)
- Check bypass valve (possibly too far open?)
- Optimize speed control so that lower speeds are reached, possibly speed limitation (heating demand of the house is 7-8 kW, heat pump modulates nominally up to 12 kW, in practice even higher?)
- Optimize hydraulic balancing (noticeably less heat reaches the upper floor than the ground floor when all individual room controllers are open)

If an error is found this way, we would save the buffer tank. A correctly adjusted, modulating brine heat pump with underfloor heating should actually also work without a buffer tank; with two neighbors with the same construction company and identical Ecoforest heating, it works perfectly well. If the problem persists, we will install the buffer tank recommended by Ecoforest to solve the problem, hoping that it will then resolve the issue. Considering the currently very small hot water tank (165 liters for 5 people), I would then opt for a combination tank ("with fresh water station") that has an internal hot water coil (no risk of legionella!).

I will report on Monday or Tuesday how it went.

Or instead of replacing the compressor for €5,000, a new, better dimensioned heat pump?

If you can afford it, I would actually consider it. Otherwise, you will have the same problem again in a few years. On the other hand, the discrepancy between your house and the neighbors surprises me. If it is similar, there must still be some possibilities for improvement.

On the ground floor, the valves are set to 22 ° because otherwise it gets too warm downstairs and too cold upstairs.

Then you reduce the flow rate on the ground floor.