ᐅ Is a buffer tank useful for a heat pump?

What is the purpose of a stratified storage tank?
Why not connect directly to the underfloor heating? For hot water production, a 3-way valve then switches to the hot water storage tank, often combined with a domestic hot water station. Keep it simple!
These stratified storage tanks often cause problems. In practice, the high flow rates of a heat pump tend to disturb the nice thermal layers in the tank.
I’m afraid you received poor advice there.

To the TO.
How was the heat pump operated then? Were ERR active? Were all rooms heated? Any hydraulic short circuits? Before investing money in a thermal storage tank, a root cause analysis should be carried out first. A heating load of 8 kW seems a bit high to me as well.

I roughly calculated it: heat load for 170 sqm (1830 sq ft), 5 people, hot water in Mainz (-12°C (10°F)) is 5.1 kW without ventilation; with ventilation, it will be even less. You probably have a combo unit where everything is integrated, meaning pump, hot water, and storage tank. Hmm. Heat pump seems oversized. Hot water storage tank: just barely sufficient, to put it mildly. How did they come up with 8 kW? From the energy saving regulation proof (EnEV / energy performance certificate)? Is the situation that bad, or is there still a chance to fix it?

Thank you very much for the responses!

Here is some additional information:

- Data
Energy standard: almost KfW-55
Annual primary energy demand Qp according to energy saving regulations: 34.5 kWh/(m²a)
Specific heat transfer loss H’T: 0.305 W/(m² K)
Heated building volume: 920 m³ (32,480 ft³)
Living area: 220 m² (2,370 ft²)
Heated living area: 170 m² (1,830 ft²)
Controlled ventilation with heat recovery is installed.

- The individual room thermostats in the entire upper floor and the guest bathroom are fully open to prevent the heating from running at full power while the valves limit everything. On the ground floor, the valves are set to 22°C (72°F) because otherwise it gets too warm downstairs and too cold upstairs.

- 7–8 kW was the estimate from an uninvolved expert based on the energy certificate and related calculations.

- Neighbors with a similar house have the same heating system but with five times fewer starts (12,000 starts vs. 60,000 starts over 7 years).

Can installing a buffer tank resolve this problem? If so, which type would you recommend? Would a buffer tank have been advisable regardless? Or is a buffer tank not useful at all for a modulating heat pump combined with underfloor heating?

Or are there other factors that could be causing the high number of starts that should be checked? Control valves in the manifold? Bypass valve?

Alternatively, instead of spending about €5,000 on replacing the compressor, would it be better to install a new, better-sized heat pump? What would the cost be (around €9,000–13,000 plus installation)? What lead time should be expected? (That is, how long might we be without hot water for showers?)

The heating system is now defective. Most likely, the compressor has failed due to 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 also declined because they cannot replace the compressor. Ultimately, a technician from the German Ecoforest office in Wangen (400 km (250 miles) away) will come on Monday to replace the compressor for about €5,000.

In the meantime, I received a list of possible causes of failure from a heat pump specialist. I have asked the company coming on Monday to check the following points:

- Check and clean the heating circuit strainer if necessary
- Check and clean the brine circuit strainer if necessary (hot gas monitoring)
- Check the temperature sensors (flow and return temperature)
- Check the bypass valve (possibly too wide open?)
- Optimize speed control to start at lower speeds; possibly limit speed (the heating demand of the house is 7-8 kW, the heat pump modulates up to 12 kW nominally, but possibly higher in practice?)
- Optimize hydraulic balancing (noticeably less heat arrives on the upper floor than on the ground floor, even when all individual room controllers are open)

If an error is found this way, we could avoid installing the buffer tank. A properly set, modulating brine heat pump with underfloor heating should actually work without a buffer tank. Two neighbors with the same builder and identical Ecoforest system manage perfectly without one. If the problem persists, we will install the buffer tank recommended by Ecoforest to solve the issue, hoping it will then fix the problem. Considering the currently very small hot water storage tank (165 liters (44 gallons) for 5 people), I would choose a combi tank ("with fresh water station") that has an internal domestic hot water coil (no risk of legionella!).

I will report on Monday or Tuesday with an update on how things went.

jx7 schrieb:

Or instead of spending €5,000 on replacing the compressor, would it be better to get a new, better-sized heat pump?

If you can afford it, I would seriously consider that. Otherwise, you’re likely to face the same problem again in a few years. On the other hand, I’m surprised by the difference between your house and the neighbors’. If they are similar, there should still be some possibilities for improvement.

jx7 schrieb:

On the ground floor, the valves are set to 22°C (72°F) because otherwise it gets too warm downstairs and too cold upstairs.

Then you also reduce the flow on the ground floor.