Dimplex SI8TU ground source heat pump - strange behavior

Since our photovoltaic system, despite O/W/W orientation, is throttled to 70% daily, I am currently dealing with how to capture these peaks. In the process, I stumbled upon a strange behavior of our heat pump for which I have no explanation so far.

Installed is a Dimplex SI8TU with deep drilling. Commissioning was carried out by the factory service. The performance of the brine circulation pump can be adjusted in steps 1-10 (on the pump itself). During installation, step 1 was set. As a result, this regularly led to temperature spreads of the brine of >9K in winter. The hot water ran at 50°C.

As part of efficiency optimization and elimination of various defects (hydraulic balancing etc.), the hot water temperature was slowly lowered and the brine pump was increased until the spread was about 3-4K, most recently to step 5.5. The COP has increased significantly as a result. Meanwhile, the factory service was here for another matter and looked at the system. He pointed out that the brine pump must be set to step 10, because the heat pump can only work efficiently that way. The more flow, the better. Meanwhile, the heat pump is in summer operation and the cooling function was temporarily on, causing the brine temperatures to rise significantly. Now the heat pump already goes into high-pressure fault at a hot water temperature of 43°C. As a test, I set the brine pump back to step 1 today and promptly the heat pump delivered hot water at 49°C until I switched it off myself.

I always thought the higher the brine temperature, the lower the necessary temperature increase and thus the more powerful the heat pump becomes? That’s what the data sheets show as well. The manual contains the (nonsense) sentence: "The lower the heat source temperature (e.g., outside temperature, brine temperature) the higher the achievable hot water temperature." I had so far considered this a typo/translation error, but it exactly describes the observed behavior. Is this related to the evaporation temperatures of the refrigerant or how can this be explained? If it really is as it is, then the only help is to manually adjust the brine pump in winter and summer.

High pressure faults are usually due to a lack of heat removal. Some closed valve? Buffer overheating?

Now the heat pump already goes into high pressure fault at a hot water temperature of 43°C.

How high is the supply temperature in this case? I assume significantly higher than the storage temperature, the return temperature is quite close to the supply temperature?

The heat pump cannot remove the heat, how large is the heat exchanger in your hot water storage?
Possibly there is a filter somewhere that is clogging up.

In the meantime, the heat pump is in summer operation and the cooling function has been on at times, so the brine temperatures have risen significantly.

But the heat pump is not running in Germany.

And "brine temperatures have risen significantly"? Never!

What brine temperatures prevail currently after 10 minutes of heat pump operation?

High pressure problems are usually due to lack of heat removal. Any closed valve? Buffer tank boiling off?

Yes, I am aware of that. The first measure was also to turn the domestic hot water circulation pump up to the maximum. Otherwise, it goes directly into the storage tank (300L) from there. The technician also told me that >50°C is not possible due to "losses at the heat exchangers and the distance heat pump-storage [approx. 2-3m]".

How high is the flow temperature? I assume it is significantly higher than the storage temperature, and the return temperature is quite close to the flow temperature?

I would have to check the exact values, but yes, from memory the flow is somewhat higher.

The heat pump can’t remove the heat, how big is the heat exchanger in your hot water storage tank?
Possibly there is a filter somewhere that is clogging.

According to the data sheet 3.5m², Dimplex WWSP335. I don’t see any serviceable filter units anywhere.

But the heat pump is then not operating in D.

And "brine temperatures significantly increased"? Never!

What brine temperature do you currently have after 10 minutes of heat pump operation?

I can only read the values that are displayed to me. At the end of the heating season, the source input was about 4-5°C after 1 hour of operation, currently it is 9.x°C. It is a borehole with 130m depth.

What is the supply temperature in this case? I assume it is significantly higher than the storage temperature, and the return temperature is quite close to the supply temperature?

And "brine temperatures have risen significantly"? Never!

What brine temperatures are currently present after 10 minutes of heat pump operation?

So, for today's 2 hot water preparations, I had the data logged. The first cycle was during the "shower phase" in the morning, hence the fluctuations in water temperature with a target value of 42°C. The second cycle was triggered at noon by photovoltaic surplus, target temperature 48°C. Hot water circulation pump at full power, brine circulation pump stage 1.0 of 10.0.

Why the brine temperature is so high again after such a short time, I don’t know. We live on a mountain, drilling into bare rock, groundwater extremely unlikely. But you can clearly see that the source return temperature rises, indicating the temperature difference decreases as the water temperature rises.