Dimplex SI8TU ground source heat pump - strange behavior

By "heat pump does not run in D" I meant, who on earth is cooling now when most have just turned OFF their heating about 4 weeks ago.

The temperature increase of the source is normal when idle.

Regarding your observations/ measurements. Something is off. If the heat pump runs for over 60 minutes, the source temperature (outlet) can never rise! With an ideal brine pump setting, the temperatures basically drop in sync. Delta approx. 3 K.

A warm water temperature increase of just 7 K in 60 minutes is not normal. It should be much faster. (Unless it might be about 1000 l).

Have you ever read the heating capacity of the heat pump? With how many kW does it "push" into the storage? (According to the datasheet and what you read!)

The gradient in the brine is also quite large, but the pump is slow as well. 130m is not insignificant, so choosing stage 1 (of 10?) as correct surprises me. My brine has a gradient of about 3K. Maybe it's worth trying to change something there. The gradient in hot water production is also considerable! How to explain that ... how do you manage 20K (!!) and still need an hour to heat the storage tank by just 7K? Strange.

I had the cooling on for 2 days as a test because we had never been able to test it since handover and then for the first time the outside temperatures were above the threshold. Not because we were sweating that much ;)

The high spread of the brine is due to the low setting of the brine circulation pump. With a higher setting, I can also get it below <1K. Only then the high-pressure fault occurs very early - that was precisely my initial question. Generally, the brine spread in heating mode has always been significantly(!) larger than in hot water mode. That was the reason back then to increase the pump setting - see the initial post.

I have now started another test run with log:

At start: hot water temp. 39.5°C
Hot water preparation runtime: 1:06h
At end: hot water temp. 48.3°C, source inlet 11.1°C, source outlet 8.3°C

Heat output according to heat quantity meter heat pump: 10kWh (only integer display, so possibly rounding errors)
Cooling circuit power according to heat quantity meter heat pump: 7kWh (only integer display, so possibly rounding errors)
Compressor power consumption after calibrated meter: 2.6kWh

I have always noticed that the temperature readings of the hot water initially drop briefly at the start of the heat preparation. Presumably, the sensor is not optimally positioned and there is mixing of the layers in the tank. The tank holds 300l. The heat pump has a 100l buffer; I don't know if it is also used during hot water preparation. The heating element is deactivated and disconnected via fuse. Circulation is installed (contrary to contract), but it is only switched on by motion control - it could very well have been activated during preparation, but it's not huge amounts of water either.

Functionally, everything seems to be running. The only question is, where do the 10kWh go?

I had the cooling on for 2 days as a test, because since handover we never had the chance to test it and for the first time the outside temperatures were above the limit. Not because we were sweating that much

Ah, ok. But it doesn't really make sense to "turn it on" just for 2 days.

The high spread of the brine is due to the low setting of the brine circulation pump. With a higher setting, I can push it below 1K as well. Only then the high pressure fault comes very early - that was my original question. In general, the brine spread in heating mode has always been significantly(!) larger than in domestic hot water mode. That was the reason back then to increase the pump speed - see the original post.

You don't have to push the spread to some specific values, but rather adjust the brine pump so that the system runs efficiently. And that is with a spread around 3 K!

At start: domestic hot water temp. 39.5°C
Duration of hot water preparation: 1:06h
At end: domestic hot water temp. 48.3°C, source inlet 11.1°C, source outlet 8.3°C

Source temperatures would be okay that way, but 66 minutes for a delta of 9 K? (Again, something doesn’t add up!)

I just looked at one hot water cycle of my VWS63/2, with the domestic hot water temperature just at 41°C. (Domestic hot water min 38°C, domestic hot water max 49°C)
Source inlet 10°C, outlet 7°C
Domestic hot water: actual 41 °C, flow 47 °C, return 43 °C.

I don’t know what your heat pump with 300 l storage and possibly 100 l storage is doing. Are both served via one mixer?

Ah, ok. But it doesn't really make sense to "turn it on" for 2 days.

I wanted to know if it even works. Unfortunately, the heating installer was our worst trade, botched wherever he put his hands. To this day, no hydraulic balancing has been done, not even the calculation exists. They built our heat pump for the first time and have no clue about the thing. That's also why there is a big distance between the heat pump and the storage tank. They just started building on the wall and then after 3 days realized that the pipe end for the heat pump comes out somewhere completely different... :oops:

You don't have to push the spread to any values, but adjust the brine pump so that the system runs efficiently. And that is at a spread of about 3 K!

I am aware of that. At the end of the cycle, 3K fits quite well. As already described, there are only 2 problems with that: In heating mode in winter, the spread is 8K, with hot water 2-3K. Therefore, in winter I set the brine pump to 5.5. That resulted in a spread of about 4K during heating operation, and 1.5-2K with hot water. And since we no longer heat and the brine temperature is higher, I had to throttle the brine pump, otherwise the high-pressure fault constantly occurs or the heat pump sets the maximum temperature to 42°C.

Are both operated by a mixer?

I can't say for sure. The buffer storage is internal, so I can't see the connection. Outside the heat pump, there is basically only direct piping and the changeover valves for heating, water, and cooling operation. But it is a confusing structure on our wall. However, the manufacturer's service found the setup okay. There are data points for mixers that are switched. But only with numerical designations without description (Mixer M21 open/close, Mixer M22 open/close, Mixer P_SK).

but 66 min for a delta of 9 K? (Again, something is not right!)

I fully agree with you! I just have no clue where to start?! The compressor runs, calls up its power and produces heat, COP is in the normal range. I find it hard to imagine a real error, because then two readings would have to be wrong at the same time (e.g., compressor power too low and heat meter shows wrong). I measure the power consumption with an external, calibrated meter. I could imagine that the hot water temperature sensor is not measuring correctly (external sensor was placed between storage tank and insulation shell) and that the water is actually much warmer. I just don't know how to measure that as close to the storage tank as possible. At the tap, the pipe losses are probably incalculably large.

In heating mode in winter, the temperature difference is 8K, for hot water it is 2-3K.

And that is not really comprehensible! The heat pump delivers 8 kW (or how much?) into hot water or underfloor heating, what does the brine care about that?