Since our photovoltaic system, despite its east/west/southwest orientation, regularly operates at a 70% limitation daily, I am currently exploring ways to utilize these excess peaks. In doing so, I came across some unusual behavior with our heat pump, for which I have no explanation so far.
The installed unit is a Dimplex SI8TU with a deep borehole. Commissioning was carried out by the manufacturer’s service team. The brine circulation pump can be set in 10 levels (directly on the pump). During installation, level 1 was selected. As a result, during winter this regularly led to brine temperature differences of more than 9K (Kelvin). The domestic hot water temperature was set at 50°C (122°F).
As part of efficiency optimization and elimination of various faults (hydraulic balancing, etc.), the domestic hot water temperature was gradually lowered and the brine pump was increased until the temperature difference was about 3-4K (Kelvin), last set at level 5.5. This significantly improved the COP. Meanwhile, the manufacturer’s service team visited for other reasons and inspected the system. They pointed out that the brine pump must be set to level 10 because the heat pump can only operate efficiently with maximum flow. The more flow, the better. The heat pump is now in summer mode, and at times the cooling function was active, causing brine temperatures to rise significantly. Now the heat pump goes into high-pressure error mode already at a domestic hot water temperature of 43°C (109°F). As a test, I set the brine pump back to level 1 today and the heat pump promptly delivered hot water at 49°C (120°F) until I turned it off myself.
I always thought that the higher the brine temperature, the lower the required temperature lift and, therefore, the better the heat pump’s performance. This is also how the data sheets present it. However, the manual contains the confusing sentence: “The lower the heat source temperature (e.g., outside temperature, brine temperature), the higher the achievable domestic hot water temperature.” I initially assumed this was a typo or translation error, but it exactly describes the observed behavior. Could this be related to the refrigerant’s evaporation temperatures or how can this be explained? If it is really like this, then the only solution may be to manually adjust the brine pump during both winter and summer.
The installed unit is a Dimplex SI8TU with a deep borehole. Commissioning was carried out by the manufacturer’s service team. The brine circulation pump can be set in 10 levels (directly on the pump). During installation, level 1 was selected. As a result, during winter this regularly led to brine temperature differences of more than 9K (Kelvin). The domestic hot water temperature was set at 50°C (122°F).
As part of efficiency optimization and elimination of various faults (hydraulic balancing, etc.), the domestic hot water temperature was gradually lowered and the brine pump was increased until the temperature difference was about 3-4K (Kelvin), last set at level 5.5. This significantly improved the COP. Meanwhile, the manufacturer’s service team visited for other reasons and inspected the system. They pointed out that the brine pump must be set to level 10 because the heat pump can only operate efficiently with maximum flow. The more flow, the better. The heat pump is now in summer mode, and at times the cooling function was active, causing brine temperatures to rise significantly. Now the heat pump goes into high-pressure error mode already at a domestic hot water temperature of 43°C (109°F). As a test, I set the brine pump back to level 1 today and the heat pump promptly delivered hot water at 49°C (120°F) until I turned it off myself.
I always thought that the higher the brine temperature, the lower the required temperature lift and, therefore, the better the heat pump’s performance. This is also how the data sheets present it. However, the manual contains the confusing sentence: “The lower the heat source temperature (e.g., outside temperature, brine temperature), the higher the achievable domestic hot water temperature.” I initially assumed this was a typo or translation error, but it exactly describes the observed behavior. Could this be related to the refrigerant’s evaporation temperatures or how can this be explained? If it is really like this, then the only solution may be to manually adjust the brine pump during both winter and summer.
By saying "heat pump is not running in Germany," I meant—who is cooling already when most people only turned their heating OFF about 4 weeks ago?
The temperature rise of the source is normal when the system is idle.
Regarding your observations/measurements: something is definitely off. If the heat pump runs for more than 60 minutes, the source temperature (outlet) can never increase! With an ideal brine pump setting, the temperatures decrease almost in sync. Delta approximately 3°C (5°F).
A 7 K (13°F) increase in domestic hot water temperature in 60 minutes is not normal. It should happen much faster (unless the volume is possibly around 1000 liters (260 gallons)).
Have you checked the heat output of the heat pump? How many kW is it “pushing” into the tank? (According to the data sheet and what you have observed!)
The temperature rise of the source is normal when the system is idle.
Regarding your observations/measurements: something is definitely off. If the heat pump runs for more than 60 minutes, the source temperature (outlet) can never increase! With an ideal brine pump setting, the temperatures decrease almost in sync. Delta approximately 3°C (5°F).
A 7 K (13°F) increase in domestic hot water temperature in 60 minutes is not normal. It should happen much faster (unless the volume is possibly around 1000 liters (260 gallons)).
Have you checked the heat output of the heat pump? How many kW is it “pushing” into the tank? (According to the data sheet and what you have observed!)
The temperature difference in the brine circuit is quite large, but the pump is also slow. 130 m (425 ft) is no small distance, so I'm surprised that stage 1 (out of 10?) was selected as correct.
My brine temperature difference is about 3 K (5.4°F).
Maybe it’s worth trying to make some changes there.
The temperature difference during hot water production is also significant! How to explain this... how can it reach 20 K (36°F) and still take an hour to raise the tank temperature by only 7 K (13°F)? Strange.
My brine temperature difference is about 3 K (5.4°F).
Maybe it’s worth trying to make some changes there.
The temperature difference during hot water production is also significant! How to explain this... how can it reach 20 K (36°F) and still take an hour to raise the tank temperature by only 7 K (13°F)? Strange.
I ran the cooling system for 2 days as a test because we had never been able to test it since handover, and only then did the outside temperatures exceed the threshold. Not because we were sweating that much 😉
The high temperature difference of the brine loop is due to the low setting of the brine circulation pump. With a higher setting, I can reduce it to less than 1K (1.8°F). However, this causes the high-pressure fault to occur very early – which was my original question. In general, the brine temperature difference during heating operation has always been significantly(!) higher than during domestic hot water operation. That was the reason for increasing the pump speed back then – see the initial post.
I have now started another test run with logging:
At start: Domestic hot water temperature 39.5°C (103°F)
Domestic hot water preparation duration: 1:06 h
At end: Domestic hot water temperature 48.3°C (119°F), brine inlet 11.1°C (52°F), brine outlet 8.3°C (47°F)
Heat output according to the heat meter of the heat pump: 10 kWh (only whole number displayed, so possible rounding error)
Cooling circuit output according to the heat meter of the heat pump: 7 kWh (only whole number displayed, so possible rounding error)
Compressor electricity consumption according to calibrated meter: 2.6 kWh
I have always noticed that the temperature readings of the domestic hot water briefly drop at the beginning of the heating process. The sensor is probably not optimally positioned, and some mixing of the layers in the storage tank occurs. The tank holds 300 liters (79 gallons). The heat pump has a 100-liter (26 gallons) buffer tank; I am not sure if it is used during domestic hot water preparation. The electric heating element is deactivated and disconnected by a fuse. A circulation pump is installed (against contract terms) but is only activated based on movement detection – it might have switched on briefly during heating, but it does not circulate large volumes of water.
Functionally everything seems to run properly. The only question is: where are the 10 kWh going?
The high temperature difference of the brine loop is due to the low setting of the brine circulation pump. With a higher setting, I can reduce it to less than 1K (1.8°F). However, this causes the high-pressure fault to occur very early – which was my original question. In general, the brine temperature difference during heating operation has always been significantly(!) higher than during domestic hot water operation. That was the reason for increasing the pump speed back then – see the initial post.
I have now started another test run with logging:
At start: Domestic hot water temperature 39.5°C (103°F)
Domestic hot water preparation duration: 1:06 h
At end: Domestic hot water temperature 48.3°C (119°F), brine inlet 11.1°C (52°F), brine outlet 8.3°C (47°F)
Heat output according to the heat meter of the heat pump: 10 kWh (only whole number displayed, so possible rounding error)
Cooling circuit output according to the heat meter of the heat pump: 7 kWh (only whole number displayed, so possible rounding error)
Compressor electricity consumption according to calibrated meter: 2.6 kWh
I have always noticed that the temperature readings of the domestic hot water briefly drop at the beginning of the heating process. The sensor is probably not optimally positioned, and some mixing of the layers in the storage tank occurs. The tank holds 300 liters (79 gallons). The heat pump has a 100-liter (26 gallons) buffer tank; I am not sure if it is used during domestic hot water preparation. The electric heating element is deactivated and disconnected by a fuse. A circulation pump is installed (against contract terms) but is only activated based on movement detection – it might have switched on briefly during heating, but it does not circulate large volumes of water.
Functionally everything seems to run properly. The only question is: where are the 10 kWh going?
Fuchur schrieb:
I tested the cooling for 2 days because we never had a chance to test it since handover, and finally the outside temperatures were above the limit. Not because we were really sweating. Ah, okay. But it doesn’t really make sense to turn it on just for 2 days.
Fuchur schrieb:
The large temperature difference in the brine circuit is due to the low setting of the brine circulation pump. With a higher setting, I can get it below 1K. But then the high-pressure fault occurs very early—that was my original question. In general, the brine temperature difference has always been significantly larger in heating mode than in domestic hot water mode. That’s why I increased the pump speed back then—see the first post. You don’t have to reduce the temperature difference to some arbitrary value; instead, adjust the brine pump so that the system runs efficiently. And that’s typically with a temperature difference around 3 K!
Fuchur schrieb:
At start: domestic hot water temperature 39.5°C
Hot water production runtime: 1:06 h
At end: domestic hot water temperature 48.3°C, source inlet 11.1°C, source outlet 8.3°C The source temperatures seem acceptable, but 66 minutes for a temperature rise of 9 K? (Again, that doesn’t quite add up!)
I just looked at a single hot water cycle of my VWS63/2 when the domestic hot water was around 41°C (minimum 38°C, maximum 49°C).
Source inlet 10°C, outlet 7°C
Domestic hot water: actual 41°C, supply 47°C, return 43°C.
I’m not sure what your heat pump is doing with a 300-liter (79-gallon) tank and possibly an additional 100-liter (26-gallon) tank. Are both served via a mixing valve?
driver55 schrieb:
Ah, ok. But it doesn’t really make sense to "turn it on" just for 2 days. I just wanted to check if it even works. Unfortunately, the heating contractor was our worst trade, with shoddy work wherever they touched. To this day, no hydraulic balancing has been done, not even a calculation exists. They installed our heat pump for the first time and had no idea about the system. That’s also why there is such a large distance between the heat pump and the storage tank. They simply started mounting it on the wall, and only after 3 days did they realize that the piping outlet for the heat pump ends up somewhere completely different... 😳
driver55 schrieb:
You don’t need to force the temperature difference to any specific values; instead, set the brine pump so that the system runs efficiently. And that’s usually with a temperature difference around 3 K! I’m aware of that. Near the end of the cycle, 3 K fits quite well. But as I mentioned, there are two problems: during heating operation in winter, the temperature difference is about 8 K, and for domestic hot water, it’s 2–3 K. That’s why I set the brine pump to 5.5 in winter. Then, during heating, the temperature difference was about 4 K, and for hot water around 1.5–2 K. Since we stopped heating and the brine temperature is higher, I’ve had to throttle the brine pump, otherwise there’s constant high pressure fault or the heat pump limits the maximum temperature to 42°C (108°F).
driver55 schrieb:
Are both controlled via a mixing valve? I can’t say for sure. The buffer tank is internal, so I can’t see the connections. Outside the heat pump, there’s basically only the direct piping and the changeover valves for heating, hot water, and cooling operation. But it’s a confusing setup on our wall. The manufacturer’s service technician thought the installation was okay. There are data points for mixing valves that can be switched, though only with numeric labels and no descriptions (Mixing valve M21 open/close, Mixing valve M22 open/close, Mixing valve P_SK).
driver55 schrieb:
But 66 minutes for a delta of 9 K? (Again, something doesn’t add up!) I totally agree with you! I just have no clue where to start troubleshooting. The compressor runs, pulls its power, and produces heat; the COP is within the normal range. I find it hard to imagine a real fault, because that would mean two readings would have to be wrong at the same time (for example, the compressor output is too low and the heat meter is reading incorrectly). I measure electricity consumption with an external, calibrated meter.
I suspect the hot water temperature sensor might not be measuring correctly (the external sensor was placed between the storage tank and the insulation), and the water is actually much hotter. I just don’t know how to measure it as close to the tank as possible. Measuring at the tap would have unpredictable heat losses in the pipes.
Fuchur schrieb:
During heating operation in winter, the temperature difference (delta T) is 8°C (14°F), and for hot water it is 2-3°C (4-5°F). And that is not really understandable! The heat pump delivers 8 kW (or how much?) to the domestic hot water or underfloor heating—why would the brine (ground loop) care about that?
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