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.
High-pressure faults usually occur due to insufficient heat removal. Is there a closed valve somewhere? Is the buffer tank overheating?
What is the flow temperature in this case? I assume it is significantly higher than the storage temperature, and the return temperature is quite close to the flow temperature?
Fuchur schrieb:
By now, the heat pump already goes into high-pressure fault at a hot water temperature of 43°C (109°F).
What is the flow temperature in this case? I assume it is significantly higher than the storage temperature, and the return temperature is quite close to the flow temperature?
T
T_im_Norden31 May 2021 06:41The heat pump is not able to transfer the heat efficiently. How large is the heat exchanger in your hot water storage tank? There might be a clogged filter somewhere.
Fuchur schrieb:
By now, the heat pump is running in summer mode, and occasionally the cooling function was on, causing the brine temperatures to rise significantly. However, the heat pump does not operate like that in Germany.
And "brine temperatures rose significantly"? Never!
What brine temperatures are you seeing right now after 10 minutes of heat pump operation?
guckuck2 schrieb:
High-pressure issues usually occur due to a lack of heat extraction. Any closed valves? Is the buffer tank overheating? Yes, I am aware of that. The first step was to turn up the domestic hot water circulation pump to its maximum. Otherwise, it goes directly into the storage tank (300L (79 gallons)). The installer also told me that temperatures above 50°C (122°F) are not achieved because of "losses in the heat exchangers and the distance between the heat pump and the storage tank [about 2-3m (6.5-10 feet)]."
guckuck2 schrieb:
What is the supply temperature in that case? I assume it is significantly higher than the storage temperature, and the return temperature is quite close to the supply temperature? I would need to check the exact values, but from memory, the supply temperature is somewhat higher.
T_im_Norden schrieb:
The heat pump isn’t able to remove the heat; how large is the heat exchanger in your domestic hot water storage? Possibly there is a filter somewhere that is getting clogged. According to the datasheet, it’s 3.5m² (38 sq ft), Dimplex WWSP335. I don’t see any accessible filter units for maintenance.
driver55 schrieb:
Then the heat pump wouldn’t be operating in heating mode.
And “brine temperatures clearly increased”? Never!
What are the current brine temperatures after 10 minutes of heat pump operation? I can only read the values shown to me. At the end of the heating season, the source inlet temperature after 1 hour of operation was about 4-5°C (39-41°F), currently it is 9.x°C (48.x°F). It is a borehole with a depth of 130m (427 feet).
guckuck2 schrieb:
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?driver55 schrieb:
And “brine temperatures have risen significantly”? Never!
What are the brine temperatures currently after 10 minutes of heat pump operation?So, I had the data logged for today’s two domestic hot water productions. The first cycle was during the morning “shower phase,” which explains the fluctuations in water temperature with a target of 42°C (108°F). The second cycle was triggered at midday by surplus photovoltaic energy, with a target temperature of 48°C (118°F). The domestic hot water circulation pump was running at full power, and the brine circulation pump was set to stage 1.0 out of 10.0.I don’t know why the brine temperature rises so much again after such a short time. We live on a hill where solid rock was drilled through; groundwater is extremely unlikely. It is also clearly visible that the source return temperature rises, meaning the temperature difference decreases as the water temperature increases.
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