ᐅ Buffer tank quickly loses temperature during domestic hot water use
Created on: 22 Dec 2025 16:40
B
Birdies
Hello everyone,
We have had a heat pump with a buffer tank "Citrin Solar Heat Pump-SKS 650-1 premium" for almost one and a half years now.
We have been observing the following issue and have been discussing it with the installer for quite some time, who has also involved the manufacturer Citrin. It feels like they are trying to shift the blame to each other, but so far we have not seen a solution.
Effect in WINTER, when the heating is running:
The lower part of the buffer tank is around 30-35°C (86-95°F). All radiators are working properly.
The domestic hot water (DHW) area at the top is heated to 50°C (122°F) starting at 3:30 pm to use photovoltaic electricity if available.
However, we can see and FEEL that the water cools down quite quickly again.
We lose about 15°C (27°F) in the DHW area within a few hours, even without any DHW consumption.
As a result, we have nicely solar-heated water around 3:30/4:00 pm, but by the time you want to shower in the evening, say around 10 pm, the hot water is no longer sufficient.
Not to mention filling a bathtub.
Together with the installer, a test was carried out where all supply and return lines and any other connections to the tank, circulation, solar, etc. were closed.
The DHW was preheated to temperature beforehand.
No DHW was drawn during the entire recording period!
(The three very thin orange bars are only markers, not DHW heating!)
For comparison, in SUMMER:
The entire tank is heated to 55°C (131°F) by the solar thermal system on the roof, both top and bottom.
Heat loss from evening (sunset) until the next midday, when the sun shines on the solar thermal panels again, is only about 3°C (5°F).
Even if there is no sun for a full day after that, the temperature loss of about 8°C (14°F) over TWO days is still reasonable.
My question:
Is such a drastic heat loss of the buffer tank normal and acceptable, or is there a fault here?
Would it make sense to use the tank only for DHW and connect the heating circuit directly to the heat pump? This was a suggestion already discussed with the installer. (My concerns are possibly an increased number of compressor starts, but the installer believes this would not happen because the system "modulates" and instead of on/off would run continuously at a lower required power.)
With this idea, there would be NO buffer in the heating circuit.
Viessmann Vitocal 250 A
The house currently has two heating circuits, which work well with the current settings:
Currently, the system is connected/set up as follows:
There is a meeting with the installer in early January, and until then I would like to understand whether the proposed solutions make sense, could cause issues, and so on.
They have already stated themselves that such a temperature loss is not normal and have reported it as a defect.
We have had a heat pump with a buffer tank "Citrin Solar Heat Pump-SKS 650-1 premium" for almost one and a half years now.
We have been observing the following issue and have been discussing it with the installer for quite some time, who has also involved the manufacturer Citrin. It feels like they are trying to shift the blame to each other, but so far we have not seen a solution.
Effect in WINTER, when the heating is running:
The lower part of the buffer tank is around 30-35°C (86-95°F). All radiators are working properly.
The domestic hot water (DHW) area at the top is heated to 50°C (122°F) starting at 3:30 pm to use photovoltaic electricity if available.
However, we can see and FEEL that the water cools down quite quickly again.
We lose about 15°C (27°F) in the DHW area within a few hours, even without any DHW consumption.
As a result, we have nicely solar-heated water around 3:30/4:00 pm, but by the time you want to shower in the evening, say around 10 pm, the hot water is no longer sufficient.
Not to mention filling a bathtub.
Together with the installer, a test was carried out where all supply and return lines and any other connections to the tank, circulation, solar, etc. were closed.
The DHW was preheated to temperature beforehand.
No DHW was drawn during the entire recording period!
(The three very thin orange bars are only markers, not DHW heating!)
For comparison, in SUMMER:
The entire tank is heated to 55°C (131°F) by the solar thermal system on the roof, both top and bottom.
Heat loss from evening (sunset) until the next midday, when the sun shines on the solar thermal panels again, is only about 3°C (5°F).
Even if there is no sun for a full day after that, the temperature loss of about 8°C (14°F) over TWO days is still reasonable.
My question:
Is such a drastic heat loss of the buffer tank normal and acceptable, or is there a fault here?
Would it make sense to use the tank only for DHW and connect the heating circuit directly to the heat pump? This was a suggestion already discussed with the installer. (My concerns are possibly an increased number of compressor starts, but the installer believes this would not happen because the system "modulates" and instead of on/off would run continuously at a lower required power.)
With this idea, there would be NO buffer in the heating circuit.
Viessmann Vitocal 250 A
The house currently has two heating circuits, which work well with the current settings:
- Underfloor heating in living/dining area ~40 sqm (heating curve level 4 slope 0.2)
- Radiators ~120 sqm (heating curve level 5 slope 0.4)
Currently, the system is connected/set up as follows:
There is a meeting with the installer in early January, and until then I would like to understand whether the proposed solutions make sense, could cause issues, and so on.
They have already stated themselves that such a temperature loss is not normal and have reported it as a defect.
J
Jesse Custer23 Dec 2025 08:26Ah, shame on me – thank you, Peter, reading educates.
I wasn’t at the heat pump SKS, I was only at the SKS.
Then I don’t understand it either.
I wasn’t at the heat pump SKS, I was only at the SKS.
Then I don’t understand it either.
J
Jesse Custer23 Dec 2025 08:30Birdies schrieb:
It has a metal sheet, which I would definitely check with a camera to see if it might be installed incorrectly or missing altogether.
The metal sheet is also shown in the diagram at the top of my first post. Based on the connection diagram from Post 1, I don’t understand how the solar thermal system produces the hot water... it only seems to feed the buffer tank for the heating system…
W
wiltshire23 Dec 2025 08:37To me (warning: non-expert), the connection initially appears correct and seems to follow the installation recommendations for this storage model.
Where is the sensor for the temperature you are reading located? If it is too close to the return line, it measures at a point where such a temperature drop is more likely to occur. In this case, your measurement would simply give a “false” impression of what is thermally happening inside the tank. An indication that the sensor position is suboptimal would be if the temperature always drops significantly when the heat pump is running.
I don’t believe it is an insulation issue; the temperature drop would not be that drastic.
Where is the sensor for the temperature you are reading located? If it is too close to the return line, it measures at a point where such a temperature drop is more likely to occur. In this case, your measurement would simply give a “false” impression of what is thermally happening inside the tank. An indication that the sensor position is suboptimal would be if the temperature always drops significantly when the heat pump is running.
I don’t believe it is an insulation issue; the temperature drop would not be that drastic.
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