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Hot water tank losing temp overnight.

Air Source Heat Pumps (ASHPs)

Last Post by Old_Scientist 6 months ago

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Craig1980

(@craig1980)

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Joined: 3 years ago

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Topic starter 02/03/2025 11:24 am

Well, i haven't been here for a while as my Samsung Gen6 12kw ashp has been working away quite well.

Now, I seem to have a problem with my hot water.

We are between switching electricity suppliers so we are on a standard rate and to save some money we have turned the heating off as the weather is good and our south facing house heats up nicely through the day. With good solar we will put the hot water on to heat up to 49 degrees at around 14:00. But, since doing this, with the heating off the hot water is at 24 degrees by the morning. With the heating on, this will stay at close to the 49 degrees.

We have a 150l Joule tank. A salamander pump that pressurises the cold water flow in to the tank when we run the hot tap. This gives us better flow on the hot water from the tap and into the shower. This was the fitters solution to boosting water pressure as we have poor flow of water supply to the house.

We have two pressurised vessels above the tank but I don't believe we have a buffer tank.

Does anyone have any idea why it would do this?

Thanks in advance for your help.

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Old_Scientist

(@old_scientist)

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Posts: 239

02/03/2025 12:48 pm

My Samsung system does the same, and it's due to the Samsung Anti-Freeze protection feature.

When the OAT is below 5C (risk of freezing) AND the system has not run for 60mins, the circulation pump(s) will run for 5mins circulating water through the DHW circuit to ensure the system does not freeze. This behaviour cannot be prevented or changed - it's a design feature and non-configurable.

Because you have turned the heating off, once the outside temp drops below 5C overnight, every 60mins the circulation pumps will run for 5mins which will leach heat from the DHW tank, until the OAT rises above 5C again.

This is somewhat frustrating as, like me, you probably have gycol in the system or anti-freeze valves fitted to prevent freezing, but as I stated above unfortunately this behaviour is non-configurable.

You can fool the system into thinking the OAT is above 5C by artificially warming the temperature sensor in the black covering on the rear of the ASHP. I warm it in my hand for 30secs causing the temperature reading to go above 5C which resets the 60mins before the pumps will next run.

I don't have a decent solution for this issue.

This post was modified 6 months ago 2 times by Old_Scientist

Samsung 12kW gen6 ASHP with 50L volumiser and all new large radiators. 7.2kWp solar (south facing), Tesla PW3 (13.5kW)
Solar generation completely offsets ASHP usage annually. We no longer burn ~1600L of kerosene annually.

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Craig1980

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Topic starter 02/03/2025 1:38 pm

Ah, that's amazing. Thanks for the answer. At least I know something isn't faulty. That would also explain why it doesn't do it in the summer when the heating is off. Thanks again.

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Old_Scientist

(@old_scientist)

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02/03/2025 3:39 pm

Posted by: @craig1980

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Ah, that's amazing. Thanks for the answer. At least I know something isn't faulty. That would also explain why it doesn't do it in the summer when the heating is off. Thanks again.

In the summer you can see another problem, where the pumps will run after 24h of inactivity to prevent pump seizure. This can be a pain if you have a buffer or volumiser, and may want to go more than 24h between reheating the DHW (for example after a legionella cycle). Similarly, the pumps running effectively transfers heat out of the DHW tank and into the buffer or volumiser.

This is not an issue if you reheat your DHW on a schedule at the same time each day, or if you reheat multiple times per day on demand.

This post was modified 6 months ago by Old_Scientist

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