Hot water making the central heating system gurgle

5 Feb 2024
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United Kingdom
Condensing Boiler downstairs, and hot water tank upstairs, with an external pump and expansion vessel. Closed S type system. 11 radiators in the house
The heating is on a timer for a few hours in the morning and a few hours in the evening during the week and all day at the weekends
The hot water is on a timer for an hour once a day.

If i turn off the hot water programme and use the immersion (electrical) hot water heating instead, so i am just running the heating from the boiler, then everything is fine - the system ran for a month like this very quietly with no issues.

However when i turn the hot water schedule back on, after a few days the central heating becomes very noisy with lots of air gurgling around the system.

I can resolve this by turning the pump on its fastest setting for 15 mins, and all the air gets pushed into the radiators, and the system become quiet again. But if i leave the hot water programme on, after a few days becomes noisy again.

I've tried leaving the hot water on for a few weeks in the hope it would settle down but it hasn't.

No idea what the cause is, or what to do other than leave the hot water off and use the immersion heater the whole time

Any thoughts and ideas welcomed
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