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Hot water then cold water then hot with conventional heating system

Posted: Sun Feb 25, 2018 7:57 am
by LeMarsu
Hi,

I see to have an odd problem, when running hot water if I turn off the tap for a few seconds and back on it comes out as cold water. The system is is a conventional boiler running with a hot water cylinder and no reservoir, it does have a external expansion vessel.
I was wondering could the cause be a loss in pressure in the external expansion vessel?

We have not had this problem since we had a plumber come in and tried to pump air in with a bicycle pump for some off reason.

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Thanks
LeMarsu