Weird Boiler/Heating issue

8 Oct 2021
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I've a Potterton promax 24 Combi Boiler which is about 4 years old. We turned the heating on for the first time this week since before summer. It's behaving really weird in that I can only get either the upstairs or downstairs to work but not both.

There are termostats both up and downstairs. If I want to get the upstairs rads to heat I need to turn the heat on downstairs first, then turn upstairs on and downstairs off. When I just turn upstairs on the boiler doesn't kick in and the rads don't heat. Likewise once upstairs is actually on if I turn downstairs on the rads downstairs don't heat up.

I did notice the boiler pressure was pretty low (just below 1) so I released the valve to increase this to nearer 2 but that made no difference. I've tried bleeding the rads etc. but that hasn't helped.

When the rads are on they heat perfectly so I don't think it's an air lock or anything like that.

Any ideas?

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Most likely a zone valve fault, you'll need to find them and see if they're opening correctly, one will be for upstairs, one down. They may be stuck closed.
I've located one of them. When I try to force the lever from one side to the other i can pull it but as soon as I release it then it springs back to it's original position

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