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My system is hot water gravity fed and a simple pumped system for heating. I just changed the pump, but all the rads upstairs are cold. Downstairs is fine. None of the pipes for the rads upstairs is hot as far as I can tell. I have drained the system and refilled it and bled all the rads OK. Is it possible I have put the pump on the wrong way round ? The arrow on the pump points to the rads rather than the boiler and the pump is on max setting. It was a like for like change.

With the hot water being gravity fed, it's not like a fully pumped system and the heating should work with the pump fitted both ways. But it is best to be pumping away from the boiler (the way yours is fitted) Turn off the downstairs radiators and see if the upstairs heat up it just sounds like an airlock.

Try turning the pump speed up to III.

If no luck, you could try turning the pump round and see if it will push any air out of the system. But put the pump back pumping away from the boiler when finished.

I turned the pump round and it worked.