When does the return get hotter than flow?

29 Dec 2018
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Pontypridd, Wales
United Kingdom
Just refilled my system and a few rads will not come on. I'm assuming I have an airlock somewhere but I just wanted to clarify something.

On a cold radiator, I use a drain off valve or bleed key to take out a reasonable amount of water. I then find one of the pipes getting hot, however, it is the return pipe. Does this make sense? In the back of my mind, I'm now starting to worry as I have a few column radiators where the flow has to be on one side. These radiators previously worked so I am getting a bit confused, but if there is a lock somewhere on the feed side, then the only place for hot water to come from should be the return, right?
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you are over thinking this, turn all the rads off, turn the heating on and only open 1 rad at a time, once that rad heats up , close it then open another until it heats up then close it and so on , once you have done this with all the rads , then open any that you are using and you should be fine, you might need to bleed some as what you are effectively doing is moving any air into the rads that can then be bled
Try bleeding from one side at a time; close the return and bleed from the flow, then close flow, open the return and bleed again. You'll soon find your airlock.
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I was over thinking it @ianmcd. After a few hours the rest of the rads started to come on. The fly in the ointment was the new rad I put on does not get warm, but I think that is because I was too liberal with the LS-X on the 10mm to 15mm reducer. The first lot of reducers leaked and the second lot I went a bit nut on. I think on the next drain down I am just going to fit proper 10mm TRV valves and be done with it.

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