Cold radiator on downstairs loop

8 Apr 2005
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United Kingdom
I have had to empty and refill the CH system. It is gravity fed. The downstairs radiators are each on a loop fed from upstairs. The radiators are full of water (i.e. if I open the bleed valve water comes out) but do not heat up fro several days. Then suddenly they become part of the system and heat up.

I presume that, although they fill with water from upstairs, an airbubble must be stopping circulation. Any ideas how to shift it quickly?

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Could you double check that the CH is gravity fed? Might you be mistaken, in that the DHW circuit (i.e. to/from the cylinder) is on gravity, but the CH is pumped?

Anyway, if you have intermittently cold radiators then you may well have sludge in the system. Try a search on "radiator cold sludge" and see what happens. If you can't find anything helpful then come back here :)
You are quite correct, there is a pump in the system.

However, it is the new radiator that is cold so I don't think it is sludge.

What new radiator? You seem to have left great chunks out of your description of what's been happening.

Have you seen this?
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Yes I had but I got interrupted halfway through writing and didn't read my post properly before I posted it.

Gas boiler, pumped CH, 15mm pipes.

I replaced a radiator downstairs yesterday and so had to drain the system (the old radiator was fine it was just in the wrong place). The original feed pipes to it were used but shortened as the new radiator position is closer to where the feed pipes drop from the ceiling.

As always when the system is drained the downstairs radiators are cold but full of water, including the new one. The water ran freely out of the feed pipes to it (I had not quite turned the header tank feed off) so I don't think they are blocked.

I am sure the system will right itself over the next few days.

If there's no blockage in the cold feed, and the rads are full of water, and the pump is running, then it should be easy...

Turn off all rads except one of the non-working ones. Get it going, then work your way through all the others.

If that doesn't work, it's likely you have an airlock somewhere, but that bridge can be crossed if and when you come to it...

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