Downstairs Rads Cold


I have recently installed some new rads downstairs and changed one upstairs. Instead of using microbore as the previous person had I installed 15mm. The one upstairs worked brilliantly and still does. The two downstairs don't seem to be flowing. I have made sure that all the valves are open but still no joy. They are all bled. I have this evening closed all the upstairs valves and now no water is flowing through the system any of the rad pipes, I have traced it all the way back to the cylinder!!
I really don't know whats the matter, I am thinking maybe the pump as it was allowed to run dry for a couple of hours by accident one evening!!! Only think is i can hear the water flowing so I am not so sure it is the pump!!!
Any ideas?
Thanks
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If the upstairs rad is getting hot then presumably the pump is still OK. I suspect that as you have installed some new rads, replaced the microbore with 15mm and opened all the lockshield valves, the system is now unbalanced. See http://www.diyfaq.org.uk/plumbing/rad-balance.html . But that shouldn't stop the downstairs rads getting hot with all the upstairs rads turned off, so there could be more wrong than just being unbalanced due to the position of the lockshield valves. My understanding is that each rad should be on a separate circuit. It should be possible for a rad to heat up when all the others are turned off, and all the rads should be able to do that. The rads nearest the pump usually get hot first, so if the pump is upstairs then the upstairs rads should heat up first. Is the one upstairs rad that does get hot the rad nearest the pump? What happens when you turn all the rads off then open each rad one at a time to check if each rad gets any hot water?
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