central heating problem

Hi All I was so impressed with the help everyone is offering, I thought Id ask if anyone had any ideas about whats the problem with mine.. Standard non-sealed sytem with Ideal classic boiler, Grundfoss 15-20 pump, Honeywell roomstat running 10 rads.
We had a problem with sludge and got a plumber to pump it round and the system has now been fully drained and refilled, and bled of air. However, last week only the upstairs rads were working, and after watching the system for a while we found the boiler only lights for about 60 seconds then switches off. Ive checked the pump is spinning ok, so Im stuck for ideas?
Could it be the impeller blades in the pump, or should I be looking at the roomstat..(can I isolate it to see if thats the problem? Any ideas gratefully received. Many thanks Maskie
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In an earlier contribution to this discussion,

We need to know a lot more about the design and layout of your system.
Is it fully pumped, or gravity hot water and pumped central heating? [Does the pump run when just heating the HW, or only when the CH is on?]
If fully pumped, it's likely that you've got one or more motorised valves. How many, and what type (2-port or 3-port)?
How is the central heating distributed? Do the main pipes go between the floors, with pipes coming down from above to each downstairs rad - or do you have separate upstairs and downstairs circuits with just 2 largish (22mm or 28mm) pipes going upstairs?
It will be much easier to suggest the likely cause if you answer the above questions.
--
Cheers,
Roger
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Roger Thanks for the help. Its gravity HW and pumped CH - pump only runs when heating is on. Problem is, I cant find any motorized valves........duh.....Ive looked along every piece of the pipes run from boiler to loft and no joy. AS far as pipework is concerned, there is a separate run going upstairs....the 2 largish pipes you suggested, going upstairs. Many thanks again Maskie
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In an earlier contribution to this discussion,

If it's gravity HW, there almost certainly won't be any motorised valves.
My best guess is that you have a problem with the pump. The shaft may be turning, but perhaps the impellor isn't.
Is the domestic hot water getting hot ok?
When the central heating is divided into upstairs and downstairs circuits, just joined by 2 vertical pipes, it's not uncommon for the upstairs rads to get hot by gravity (convection) circulation, without the pump running - but the downstairs rads won't get hot.
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Cheers,
Roger
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