Downstairs radiators cold?

We have got a problem with the downstair radiators being cold.
System is a standard boiler & tank, house is a two storey (the top story was added after the heating system however heating used to work ok)
Upstairs gets warm, downstairs doesnt. If the rads are turned off upstairs then downstairs gets heat.
british gas (three star cover) have been many time but cant solve it (fitted 3 new pump and powerflushed the system at great expense). They have also said that it cant be the boiler or boiler size?
Can anyone suggest what it can be?
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I would say that the radiators are just not balanced. This is shown by closing off the upstairs radiators. The problem is that the upstairs radiators are plumbed in closers to the boiler and have their lockshield valves too far open. This is giving the hot water a nice short, easy route back to the boiler. The radiators downstairs are likely to be the furthest from the boiler and therefore offer the highest resistance. What you need to do is to adjust all the radiators' lockshield valves so that the ones closest the boiler are only just open (1/4 turn on our ones!) and the furthest ones are fully open.
We had this problem and 20 for 2 clip on radiator thermometers and an hour or so and it was working fine after that. You can do it (roughly) without the thermometers if you want. Basically just close the lockshield (ie. not the one with a handle) valves on the upstairs radiators and then open them small amounts. Give them a little time to heat up and see if its acceptable. As I said, ours only required 1/4 turn from off, so don't think you need to give it much. Once you've started to get some sorted, you'll find the downstairs ones improving.
There's quite a few FAQ's out there on how to balance the radiators.
D
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radiators are full of sludge or the system is not balanced.
I would test for the latter first by turning off all upstairs radiators and see if the downstairs ones start to warm up. Even turn off all the downstairs ones bar one. When you get one working, feel along the bottom centre area of the radiator. If you find a triangular cold area then there is a good chance that there is sludging.
Balancing is easy to do but time consuming, which is why installers often don't do it and BG on a service contract are not likely to do much. There is info. in the group FAQ on balancing.
.andy
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Someone did come to "balace" the rads, but still has not helped (unless they didnt do it right!)

Yes they do.
Even turn off all the downstairs ones bar one. When you get one working,

Will check this.

Will have a fiddle then. cant make it much worse!
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