hi ,my central heating system seems to make a lot of noise when it starts up followed by the sound of occasional dripping/trickling waterthere isn't a leak as have checked all radiators and after a while no sound at all only when it switches .have found downstairs ones working fine but upstairs the tops are cold/warm and bottoms hot.I think it maybe trapped air but I don't know how to bleed the radiators as the radiators seem to only have the valve at the bottom and a nut at the top.We had a plumber in a couple of months ago to fix some taps so think this started around then when he switched the water off.we have a main boiler in the spare room and a combi boiler in the kitchen extension if that helps.
Any help would be much appreciated as this is my first time dealing with anything like this
ps it doesn't seem to make as much noise in the evening when it starts up
A cold top and hot bottom is typical of trapped air. The bleed valve is usually at one end, and is in the form of a small nut about 1/8" square. You need a special key to fit. Sometimes the valve is at the back, and covered witha little plastic cap. See the DIY project section on how to bleed rads.
This problem has two potential causes, the first is fairly common and easily remedied by radiator bleeding to release trapped air as Martin has suggested, The second possibility is far more serious and potentially dangerous. The noise suggests air in the pump and the trickling noise could be a partially empty system. It's remotely possible that plumber could have isolated the supply to the header tank, this would prevent the system from filling when required. Check the level in the header tank, if there is water in the tank push the float valve down to ensure that the supply is present. If all is well in the header tank proceed with the bleeding, start with the upstairs rad that is furthest from the pump and work backwards. Go round and round the system every day for a week or so as the trapped air will find it's way into the rads.
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