Cold radiators

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Ipswich
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United Kingdom
Hope someone can help as we have exhausted every avenue (and most plumbers) in our area.
In June this year we had 2 new chrome towel rails fitted to our oil fired central heating system; one worked fine, the other one did not. The plumber who fitted them drained the system down but it would not refill as the header tank valves were scaled up. He replaced these plus another valve to vent the central heating pump and the heating started to work again (with the exception of the second towel rail and the radiator in the lounge - which I gather is the closest to the boiler). He promised to come back but did not. I then contacted another plumber who suggested that we flush the system out completely as there may be sludge and air in the radiators. As he was going to drain down the system I asked him to replace 2 of the "older" style radiators while he was at it. He did this and then put in Sentinel 200, flushed the system through and then put in an inhibiter. After 3 or 4 hours of going back and forth from boiler to radiators throughout the house he managed to get all of them working - albeit that 2 of them (the towel rail and the one in the lounge) were barely warm while the others were red hot. He said the system needed balancing and came back another day. Again, after another few hours he managed to get them all working but still the same 2 were barely warm. He has turned up the thermostat to 30 degrees, the boiler is on maximum and the pump is on its highest speed. The main problem is that although the radiators work whilst the plumber is there (and while it is switched on) the following day only one or two radiators are working properly - the heat just seems to fade away! This latest plumber says he has done everything possible and that he cannot understand why the system is not working properly. Now only 2 or 3 radiators out of 9 are hot, the remainder are stone cold. We have now contacted another plumber who has suggested replacing the valves on the radiators that don't work (although one of the rads, the towel rail, has brand new valves) and also putting in a new pump. Honestly, at this rate, we will end up with a completely new central heating system - just waiting for someone to suggest we replace our boiler aswell. Does anyone have any suggestions at all before I go ahead and replace the pump? :(
 
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Try turning all off the rads off apart from one of those that doesnt work. All the power from the pump should then push the water solely through the open rad. This should push out any blockage of air.
If this trick doesnt work then you will probably have one or more physical blockages that will require some further investigation.
I did have a similar job to this last week and found that someone had fully opened the bypass valve. Therefore make sure the bypass valve if fitted is in the closed position when you try this. When all the rads are working again it should be opened just slightly or in accordance with any instructions that came with the boiler.

EDIT

Now only 2 or 3 radiators out of 9 are hot
Just had a re-read and heres some more thoughts

1. It could indeed indicate the demise of your pump

2. There could be air getting in to the system This is usually from one of the following sources.
a] Sucking in through the vent. Try running the pump at a lower speed. Fitting a de-aerator is a guaranteed way of eliminating this. Temporarily fitting a stop end on the vent pipe will help you prove if this is happening. Dont be tempted to leave it on for more than a few days as a restriction in the feed pipe potentially creates a pressure bomb which will at the least cause damage to your system.
b] Aeration of water that is pumping over and getting drawn back into the system through the header tank. Again fit a de-aerator
c]Air getting in through the pump. Not a common occurence but I have come accross this a few times. It is often an old Grundfoss pump that is the culprit. If you have a grundfoss pump then this is a feasible suspect.
An easy way to check is to isolate the pump and take the head off it. Inspect the impeller chamber. A black magnetite inner and rusty red outer will prove that this is the source of aeration
d] Air getting in through a 'leak'. If water can leak out when the system is cold then air can get in when the system is running and the 'leak' is in the pumps suction zone. Have a good look about for visible leaks.
 
Go with slugs suggestion, especially 2) a;

Go into the loft with a jar of water and place this over the open vent and then get someone to turn the heating on. If the water is sucked from the jar then there is your problem.
 
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Thank you so much everyone for your suggestions - I am going to get the plumber back next week. Will keep you informed of success or otherwise. Fingers crossed!
 

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