1rad goes cold

[pandaman]

I know this problem has been asked before and I have looked at alot of replies but can I pick someones better knowledge than mine. I have open vent system which has been running fine for some years now. It had a power flush about 7 yrs ago and have not had to bleed a radiator since, before that I was bleeding 2 or 3 upstairs rads every week. There are 10 rads or 11 if you count a double in the main lounge in a 3 bed house. The system had an inhibitor top up last year. Yesterday the rad in the kitchen and one the other side of the wall went cold. I turned off all the other rads, no trvs on them, and the heat returned, great I thought, later though the one in the downstairs toilet went cool, repeated procedure as before, rad got hot, opened all valves again later on just before retiring it had gone cool again. Heating off overnight, after system had heated up this morning the problem rad got some heat but not as hot as all the others. Later it went cool again, did the valve closing on all the others and the rad got hot again, I could hear a tinckly rattly sound, ?air, for a few minutes but stopped after 5 mins. Turned all rads back on but it has now gone cool again.
Why has it suddenly decided to do this, if airlock why does rad get hot then go cool again when system fully open? Have tried bleed valve but no air in, when hot it is hot all over. Any advice would be welcome.

 

[terryplumb]

Sounds like the system isn't balanced .Rads have two valves ,one for user to operate ,the other is the lockshield ,which is set on each rad to a specific position,in order to balance the system. Once you start altering them the system is no longer balanced.

 

[pandaman]

Yes but as I have not fiddled with any valves why should it suddenly start doing this after all this time? Thanks for your time.

 

[terryplumb]

Yes but as I have not fiddled with any valves why should it suddenly start doing this after all this time? Thanks for your time.

Your rads don't have TRV's ,could you have altered a lockshield inadvertantly ?

 

[pandaman]

Definitely not touched any lockshield valves before this, about half an hour ago I opened the lockshield valve fully to see if any difference. Rad is barely warm while others are hotter. Does this point to an air lock? I opened the drain valve next to lockshield valve, water came out clear after a few drops of black stuff. Short of taking rad off and flushing it is there anything else I can try?

 

[terryplumb]

If the cold rads heat fully ,when other rads are turned off they can't be air locked. Your system needs balancing.

 

[pandaman]

I do not doubt your expertise but why should it suddenly need this after all this time?

 

[terryplumb]

Some fundamental change ,that I am unaware of. Pump not as effecient ,pump speed changed , MPV not fully opening to CH port ,new radiator fitted somewhere ,lockshield valve/s adjusted ,partial blockage to flow or return Pipework on colder rads .
Water follows the path of least resistance when circulator moves it around the system, it has an easier path around those radiators that heat fully. You need to restrict those rads by closing ( partially) their lockshields ,and force some of the flow through those that are cold.

 

[pandaman]

Ok thanks for that, I will try that and see how it goes. Thanks again

 

[pandaman]

Just opening the lockshield valve fully seems to have cured it, so in a way I suppose it was a balancing problem even though I did not have to adjust any of the others. Thanks again.