Cold radiators...

System as is follows: - 17 radiators of various sizes - approx half upstairs, half downstairs. - 1 year old boiler - Worcester Bosch Greenstar 34CDi Classic (a combi)
Problem is that 2 radiators upstairs are cold...
When the boiler was installed I had them both replaced, single panel for a double inc replacing the TRVs, as they had always been much colder than the rest. Both seem to work fine initially, but over the last year have gradually started to get colder and colder (when the system is on) until they are now totally cold.
Not a clue what is wrong, but these are the things I observed:
- Original system (inc. radiators) is plumbed using 15mm copper. - We had an extension built several years ago, and several radiators were added. For those, the pipes running into/from the new radiators (i.e. last ~0.5m) are flexi 10mm pipes (the lazy bu**ers couldn't be bothered with soldering straight joins...). - In the case of the two cold radiators, the 10mm pipes attaching them to the 15mm pipes are much longer - approx. final 2-3m. - When system is on. the feed into one of the cold radiators is warmish, outlet is cold. For the other radiator both pipes are cold.
Any idea what is going on here?
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On Wednesday, February 26, 2014 1:34:18 PM UTC, JoeJoe wrote:

Do they need bleeding?
Jonathan
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Jonathan brought next idea :

Bleeding and/or air-locked.
Turn all of the other radiators off, set heating to max and bleed any air out from the two radiators.
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Regards,
Harry (M1BYT) (L)
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On Wed, 26 Feb 2014 05:52:24 -0800 (PST), Jonathan wrote:

cold.

Even if virtually full of gas there still ought to be flow through the bottom, so the pipes should be hot even if the majority of the radiator is cold.
I'd go for a "blockage". This could be stuck TRV's (common), disintegrated washer in a valve or possibly a build up of crud in the pipe work/valves.
I'd start with bleeding, if that doesn't help move to the TRVs and make sure they are open with the control heads off if that doesn't help move to excercising all the valves to fully open first then off repeat a few times and retrun to the orginal setting, this with the system on.
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Cheers
Dave.
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On 26/02/2014 14:25, Dave Liquorice wrote:

Agreed. And *when* you bleed, do each side of each rad separately - i.e. bleed with one valve open and one closed, and then the other way round. That way, you'll make sure that both pipes are bled and - in the event that no water comes out of the bleed valve - you'll know which side the blockage is!
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Cheers,
Roger
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Roger Mills wrote:

    Good point.
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Roger Mills explained on 26/02/2014 :

I've never though of that before - good idea!
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Harry (M1BYT) (L)
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On 26/02/2014 13:34, JoeJoe wrote:

It sounds like the system has never been properly balanced. It could simply be a case that the new rads just don't get any flow since the rest of the system poses a much lower flow resistance.
Have a read of:
http://www.diyfaq.org.uk/plumbing/rad-balance.html
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John.
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On 26/02/2014 16:40, John Rumm wrote:

Agreed, but the change over time does suggest a developing blockage.
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On 26/02/2014 20:54, newshound wrote:

or the weather is getting warmer, and hence the TRV is shutting sooner...
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John.
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It might simply be trapped air (worth checking) or it might be stuck TRV valve actuator pins. You can check this quite easily by removing the TRV heads to reveal the actuator pin which should be in the fully raised position.
You can easily check this by using a pair of pliars to push the pins down and check whether or not they will return back to fully raised immediately the down force is removed.
They may only move a millimetre or so before 'bottoming' which is indicative of a sticking actuator pin in which case those pliars come into their own for pulling the pin to the raised position (but don't get too carried away with the applied lifting force!).
If this is the case, they usually free off after a few cycles of push pull activity and you should find the radiators will warm up quite rapidly unless, of course, they are in need of bleeding as well (not uncommon with top floor radiators).
I once experienced a similar problem with our top floor radiators after refilling this gravity pressurised system due to air locks in the floor distribution manifolds which, afaicr, required me to set the pump to its max setting and shut all the other rads off using their TVR valves to force the trapped air 'round the bend' and into the top floor rads where it could then be bled from the system.
I think I left it running this way for 20 or 30 minute periods between each bleeding attempt before there was no more sign of air accumulation. Once the system had been fully primed, I opened the TRVs on the other rads and knocked the pump speed down a notch.
Now it's just a matter of checking for air in the top floor rads by the expedient of checking that they're equally hot from top to bottom. Opening the bleed valve as necesary is all that is required. Since I can't recall the last time I had to bleed these rads, trapped air is not a problem.
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Regards, J B Good

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On 26/02/2014 13:34, JoeJoe wrote:

If you turn off all the other radiators do they both stay cold?
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On 26/02/2014 13:34, JoeJoe wrote:

- Closed all the radiators apart from the two offending ones. - Turned boiler on. - Waited until the started to warm up, and bled them both. One had no sign of air, but the other (that was always completely cold) let out some air (with water) from one of the panels. - Opened all the other radiators around the house, and so far at least, everything works fine, including the two radiators that have been all but dead for the best part of a year.
Many thanks for all the very useful advice - much appreciated! ... and I definitely learnt a thing or two.
PS: Will check to make sure the pressure is still OK once the radiators cool down.
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On 27/02/2014 11:50, JoeJoe wrote:

Was celebrating too early... :-(
Checked the pressure of the system once the radiators were cold, and it was still OK.
However, tried to run the heating for a little while again to double-check that the two radiators were still working (still couldn't believe that they came back to life after such a long while...), and they are already noticeably colder than all the others. If I remember correctly, then this is how they gradually stopped heating up last year.
What does the learned panel think? Time to try to balance the system next? I already ordered an IR thermometer - looks like a nice toy if nothing else...
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