One Bad Rad

[halton1957]

One of the rads on our microbore open vented and pumped system has never really heated up properly.
It is slow to heat and the bottom stays just warm. My first thought was sludge so took the rad off and flushed, then balanced the system as well as possible - no improvement.
Next I changed the TRV as it was very hot and the rad still cool - no difference. New TRV also hot.
Yesterday I changed the rad for a new double one and rebalanced - still no improvement. TRV still hot. The lockshield operates freely and is in the fully open position.

Please help.

 

[TCCHeating]

I would be checking the microbore flow and return pipes to the rad for blockage. You have pretty much eliminated everything else

 

[halton1957]

Thanks for the prompt reply, unfortunately the microbore is under the carpet and floorboards fed from 22mm (I think) behind the plasterboard.
Hopefully the TRV being v hot would indicate a problem in the return? Possibly a kink in the microbore.

 

[Stadan20]

Have you tried shutting down all rads apart from this problem one ?

 

[halton1957]

Have you tried shutting down all rads apart from this problem one ?

Thanks, I'll try that, presumably the boiler will overheat if there's a blockage?

 

[Stadan20]

More than likely yes but by shutting down all rads bar your problematic one will force out an air lock ( if any ) also make sure your pump is on the highest speed

 

[halton1957]

More than likely yes but by shutting down all rads bar your problematic one will force out an air lock ( if any ) also make sure your pump is on the highest speed

Thanks I tried it but no luck. Boiler went through it's start, run, shut down on thermostat cycle. Let it do that 5 times but rad still lukewarm although pipe and TRV v hot. Lockshield end cold.
One other rad heated up, I guess TRV passing. Closed lockshield on that one to give max pressure at problem rad. Pump was set to max.

 

[TCCHeating]

It will be a blockage. You need to test the flow from each rad valve to see which pipe is the culprit. Search the faq for the bucket test

 

[halton1957]

It will be a blockage. You need to test the flow from each rad valve to see which pipe is the culprit. Search the faq for the bucket test

I know this is going to seem pretty stupid but I can't find the FAQs. I've found reference to bucket test being No 52. Perhaps I'm unusually thick this morning.

 

[lloyda]

Here.

 

[halton1957]

Here.

Thanks a lot!

 

[Tay7]

Have u tried seeing if there is flow from the lockshield with rad disconnected?

Could be lockshield, if not its a blockage.

 

[halton1957]

Have u tried seeing if there is flow from the lockshield with rad disconnected?

Could be lockshield, if not its a blockage.

Thanks for that thought, I haven't actually tried that as I drained down to change the rad and TRV.
The lockshield does operate freely over it's full range.
I'll check it out when I do the bucket test.

 

[alan333]

I had similar and paid attention when the Scottish Gas guy fixed it (after power flushing didn't cure it). I also have an open vented system with the header tank in the loft.

When the heating is switched off the water will flow through the pipes in either direction via gravity. So...
Turn the heating off. Turn both rad valves off. Drain the rad if you want, or catch the water which will slowly escape from the rad in a deep tray as you go (my guy did it this way with no mess/spill). Disconnect one valve from the rad and turn the valve on into a bucket. It only flows by gravity so there's no high pressure escape and it's easily stopped by putting your finger over the end. Do the same with the other valve and compare. The one with the slow flow (or no flow) is your culprit.

One of mine had virtually no flow, so to prove whether it was the valve or the pipe he removed the valve from the pipe...but still had no flow from the pipe. Therefore my problem was a pipe blockage.

To flush out the pipe he blasted air back up through it using a pressurised garden sprayer. First he turned off the water supply to the header tank and drained about half the water out so that the forthcoming big bubbles wouldn't make it overflow. He connected his sprayer to the rad pipe using rubber hose and a jubilee clip, pumped up the pressure in the sprayer and released it all in one go up the pipe. Loud bubbles could be heard as they travelled up the pipework and into the header tank. He then disconnected the sprayer and caught the dirty lumpy water coming back out in his tray. He did this a couple of times until the flow was good and the water clean, then re-fitted and connected the valve but didn't turn it back on. First he checked for debris in the header tank, then turned the header tank supply back on, then turned the rad valves back on and bled the rad. Bear in mind you can't let the header tank get empty, so check it regularly if you let a lot of water flow out your pipe as you go. He then checked and bled all rads, then turned the heating on and let everything heat up. He then left it on to settle for little while then checked and re-bled all the rads again. Everything now worked perfectly.

The garden sprayer he used was similar to this (but with the spray nozzle removed)...

http://www.screwfix.com/p/pressure-...nd Gardening&gclid=CK_ZufPnzsoCFdW4GwodGywM3g

 

[halton1957]

Alan, many thanks for taking the trouble to write such a detailed reply.
It sounds like a system that could work for me. All I can see of our pipework is 6" of 8mm feed and return emerging from under the floorboards which are fully carpeted. You can imagine that I'm reluctant to tear everything apart for access as it's only one inefficient rad of the 2 in the room.
I can see 22mm pipe from the CH pump disappearing into the wall of the boiler cupboard so I assume the 8mm is teed off at various points and the return is also 22mm.
I don't have a garden sprayer that I can adapt, but I do have the opposite - a vacuum pump designed to suck the oil from a car engine. It is quite powerful so hopefully will suck out a blockage. If not it's off to get a sprayer. The only problem I can forsee is that I can't hold back the suction and release it suddenly
I might even have an advantage using suction as the debris should be removed rather than blown back into the system.
Either way I'll report back here in a few days, thanks again for the suggestion.