One cold rad... dodgy pipework

[Daddy Boots]

Old CH system and always had problems with one rad not getting hot. Flushed the system today and did a sort of bucket test on both ends of the pipes feeding the suspect rad... no problems. Flushed rad, a bit of gunge.Normal valve one end, TRV the other ok.
Put it all back together and left only suspect rad valves open and pump on highest speed setting.... doesn't heat up. Opened the valves on the other 3 rads in the downstairs circuit one at a time and they are scorchio. Some head scratching. Closed the return valve on the rad and with the TRV fully open I opened the bleed valve... seemed to have full pressure ( gravity fed) and after bleeding 1/2 a bowl full I was getting warm water and the pipe below the trv was hot....hmmmm. Opened the return valve and it still doesn't get hot.
So I go into the basement to inspect the flow/return layout. The hottest pipe I reckon is the flow from a temp test on the pipes on the other rads.
The suspect rad is fed off a tee . One other rad on the same downstairs circuit is fed off a tee and that gets hot ok. What seems to be the problem is that the tee does not come off horizontally... it is about 25degrees below the horizontal so the pipe drops about 5 inches before turning vertical to go into the TRV.
......................TRV
.......................¦
T-._.................¦
.......-.._...........¦
..............-..._...¦
.....................-.¦ not to scale !

I can't imagine why the installer dropped the pipe rather than go horizontal. As I have flushed there shouldn't be any gunge in the below horizontal bit of the pipe to stop flow. The pipe at the tee is hot but drops to cold just a few inches along the 15inch run, so the water is clearly not flowing downhill
Can anyone confirm that the water would not like flowing downhill and thats the reason for the rad not getting hot or have I got a weird TRV ? The bleed test would suggest the TRV is letting water flow.

 

[Hugh Jaleak]

May be air locked in the downward loop. Would try turning ALL the rads off bar this and it should force water through shifting any trapped air. Opening bleed screw may not allow enough water through to shift air lock. Possible TRV is on wrong way round, some are reversible, if not arrow will indicate direction of flow.

 

[GodKnows]

Alot of TRV's after awhile cease to function properly, some where the pin sticks in the main body of the TRV, often after when heating off for a spell! You need to free the pin and gently tap the body fella, the flow through the rad won't happen if the TRV isn't open. Yes it can be full of water, but, no flow

 

[garagerob]

you should contact the Vatican as water not flowing down hill can be classed as a miracle!!

 

[Agile]

The Bucket Test requires that you take most of a bucket from EACH of the rad valves in turn.

Did your "sort of" test do that and was the flow rate about 6-10 litres per minute minimum?

Tony

 

[namsag]

Can you trace both pipes from the rad. not by any chance teed into the same pipe

 

[david17uk]

Sounds to me like one of the pipes to the rad could be blocked. E.g. the return pipe.


How to test:

Turn off all the valves in the house (including the cold rad). Then drain the cold rad.

You now should have all the valves shut off and the cold rad fully empty. Now open the bleed vent on the cold one (nothing should come out so leave it open).

Next turn on one valve of the cold rad (e.g. the trv) see if you get anything through, if you do shut it again. Now do the same on the other valve (lockshield).

You might find that one of the valves is now letting nothing through and this one is your blocked pipe. If so, you have 2 options - you could try a powerflush to unblock it, which is expensive. Or you could try and find the blockage in the pipework.

Hope this helps you.

David.

 

[Agile]

I cannot recommend that because it may not give meaningful results.

Only the Bucket Test as described int eh FAQ on this site will confirm that both feed pipes are free flowing.

It will not identify if the pipework is incorrectly connected but that would have been a problem from the installation date.

Tony

 

[david17uk]

I haven't seen this "bucket test" has anyone got a link please

 

[Daddy Boots]

Bucket test.. //www.diynot.com/forums/viewtopic.php?p=1140570#1140570
Number 52 in the FAQ .
I'm sorted... looks like a visit to the Pope 'cos the water wasn't flowing downhill
I discovered both flow and return dropped 5 inches from their Tee's to miss an old gas pipe... presumably in use when the rads were installed. Knack**ed a cutting blade on my angle grinder 'cos it was very awkward to cut. Put in two straight compression joints on the run from the Tee's and was able to take around 4inches off the top of the pipes to the valves. Refitted the valves, filled the system and now the rads scorchio from top to bottom