One cold radiator in a bungalow central heating system

[Baudinot]

Hi Guys. I'm a newbie, and need your help if you would be so kind. I have a 2 bed bungalow with 4 radiators and a towel rail. One day, after two years of untroubled use since I moved in, the lounge rad went cold. All the rads had Servowarm inlet taps on them. I tried bleeding at the top but nothing happened. Warm for a bit, then cold again. I took off the thermo tap and put an ordinary one on as I thought it might be U/S. Masses of muck in the system. Hosed out the rad in the garden. Reassembled. Result - rad scalding - then went cold again. Took off the pipe at the thermo side, as I thought there was an air lock, filled 4 buckets of water out of the inlet pipe, making sure not to empty the header tank, connected up the rad again. For a while, scalding - then cold again. Meantime, all the other rads are hot. Need to bleed the towel rail now and again though. The water is making rushing, bubbly noises through the system. Boiler is a Servowarm Elite 50. Another thing I have just noticed. The pipe for each radiator comes down from the ceiling, although the boiler and immersion tank is on ground floor. On the offending rad, the pipe that has the control valve side on (the one you don't adjust, although I have turned it off in the past to get the rad off, and it is wide open now, which was the way I thought they always were) is very hot. The other pipe which had the thermo on is stone cold. Isn't that back to front? One other thing that might be a clue. In the loft, there was a six inch bit of pipe sticking up, with a fatter bit on top, and a black knurled plastic knob the size of a tyre dust cap. Water was coming out of this cap, dripping, and it soaked the ceiling and ran into a light fitting, blowing a bulb. To stop it, I screwed down the black cap finger tight. It stopped the leak and everything dried out. Cap is still closed
So, what have I done wrong, lads, and how do I fix it? How do I get this one rad to work?

 

[croydoncorgi]

The black 'thing' in the loft with the screw-cap i an automatic air-vent - which you have now turned off so that no air can be vented from that part of the system.
From your desription, an air-lock would be favourite for your radiator problem too.
IF (and only if) the AAv has 'Brauckmann' written on the side, it is possible to rotate the valve body to the closed position then (carefully) remove the big screw top and clean the float and needle valve, which are probably fouled.

 

[ChrisRoberts]

Or the thermostatic valve could be stuck down.

 

[workbench]

It's a possibilty but there may be gas in the system (not the stuff that fires the boiler but caused by contaminated water). Give this a try - undo the bleed tap at the top of the cold radiator, get a match and when you hear hissing strike the match and hold to the bleed tap. If there is a little pop or a small flame then there's a build of 'gas'. The whole system will need drained down, flushed through with clean water and then refilled, adding inhibitor.

 

[Expertboy]

Had exactly the same problem as this years ago when i was training, the plumber drained down did the work then filled up again and 1 cold rad which died all the time.

Turned out to be a bit of dodgy pipework under the floor boards which swan necked badly and for some reason was causing an airlock to the rad, he cut out the bad pipework and it worked fine.

Not saying this is your problem though.

 

[Nige F]

There are certain principles in laying out C/H pipework that need to be followed when installing.......I loved Bungalows because usually there was room to lay it all out in the roof and drop to each rad.......but some installers liked to use the layout as an opportunity to get some bonus £££and get out quick. Now I don`t wish to cast doubt on the quality/accuracy of your installation....particularly as I can`t see it...but with the large amount of corrosion you mentioned, you might well consider having a Heating Engineer appraise the system and @ least change the auto vent for a manual one.

 

[Baudinot]

Hi Guys.
Took the advice, turned off all rads but that one, and ran the system. You have never heard so much bubbling. Anyway, now the rad works great, but I don't know where the air went. I'm still looking. Thanks for all your help. I'll get that vent changed as well.
Cheers to you all.