One cold radiator

[bodman]

fully pumped gas fired central heating system (S-type) in a two-storey house.

six radiators downstairs, 5 upstairs.

Problem:
One radiator downstairs doesn't heat up properly, all others are fine.
This radiator gets warm at the top, stays cold at the bottom. The input pipe (bottom right) is very hot to the touch, the output pipe (bottom left) is cold.
This is the furthest radiator from the boiler (downstairs) & pump (upstairs).

Attempts so far:
Assuming the radiator was sludged up, I removed it outside & flushed it through with hot water, top to bottom, bottom to top, all ways up, and got no sludge to speak of.

Whilst the radiator was disconnected, I opened the input valve and got a good pressure of very hot water. I opened the output valve and got a good pressure of cold water, which after a few second also got very hot.

Is it therefore safe to assume no airlock in return pipework / manifold ?

Refitting the radiator, problem still persists.

I'm now thinking that there may be too much back-pressure on the return valve, preventing proper circulation through the radiator. Does this seem likely ?

I'm planning to close all the other return valves except this one, to try to force a circulation through this radiator - is that a safe thing to do ?

Any advice or better ideas would be most welcome.

 

[JPC]

yes..for a short while..

if u think there is no sludge in the rad. if u state it was cold at the bottom and hot at the top and u got no sludge out of it .....it might still be sludge up ?

 

[bodman]

if u think there is no sludge in the rad. if u state it was cold at the bottom and hot at the top and u got no sludge out of it .....it might still be sludge up ?

Fairly sure it's not sludge - water was gushing from the return outlet when I had the garden hose connected to the feed side, so water flows easily through the radiator when offline.

Last night I closed the return valves on all the other radiators except this one - after about an hour the radiator was much hotter, but the return pipe was still cold, so I guess there must be an airlock in the return pipe from this radiator ?

But wait - I was trying to 'force' a return from this radiator, but forgot about the bypass circuit - would that have scuppered by cunning plan ?

 

[outinthestix]

Have you balanced the rads ?

 

[bodman]

Have you balanced the rads ?

Yes, had to balance them after having all the return valves shut.

 

[jobloggs]

Bodman

Your latest suggestion about an air lock in return pipework does not square up with what you said previously about getting good pressure of hot water from both valves when the radiator was removed.

Also how can the rad be hot unless you are getting flow through it? Return pipe should be about 10 degrees cooler than flow pipe into radiator but that will still be hot to the touch. Should not take an hour to heat up one radiator.

If you have an automatic bypass valve, note reading and screw it down a bit. Then run system with all radiators shut off except the one in question and see what happens.

 

[bodman]

Your latest suggestion about an air lock in return pipework does not square up with what you said previously about getting good pressure of hot water from both valves when the radiator was removed.

Yes, I know. I'm clutching at straws here. It's quite a long pipe run, and I wondered if I hadn't left it flowing long enough to clear any airlock in the manifold, or maybe even introduced a new airlock when I had the radiator off.

Also how can the rad be hot unless you are getting flow through it?

I can't answer that, either. Convection, maybe ? The radiator is hot at the top, all the way along, getting cooler towards the bottom, and the bottom inch or so is barely warm. The return pipe is cool to the touch, the feed pipe is too hot to hold.

If you have an automatic bypass valve, note reading and screw it down a bit.

Will check tonight. btw, I did remember to shut off the DHW at the controller, but completely forgot about the bypass.

Many Thanks for your help with this.