Central heating flow pipe very hot but return pipe stone cold

diesel_doglet

Hi All,

I hope someone can help and I appreciate you looking. Got a problem with my CH that has got progressively worse over the past 4 months or so.

I live in a small flat with 6 rads and an old Marathon boiler (Pumper CH and Gravity HW).

Issues initially started a few months ago when the flat would take ages to get warm. It appeared that the boiler was running for a short while and then cut out as it appeared to overheat. The pump started making clanging noises so I bought a new pump to replace it. We replaced the head only and that seemed to resolve the issue for a time however steadily the issue has come back.

If I turn the CH on, the boiler fires up as does the pump. Five of the Six radiators get warm but not red hot. One (kitchen) does not even get warm. The pump occasionally clangs and bangs and the boiler cuts in and out more frequently than usual, even though the rooms are not at temperature.

The odd thing I have noticed is that the return pipes on all of the rads remains stone cold. I have checked the return pipe to the boiler and that is also stone cold - never gets warm.

I have turned off all the rads bar one of the good ones and although that rad will get hot along with the flow pipe, the return is still stone cold. I have tried this on all the five rads that get warm and all do the same.

I have tried this with just the rad in the kitchen on and this does not even get warm, the boiler just runs for a short while before cutting out.

I am certain that the boiler is cutting out because of a lack of circulation and hence it is reaching max temperature and then cutting out until that temperature recedes.

I am however not sure where to start with this now. I would think that if there is a blockage in the return pipe of one of the rads, the others would still function OK and the return pipe where it goes back into the boiler would eventually get warm but it does not.

I can only imagine that perhaps there is a blockage in the last 7-8ft of 22mm return pipe before it gets to the boiler and before the first Tee (the first tee is the return from bedroom rad)?

Someone I know has suggested that perhaps the pump is faulty. Although it is making a knocking noise from time to time, heat is getting to five of the six rads to a point if left on log enough, the rads do get pretty hot as do their flow pipes, but no head on the return pipes, they always stay bone cold.

Incidentally the hot water (gravity fed with cylinder) seems to be unaffected. I was thinking of getting some X800 into the system and/or replacing the whole pump but any advice would be greatly received as I am not 100% where to start.

Some photos which may help: https://ibb.co/album/co54WF

Thank you very much in advance.

DD

FreeBear

Check the two way valve is operating as it should.
Replacing the pump (not just the head) is also worth doing if it is banging & making other strange noises.

Side note - Make sure those cables from the valve & boiler (?) are well away from any hot pipes. Whilst they may be heat resistant cables, they don't want to be making contact with hot pipes.

Alfrescodave

You've probably checked this out but is it possible that air is trapped in the system preventing water circulation. Isolating individual radiators (as you have done) should shift it but might be worth continuing with just the kitchen radiator.

Obviously you've bled the radiators

coffeehound

Perhaps undo the brass centre cap on the pump and see if it is spinning?

panam

This reminds me of an issue I had. The boiler would fire for 40 secs then go out, then relight for 40 secs etc. The feed from the boiler was very hot, the return stayed cold.
The plumber was going to do a complete diagnosis for a suspect system blockage, but he first took off the front cover to the boiler... and the system stayed on. From that he deduced that it was a problem with restricted air intake (with the front cover off, it was free to draw in air from the room, rather than through the intake pipe).
A long overdue service, with particular attention to the air intake (mine is routed through the middle of the exhaust flue) cured the problem and it has been working happily for months now.

Myser

Rather than X800, run X400 through the system for a few weeks. It holds the sludge in suspension. X800 is generally for use with power flush systems.

southcoastrgi

For a start you haven't got pumped heating & gravity hw, you have a fully pumped system, what happens if you run just the hot water if it's ok then I'd look at the pump or the heating circuit, looking at the center screw on the pump shows you have a dirty system,

Just as a side note what is connected to that silver 4" pipe at the back of your pic ?

diesel_doglet

Hi All,
Thank you everyone for all your advice.  On the advice of Myser I have got some X400 and have ran that through the system since Friday and have also (to the best of my ability) re balanced all rads and now they are all getting hot.  The pump noise is still there but is less than it was.  I have a new pump on the way to replace it and plan to drain the system this weekend, replace the pump and then refill the system adding X100 to the mix.
Thanks again for all your help and I will keep you posted.
DD

markin

Is the return pipe warm now?

gary_99

Sorry to hijack the thread. But I have a similar problem . Iv just had a new boiler fitted.  On a gravity feed s plan system. All pumps and valves are less than 2 years old.
When hot water is on all is fine. Same if hot water and central heating.  All is good.
The issue is if just central heating is on. Boiler starts and pipes start to get hot. After a while you can hear what sounds like boiling water in the pipes in the airing cupboard.  touching the pipes they are red hot apart from the boiler return that seems cool. The boiler then cuts out, guessing getting to warm. Main pump and valve stays open. 
It appears to me the radiator return is getting reclyed back into pump rather than returning to the boiler. If I open hot water valve,  it appears to force water back to boiler as the return gets hot .
Do hope iv made sense 

neilmcl

gary_99 said:

Sorry to hijack the thread. But I have a similar problem . Iv just had a new boiler fitted.  On a gravity feed s plan system. All pumps and valves are less than 2 years old.
When hot water is on all is fine. Same if hot water and central heating.  All is good.
The issue is if just central heating is on. Boiler starts and pipes start to get hot. After a while you can hear what sounds like boiling water in the pipes in the airing cupboard.  touching the pipes they are red hot apart from the boiler return that seems cool. The boiler then cuts out, guessing getting to warm. Main pump and valve stays open. 
It appears to me the radiator return is getting reclyed back into pump rather than returning to the boiler. If I open hot water valve,  it appears to force water back to boiler as the return gets hot .
Do hope iv made sense 

Please start your own thread. It's really annoying to have to read through a whole thread only to find out it's from the beginning of the year. Thank you.