Have noticed that on rad seems prone to getting air in it. Patchy heat distribution until opening the bleed valve, and a satisfying long hiss. Then it all heats up again.
Pressure in system seems to be holding up and all other rads are trouble- free.
Any obvious answers ?
All guesswork, but:
"Pressure in system seems to be holding up" - so I presume it is a sealed system and air cannot be drawn in anywhere. So it won't be air that is bleeding out, but likely Hydrogen from rapid corrosion of disimilar metals. Have you got inhibitor in the system?
I had a similar problem. It was always the same radiator that trapped air. Plenty of inhibitor in my system and no leaks so I went for automatic bleed valves. Possibly trapped air in my system that was just circulating. Certainly we used to get odd audible gurgles.
These have been brilliant. No gurgles, no more radiator bleeding.
Completely drained, flushed and refilled in October ...
Now that/they (I like the 10 pack) looks like just the ticket.
Not sure how you fit them without draining the system.
The highest rad in your system?
Turn off both valves to the radiator, making a note of how many turns it takes to shut off the balancing valve. Change bleed valve. Turn valves back on, resetting the balancing valve to its original position. Expect some water to come out of the bleed valve until the washer inside swells enough to cut it off.
Nothing so simple. I have 2 upright ladder radiators in the bathrooms that are the highest (it's where I put the inhibitor in) and they only needed a quick bleed.
Easy. Just close the valves at each end of the radiator.
I seriously wouldn’t bother fitting them to all your radiators. I only had a problem with the tallest upstairs towel rail radiator, fitted a valve to it (and the nearby bathroom only only because there were two in the pack) and my whole system has remained air free.
Tim
They are complete and utter crap. They may work when first installed but get easily blocked if the system has any disturbed sludge when a system is re-filled. They have a pinhole to a chamber that has a membrane that swells up and forms a seal when wet.
When first fitted to a drained system the radiator is full of air and the membrane is dry allowing air to pass from the pin hole to the outside world. Once the water reaches the pinhole it enters the chamber and the membrane swells stopping the water reaching the outside world. If the sludge in the bottom of the radiator is disturbed when refilling the contaminated water reaching the pinhole will likely block it. Furthermore if you want to drain a radiator or system for a few hours or days the membrane in the Aladdin auto-bleed will remain wet and remain sealed for both air and water. On a subsequent refill it will not auto-bleed. It requires the top of the radiator to be hot to dry out the membrane which is a bit difficult to achieve unless the radiator has been bleed.
Aladdin did/do two versions of this valve, one with a non-replaceable membrane module and another identical looking model with a replaceable membrane module. With the latter they recommended module changing every few years.
I had these fitted for a couple of years. I no longer have them in my system.
They may be slightly different now for the Mk3 version but probably have exactly the same problems as indicated in the following video.
They worked for me. Job done, no more issues after 18 months. If their action was as transient as you suggest I should have had trapped air issues again by now. Given the small outlay I would suggest that the OP try them and see how they work for him.
Tim
It's simply where the air/gas collects. Why the air is there, is another matter. My CH slowly looses pressure over time. I can find no leaks
Probably therein lies your problem
Trapped pockets of air are gradually finding their way to that particular rad.
Which would make sense if it was the highest ... I wouldn't have posted :)
Maybe I'm a bit more aware as we have been running the heating for a few days now - I may not have noticed if it was once a week.
There's always the possibility of a leak that lets air in, but not water out.
Plus I guess dissolved gases in the water ?
Under pressure?
Yup. I think that is more likely.
And is consistent with the fact that its happening less and less. As each refill since the pressure vessel was replaced is smaller, and less frequent..
No. its NOT always the highest.
In my case its one bathroom rad - there are two others just as high, and another rad downstairs!
And pipework in the roof that is even higher but doesnt collect air. IT started with an airlock, and I put bleed points on it, but I have never since then had to bleed it again.
With bottom fed rads, *any* rad can collect air.
Its more a question of which ones have a high flow that will promote bubble formation and mixing and onward transport, and which ones have a smoother flow that allows gases to settle out and accumulate.
Once you have flow with bubbles it's a toss up as to where it ends up.
Possibly a partial airlock in the pipes gradually making it's way to the nearest radiator. Although an open vented system, my parents once had a CH system that always had a airlock in one of the pipes that lead to one room. On a drain down/fill the only way to totally clear the the air from the pipe was to turn the pump to full speed and close down all radiators except the one that had little flow to it. After running the pump for a few minutes the radiator could be bleed, the pump turned down and all the other radiator valves turned back on (no rebalancing was necessary)
On modern boilers there is also an purging cycle of perhaps 10 minutes that should be selected after a refill. This tends to run the pump intermittently and/or at different speeds without the burners being turned on.
Selecting this mode is usually detailed in the installer instructions or check on Youtube for a particular make/model of boiler.
I have an automatic bleed valve, not this model, that's still doing its stuff after more than 20 years.
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