Rayburn hot water problem

[bower1]

Penners, thanks for the ideas, an amusing image!

Unfortunately, I don't have mains water, so I'll need to think of something else.

 

[Lime]

bower1 I think you have described why you have the problem.
The fact that the hotwater cylinder is slightly higher than the vent means that once air is trapped in that area of pipework it has no where to go except down which it is unlikely to do.
Even Penners' method is unlikely to shift it.
When you switch on the pump the movement of water through the boiler for the central heating causes some movement of water in the gravity fed system.
That moves the trapped gas/air enough so that some hot water from the boiler reaches the hotwater cylinder.
However, when you switch the pump off, the gas rises again to the highest point and causes an air lock as before.

To understand venting it is important to realise that gas trapped in piping tends to follow the flow of the water.
When an air bubble reaches a vent pipe it doesn't immediately go up the vent but continues on with the water flow.
It doesn't go up the vent pipe because the vent has no flow of water through it.
In order for the air to go up the vent pipe requires it to displace the water up the pipe which is a slow process, much slower that the time it takes to pass it.

Only when a gas bubble is big enough and the flow almost stationery (or the pipes are large) does it rise up through pipe pushing the water out of the way as it does so.

However, none of this will help your situation with air trapped in the hot water cylinder.
Even if you manage to dislodge some of it and get the hot water cylinder working again some gas/air will remain slowing the gravity system.

Can I make a suggestion since you did the pipework?
If you install an extra pipe rising between the point where the boiler flow enters the cylinder and the vent pipe you will have a permanant bleed so that the gravity system never has an airlock again.
You will also have the added advantage of having absolutely no air trapped in the top of the cylinder so that the gravity system works at 100% efficiency.

 

[bower1]

Lime, thanks for your description. The one bit of pipework I didn't do was the feed to the cylinder and the vent pipe, so I (obviously wrongly) assumed that was OK.

I have just tried lifting the feed pipe to the cylinder and I can move it temporarily so that it slopes the other way. I think that should be enough to clear any gas.

If that works I will look at moving the vent pipe to where the pipe enters the tank then refilling with a better inhibitor. Thanks for the advice.

 

[Penners]

I'm sure Lime is right about the location of the airlock. One other way of preventing this in the future would be just to install a little air bleed valve into the highest point of the offending pipe-run, enabling you to bleed off any accumulated air as and when necessary.

You can get little bleed valves which are finger-operated (no key needed) for precisely this purpose.

 

[bower1]

One other way of preventing this in the future would be just to install a little air bleed valve into the highest point of the offending pipe-run, enabling you to bleed off any accumulated air as and when necessary.

Good idea

 

[NT]

You can also get automatic bleed valves that are porous to air but not water. Whether theyre long term reliable I dont know.


NT

 

[Penners]

There's one in our system. Never been any good whatsoever.

 

[Lime]

There's one in our system. Never been any good whatsoever.

I agree.
The automatic vents look good on paper but often fail miserably in practice.
The manual one's are ok if you don't mind bleeding air off whenever necessary.
Certainly in the OP's case something is required to release the trapped air/gas.