Blocked HW vent pipe!

[Agile]

I have to replace a cylinder shortly however the hot water vent pipe is well blocked. The old cylinder is bare copper so its about 30-40 years old. I have applied about 1.5 Bar to the cylinder but the blockage withstands that pressure and does not let any flow pass it.

Due to the construction of the property the vent pipe is not physically accessible between the downstairs ceiling and the vent above the cistern on the floor above.

Has anyone encountered this AND knows what the blockage will actually consist of?

Tony

 

[HERTS P&D]

Tony can't you push a lenght of 15mm Hep pipe up the vent pipe?

Andy

 

[gas112]

So what have you already cut out and how did you apply this pressure

 

[mysteryman]

Tony, you are usually such a stickler for doing things properly! You know what you need to do.

 

[Agile]

Yes, it should be replaced but was intalled before the cistern and is not a straight run up anyway. So poking at it is not going to be possible. In any case as it resists 1.5 Bar its going to be pretty solid!

Its basically a bungalow with a singe central loft room and the cistern is squeezed into the space under the roof slope.

 

[Onetap]

Surely it can only be limescale, there shouldn't be anything else in the HW pipework, unless it was once a primatic cylinder?

This usually happens with the cold feed connection on a leaking heating system ( as I know you know) where the limescale in the cold feed water precipitates in the tee when it hits the hot water; it suggests there may be cold water coming down the OV pipe. I'd hazard a guess that the open vent pipe might be actually dipping into the water in the cistern and siphoning cold water out of the tank. No head-room above the cistern in the apex of the roof? Open vent pipe fitted blind?

Cut out the OV tee and have a look, as you'd do with a heating system? Alternatively, seal the cylinder end of the OV pipe with a compression fitting ( where it has been disconnected from the cylinder) or with a push-fit fitting and pour in some DS-3 descaling solution from the loft. If the cylinder is out, maybe connect the cold feed to the open vent, fill the cistern with DS-3 and leave to brew for several hours; you'd need to get some down the OV pipe or it will be air-locked and the acid won't get to the blockage from below.

Another possibility is the installer had some problem with the installation (sucking in air?) which he 'fixed' by capping the OV in some void. Shower pump fitted?

 

[Agile]

This is a standard indirect system on a sealed system. Could have been a primatic before that but I doubt it.

Vent does not dip into water.

Max headroom is a max of about 800 mm sloping down! Cistern must have been installed before the internal upstairs walls.

System had been power flushed by others and when BG refused to work on it I was called in to sort it out.

I found that the heating coil in the cylinder was leaking slowly at first into the cylinder and then after a couple of weeks much faster to the point that it was tedious to drain and isolate the heating coil with about 3 li/min coming out with just the head of water from the top of the cylinder!

Have you actually found a lime scale blockage in a senario like this?

I expect that I will use 10% HCl poured from above to dissolve the blockage when I remove the old cylinder. Its much faster than DS3 ( when cold ).

Tony

 

[gas112]

will ask again what have you cut out . limescale on outlet of cylinder is not unusual

 

[Onetap]

Have you actually found a lime scale blockage in a senario like this?

No; I can't imagine any other means by which you could inadvertently get an immovable blockage in the OV pipe. I've had the usual bits of loft insulation and foam cylinder insulation partially blocking water pipes, but not the open vent.

Can you be sure the OV pipe from the cylinder connects to the OV pipe over the cistern? Someone may have saved themselves a few yards of tube. I have seen that (or something similar) done.

 

[Agile]

All I can say is that the vent pipe can be seen disapearing into the corner of the roof and going up from the nearby corner of the kitchen! Its not accessible to touch or work on unless the built in cistern was removed!

I have never found a vent pipe totally blocked to 1.5 Bar like this before but there is always a first time. Hence asking for the broader experience of everyone who posts here.

Gas112, the pressure was applied by closing the cold feed and letting the leaking coil transfer the pressure into the cylinder! I cannot access any pipework between downstairs ceiling and the vent pipe at the cistern.

Tony

 

[Onetap]

I have never found a vent pipe totally blocked to 1.5 Bar like this before but there is always a first time. Hence asking for the broader experience of everyone who posts here.

I've never seen one totally blocked either. I'm starting to incline towards the capped OV and away from the limescale theory. Could you get a push fit fitting and a bit of flexible tube onto the cistern end of the OV and blow down it? If it's a dummy, the other end is probably open. Let us know what you find, preferably with a picture, it's more interesting than the cryptic crossword.

 

[Agile]

I can with difficulty access the loop and will need to cut it off to pour the acid down it.

I will arrange to take a tube connection and blow down it and see if its open at the other end. Doing that would be prudent anyway as I dont want to be pouring acid into an open pipe.

Because I have not had experience of working on plumbing jobs as an employee, I had rather discounted that being likely but as you point out its a distinct possibility in view of the very limited access!

Thanks for the suggestion!

Tony

 

[gas112]

So what exactly are the symptons you are getting ? no water out taps ? and what makes you think the actual vent is blocked

 

[Onetap]

Its basically a bungalow with a singe central loft room and the cistern is squeezed into the space under the roof slope.

Vent does not dip into water.
Tony

Thinking more about it, if the loft room is a conversion, the pipe blockage by limescale may have happened with the earlier cistern installation. The only way I can think of that happening was if the OV had been submerged.

 

[Agile]

Its difficult for me to come to any conclusion about the loft room.

But the floor of the loft room may be on 4 x 2 joists laid perhaps across the ceiling joists. Its certainly at a higher level than the base of the cistern!

Unfortunately its a street of houses where they are all of slightly different different designs.

But having just looked at the street on Google earth I am very strongly now thinking that its a loft conversion from many years ago. Still cost 367k though.

Tony