Oso 20RI unvented indirect HW cylinder tundish discharge

[plumbermitch]

The cylinder is discharging cold water to the tundish. I have followed the instructions to correct this and have also had it done at the annual sevice by a competent technician.
However this discharge has been happening on a regular basis (3 or 4 times a year) since it was new in 2007, and is a nuisance, apart from the waste of hot water when it has to be done.
I understand it is something to do with a 'pressure bag'? inside. Any advice would be much appreciated.

 

[dcawkwell]

Cut off supply to unvented cylinder and open a hot tap and allow any water in the pipes to drain.

Check the pressure in the white expansion vessel of the hot water tank.
It should ready about 2.5 bar if not use a foot pump to top up to 2.5 bar.
Check the tyre valve with a small amount of washing up liquid water to see there is no leak on the tyre valve.

If there is any sign of water coming out of the tyre valve when you check
the pressure the diaphragm in the expansion vessel has failed.


Sorted.

 

[oldbuffer]

I've had one or two of these where the "commissioning valve" is blocked, so that the procedure to recharge the expansion space does not work. The way round this is to drain the entire cylinder and replace the commissioning valve, after which the normal recharge procedure should work.

 

[CBF]

I've had one or two of these where the "commissioning valve" is blocked, so that the procedure to recharge the expansion space does not work. The way round this is to drain the entire cylinder and replace the commissioning valve, after which the normal recharge procedure should work.

what the hell is a commissioning valve

 

[oldbuffer]

For cbf. The commissioning valve is actually just a doc used to drain off any water which may have found its way into the air space above the water level. Its item 12 in the diagram of OSO MIs for the 20 RD / 20 RI.

 

[Dan Robinson]

Having the cylinder installed in the loft can cause problems with this happening. Easiest solution is to remove the dip tube (getting more hot water in the process) and fit an external expansion vessel.

 

[plumbermitch]

I've had one or two of these where the "commissioning valve" is blocked, so that the procedure to recharge the expansion space does not work. The way round this is to drain the entire cylinder and replace the commissioning valve, after which the normal recharge procedure should work.

Thanks for all your replies. Only just read them 'cos my request for email notification didn't work!! (Just realised I hadn't updated my email address, so not surprising!! Us OAPs struggle with this sort of thing!!)
So:-
My tank is not in the loft but outside in a barn at ground level.
It has an internal expansion vessel, whatever that actually is!
Commisioning valve was replaced by engineer in July because he could not get it to run properly and he drained tank completely as described by oldbuffer.
However, it's still discharging on a daily basis despite me doing the recharge as instructed in Manual.
Can anybody tell me how much water should normally come out of the Commisioning valve when doing a recharge?
Is it better to open the highest hot tap in the house?
If I get somebody to remove the dip-pipe, and get an external expansion vessel will this solve the problem?
Can you see the water in an external vessel, and does it lose heat here?
Should I move to another house???

 

[Dan Robinson]

Sound like one of the relief valves is shot.

 

[plumbermitch]

As none of you could/would give me answers, I'm just replying to myself for the benefit of any other poor soul who has one of these tanks.

I recharged the expansion vessel again.
I opened several hot taps!
The commisioning valve was blocked again but eventually released about 80-100 litres of hot water over nearly 2 hours!. I could have run it off first into the bath, if I'd known!!
Turning pressure Relief Valve (Number 5 in instructions) to left and holding, released another 15 litres approx.
A week has passed and so far so good!
I will post again if/when it needs doing again.