Internal bubble expansion

[Johntheo5]

I has an OSO one too and always open the commissioning valve as instructions state but there has never been anything come out of it. When the cold water supply is off and a hot tap running it just runs slow out of the hot tap. The gurgling stops after about 30-40 mins and the job lasts about 8 months like yours.
When I see the dripping in the tundish its when the hot water is on - boiler I know roughly when to expect it so when I see it I stop the hot water heating and then just use up the tank normally- just so I am not needlessly wasting hot water when I do the recharge.

Do you open the T&PRV?

 

[aveatry]

Do you open the T&PRV?

Yes - as per the instructions sorry I wasn’t attempting to describe the whole process it was more about the instructions saying open the commission valve until the water stops no water has ever come out when I open it .

 

[Johntheo5]

Yes - as per the instructions sorry I wasn’t attempting to describe the whole process it was more about the instructions saying open the commission valve until the water stops no water has ever come out when I open it .

If you get fed up with re doing the bubble and want another 20% or so greater HW cyl vol, you can remove the OSO's dip tube and install a expansion vessel.

 

[aveatry]

If you get fed up with re doing the bubble and want another 20% or so greater HW cyl vol, you can remove the OSO's dip tube and install a expansion vessel.

I also have an expansion vessel.

 

[Johntheo5]

I also have an expansion vessel.

IF sized to 10% of the UVC volume and with pre&filling pressures of 2.8(nominally 3.0bar) & 3.0bar then the expansion relief valve should never lift even if the bubble is completely lost.

 

[aveatry]

IF sized to 10% of the UVC volume and with pre&filling pressures of 2.8(nominally 3.0bar) & 3.0bar then the expansion relief valve should never lift even if the bubble is completely lost.

So its been fitted as a belt and braces / best practice ?

 

[Johntheo5]

So its been fitted as a belt and braces / best practice ?

Why is this happening then??,
"When I see the dripping in the tundish its when the hot water is on"

 

[Rodders53]

Hmm...
MY T&P relief valve has never let water by... I know the bubble is 'gone' by the HW pressure when opening a hot tap being higher than normal / faster flow and then slowing. I guess if I left it like that long enough it might?

Nothing out of the commissioning drain cock suggests to me it'll be blocked with limescale crud or the rubber washer is stuck in place to the same effect.

The external (potable white) expansion vessel needs checking with water off / hw tap open. But why have one without removing the dipstick and having the greater HW capacity?

 

[Johntheo5]

This just might be of interest, I "made" it decades ago but added the air bubble calcs, hopefully correct.
The OSO commissioning valve seems to be at the 50% level so should have a air bubble of ~ 22L after a full reheat vs the megaflo's 9.5L, maybe last far longer between remaking the bubble, if its (or the Megaflo) fully drained then ~ a 47L bubble, everlasting?.