Internal bubble expansion

[Matt887]

Got a pressure relief dripping on an internal bubble hot water cylinder. To recharge the bubble it's close the cold fill to cylinder. Open a hot tap and run till it stops and open pressure relief and refill?

 

[muggles]

It's follow the instructions in the instruction manual. Different brands will have different procedures

 

[Matt887]

Thanks it's one of these.

 

[Matt887]

I've found the instructions, how long should you leave the immersion/boiler off before draining?

 

[muggles]

There's no need for a delay

 

[Rodders53]

Step 2 in the instructions of the manual applies, surely?

Step 1: Turn off the boiler and immersion heaters to the cylinder.
Step 2: Allow the water to cool through your normal usage. Alternatively run off all the hot water by opening the lowest hot tap in the property until the water runs cold. Once complete turn the tap off.
Step 3: Turn off the water supply to the cylinder, via the stopcocks.
Step

Not that I bother with my OSO air bubble unvented. Just wastes a fair bit of hot water.
If I remember I replenish the bubble on return from a holiday away (and it's been long enough since the last time).

 

[Matt887]

Thanks so there suggesting turn boiler off/immersion and run hot tap till water cools?

 

[Johntheo5]

I've found the instructions, how long should you leave the immersion/boiler off before draining?

Presume it states this as its very important, that you also open the cylinder T&PRV when the hot water has finished draining and leave it open until water stops coming out of it, if any, then reshut it.

I'm not too sure if its necessary to let the cylinder cool down before recharging the bubble, I've recharged a neighbours Megaflo a few times without letting the cylinder cool down and the bubble lasts ~ 10 to 12 months.

 

[Matt887]

Thanks. Once isolated and running the lowest hot tap. It should stop pretty soonish?

 

[Matt887]

In essence then by opening the tprv water is only getting drained from the cylinder to this point

 

[Rodders53]

Thanks so there suggesting turn boiler off/immersion and run hot tap till water cools?

Indeed they are. But many people would skip that step 2.

Thanks. Once isolated and running the lowest hot tap. It should stop pretty soonish?

With the T&P valve open (45 degree twist) you should hear 'gurgling' that will be fast at start and then slow -- gurgling and water flowing out should stop eventually. Will take some time as the air gets in via the T&P valve to the cylinder in the main.

The air bubble is above that T&P valve --- closing it sooner than water stopped completely just means a slightly smaller bubble I think.

My OSO instructions are different and reference using a 'commissioning valve' lower in the tank. But that involves a hosepipe and a dreadful leak everywhere draincock valve. So I shortcut using the T&P valve only.

Here's Kingspan's video on doing it - I haven't watched it in full

 

[Johntheo5]

In essence then by opening the tprv water is only getting drained from the cylinder to this point

Yes, but if the expansion relief valve has lifted then there could be up to 40L of water (assuming a 200L UVC) still above the T&PRV when the water stops running out of the hot tap as the bottom of the dip tube extends down to ~ 25% of the cyl height/vol. so could take a pretty long time to drain off through the T&PRV.

 

[Johntheo5]

Indeed they are. But many people would skip that step 2.


With the T&P valve open (45 degree twist) you should hear 'gurgling' that will be fast at start and then slow -- gurgling and water flowing out should stop eventually. Will take some time as the air gets in via the T&P valve to the cylinder in the main.

The air bubble is above that T&P valve --- closing it sooner than water stopped completely just means a slightly smaller bubble I think.

My OSO instructions are different and reference using a 'commissioning valve' lower in the tank. But that involves a hosepipe and a dreadful leak everywhere draincock valve. So I shortcut using the T&P valve only.

Here's Kingspan's video on doing it - I haven't watched it in full

I think that commissioning valve was installed about a 1/4 way up the cylinder which meant that (if drained down) you start off (assuming a 200L UVC) with a 150L bubble which becomes 37.5L on pressurizing the UVC to 3.0bar and is still 35L after a full reheat to 60C and a final pressure of 3.2bar, these valves gave endless trouble, getting choked/leaking etc, some used to fully drain down the cylinder through the drain cock. If only drained to the T&PRV level then, like the Megaflo, will give a initial ~ 50L bubble, then 12.5L on pressurizing to 3.0bar and 9.5L and 4.3bar after a full reheat. So wonder does a OSO which has no baffle, and a 35L bubble give the same time between bubble recharge as a baffled Megaflow
with a 9.5L baffle or if this baffle does little or nothing then one might get say 1 year recharge intervals with a Megaflo but a possible ~ 3 year OSO interval?.

 

[Rodders53]

So wonder does a OSO which has no baffle, and a 35L bubble give the same time between bubble recharge as a baffled Megaflow with a 9.5L baffle or if this baffle does little or nothing then one might get say 1 year recharge intervals with a Megaflo but a possible ~ 3 year OSO interval?.

No idea. The OSO commissioning valve is also the secondary return location (not fitted on mine) and about 1/2 tank height. The T&P valve is nearer 3/4 from bottom. Oso say my 210 litre nominal can hold 250 litres if the HW dip pipe is removed and an external pressure vessel installed.

The only time I have done the OSO bubble as per the MI was when I changed the commissioning drain cock due to it weeping... it was well 'crudded up' with soft limescale despite the previous owner fitting a water softener, so no water or air could get in/out of it. That bubble lasted me 13 months 12 days.

Since then it has been very roughly 8 month intervals... I've not recorded every replenishment.

 

[aveatry]

No idea. The OSO commissioning valve is also the secondary return location (not fitted on mine) and about 1/2 tank height. The T&P valve is nearer 3/4 from bottom. Oso say my 210 litre nominal can hold 250 litres if the HW dip pipe is removed and an external pressure vessel installed.

The only time I have done the OSO bubble as per the MI was when I changed the commissioning drain cock due to it weeping... it was well 'crudded up' with soft limescale despite the previous owner fitting a water softener, so no water or air could get in/out of it. That bubble lasted me 13 months 12 days.

Since then it has been very roughly 8 month intervals... I've not recorded every replenishment.

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.