Unvented Megaflo Cylinder (Solar), why does it have an expansion vessel?

[Doug0205]

Have a fairly old (2007) Megatech Solar unvented system that has worked fine since I've been here. However, the tundish was leaking (for how long I'm not sure), so I repressurized the air gap, which didn't solve it as it dripped with frequency shortly after turning the heating back on. There is an expansion vessel, or what I assume is one, attached to the side. It has no pressure in it at all, which seems strange. I looked at the manual, and it does make mention of the expansion vessel and its need to be repressurized when necessary; so is it an integral part of the system? Because if it is then, with it having no pressure in it all, then that seems a clear indicator as to why the tundish leaks; but then I was under the impression these things didn't need one as they create their own air gap in the main cylinder?

Will inevitably get a plumber in but would just like to get some background info on what's happening, if anyone can help.

 

[Rodders53]

Unvented cylinder can have an air bubble to accommodate the HW expansion via air compression ... mine is an OSO brand, Megaflo make them, too.

That can also have a separate pressure vessel that does the same via a 'rubber membrane' separator and air pumped into it...

In my case OSO say when and external EV is fitted one removes the HW dip tube and you get extra HW volume in the cylinder; so then no air bubble. You'd need to read the Install manuals for your cylinder to see what your situation is wrt external EVs etc.,..

The Pressure Relief valve is leaking water possibly due to muck and not reseating properly. Or doing it's designed job.

Yes you need to (reasonably urgently) get in an unvented qualified plumber to service and check it all over... and explain what you can do yourself and what needs done during a regular service.

 

[Madrab]

Nope as far as I remember the mega tech didn't have an internal bubble and came with an external expansion vessel (EV)

Only certain models of the Heatrae Sadia mega range had a built in internal expansion bubble.

Get it scheduled for a service by someone that's qualified, as part of that the EV pre-charge should be checked and any remedial work performed. An unvented cylinder should be serviced yearly.

 

[Doug0205]

Ah okay, so the expansion vessel isn't just some backup or anything. It definitely needs to be pressurised.

 

[Madrab]

Ah okay, so the expansion vessel isn't just some backup or anything. It definitely needs to be pressurised.

Yep. In a lot of cases with older UV cylinders with internal bubbles, it's capability to retain the bubble becomes compromised. That doesn't mean the cylinder needs replaced, it just means and external EV is fitted.

In this case though the cylinder doesn't have an internal bubble and has an external EV so yes, the vessel needs to be properly pre-charged. What that charge will be will be defined by the system setup, so you need to have someone that is qualified and experienced

 

[MeldrewsMate]

Perhaps @Madrab has been inadvertently misled by the OP's lack of attention...he quotes a Megatech in the post, but shows a photo of a Megaflo which, in all the cases I have seen, has an internal expansion space.
Has the OP correctly followed the user instructions regarding regeneration of the air-gap; mentioning this because it appears the cylinder is located in a plant room type area, probably on a ground floor, or even a cellar. In any case patience is required to fully drain through the T&P valve until the flow stops at the tundish with a nearby hot tap open.
The T&P valve may, as previously suggested, have debris in it, or otherwise be damaged by the longstanding leak past its seal.

 

[Doug0205]

Hi,
Thanks for responding.

The room is upstairs. The procedure I went through went as follows: turned off mains water to it, opened up hot tap in kitchen downstairs and let drain fully, turned the PRV (the blue dial located at top of cylinder with pipe running to it) anti clockwise; first time I over did it, and it clicked (presumably back to default position), did it again and the gurgling noise commenced, and held it there until the gurgling finished, and it made no more noise (this didn't take that long - perhaps a few minutes for the last 'plop' sound and then turned the dial back clockwise). Turned mains water back on, let hot tap splutter out and run solidly. Turned the top element on (the "boost") and waited. The tank was cold, so it took about 30 minutes for stuff to start happening, the tundish then started dripping frequently

The system is now entirely off and cold again as the drip went into a run-off pipe that's killing a shrub outside the house. I assume that run off pipe is from the drip at the tundish? Should you hold the PRV for longer even if there is absolutely no more noise?

Here are some more pics to try and indicate what the cylinder is.

 

[Johntheo5]

Is there a label on the Cylinder instructing how to recharge this bubble??,
Around 20 to 25L should be drained off through the hot tap and the Temperature & pressure relief valve (T&PRV), this usually red with a temperature and pressure written on it, say 95C and 8bar, what is written on the blue one you show in the photo, is this the one you are opening??.
If the photo in your post #1 is yours with a expansion vessel adjacent and the outlet of that expansion vessel is teed into the cold water supply to the cylinder then there is NO air bubble (at least required). but the EV then requires attention.

 

[MeldrewsMate]

This is indeed a Megaflo Megatech cylinder , my apologies to @Madrab.
There should be a separate grey or white expansion vessel on the cold water inlet pipework or inlet control set. This vessel will have a schraeder (car tyre) valve at the opposite end to the water connection.
1. Turn off the mains and open the hot tap as before.
2. Using a car or cycle pump, pump air into the expansion vessel to a pressure of 3.5 bar....whilst pumping you should see water issuing from the open hot tap.
3. Close the hot tap and re-open the cold mains.
4. Re-test as before with immersion heater.

If it didn't pass when under pressure but cold then the T&P seal is likely good.

 

[Johntheo5]

I would think, looking at that EV position, that the whole cylinder has to be drained down, at least to below the bottom of the EV and precharged at the air end to 3bar, then refilled.

 

[Madrab]

Yeah the original Megaflop wasn't solar ready the Megatech was.

EV pre-charge - as per the units MI and on the side of the unit in pic 1 - 3.5bar. PRV as part of the control set is factory set to 3.5bar.

Even still, get the unit serviced and they should set it all up properly. If it's running the solar side too then that will need checked properly, ensure the engineer has the relative qualis in solar as well as unvented.

 

[Doug0205]

This is indeed a Megaflo Megatech cylinder , my apologies to @Madrab.
There should be a separate grey or white expansion vessel on the cold water inlet pipework or inlet control set. This vessel will have a schraeder (car tyre) valve at the opposite end to the water connection.
1. Turn off the mains and open the hot tap as before.
2. Using a car or cycle pump, pump air into the expansion vessel to a pressure of 3.5 bar....whilst pumping you should see water issuing from the open hot tap.
3. Close the hot tap and re-open the cold mains.
4. Re-test as before with immersion heater.

If it didn't pass when under pressure but cold then the T&P seal is likely good.

Yes, there is a grey cylinder on the side. I've plugged a bike pump in and the thing is totally flat. I did tentatively give the pump a squeeze to see what would happen, and sure enough, water came out the hot tap in the kitchen.

I would think, looking at that EV position, that the whole cylinder has to be drained down, at least to below the bottom of the EV and precharged at the air end to 3bar, then refilled.

Yes, that's my cylinder cupboard. I've just recharged the air gap again, this time holding the PRV for a longer time. Oddly, after the gurgling has stopped completely, the tundish then starts to release a drippy stream of water and continues to do so. Ive stopped it for now by clicking the PRV closed, because this was going on for half an hour. There is no label on the cylinder with instructions with regard to recharging the air bubble.

 

[MeldrewsMate]

Yes, there is a grey cylinder on the side. I've plugged a bike pump in and the thing is totally flat. I did tentatively give the pump a squeeze to see what would happen, and sure enough, water came out the hot tap in the kitchen.

There is no label on the cylinder with instructions with regard to recharging the air bubble.

Carry on then, pump up the EV to the 3.5 bar as stated on the data label.
There is probably no air bubble in the cylinder, hence the supply of the EV and the data sheet showing its pre-charge pressure.

 

[Rodders53]

THERE SHOULD NOT BE AN AIR BUBBLE in that cylinder !!!!!
Don't keep playing with the PRV, just close it firmly.

Set the EV pressure right with hot tap open to release water in EV and all might come good.
Or not. The PRV may continue to let by and need replacing.

Call in an expert to service it properly. Ideally annually.

Manualslib has an install manual for the unit if needed and it can't be found on the Heatrae Sadia website.

 

[Doug0205]

Carry on then, pump up the EV to the 3.5 bar as stated on the data label.
There is probably no air bubble in the cylinder, hence the supply of the EV and the data sheet showing its pre-charge pressure.

It's quite stiff with the bike pump (foot). Decided to drain the system and do a reboot, as it were.

THERE SHOULD NOT BE AN AIR BUBBLE in that cylinder !!!!!
Don't keep playing with the PRV, just close it firmly.

Set the EV pressure right with hot tap open to release water in EV and all might come good.
Or not. The PRV may continue to let by and need replacing.

Call in an expert to service it properly. Ideally annually.

Manualslib has an install manual for the unit if needed and it can't be found on the Heatrae Sadia website.

As above, decided to a "reboot" and drain cylinder, try and recharge EV, then refill and get a service before it all goes back on. Got a feeling the EV might be bust, but who knows.