How much water to expect from the overflow during air gap recharge of Megaflo cylinder?

9 Aug 2022
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United Kingdom
Thinking about getting a Megaflo cyclinder. Bit worried though because the overflow would likely head out onto the flat roof outside and I don't want to regularly be dumping a lot of water onto that (doesn't seem like a good idea).

Does the air gap recharge process end up dumping a lot of water out of the overflow? I've seen conflicting YouTube videos on the subject that have left me a bit confused about what I should expect.

My assumption is that if I leave the hot taps open until they stop flowing; when I then open the temp/pressure valve I should get little to no water running into the
Tundish and out onto the flat roof, all that's happening there is air would be flowing in that way. The only way I'd end up with a lot of water coming out via the overflow is if I was impatient and opened the temp/pressure valve before the water from the hot taps had stopped flowing.

Am I assuming right and should not expect a lot of water out via the overflow during recharge or I got that wrong and it's probably a bad idea to fit a Megaflo with overflow out onto the flat roof.

Really appreciate any thoughts/guidance there. Just left a bit confused by the various YouTube videos that seem to contradict each other.
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Shut off the cold mains to the cylinder, open the hot tap to depressurize the cylinder, open the TPRV, you will then dump ~ 30/50L of water from the cylinder to re establish the air bubble but remember, you have control over this, you can open/shut the TPRV to give a controlled (slow) flow onto the flat roof, when no more water from the TPRV, shut it and reopen the mains supply, The air bubble will be lost slowly over time and need recharging perhaps once/twice/year, even if its lost completely then each time the cylinder heats up, the expansion valve will dump 2.0/5L of hot water but over the reheating time which may or not be a problem for your flat roof. As the air bubble is being lost you will initially see a very slight discharge from the tundish or whatever so plenty of warning to do your (controlled) air bubble recharge.
Thanks, Johnthe05, really appreciate the thoughts/help!
You will dump the amount of water from the top of the cylinder to the T&P valve level. Depends on the size of cylinder as to how much water is actually emptied. Of course, no reason not to fit another type that has an external EV therefore no need to replensich any air bubble.
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From an owners perspective, it’s a job that I schedule doing twice a year. I don’t wait till it starts losing pressure, and always one or two nights before, turn off the heating so the water is cool and not lose energy. I’ve found that when I take the time to let the hot taps drain fully, there’s very little if any water that flows out onto the tundish.
Its interesting that you say "I don’t wait till it starts losing pressure" by which I presume you mean HW flowrate, I've seen this mentioned in other threads but can't figure out the reason for this.

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