The reason the air gap in the Megaflo (and othrs like OSO) "needs recharging" periodically is that there is no rubber diaphragm to stop the gas being dissolved. A separate PV, DOES have one so you will not get the same situation, for perhaps several years (like in a combi).
Yeah but you dont need to drain and refill every 6 months, just do it when it starts dripping from the pressure relief valve, all you do is turn off the stopcock next to cylinder open your hw taps then drain and turn stop cock bk on and your bubble will reform, you dnt even have to be qualified to do that, but you do however have to have a seperate qualification other than gas safe, plumbing/gas nvq's to fit these cylinders something to makesure your installer has, anyway your expansion vessel will go eventually and need replacing at a cost to you, where as i said the air bubble type can be reformed at no cost to you, so for the extra it costs, it will save you a little in repair costs over the years
use a gas safe engineer who is register to commission an Unvented hot water supply, gas safe site will list all such operative in your area. jon wells. gas safe 186229
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