Megaflo - is a design flaw built in?

robatwork

Inherited a Megaflo when we moved last time, in the loft, which I thought was a great result.

After noticing some water coming out the overflow and then through the tundish, a 10 minute google later I was doing a DIY fix which Heatrae Sadia actually put on the side of the Megaflo, which was to recharge the airgap.

As there are lots of posts and youtube videos about doing this - for example https://vimeo.com/168337066 - I did some more reading on forums, and some plumbers hate these kind of unvented cylinders as they are manufactured with a "design flaw". Having spent about 20 minutes holding the valve open I'm inclined to agree with them.

fred246

You just have to recharge the air gap occasionally. Don't think it's a big deal. They are a very good cylinder. They are very expensive which is why some plumbers don't like them. Plumbers can often make more money installing a combi. Once they have bought a megaflo and a boiler they can't add a silly profit margin. With a combi they just buy a boiler and then they can add a crazy installation fee. 

nofoollikeold

1. The MegaFlo has an internal baffle at the top, and an air space above it.  In that way it can absorb the expansion of the water as it is heated without the need for an external expansion vessel.  
2. Because of the large surface area of the expansion space, over time the air gets replaced with water, which is why the gap needs to be regenerated.  Yes it takes a while, but no special skills are required. 
3. The air gap reduces the volume of water which can be stored in relation to the total size of the cylinder.
3. The alternative is an external expansion vessel.  This is a small cylinder with a rubber diaphragm inside it.  One side (should be the top) is pressurised with gas (nitrogen or air), the other side is open to the water in the cylinder.  As the water expands it compresses the gas, thus taking care of the increased volume.
4. If the cylinder goes (faulty, gas goes over time) it needs to be recharged or replaced.  Only a G3 qualified engineer is legally allowed to do this.  
5. I personally prefer the OSO cylinders to MegaFlos, and many others prefer Joule cylinders.  It's nothing to do with money, its to do with experience of installation and maintenance.  Supplying and installing a boiler and unvented cylinder, I would always add a percentage to the trade price and charge fully for the labour involved.  The uplift on the parts makes some contribution to the (fortunately rare) costs of warranty claims. 
6. I'd much rather fit a heat only boiler, with or without an unvented hot water system, because they are less complicated and don't rely on so many manufacturer's parts at manufacturer's prices to maintain.