Megaflo/expansion vessel not relieving through cold mains

[andy_g]

Hello

I have been using a Megaflo for 17 years now. For those that do not know the Megaflo is a pressurised, steel, hot water cylinder. Internally it has an air gap and when the hot water gets hot and expands, the air inside the megaflo gets compressed.

When it heats up there is a significant pressure increase inside which can only be relieved through the hot water taps.

In practice this means that when we turn on any hot water tap in the house, we get something like a Yellowstone geyser until the pressure drops and then we can use the mixer taps normally.

This is dangerous (as the water that comes out is terribly hot even though the mixer tap is in the middle position), it is also very wasteful as you sit there letting the hot water run freely until its pressure more or less equalises with the cold's, and it is time consuming.

In another county where I have lived in the past, the hot water cylinders do not have a check valve at their cold feeds and they let their pressure back into the cold so that inside your house the pressures are equalised. That means no more surprises when you turn on a tap.

Is there a reason why in the UK the Megaflo requires the check valve at its cold water feed? Is it to prevent contaminated (hot) water into the cold mains? Assuming the house has a check valve at the water meter, is that really a problem?

 

[wannocks]

Might be worth refreshing your air gap. There's some instructions on the side of the cylinder explaining how to refresh it.
The temperature is controlled by the cylinder thermostat. It may be set too high or there's a controls fault. Check the immersion heater isn't on and left set too high.

 

[andy_g]

Thank you for your reply.

We regenerate the air gap every so often. Right after a refresh is when we get the almost explosive pressures. As time goes on (months) the pressures get smaller and smaller, until the next air gap regeneration. This can be a few months, typically I think we do it 2 or 3 times a year.

The cylinder thermostat seems to work OK and is set to 65C or thereabouts, I believe max is 70C.

The immersion is permanently off from the consumer unit, until and unless we have issues with the gas boiler and only then do we ever use it.

I am wondering if it would be better to remove the cold feed check valve and allow the hot water pressure equalise with the cold mains so that when we turn the mixer taps / showers on we do not get greeted with karcher style jet!

 

[wannocks]

65 is too high, if the water at the tap is too hot turn it down, the water at the outlet at the top of the cylinder will be hotter than the stat measures at the bottom.
There will also be less expansion if you turn the temperature down.