Megaflo and mixer valves

[Lower]

We've had ongoing problems with a megaflo that loses its air gap approx every 4 weeks or so and have had a unvented qualified plumber looking at it.

So far he has cleaned the pressure reducing valve, replaced the the expansion relief valve and fitted an external expansion vessel.

Since that work, the system will work fine a similar length of time as it used to take for the airgap to go and then we start to get a banging and 'trumpeting' noises when the hot tap is turned on and off. Following the airgap replacement procedure fixes it again.

I've had the plumber back and he says that its because we've got 'mixer taps' with the hot water supplied via the megaflo and the cold water coming straight from the mains. He says that he thinks cold water is leaking through the mixer taps and bypassing the pressure reducing valve. He doesn't think he needs to replace the pressure reducing valve but wants to re-plumb the mixer taps.

All sounds fine, except the mixer taps he refers to are not what i would call mixer taps. They are normal taps that share an outlet but have two separate ceramic valve inserts that control the water flow.

Surely if one of these ceramic valves were leaking the water would just come out of the tap and couldn't pressurise the megaflo?

Advice appreciated.

 

[Madrab]

It could be the unbalanced supply is backfilling to the cylinder, I guess have quite a high mains pressure. I'd be testing that first to see what the difference is. Your plumber is probably suggesting he takes the cold feed from the balanced supply at the combi valve.

The other option & probably a lot less hassle with the same outcome would be to put a similarly rated PRV on the mains supply.

 

[Lower]

I can see the logic of what he’s suggesting, but can’t see how it could back feed through the taps from the unbalanced supply as they’re not actually mixers. The plumber's answer to that questions was to say that it just happens. The taps in question are this type of tap:

I can turn on the cold water independently of the hot and if i do so water flows from the spout. If it did leak, water would just come out of the spout so i can't see how this could back feed into the megaflo.

Replumbing the taps in question is possible but disruptive so I want to be sure he’s right.

Fitting an PRV on the incoming supply would be substantially less disruptive.

 

[Madrab]

It's all about hydrodynamics, 2 streams of water at different pressures will always try to balance each other out. Even though the water flows out of the spout, there can still be a push back effect against the hot water supply by the higher colds mains pressure. That does need to be confirmed by a quick pressure test though.

If there is an imbalance then if you fit a pressure reducing valve on the mains as it enters the building then both supplies will automatically be balanced by the one valve.