Cold water Crossover problem?

[ColinS1]

I have a Megaflo cylinder which is probably 20-25 yrs old. I recharge the air gap around every 6 months. All my taps and showers are direct-fed mixers. Thought I might have a problem when hot water from cylinder (pipes there are both hot) is coming out of mixer taps either cool, warm, sometimes hot and sometimes just cold, intermittently when set to hot. From what I’ve read so far, it may be a failure at one of the mixers/showers, leading to a crossover? I went to recharge the air gap today and having closed off the cold supply to the cylinder, the water was draining outside continuously forever via the tundish? Gurgling never stopped. And nearest single mixer tap which was set to run hot simply wouldn’t stop running at all. Gave up. As the cold water feed to the cylinder was isolated, I couldn’t understand why the cylinder didn’t just run to empty unless the isolator has failed? Or was the cylinder filling up via backflow from crossover? (If that’s possible?) I will most likely end up getting an engineer in but would like to narrow the issues to speed up any necessary repair. I’ve got 2 washing machines and 2 dishwashers in different parts of the property but all are cold water feed only so don’t think there will be any crossover/failure with those.

1. Not sure what’s the best way to try and identify which mixer tap/shower might be at fault? - Main issue

2. Should there be a non-return valve fitted to hot supply to each tap to stop cold getting in to hot? (Or is that pointless?)

3. Could it be a problem with the Megaflo cylinder? I had mains feed PRV replaced about 2 years ago after it had failed and thermostatic safety switch replaced last year after it kept cutting out. CH zone valve was replaced about 6 weeks ago. H/W zone valve, pump and gas boiler all activate correctly when switch calls for heat or if cylinder thermostat temp increased. Open to all suggestions…

 

[Notch7]

I think Urban plumber on YouTube came across this problem, a shower was refilling the tank by crossover

I think showers have a not return valve fitted to prevent, maybe you've got one that has failed.

I'm not a plumber so could be completely wrong, I'm sure a professional.wi be along to advice

 

[jurassic sparkk]

Someone could test for boiler PHEX blockage for fouling first, turn the DHW temperature to max on the boiler, open any one hot water tap fully, if it remains hot, open any remaining hot taps fully, if the water then runs cold, quite possibly the PHEX. Set the shower to its hottest setting and open the flow control to give a fairly low flow or until hot, then open it up to give full flow, does it then run cold shortly afterwards?


As Notch has mentioned above shower could be the culprit.