Mystery cold water tank overflow

[Jobie]

I have an oil fired boiler supplying ch & hot-water cylinder coil-heating via a pressurised primary circuit. I have two venturi type showers located on the first floor supplied by mains cold and hw from the cylinder located on the ground floor. The other taps (mixer) in the house are fed from the main supply running at 3 bar and hot water fed from hw cylinder.

Just recently the cold water tank in the loft started to overflow. I replaced the ball-cock valve with a new unit - still leaking. I changed again - still leaking. have just one cold water tank in the loft.

Overnight the gravity fed cold supply to the hw cylinder becomes warm, but the hw out pipe remains cool. Assuming my second new replacement ball-cock valve is working well, is it possible that water is returning back up to the cold water tank via the hw cylinder tank cold feed down pipe? Or maybe the secondry circuit is venting into the tank?

Some time before I noticed the overflow running I increased the temp setting on the hw cylinder thermostat to obtain a better temperature range on my showers.

What diagnostic route should I take to solve this problem please?

 

[Neil K]

You've got unequal pressure at your showers. The cold water mains is mixing with the hot at one or both of the showers, pushing back up the hot, through the cylinder, back up the cold feed backfilling the cold water tank making it eventually overflow

 

[Mike Jackson]

I'm assuming that you've got Treviboost showers or similar. Check the check valve on the hot feed to each of them is working correctly. If you run the shower until the feed pipe gets hot and then turn it off. If cold water creeps back up the pipe the check valve is faulty.

 

[Jobie]

You've got unequal pressure at your showers. The cold water mains is mixing with the hot at one or both of the showers, pushing back up the hot, through the cylinder, back up the cold feed backfilling the cold water tank making it eventually overflow

Thank you for your advice. J

 

[Jobie]

Thank you Mike for your advice. I do indeed have Treviboost showers. (1) Is the check valve fairly obvious to locate and how do I actually check it? (2) If I isolate the showers totally from my system, and the check valve/s is at fault I assume the overflow pipe will stop dripping. Is that a reasonable first step?

 

[PhoenixServices]

Exactly as mike says, had the same thing on a kitchen tap today.