Hot water cylinder venting for first time in years- why?

[MattyD]

Hi,

I've got a gravity fed heating system, with a potterton boiler, cylinder in airing cupboard and tanks in the loft.

For the first time in years there has been a discharge from the hot water cylinder vent pipe (which didn't quite vent into the cold water header tank, which is how I know about it).

No settings for water temp have been changed.

My question is - what's the likely cause? - could it be extremely cold water feed into the cylinder causing more expansion than normal? Or is it something more sinister like the thermostat in the boiler not working and overheating the cylinder?

thanks

Matt

 

[petit_pablo]

If the hw is on gravity circulation then its likely to be the boiler stat. Whats the actual temperature of the hot water?

 

[45yearsagasman]

Agree boiler stat likely.Immersion heater on by mistake?

 

[MattyD]

Sorry My original post was slightly inaccurate. javascript:emoticon('') It's not gravity fed, it's an open vented system with a circulating pump.

The immersion isn't connected, but I'll check. The boiler thermostat is possible but in the past it's proved very oversensitive and has had a tendency to cut out at the slightest provocation I'll check the water temp.

Thanks

 

[YELLERBELLY]

is water coming from the vent in the small or big tank?

 

[MattyD]

It's venting from the HW cylinder into the large cold water tank, not the heating circuit venting into the heating header tank.

 

[jackecole]

How much water is coming out of the vent-pipe? Is it a drip or a half decent flow?