John Smith wrote:

Our old Valliant Turbomax is humming away with the central heating
light on, and every 5 mins or so firing up for a few secs and then
firing down, but both heating and hot water are turned off at the
controller.

I've noticed that the boiler pressure is over 2.5 bar.

It's an unvented system with a hot water tank and (blue) expansion vessel.

Am I right to suspect the expansion vessel - it either is failing or
needs recharging. I guess not a DIY job.

You have a problem with your control system or wiring, as this should
not be happening if both hot water and central heating are off. Do you
actually get sustained hot water if you turn on the tap while the hot
water is nominally switched off.

It seems likely the expansion vessel has failed, but to confirm this it
would be useful to turn of all electrical supply to the boiler, let it
cool down, and confirm the pressure falls.

--

Roger Hayter