On 2020-04-24 11:58:39 +0000, Davey said:

On Fri, 24 Apr 2020 12:28:03 +0100
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.


Check that you don't have a motorised valve that has failed, leaving a
microswitch closed, faking a demand condition. Do you have a test meter
and a wiring diagram, and a suitable junction box for testing power at
different points of the wiring?

I replaced both the hot water and heating motorised valve motors as
they were failing last year so I would hope they are OK but I'll check.

Yes I can test things with a meter and can get at the wiring box but
it's awkward.