On 24/04/2020 12:28, 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.

Replacing the Expansion vessel is actually a really easy job but you'll
have to remove the flue pipe. (This is on my Turbomax Pro 828E)
My Flue pipe was already falling to bits on the outside of the house so
it made sense to get a new flue kit along with the expansion vessel.

Removing the vessel is simply the one wet connection at the bottom of
the vessel.
Take a picture of the air-pressure switch top right of boiler and remove
that and poke air pipes through the back plate (making a note of which
pipe goes to which connection). The expansion vessel then simply slides
up and out. I say simply but it's quite a heavy lump especially if it's
still got water in it (on the "dry" side of the membrane) so you have to
push it up as far as possible from the bottom then grip it from the top
and pull upwards.