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.

Not necessarily exceptional for a "hot" system. How many rads on the system?

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

So you have an unvented primary (aka "sealed") system for the heating
circuit, and you also have an unvented (mains fed) hot water cylinder?

(In which case there will be two separate expansion vessels - one
(probably in the boiler) for the primary CH system, and another
connected to the unvented cylinder.

I take it the cylinder is an "indirect" type?

http://wiki.diyfaq.org.uk/index.php/...rect_sys tems


Could you explain what type of control system your setup uses? i.e. Does
it have a time switch or "programmer", room stat, cylinder stat, one or
more two or three port valves?

If you refer to :

http://wiki.diyfaq.org.uk/index.php/...ols_and_Zoning

You should be able to identify the layout - e.g. Y plan or S Plan.

Once you know the layout and the control system, you now need to
logically step through the controls, and work out why a call for heat is
reaching the boiler when it should not be.

Am I right to suspect the expansion vessel - it either is failing or
needs recharging.
Its possible, however it does not explain the cycling boiler problem.

I guess not a DIY job.

Its a fairly easy DIY job IME.


--
Cheers,

John.

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