The mid position valve which controls the flow of hot water to the tank and
the central heating needs replacing.

I've changed a couple of control heads but they break after a few months.

When I tried manually moving the axle on the valve, it's very stiff and my
guess is this is causing the motor in the power head to burn out.

Anyway, I'm pretty competent with most DIY but I haven't done much plumbing
(fitted a shower, taken raditors off for painting, fitted a sink).

Is replacing the tri-valve something that relatively easy to do. The valve
is easily accessible, but I'm nit sure what the process is and what parts of
the system need to be drained.

Any opinions, advice, pointers to helpful sites would be most welcome.

Cheers,

Nigel