Replacing a SW400 controller with a HoneywellST6400

There's usually a switch or jumper of some kind to do this. RTFM. And have a close look over the new controller.

If there is no option, then an external relay ...

I can't find a switch which does this. Reading again the installation it looks as if I will have to find a different controller. Not sure what one yet though.

So what is your actual switching requirement?

You want the hot water timer to turn on just the hot water, and you want the CH timer to turn on both the CH and the HW?

If you do want this, the next question would be why - and if required by the existing control wiring, would changing that be an option instead?

There are detailed diagrams and circuits for all the common wiring schemes here:

(link to a downloadable version at the bottom of the article)

Sounds like he might have a gravity HW and pumped CH system. Does any such animal still exist? I thought they were last installed in the Ark!

Yep.

Mine certainly was.

We hd such a system until last year, when I converted to fully pumped.

not sure of installation date, no idea when fully pumped became common. Boiler was installed in circa 2000, but that seems to have been a replacement. Rads are old non convector finned ones, old TRV's, mid to late '70's maybe?

It's what I have. It was installed in 1977.

It was certainly the norm in the 60's. The system which I installed in my previous house in about 1969 was gravity HW and pumped CH (with a home-made "programmer"!)

But when I moved into my current house in 1977, it already had a fully pumped system installed - albeit W-Plan.