Heating needs hot water on to get it to work

[SlimD]

After advice:

Got a sealed heating system run off an old Potterton Suprema system boiler.
Can only get heating to come on by turning on both heating and hot water. Then once boiler fires turn hot water off and heating kicks in after a few minutes but Heating won’t come on by itself when first turned on. I’ve replaced 3 port valve already as old one was a bit stiff and wired it back in like for like. Is it likely that system is wired wrong in first place or something else? I checked wiring of 3 port against a y plan diagram and seemed ok but wondering if I missed something.
Wondering whether I’ll have to pull all wiring out of wiring centre and start from scratch.
Bit baffled so any advice appreciated.

 

[SJB060685]

Have you checked from the beginning with a multimeter? Ie, from programmer, through stat and to white wire?
Did you replace the whole valve?

 

[SlimD]

Have you checked from the beginning with a multimeter? Ie, from programmer, through stat and to white wire?
Did you replace the whole valve?

Not yet. Thought I’d seek some advice first.
ive just been told that on these old systems HW has to be on for CH to work but there might be a dip switch on back of timer to change setup.

 

[SJB060685]

Not yet. Thought I’d seek some advice first.
ive just been told that on these old systems HW has to be on for CH to work but there might be a dip switch on back of timer to change setup.

You are right that some older systems only give CH when HW is on but this is on gravity systems, typically not a fully pumped Y plan.

 

[John.g]

Have you checked for power (230V) on the orange wire which provides the run signal only when heating only is called for, there is also power on the white wire (CH signal) and the grey wire (hot water off signal)

 

[SlimD]

You are right that some older systems only give CH when HW is on but this is on gravity systems, typically not a fully pumped Y plan.

That’s what threw me. It’s an old boiler (20+ years) but fully pumped. I’m wondering if it just wasn’t set up right originally?
[automerge]1603636107[/automerge]

Have you checked for power (230V) on the orange wire which provides the run signal only when heating only is called for, there is also power on the white wire (CH signal) and the grey wire (hot water off signal)

I did put my multimeter on them and couldn’t find a problem. Will be checking again though when I go back.

 

[SJB060685]

That’s what threw me. It’s an old boiler (20+ years) but fully pumped. I’m wondering if it just wasn’t set up right originally?
[automerge]1603636107[/automerge]

I did put my multimeter on them and couldn’t find a problem. Will be checking again though when I go back.

I'm certain the programmer is not set to gravity as in that mode you will always have HW with CH, the HW light will stay on during this time.
I would check it all from the beginning. Make certain all connections are nice and solid everywhere.