Valiant Ecotech Pro 28 & Drayton Digistat 3

[HarmlessSaucer]

Hey people,

I had a quick look at this site for the issue I'm having and I found this page:
//www.diynot.com/diy/threads/valiant-ecotech-pro-28-drayton-digistat-3.228065/

I had a custom heating control system installed and I'm looking to revert back to the original from the house now that I'm moving.

I have a Valiant Echotech Pro 28 boiler and a Drayton Digistat 3 that I want to get back installed to it.

The cabling in the wall is still there and correct. I've tested that if I bridge/short the two cables at the Digistat end, the boiler stays on constantly, so the cabling looks good.

However when I wire in the Digistat it's not triggering the boiler.

Is there a way I can test the Digistat? My logic would be if I put on the heating on the stat with it off the wall and on it's own, surely it should bridge two pins on the back of it, which I can prove with a continuity test on a multi meter? There seem to be two pins constantly bridged even when the heating is set to off - is this correct?

Which pins on the stat should correspond with the boiler? What I've read is 3 to common and 4 to on?

Thanks in advance.

 

[stem]

Your reasoning is pretty much correct, but there are several versions of the Digistat. Is yours just a Digistat 3 (can't remember ever seeing a 3) or is it a Digistat +3, or +3RF? and they are all different.

If you could post details of what each of the terminals actually does rather than a number [eg Com, call for heat, satisfied, NO. NC etc.,] that would be helpful.

If it's mains powered, you may have to make a link between the live that powers the thermostat and one of the switching contacts.

Normally for switching purposes there is a 'common', and two other terminals, the 'common' will be connected to one when the thermostat is calling for heat, and the other when it isn't.

 

[HarmlessSaucer]

Thanks for the reply!
It's definitely just a straight up Digistat 3, but then it is pretty old... the sticker on the back says the warranty ran out in 2005!

It looks like this:
https://c2.staticflickr.com/4/3189/2907168160_93089eff49_z.jpg?zz=1

On the back it says
pin 1 - C
pin 2 - SAT
pin 3 - CALL

 

[stem]

Thanks. I recognise it now. It's battery powered.

So, when the thermostat is calling for heat, pins 1 and 3 should be connected together, and when it's not calling for heat (satisfied) then 1 and 2 should be connected together. You should also hear a click as it changes between the two positions.

I have known Digistats to stop switching when the batteries are getting low, or not making good contact. There is a low battery indicator, but I have known one stop working before it appears. So if the batteries have been in for a while, it might be worth replacing them.

 

[HarmlessSaucer]

Thanks for that! I hadn't considered that a low battery voltage could be the issue.
I've just tried 3 sets of them, only one of which showed the low battery indicator.
Unfortunately though, whatever I did 1 and 2 stayed connected together. I heard a click as it changes between the two positions, but the pin connections stayed the same.

I guess there's a good chance the relay in there has died or something.

I'll try again with a brand new set of AA's just to rule that out, and if not looks like I'll be getting a new one!

 

[stem]

If the thermostat has been wired up incorrectly at some point and a short circuit has been created, it's quite common for the switching contacts to become welded together, although if that happened I wouldn't expect to hear the 'click'

 

[HarmlessSaucer]

Well blow me. I worked it out.
It's fussy about the "type" of battery!
After you mentioning the batteries on here, I did a bit of digging online (I don't have the original manual for the stat anymore, only the boiler) and found stuff referring to it only working with alkaline batteries above a certain voltage.

Being 2016 and all, I had only used some new fangled rechargeable shenanigans which was powering it, but obviously not enough for it.

I dug out some alkaline batteries and the click of the relay was SO much louder- quick test with the multi meter and it worked! The pins changed!

Now I just need to wire it back into the system to test.
Just to check my sanity-
3 on boiler to Common
4 on boiler to ON
That's what I've found online and what I've deduced from the diagrams in the instructions.

Thanks again!