replacing analog with digital room thermostat

dxtans · 3 Oct 2018

Hello,
I wish to replace my current room thermostat ( WS4) with a digital one. (Honeywell DT90E)
But am confused with the wiring. I need help please. What needs to be wired into the new one.

Thanks
Dxtans

Current: British Gas WS4
N: Black
L: Brown
3: Grey
( plus earth)
The DT90E has the following terminals, ( is it like for like) ie: N to N and L to L ?
L and N.

thanks any guidance welcomed

SFK · 3 Oct 2018

DXtans
Manual is here: http://www.free-instruction-manuals.com/pdf/pa_457920.pdf
Your Setup is Diagram A.

Your new thermostat uses Batteries and so does not need any connections to actually power it.
It is simply providing a switch to switch on and off a signal (in this case 240V) to the boiler.

Your wiring is:
Live: Brown
Neutral: Black
Switched Live: Grey
Earth

So on your new Thermostat
A: Live In: Your Brown Wire
B: Live Out (Switched Live): Your Grey Wire
C: No Connection: no wires connected to this
Neutral - put in a termination block and place safety away from other cables.
Earth - put in a different termination block and place safety away from other cables.

Now when Thermostat calls for heat, the Switch in the thermostat is moved from C to B, and the Live IN cable is connected to the Switched Live cable, making it live and activating the boiler.

SFK

EFLImpudence · 3 Oct 2018

The N on the diagram is after the rectangle which represents the boiler or valve - whatever it turns on.

Echo the husky · 3 Oct 2018

I wonder how many people have put a neutral wire in terminal B and had a loud bang when the stat called for heat.

DetlefSchmitz · 3 Oct 2018

...and has to buy another thermostat...

dxtans · 3 Oct 2018

Thanks SFK,
Your description and explanation is very lucid.
I will attempt the replacement tomorrow and post back the results
Thanks
dxtans

dxtans · 4 Oct 2018

Now replaced the analogue with the digital one. As per SFK instructions. Tested the temp settings. All working as expected.
Thanks for your help SFK.

thanks
dxtans

SFK · 4 Oct 2018

Dxtans,
My pleasure.
Always great to hear how it went.
SFK

Mottie · 4 Oct 2018

I have the DT90E and what I don't like about it is that when the heating first comes on it never goes straight to the set temperature. It normally switches off 1 degree before the set temperature for a few minutes, then starts again until .5 of a degree away from temperature, then switches off again then switches on. Finally when it gets to the set temperature it stays at that temperature but switches on for a minute or so every 10-15 minutes. I phoned Honeywell up at they say that is normal. Is it?

SFK · 4 Oct 2018

Motman,
Never used this thermostat, but looking at the details on page 8 of the manual and the following document, yes it is normal. Presumably this is to minimise Overshoot.
If you wish to change this you need to modify the parameters including the "Proportional band width" and the "Minimum On/Off time".

Manual is here: http://www.free-instruction-manuals.com/pdf/pa_457920.pdf

Proportional band width: https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=2ahUKEwip8YvF1-zdAhWoL8AKHXuGCyEQFjAAegQICRAC&url=https://www.honeywelluk.com/documents/All/pdf/TPI%20Functionality%20explained.pdf&usg=AOvVaw3cbVsv9wfDRixp-I1TsdpH

And some details of all the Parameters and possible settings, (mostly suggesting that you should not change these parameters in normal use) is here:
https://www.diynot.com/diy/threads/honeywell-dt90e-thermostat-setting-up-help-needed.217838/

ban-all-sheds · 4 Oct 2018

Echo the husky said:
I wonder how many people have put a neutral wire in terminal B and had a loud bang when the stat called for heat.

As many as those who fall short of the competence requirements in the installation instructions.

Scot Maclachlan · 3 Jan 2020

My existing stat looks like this. Can anyone tell me how I should replace it with a DT90E1012?

AndyPRK · 3 Jan 2020

Not seen one of them before

EFLImpudence · 3 Jan 2020

What model is the old thermostat?

Is there a wiring diagram on the inside of the cover?

EFLImpudence · 3 Jan 2020

It looks very complicated for just a thermostat.

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replacing analog with digital room thermostat

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