Replacing a basic room stat.

Phill Most Chill · 22 Nov 2023 at 19:44

Hello
I am trying to replace old mechanic Honeywell thermostat for what the manufacturer says is a universal Honeywell DT90E and should be suitable for all boilers. However after swapping the stats, DT90E did not communicate with the boiler and I had to swap back to the old stat. The manual states that DT90E is either 24V or 230V. Therefore I just left the DT90E connected to the 24V connection on PCB board as it was with the old stat, please open the link: https://drive.google.com/file/d/1VF8SHWkA6Et-bOvm_1psXnU82OBj1Jr2/view?usp=sharing. Yellow circled is the white "24V terminal" connection. Sadly for some reason there is no communication between the DT90E the boiler, with this connection. Thinking about wiring it to 230V terminal right next to it, encircled in green. Now I am not sure which terminals on brown, 230V box to use. On the 24V both wires are connected to two RT terminals. But on 230V there is only one RTterminal. Does that mean I have to use L and N terminals? Any ideas why is it not working on 24V? The Boiler is Thema Condens F30E if that helps. Thank you very much for your help and wisdom...

ZG002 · 22 Nov 2023 at 22:55

It's just a "volt free" switch. Basically, if you removed the blue and brown wires from the RT terminals on the 24v connection and put a link in place the boiler would fire up for heating.

The stat is just a switch that opens or closes in relation to it's set temp and the temp in the room it's installed (there's a clock in there also but it's all built on so ignore that for now)

Are you able to identify those two wires at your stat position?

Phill Most Chill · 23 Nov 2023 at 18:08

Sorry boss, I feel stupid now :confused:

. What do you mean by "Are you able to identify those two wires at your stat position?"

ZG002 · 23 Nov 2023 at 22:25

The two low voltage wires should be at your stat.

Probs best to get some photos of the wiring at your stat.

Phill Most Chill · 24 Nov 2023 at 19:16

Yes, of course. I f you are asking me if I know where the other end of the cable is, then yes. I already tried and swapped the stats. But it did not work...
Re-reading what I wrote in my first post, I think I should have made myself clearer: "Therefore I just left the DT90E connected to the 24V connection on the boiler's PCB board as it was with the old stat" and as seen on the picture. So on the boiler's side I had left the wires as they were and on stat's side connected them accordingly to the manual provided which is just L to terminal A and N to terminal B really. No idea why it is not working...

dale1uk · 24 Nov 2023 at 22:50

Seems strange that its not working when wired into the 24v connections. Do you have a test meter? If so you could test for continuity across the A and B stat terminals to check that the relay is latching when its calling for heat.

Richie · 25 Nov 2023 at 07:16

I looked at the manual but there's a few different wiring installations available.

Can you take a photo of the wiring with the original thermostat in place and working and also an image of the terminals on the new thermostat?

Then we can see what the corresponding Terminals should be

Phill Most Chill · 25 Nov 2023 at 20:16

First, let me thank you for your time you're devoting to this.
Here is the photo of the old functional stat:
https://drive.google.com/file/d/1n8PdRas-6p8uLg3mOAutCykpfCtPX24E/view?usp=sharing
And here is the backplate of DT90E:

backplate of DT90E.jpg

drive.google.com

Phill Most Chill · 25 Nov 2023 at 20:17

I could surely try and do connectivity test have thought about yet.. :rolleyes:

ZG002 · 26 Nov 2023 at 08:17

There is of course a connection somewhere, if you can find that next it may help, unless you do have a way to test the connections.

Richie · 26 Nov 2023 at 09:51

Going by the images.

A - Red Wire
B - Blue Wire

It's a simple switched circuit. When the thermostat switches on, it connects A with B sending a live signal to the boiler and switching it on.

How have you wired the new thermostat up before?

Adam_151 · 26 Nov 2023 at 11:22

Or other way around really, just put your brown and blue from the 24v stat header to A and B on the stat end and ignore C. Luckily the logic in your new stat is powered by batteries (have you fitted them/ removed the packing tab?) - the stat should have a display on it even with the boiler turned off. If you reconnect it, put the boiler PCB cover back on, and give it a try and don't get any response from boiler, then unclip the stat from the backplate and just bridge the contacts out and see if the boiler fires (It'll be 24v unless you have moved it from the 24v header so holding a jumper on to see if boiler fires shouldn't be an issue)