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Thermostat doesn't switch on

PostPosted: Sat May 04, 2019 9:25 pm
by martenjacobs
Hi everyone,

I recently moved house and took my (previously working) OTGW with me. I bought this from Kiwi electronics in 2017 and it worked fine previously. Now, I'd like to connect it in the new house and I can't seem to get it to work. I have a Bosch HRE 35 boiler and an (old version) Honeywell Round thermostat (T87M1003). When I connect the thermostat directly to the boiler everything is fine. I checked the voltage to the thermostat and it looks like it's about 31 V when the thermostat isn't connected, and 6 when it is, so that seems correct. When I connect the thermostat, on the serial bus, I see a "Thermostat connected" message and after that I get "T80190000" every second. If I remove the thermostat that's reported as well. So everything seems pretty normal (although I'd also expect some other messages than T80190000).

The problem is that the thermostat doesn't switch on (at least: there's nothing on the display and I don't see any messages after turning the knob). It works when it's connected to the boiler directly, but never when connected to the OTGW. I've pretty much tried everything I could think of, but I'm probably missing something. I tried connecting it to the OTGW with very short leads (guessing the OTGW may not be able to get the power to the other side of the long leads as well as the boiler) but that didn't help. I thought I may have a broken cable, but that's not it (I connected it with some splice clamps in place of the OTGW and the thermostat works just fine).

Is there some command I need to send to the OTGW to get the thermostat to work or am I doing something else wrong?

Re: Thermostat doesn't switch on

PostPosted: Sun May 05, 2019 3:52 pm
by hvxl
This is normal for the Honeywell Round Modulation. It doesn't turn on its display until it has received some response. So, you are investigating the wrong device. You just have to connect the boiler to the OTGW. Or, if you already have, look into why it is not producing any responses.

Re: Thermostat doesn't switch on

PostPosted: Sun May 05, 2019 9:12 pm
by martenjacobs
hvxl wrote:This is normal for the Honeywell Round Modulation. It doesn't turn on its display until it has received some response. So, you are investigating the wrong device. You just have to connect the boiler to the OTGW. Or, if you already have, look into why it is not producing any responses.


Okay, that's good to know! I just did some more digging. I measure a voltage of about 5.1V on X2 and the same on the anode sides of D7 and D8, so it doesn't look like my solder snapped due to unscrewing and re-screwing the terminal. So it looks like the wiring from the boiler to the OTGW is fine. What I am seeing is that when I start up the boiler, I get some "Error 3" messages on the serial bus. I haven't seen these when the boiler is not connected or switched off.
I then installed the diagnostic firmware and performed some tests:
Code: Select all
Opentherm gateway diagnostics - Version 1.1

Enter test number: 2
530,502,540,1026,532,514,536,512,542,512,538,517,537,512,538,515,541,513,539,508,540,513,1065,504,538,1029,529,515,1063,1031,529,515,537,513,541,513,537,515,537,513,541,513,537,513,539,513,537,513,541,511,539,515,539,511,545,509,541,509,541,513,1065.
524,501,539,1032,534,508,538,518,538,510,538,514,540,510,542,510,542,515,539,508,542,525,1053,502,542,1025,529,519,1059,1029,530,512,544,508,544,508,540,517,534,518,536,513,542,508,540,513,541,510,540,515,539,513,537,515,539,511,541,515,537,511,1065.
522,506,538,1028,534,510,542,512,536,512,544,510,538,514,542,508,540,519,533,514,538,515,1061,508,534,1031,529,515,1063,1043,519,513,535,515,537,513,544,510,540,514,538,512,544,506,544,508,540,512,540,512,544,508,540,514,538,512,540,512,545,509,1066.
530,501,539,1032,528,510,540,516,536,514,539,515,535,513,541,511,541,511,543,511,539,513,1061,506,538,1031,529,511,1063,1033,527,517,537,513,537,513,543,511,539,515,535,515,537,515,539,513,537,512,540,517,535,515,539,510,540,514,540,510,542,512,1061.
524,504,538,1030,530,517,535,512,540,512,544,512,536,515,537,512,544,508,544,510,540,511,1065,506,538,1029,531,511,1063,1033,525,513,541,511,539,515,537,513,542,510,544,508,541,513,542,512,538,514,538,510,539,513,540,512,541,511,539,518,534,515,1063.
526,504,536,1032,530,512,540,510,546,508,540,510,542,510,542,510,542,512,538,513,541,510,1066,506,534,1031,531,512,1064,1031,527,513,541,510,539,513,542,512,539,513,541,509,541,513,537,515,537,519,535,511,539,513,541,515,537,513,541,513,535,515,1063.
523,502,538,1030,530,513,539,512,544,512,538,510,540,514,538,512,542,514,540,508,540,512,1066,504,535,1033,531,511,1065,1031,525,515,539,511,539,515,541,511,537,513,541,511,539,513,539,515,539,515,535,517,535,513,541,513,541,509,541,513,537,515,1063.

Enter test number: 3

Enter test number: 4
### Error: Interfaces don't appear to be looped
Opentherm gateway diagnostics - Version 1.1

Enter test number: 5
Thermostat: 0.75, 2.01
Boiler: 0.00, 0.85
Reference: 1.22

Enter test number: 6



During test 3, I waited for about half a minute, then restarted the boiler and waited about as long again, but I got no output at all.

From this I think I see that the boiler's logic HIGH is only 0.85V while the thermostat's logic LOW is 0.75V. This tells me I may have to set my reference voltage to 0.90V, but it seems a very small margin to me. Is this normal?

Re: Thermostat doesn't switch on

PostPosted: Tue May 07, 2019 9:32 pm
by martenjacobs
I just read a similar issue in this post, and checked out the voltages mentioned in that thread on my board. They're all a bit different from the values they're supposed to be, but are still quite similar. I'm wondering if these could be the issue. Of course, I am using a different boiler now from the one in the old house, so if this one is more critical of the levels or something I can see it not working now while it did work before.

Resistance between OK1 pin 1 and pin 2 of the IC1 socket => 989 Ohms (should be around 1500 Ohms)

With X1 and X2 looped and IC1 socket pin 2 connected to pin 14:

- Voltage across D11 => 2,99V
- Voltage between pin 7 and pin 4 of OK1 => 2,98V

Relative to OK1 pin 4:
- OK1 pin 8 => 3,1V (should be 4,5V)
- Cathode of D10 => 3,1V (should be 4,5V)
- Cathode of D9 => 1,25V (should be around 1,7V)
- Both sides of R1 => 3,1V | 3,77V (should be 4,5V | 5,1V)

Relative to IC1 pin 5:
- OK1 pin 1 => 1,12V (should be 1,2V)
- OK1 pin 2 => 0V (should be 0V)
- IC1 pin 2 => 5,03V (should be 5V)

I also did a measurement of X1 (and X2, they're connected)
- With IC1 pin 2 connected to pin 14: 5,17V
- Without these pins connected: 16,88V

By the way:
I'm also getting "### Error: Interfaces don't appear to be looped" when I try to run test 4, while they most definitely are looped.

Re: Thermostat doesn't switch on

PostPosted: Wed May 08, 2019 4:34 pm
by hvxl
The voltage across D11 is much too low. That should really be 4.3V or thereabouts (anything below 4V is no good). As a result the voltage on the collector of Q1 is too low to allow enough current to go through R3 to activate OK1B. Without enough current, OK1B cannot pull pin AN1 of IC1 high enough. That is reflected in your diagnostic test #5 results. So, it looks like there is an issue with or near D11.

I have no explanation why this worked with your previous setup and now suddenly is broken.

Re: Thermostat doesn't switch on

PostPosted: Wed May 08, 2019 9:52 pm
by martenjacobs
Do you think I should replace D11? I've previously used this diode for D11. Should that one work?

Re: Thermostat doesn't switch on

PostPosted: Wed May 08, 2019 10:30 pm
by hvxl
The data sheet for that one specifies the voltage to be between 4.0V and 4.6V @ 50mA. However, we're dealing here with only 2mA, which may be the reason for the deviation you're seeing. You would be better off using a zener designed for lower currents. The one I suggest on the site is specified to produce the same voltage, but then at 5mA.

Re: Thermostat doesn't switch on

PostPosted: Tue May 14, 2019 7:11 pm
by martenjacobs
hvxl wrote:The data sheet for that one specifies the voltage to be between 4.0V and 4.6V @ 50mA. However, we're dealing here with only 2mA, which may be the reason for the deviation you're seeing. You would be better off using a zener designed for lower currents. The one I suggest on the site is specified to produce the same voltage, but then at 5mA.


OK, I replaced all the zener diodes with versions made for lower amperages, and it works now! The voltage across D11 is still on the low side (still below 4V) but at least it's working now and I'm not seeing any error messages. Thanks a lot for your help!

Re: Thermostat doesn't switch on

PostPosted: Tue May 14, 2019 7:53 pm
by martenjacobs
Literally 5 minutes after posting my gateway stopped working (after working fine for about 24 hours). I've just ordered a preassembled one to replace it.