off-topic: modulation instability

This Forum is about the Opentherm gateway (OTG) from Schelte

Moderator: hvxl

off-topic: modulation instability

Postby rpav » Wed Feb 08, 2017 12:37 pm

Sorry for OTGW off-topic in this forum, but OTGW users can help me with diagnostic of this issue.

About month ago I noticed on OTGW graph modulation instability of my boiler.
The boiler is Geminox THRi 2-17 DC (Siemens LMU64 ver. 3.03). As you can see in attached graph, modulation level (and boiler water temperature) oscillate a little bit. Sometimes more, sometimes less. Question is why.

modulace.pdf
(29.89 KiB) Downloaded 59 times


My guesses:
1) there is something wrong with boiler ventilation (it is EBM RG128/1300-3612 and is known for problems with capacitors)
2) it is normal behaviour
3) something else is wrong

After few weeks of thinking I decided to change the ventilator. It was done today, but the behaviour is the same:
modulation_instability.pdf
(27.78 KiB) Downloaded 56 times


To be sure that this really is the issue and not the correct behaviour I would like to kindly ask other OTGW users to look at their OTGW graph and send me their experience with modulation level (in)stability. Any idea what can be wrong is of course very welcome.

Thank you,
Roman
rpav
Starting Member
Starting Member
 
Posts: 19
Joined: November 2015

Re: off-topic: modulation instability

Postby hvxl » Thu Feb 23, 2017 2:02 pm

I don't see much to worry about with the modulation level. Sure, it goes up and down a little bit, but not wildly. My boiler doesn't behave like this, but that's because it doesn't have to strain as much as yours to keep the house at the desired temperature. Mine only runs at its minimum modulation level for about 25% of the time. Yours is constantly on and needs to adjust its modulation level to produce the necessary water temperature. Due to the slow feedback in a central heating system there can a bit of oscillation. It looks like either your house is not well insulated or the heating system is quite minimal.

One thing that is unusual is that your boiler seems to regard the control setpoint as the average between boiler water temperature and return water temperature. It normally specifies the desired boiler water temperature.
Schelte
hvxl
Senior Member
Senior Member
 
Posts: 1016
Joined: June 2010

Re: off-topic: modulation instability

Postby rpav » Fri Mar 17, 2017 4:40 am

hvxl wrote: One thing that is unusual is that your boiler seems to regard the control setpoint as the average between boiler water temperature and return water temperature. It normally specifies the desired boiler water temperature.


I believe it is because the boiler is dual circuits system. It has the underfloor heating circuit as well as the radiator circuit. Boiler water temperature is a function of control setpoint circuit 1 and control setpoint circuit 2.

But it looks like there is another issue when OTG is in use with this system. Look at the dump of OpenTherm communication:
Code: Select all
20:42:15.517283  T1081020A  Write-Data  Message ID 129: 522
20:42:15.885969  BD081020A  Write-Ack   Message ID 129: 522
20:42:16.505755  T80000200  Read-Data   Status: 00000010 00000000
20:42:16.874543  B40000200  Read-Ack    Status: 00000010 00000000
20:42:17.544392  T10101580  Write-Data  Room setpoint: 21.50
20:42:17.863048  BD0101580  Write-Ack   Room setpoint: 21.50
20:42:18.533021  T90171580  Write-Data  Room Setpoint CH2: 21.50
20:42:18.851678  B50171580  Write-Ack   Room Setpoint CH2: 21.50
20:42:19.521556  T901815F4  Write-Data  Room temperature: 21.95
20:42:19.840078  B501815F4  Write-Ack   Room temperature: 21.95
20:42:20.509922  T80800000  Read-Data   Message ID 128: 0
20:42:20.829965  BE0800000  Data-Inv    Message ID 128: 0
20:42:21.032454  T01000100  Read-Data   Status: 00000001 00000000
20:42:21.422388  BC110D21B  Read-Ack    Room setpoint: -45.89
20:42:21.634796  T02100000  Read-Data   Room setpoint: 0.00
20:42:22.015963  BC210001E  Read-Ack    Room setpoint: 0.12
20:42:22.183218  T83100000  Read-Data   Room setpoint: 0.00
20:42:22.475643  BC3101E08  Read-Ack    Room setpoint: 30.03
20:42:22.650418  T83100000  Read-Data   Room setpoint: 0.00
20:42:23.003959  BC3101E08  Read-Ack    Room setpoint: 30.03
20:42:23.176466  T83100000  Read-Data   Room setpoint: 0.00
20:42:23.463774  BC3101E08  Read-Ack    Room setpoint: 30.03
20:42:23.654906  T83100000  Read-Data   Room setpoint: 0.00
20:42:23.992343  BC3101E08  Read-Ack    Room setpoint: 30.03
20:42:24.183368  T83100000  Read-Data   Room setpoint: 0.00
20:42:24.451901  BC3101E08  Read-Ack    Room setpoint: 30.03
20:42:24.620655  T02100000  Read-Data   Room setpoint: 0.00
20:42:24.980607  BC210001E  Read-Ack    Room setpoint: 0.12
20:42:25.149360  T83100000  Read-Data   Room setpoint: 0.00
20:42:25.440421  BC3101E08  Read-Ack    Room setpoint: 30.03
20:42:25.626411  T83100000  Read-Data   Room setpoint: 0.00
20:42:25.970219  BC3101E08  Read-Ack    Room setpoint: 30.03


It seems that Room setpoint is always pair of Write-Data, Write-Ack datagrams. But in this situation, OpenTherm master works in "Boiler Management Unit configuration mode" and use OT bus probably for sending OEM-specific datagrams(?). I have not read OpenTherm protocol specification so can't say if it is protocol violation or not, but the real semantics of those datagrams was not Room setpoint at all.
We can ignore it if it is only wrong interpretation of OEM-specific datagrams. More serious would be if the OTG can harm OEM-specific communication on OpenTherm bus. Look at this log:
Code: Select all
16:59:15.649827  T9014F101  Write-Data  Day of week and time of day: Sun 17:01
16:59:16.638688  T9014F101  Write-Data  Day of week and time of day: Sun 17:01
16:59:17.631027  T901607E1  Write-Data  Year: 2017
16:59:18.668408  T901607E1  Write-Data  Year: 2017
16:59:19.657196  T901607E1  Write-Data  Year: 2017
16:59:20.640789  Error 03
16:59:21.618544  T9014F101  Write-Data  Day of week and time of day: Sun 17:01
16:59:22.606165  T9014F101  Write-Data  Day of week and time of day: Sun 17:01
16:59:23.643714  T9014F101  Write-Data  Day of week and time of day: Sun 17:01
16:59:24.632427  T9014F101  Write-Data  Day of week and time of day: Sun 17:01
16:59:25.619850  T9014F101  Write-Data  Day of week and time of day: Sun 17:01
16:59:26.608557  T9014F101  Write-Data  Day of week and time of day: Sun 17:01
16:59:27.579800  Error 03
16:59:28.607372  T9014F101  Write-Data  Day of week and time of day: Sun 17:01
16:59:29.594825  T9014F101  Write-Data  Day of week and time of day: Sun 17:01
16:59:30.582216  T9014F101  Write-Data  Day of week and time of day: Sun 17:01
16:59:31.590837  T9014F101  Write-Data  Day of week and time of day: Sun 17:01
16:59:32.578327  T9014F101  Write-Data  Day of week and time of day: Sun 17:01
16:59:33.565737  T9014F101  Write-Data  Day of week and time of day: Sun 17:01
16:59:34.564532  T9014F101  Write-Data  Day of week and time of day: Sun 17:01
16:59:35.552149  T9014F101  Write-Data  Day of week and time of day: Sun 17:01
16:59:36.589608  T9014F101  Write-Data  Day of week and time of day: Sun 17:01
16:59:37.559517  T9014F101  Write-Data  Day of week and time of day: Sun 17:01
16:59:38.546858  T9014F101  Write-Data  Day of week and time of day: Sun 17:01
16:59:39.534474  T9014F101  Write-Data  Day of week and time of day: Sun 17:01
16:59:40.558116  T9014F101  Write-Data  Day of week and time of day: Sun 17:01
16:59:41.545767  T9014F101  Write-Data  Day of week and time of day: Sun 17:01
16:59:42.533378  T9014F101  Write-Data  Day of week and time of day: Sun 17:01
16:59:43.517044  T001B0000  Read-Data   Outside temperature: 0.00
16:59:44.554655  T001B0000  Read-Data   Outside temperature: 0.00
16:59:45.543424  T001B0000  Read-Data   Outside temperature: 0.00
16:59:46.579694  T801C0000  Read-Data   Return water temperature: 0.00
16:59:47.517143  T801C0000  Read-Data   Return water temperature: 0.00
16:59:48.504813  T801C0000  Read-Data   Return water temperature: 0.00
16:59:49.486051  Error 03
16:59:50.523406  Error 03
16:59:51.500911  T00110000  Read-Data   Relative modulation level: 0.00
16:59:52.488441  T00110000  Read-Data   Relative modulation level: 0.00
16:59:53.455793  Error 03
16:59:54.493168  Error 03
16:59:55.470707  T80130000  Read-Data   DHW flow rate: 0.00



Again this is the situation, where OT Master is in "BMU configuration mode". Again the semantics of the datagrams is OEM-specific. But now Error03 appears there (normally I can see only one or zero E03 per day). 10ms tmeout is probably not enough for this type of communication but I have no idea what the impact could be.

All this was done with Siemes (Landis & Staefa) LMU64 Boiler controller http://www.mhgheating.co.uk/wp-content/ ... p7494e.pdf which is widely used by a different boilers producer.

Can you pls. let me know if this should be analyse or we can just ignore it?
Thank you,
Roman
rpav
Starting Member
Starting Member
 
Posts: 19
Joined: November 2015

Re: off-topic: modulation instability

Postby hvxl » Fri Mar 24, 2017 10:02 pm

This is not an issue with the OTGW, as you claim. Those are not valid Opentherm messages. According to the spec, the lower 4 bits of byte 1 are spare and "should always be ‘0’". In your first piece of log they are not. But even so, the OTGW seems to pass them on unaltered.

The greatly increased frequency of error 03 occurrences in your second log seems to indicate that the messages in this mode even deviate so much from the opentherm spec that the OTGW is unable to decode them at all. You can try to run test 2 and 3 of the diagnostic firmware to get an idea what the signal looks like (if at lease it uses the Opentherm signal levels).
Schelte
hvxl
Senior Member
Senior Member
 
Posts: 1016
Joined: June 2010


Return to Opentherm Gateway Forum

Who is online

Users browsing this forum: No registered users and 1 guest