LMU64: strange Burner fan control (modulation instability)

Hi,

Application is Geminox Boiler, LMU64 ver. 3.03. The issue is with modulation instability of the burner.

In the attachment, there is graph of values obtain from OpenTherm bus during communication between OpenTherm master (qaa730.110 ver. 1.06) and OpenTherm slave (LMU64).
As you can see in attached graph, modulation level (and boiler water temperature of course) oscillate a little bit. Sometimes more, sometimes less. Question is why.

Changing boiler ventilator do not solve the issue.

Can it be the result of wrong parameters / fault of LMU64? I noticed, that fan control parameters of LMU64 are described in section "Burner control fan" of LMU64 2.08 documentation, but only few of them can be changed from QAA73, most of them are OEM specific.

Or is it known behavior of some fault of LMU64?

Thank you for help.

Hi,

would you mind to explain "The issue is with modulation instability of the burner." a bit more in detail. Does this happen only in DHW preparation. If yes which type of DHW System do you have (storage tank, instantaneous plate heat exchanger, stratification tank)?

with best regards

Thomas Born

thank you for your interest.

The issue appears in both boiler modes - central heating mode as well as DHW mode.

By the term "Modulation instability of the burner" I mean the following observation:

1. There is "a listener" on the OpenTherm bus that logs every OpenTherm datagram exchanged between LMU64 and QAA73.110. Most of them contains values of current state of boiler reported by LMU64. These value is recorded and drawn in the graph. We can see how the values changed in time. One of such value is "Relative modulation level". By this LMU reports value of burner modulation (between 0 and 100%). The issue is that this value changes quickly as you can see in the graph attached on my original mesg (black line in the end of graph, time between 2.55 and 3.10). The application is Geminox 2-17DC boiler - the modulation depends only on fan speed. Fan speed is controlled by LMU64.
   
2. Listening next to the boiler, one can hear that fan really changes speed. The sound of boiler's fan is going up and down. These "human" observation correspond  with values from the graph  - sometime it is better (less changing) sometime worse.
3. It seems that the burning process is less efficient, but I haven't got enough data to confirm this.
   

I assumed that there was a problem with boiler fan (ventilator). But as I wrote, changing the ventilator does not solve the issue.

Now I'm not sure what is cause and what is consequence. It is clear that the fan speed control is incorrect. Should it be fault of LMU64? Or is there something else on which LMU64 does the decision about fan speed and that is wrong? Unfortunately, there is only very limited information about the process of fan speed calculation by LMU64 (or I haven't the right documentation). That's why I asked here, any suggestion will be very appreciated.

Of course I'm ready to answer any question about the issue.

Thank you,

Roman

PS: Attached graph represent the issue even better: the relative modulation (black line) changes quickly from 15 to 27 % and vice versa. It is not expected fan control by LMU64 :(

Hello Roman,

the recording show two problems: instable modulation level and instable boiler flow temperatur (Boiler water temperature).

Since you already changed the fan my next guess would be a problem with the PID Setting. This is of course based on the assumption that the PID Settings were changed by someone. But since Google a standard password isn't that safe anymore.
After that there are other components which might have an influence: boiler pump, temperature sensor, gas valve, flue pipe (exp. horizontal part partly filled with water), heat exchanger. But none of them really fits to the problem.

An analysis is really difficulty due to the poor resolution of the OT recording.

best regards
Thomas

thank you very much for your reply. Pls. let me know if you agree with the following conclusion (for now :)

1. Described boiler behavior is real issue and should be investigated and fixed.
   
2. OpenThem recording can be use to detect problems, not to analyze it (because of poor resolution of OT data)
3. OCI420/ASC420 is required for investigation of the issue.

I don't believe it is PID setting issue (nobody play with it). My guess is boiler pump. It is Grundfos UPER 15-50 N1 AOS with PWM control. Anyway, analyze with ASC420 should give the answer.

Thank you, regards,
Roman

Hello Roman,

indeed the usage of ACS420 will allow a depper analysis of boiler behaviour. Please understand that I can give only a recommendation what might be the problem with 3rd party devices.

best regards
Thomas

I've call Geminox engineer to connect service tool and analyze the issue. Unfortunately, after one hour of  parameter checking, there was clear modulation instability issue but no error of any device was found.

I've force the engineer to change the boiler main pump, but the issue is still the same.

I believe that next step should be to log LMU parameters for longer time. Is it possible to connect Web-Server OZW672.01 to LMU64 and run ACS790 to log trends there? Is it necessary to use OCI420A109 clip-on?

Thank you for advice,
Roman

Hello Roman,

yes it is possible to connect Web-Server OZW672.01 to LMU64 via OCI420A109 clip-on. ACS790 is not necessary since OZW is able to log data on its own.

update:
To Access LMU64 via OCI420 it is neccessary to use a hydraulic schematic with a zone controller (64+). A Zone Controller must not be present. In General add 16 to the existing schematic.
Example: schematic 60 --> 76

best regards
Thomas

Thank you for advice. Web-Server OZW672.01 is now connected to LMU64 via OCI420 and data is collected. There are first results and questions. All data have been collected in 5s intervals on OZW672.

1. The attached graph shows correlation between fan speed and Current control difference. The question is why is current control difference changing so quickly.
   

next graph follows....