De Dietrich boiler regulating weirdly

ThomasVDB

Hi,

(sorry, I'm not a native speaker). I have a modulating De Dietrich MCA25 boiler that takes care of our sanitary warm water, the radiator heating on our first floor, and the floor heating downstairs.
Last week, I connected an RS485 Modbus connection to the boilers service connector, to hook it up to our home automation system (Home Assistant). I am not a noob in automation (I am a developer in machine automation), and I hooked it up. I just read some temperature values from the bus.
However since then (or maybe before that), our floor heating doesn't work correctly anymore. It only heats up our home to about 20°. (For testing, I've set the setpoint for night and day to 25°), but the measured home temperature fluctuates between 20° and 21°.
I looked into it and this is what I am seeing :
- there are no error messages on the boiler.
- The sanitary warm water is working correctly (we have warm water).
- The setpoint for the water temperature for the floor heating is set to 30°.
- The water from the boiler goes via the hydraulic balance tank to the 3-way valve for the floor heating.
- Now what I am seeing is that the boiler is only heating half of the time. I've logged the boiler temperature and I am seeing this picture :

- So the boiler correctly regulates a certain time to to 30° (1 or 2 hours). Then there's always a spike and after that it stops heating (falls back to room temperature). During this low boiler temperature, the boiler also maxes out the 3-way valve to max hot water. But logically, the floor heating water falls to 20° and the house gets colder. Then after a few hours the boiler kicks back in and regulates again to 30°, and the floor start to heat again.
- Also I am seeing that when we activate the radiator heating for the first floor, the boiler doesn't go to 75° correctly, and so the radiaters don't warm up correctly.

So for the root cause : I don't think that I've done something wrong in the communication with the boiler (set a bad value to a certain register, but of course I can't be sure). I rebooted the boiler a couple of times but it didn't help.
Can anyone explain these weird boiler temperature fluctiations? The thermostat temperature reading always remains at 20° tot 21° (not at setpoint (25°) yet) , so there should not be a reason for the boiler to stop heating.

thx,
Thomas.

ratio

Did you unplug the Modbus connector to see if that fixed it?

Are you just interpreting data that appears on the bus, or are you sending commands\requests to the controller? Maybe a malformed packet could be confusing things?

ThomasVDB

Hi,

I unplugged the modbus controller and rebooted the boiler. I evaluated this for 2 days. Made no difference.
I am only reading from the bus. No writing/sending requests.
The data that I read via modbus, matches the readings on the display.

ratio

I'd guess that the unit's hitting a high limit somewhere & locking out for a while. Seems strange that the timing is so regular, though. Check the internal sensors for correct calibration & check for a loose/crummy connection somewhere that could be interfering. Might want to try to get tech support on the phone, although not all places will deal with a homeowner.

ThomasVDB

thx.