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Outdoor temperature sensor reading wrong on Buderus system

jnejne Member Posts: 1
I have an approx 4 year old Buderus Logamax plus GB142 gas boiler heating my house in the Boston area.   It is forced hot air with 3 blowers and 4 zone plus a few radiant floors.

A few days ago, the heat stopped working.  The blowers would run and run when I turned up the thermostat(s), but the rooms wouldn't get hot.   

The display on the GB142 showed about 66 degrees and 19PSI, but not running.

The outdoor temperature sensor (AM10?), however, is showing the wrong temperature.  It is in the 50's out and the display is showing from around 79 to 82 degrees as the outside temperature.   Reading the manuals, I guessed the WWSD was kicking in and the Buderus thought it was too warm outside to turn on the boiler.   I tested this hypothesis by raising the WWSD temperature from the default of 70 to 85 degrees, and the boiler suddenly started working.

So, I'm presuming the outdoor thermostat is bad or the wiring is having problems or some such thing.  The outdoor thermostat says Viessmann 7405121.

My question is -- is there anything I can do to further diagnose or fix or replace myself before I call in the pros and pay for a service call.   Called Buderus already and no longer under warranty.

Thanks in advance.



  • EricEric Member Posts: 261
    you can test...

    most thermistors using resistance at the temperature. There is a chart that will show what the resistance should be as the temperature changes. If you can not find the chart yourself, you can test the thermistor, verify the exact temperature when you take that reading, and call Buderus and let them know your readings and they will tell you if yours is within spec or not.

    You will need a DMM that reads resistance in OHM's for the test.
  • AnaxasAnaxas Member Posts: 1
    Remove Thermistor

    On the Buderus R2017 you can completely remove the the thermistor and the system will produce an "Outdoor Temp Sensor Error" and default to the outdoor to -10 C .  Your system should work the same way.    If you just elevate the WWSD temperature the system may not heat up the water enough because it's lowered the operating temperature based on the false reading.

    Order a new sensor (or try to find a thermistor to replace the one in the case) . 
  • heatpro02920heatpro02920 Member Posts: 991
    edited March 2013
    I should have that sensor in stock

    You pay shipping and my cost Ill toss one in the mail for ya... its the older tombstone shaped model correct? I usually unplug the sensor and control back to non odr when the sensor goes bad, then replace the sensor when it came in, but then they were discontinueing them so I bought a few for stock, Im sure I have some still in stock... Le me know, it will only take a few minutes to have someone go check the shelf and pull up my cost...
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