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low thermocouple voltage

jbalint
jbalint Member Posts: 1
in Gas Heating
I'm working on an old residential hydronic boiler. burner kicks off after 1-2 minutes. replaced thermocouple. but old and new thermocouple voltage don't cause closed circuit readings above 10mV. is this too low? I'm seeing the voltage fluctuate when the burner is running. it eventually goes below about 7mV and the gas valve closes and shuts off the pilot. I noticed that the pilot is a little far away from the thermocouple. I could maybe get it 1/4" closer but not sure if this would make a difference. when the burner is running the flame is close enough to the pilot that the airflow/flame directs the pilot flame slightly off center of the thermocouple. thanks.

Comments

  • HVACNUT
    HVACNUT Member Posts: 6,337
    The pilot assembly should be a fixed piece. How could you bring it closer to the thermocouple?
    Have you checked the pilot tube and orifice?
    How old is old? Does the pilot tube come off the gas valve or the piping at the cock?
  • Jamie Hall
    Jamie Hall Member Posts: 24,876
    The voltage from a thermocouple is determined by the two metals involved and the temperature. Therefore, for a given thermocouple -- like the one you have -- it's just temperature. The thermocouple has to be really in the hottest part of the pilot flame.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England

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