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  1. #1
    Join Date
    May 2007
    Sandyston, NJ
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    Lennox/Dunkirk GWB9 or Q90-100

    This boiler is an earlier model installed in 2001. The boiler is going off at 120 degrees F and is always short cycling .
    I checked gas pressure and it was 2.5" wc. I checked all safetys and high limit stat and found all ok. The dual acting hot surface ignitor/flame sensor was replaced several months ago. To check the HSI for the .6 microamps I need the special tool which i was told neither provided by Lennox or Dunkirk. I ran a time check on the boiler staying on to see if it was a temperature control issue. If I let the boiler run approaching 120F it short cycles. If I let the boiler watertemp cool down and then start it up it will run steady on until it reaches 120. I cannot see any temperature control to check other than the High limit control. How is this boiler water temp controlled and is this a temperature control problem or something else?

  2. #2
    Join Date
    May 2004
    Salt Lake City/Tooele
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    Hard to say. But these boilers should have a factory installed adjustable high limit aquastat that is adjustable. Think it comes default at 180 degrees but should not be adjusted above 200 degrees. The newer GWB models (with the inducer on the outside of boiler have the same aquastat but max setting is 190 degrees. Both aquastats should have differential dial to help with the short cycling. These boilers should operate on a thermostat of sorts. Thermostat would install on terminals P3-7 and P3-9. Could be a wall mounted thermostat if using boiler for radiant heating needs, or a well mounted thermostat to simply maintain water temperature.
    An oversized boiler or low water flow through the boiler could cause the short cycling your experiencing. Would need to verify flow rate vs manufacturer's requirements. And also verify it is properly sized for the job.
    These units are sensitive to high altitudes so they should be adjusted as per manufacture specifications.

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