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When firing the lochinvar knight boiler it went to "man reset high limit" on computer and says LOCK OUT !?!? is it air locked? the pump in on the supply side instead of the return side. does that make a difference?! please help asap! thank you

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This kind of Manual High Limit issue can most likely be caused by two things :

1) If it's a Knight XL (399MBH and up), it's equiped with a manual high limit with is located in front of the boiler, behind the front panel just under the display bezzel. It has to be adjusted and reset (with the manual reset button on the control display).

2) If your unit is NOT equipped with that kind of Manuel High Limit, this message means that there was a low voltage wire that was shorted to the ground. Either the flow switch or the thermostat wire. The 24v fuse located on the main control board burned to protect the components. Lucky, Lochinvar provide replacements in the small plastic bag attached to the connection board.

Hope this help. Even if i'm late.

Posted on Nov 11, 2010

  • willer resende
    willer resende Nov 12, 2015

    thank you guys.that was exactly what was happening with my boiler. a u$3,00 fuse solved the problem


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Lockout is usually non confirmation of ignition. The control goes through a routine to verify flame. When none is detected the control locks out the ignition sequence and usually declares it only after a number of try's. 99% of the time the flame sensor is contaminated to the point it cannot sense the heat fast enough for the control. This video is of a high efficiency furnace with a lockout problem:

Posted on Aug 24, 2010

  • Dan Webster
    Dan Webster Aug 24, 2010

    If the flame sensor is ok then the next thing to look for is safety rollout switch that has popped. Rollout happens when something blocks the ignition from properly lighting. Rusty burners allowing too much flame or partially blocked smokestacks. Rollout switches are mounted all around the burner area. Their is no set number of them. One may be in the stack, 2 near the flame itself. 1 under the furnace.

  • QcTech Nov 11, 2010

    Sir, these units are scealed combustion. They have no rollout switches at all. And a manuel reset high limit is due to a water temperature, not a flame sensor or flame issue!!



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