McDonnel & Miller LWCO Switch Malfunctions

The following is a list of what causes a LWCO switch to malfunction taken from the McDonnell & Miller Service Guide.

   1. Burner motor having greater power requirements than the LWCO switch

   2. Feed pump is not properly balanced for the required fill rate resulting in rapid cycling of the switches.

   3. Shorting of power wiring in control circuit.

   4. Switch submerged in water

   5. Lightning striking electrical service to building, causing electrical overload

   6. Overloaded circuit in building, resulting in low voltage conditions which in turn causes higher amperage draw and consequent switch failure

   7. Other limits like pressure controls, relays, thermostats, etc., may short circuit, overloading all switches in the same electrical line.

   Pump motor having a dead spot, may stall and generates heat, causing overloading of switch

The most common cause of switch overload is incorrect application. Check the electrical ratings of the switch against ratings of the equipment controlled.

If ypu have any other questions about McDonnell & Miller controls please call Stromquist at 1-800-241-9471 in Atlanta or 1-800-678-7828


Over 35 years in the controls industry. Mr. Jones, Mr, Jones, Mr. Jones has a good thing going on... and lovin' it. Avid photographer, motorcyclist, and antique motorcycle restorer.

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  1. john says

    have a McDonald-Miller 851 24V. manual reset. when wired up and installed lights do not come on and niether button on top will shut off burners. when the left hand button is pushed the amber light lights up but the red light never has lit. boiler fires and runs but lwco does not seem to work. i need to know how to test lwco and trouble shoot.

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