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


9 Responses

  1. 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.

  2. Why does Seneca place their LWCO between the top take off, & the side takeoff? Whereas the McDonald Millers mostly are placed with the make up water feed coming in the top of the LWCO which washes the float switch out with the delta P between makeup, around 20psi, and the existing boiler pressure. Seneca has no pressure difference accross the LWCO by feeding into the loop line.

  3. swapped out old burnham steam boiler for a new one just like it . power to mcdonnell&miller ps-802 low water cut off goes in but does not go out with green light flashing and red light flashing 4 times and red light stays on and then both start flashing again ,needless to say it will not fire and yes water level is full .nowhere in my operations book does not even mention lights or flashing light on this devise help, thankyou

  4. CRTD-101, part #310453 (LWCO cartridge) seems to have a slow leak. This over fills the steam boiler. What could cause it? Does it need to be replaced? What do you recommend to keep it clean and working well? It does not have too many moving parts. Please help!

  5. I have a McDonnel Miller PS802 and hooked up a water feeder and have 24v between hot & neutral but only 13v to to feed same thing in on the Lwco only 13v between hot & w what Component Is bad

  6. My steam boiler overfills. I isolated tankless,burner guy replaced float, still filling. Replace cartridge?

  7. I have a steam oil boiler which has low water cut off valve ( Mcdonneld Miller No. 67)and has a problem, gauge glass shows water is surging when its completely off ( not using the boiler now) and its connected to automatic water feeder. This problem started recently means after 20 years. when heat is on I flush the water every week. I will be appreciate If you can help me to give some advice to fix this problem. Thank you for your time. Duleep

  8. I have McDonnell & Miller model PS 802 RX2 24 everything works perfectly won’t turn on till the water reaches past the censor if I slowly drain it it will shut off just below the sensor the unimatch feeder comes on but if I push the test button the yellow light comes on it will not go into test and stop the boiler everything test 25 volts would just like to know

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