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Weil-McLain HE II - Prepurge will not activate after first cycle - any advice?

pitamandan Member Posts: 2
Hi all,

As mentioned above, have a Weil-McLain HE II.

One day unit doesn't seem to be heating. Come down and as soon as I touch inside the unit kicks on. I eventually figure our there's a contact issue with the wires, I remove them all, no corrosion, issue remains. Then I use a toothpick to slightly expand the metal male ends inside the molex connectors to "make a better fit" when inserted. This completely resolves the connection issue. Unit stays on from here on, no "click" if I jiggle all the wires.

But now the current issue is that when the Thermostat calls for heat, the blower will come on, but the "pre-purge" stage never activates. Check this PDF, page 4, for all the stages.

If I remove the main power connector, then plug it back in, it will always work *once*. Therm calls for heat, blower blows for 20 seconds and then prepurge light comes on, then igniter and igniter light comes on, then valve light comes on and gas flows, the house gets warm. The whole cycle takes about 45 seconds until heat is flowing. Once it ends all lights go off as normal.

BUT, then when the next heating cycle starts it will do the above issue again. Therm calls for heat, blower goes for about 60ish seconds, but the PrePurge light never comes on. If I unplug the power and plug it back in, it'll work fine again for just one heat cycle.

As always suggested I do have a tech coming to look at it, but they can't get here for 3 weeks, so I'm trying my best first while I wait for that day to come.


  • pedmec
    pedmec Member Posts: 827
    Sounds like you might have a defective pressure switch. Its just a little switch that will confirm that the blowers on during the trial for ignition. Needs to switch from normally open to close. Also could have a leaking hose.
  • pitamandan
    pitamandan Member Posts: 2
    edited October 2022
    Thanks for the idea, I'll check on a pressure switch.

    Edit: Any suggestion on where that would be? Near the blower or the tubes perhaps?
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 6,761
    edited October 2022

    You might check the hoses (#5 & #6 in the illustration) that connect the pressure switch to the barbed fittings on the flue collector box. they sometimes can get clogged with condensation (water)

    I hav also found that on many HE boilers have a problem with the Barbed fittings get rusted over or blocked with corrosion over several years of use. poke something in those holes. They should be clear on the other side (inside).
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics