Furnace won’t start, fan runs constantly

[Cary Joe Needham]

Lennox heater G20Q3E-75-2
Furnace won’t fire, fan runs constantly regardless of settings. Seems to be the control board. Limit switches are ok, but no apparent voltage to t-stat. Question about ignigion – according to manual this is supposed to be a standing pilot unit, but there is a Johnson G770 NGC-7 intermitent pilot ignition unit on here (obviously replacement). Has evidently been working okay (til now), but is this a problem?

[Selgas]

Have you chesked out the ignition system yet? Is it getting power to supply the spark to light? Check also with some models of Lennox units the combustion fan must run at a specific speed to apply sufficient air flow to activate the pressure switch and start the iginiton sequence, if this is not achieved it will not start up and the room fan will run constantly.

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Selgas Services Ltd
Craftsman Gasfitters, Plumbers, Electrical Service Technicians

[Keithiscrazy1]

No combustion fan. Blower starts immediately when breaker is turned on, regardless of setting on t-stat. Transformer is okay. Limit switches check okay. I think the control board is mistakenly sensing an open safety circuit and defaulting the fan to “on”, and keeping the ignition sequence from starting.

[robertgf]

the blower motor is coming on to tell you that the furnace is in a locked-out mode, does the furnace have a fan inducer on the exhaust and if so is the pressure switch stuck closed? it must start in the open position to start the firing sequence

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