Lennox RTU

[mickyj]

Working on a Lennox Rooftop that keeps cycling on and off in heat mode.As soon as the blower kicks on,the flame shuts down.Draft inducer stays running, and the prossess starts allover again. When I opened the cover to inspect the heat X the unit will stay running.Even after Iput the cover back on the heat X it will keep running. But when I shut off the disconect and turn it back on (to simulate it being satisfied)The problem starts allover again. Iam thinking faulty high limit switch but when it stays running after the door comes off and then reinstsalled, I run out of answers. Any suggestions from anyone??

[mdp]

Dig out your trusty voltmeter from the bottom of the bucket and start tracing the control circuit. Find what's opening and shutting you down.

[valdelocc]

Start by jumping all the safety switches one at the time until ,you find the culprid,of jump 'em all including t-stat and see what happens if the unit acts up again then focus on flame control and rectification.

[mickyj]

Thanks all!
I'm still new and sometimes it gets a little overwelming!
In regards to tracing down the control circuit,I tried that but when the blower shut down,everything shut down.
(24V circuit)
jumping out each safty has got to do it.
Thanks again.

[service guy]

Stupid question here goes, Does this unit have a smoke detector? This could be tripping and shuting down the fan. Just a suggestion it's happen to me before let us know what you find.