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Ask Mr. Jeff Pearson Your Own Question
Mr. Jeff Pearson
Mr. Jeff Pearson, Mechanical Engineering Technologist
Category: HVAC
Satisfied Customers: 155
Experience:  25 years as Refrigeration Contractor,
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We have a Lennox furnace [ Model G24M3/4-100A-2; Serial #6395C31989

Customer Question

We have a Lennox furnace [ Model: G24M3/4-100A-2; Serial #6395C31989 ] with a weird problem. After many visits from our plumbing/heating company over the past year in which we essentially rebuilt the unit with new motors, new mother board, new pressure switch, new flame sensor the furnace finally started operating normally until yesterday. Now it runs normally BUT only with the front panel removed & not screwed into the body of the furnace. When it went off the red & green trouble lights were flashing alternately indicating the burners were not igniting. So I turned off the main power to the furnace ( it is on its own circuit) then back on with the front panel off. The furnace started operating immediately with both lights flashing together, more rapidly when burners on. I left it like this for the last 24 hrs, and had no problems. Today I attached the front panel back on and immediately the problem came back ie lights flashing alternately & no burner ignition. Took panel off again and all running smoothly. Any idea what is happening???? Thanks
e-mail to Jack at [email protected]
Submitted: 6 years ago.
Category: HVAC
Expert:  Mr. Jeff Pearson replied 6 years ago.

NEVER run your furnace with the front panel removed. Are you talking about the upper cover that covers the gas valve, burners etc., or the cover for the blower/filter compartment? If it is the lower access door for the blower compartment, the furnace can and will draw in combustion gases and send them into the house which

can cause health problems or even death.

If you are talking about the other door, this wont happen however you should not

run the furnace with any doors missing.


The furnace has a "Door Switch" that is designed to prevent the furnace from operating if the door is not fastened correctly. If replacing the upper door is causing the furnace to stop running, it may be somehow interfering with the door switch causing it to disable the furnace.


Locate the door switch and observe it while installing the upper door, does it cause some deformation when installed, or does it cause the lower door to not seal properly? In some applications the upper door must be installed before the lower door and visa versa in some furnaces.


This door switch is most likely the source of your problem, however the upper door may be in contact with some furnace wiring when installed. You need to observe these things when installing the door and you should be able to determine exactly what is causing the furnace not to operate.

Customer: replied 6 years ago.
<p>Thanks. I will go and have a look this morning but our furnace has only one top to bottom, louvered, front panel that when removed exposes both the burner area as well as the blower motors, mother board etc area. Would this style of furnace still have a door switch? Previously I have not noticed one but will look again. I gather from your comment that none of the wiring should come in contact with this panel, correct? Look forward to your reply.  BTW it appears that there is a 2 hr time difference in our locations, I am MST.</p><p>Some additional information:  I have checked all wiring & narrow tubing to make sure nothing touching front panel when installed.  Their is no door switch on this model of Lennox furnace.  Have tested some senarios this morning.  Themostat set at 24C.  With panel off & leaning against furnace with no contact on 3 sides, burners ignite at 23.5C & stay on continuously until thermostat reaches 24.5C then off.  I then put panel back on & waited for next heating cycle.  Thermostat down to 23C with "heat on" indicator showing but furnace did not start.  Took panel off, red & green lights flashing alternately.  Turned main power to furnace off for 10 sec. then on.  Furnace started operating immediately, venting fan, followed by burner ignition, followed by blower fan. Left panel leaning up against furnace with approx 1/2" to 1" gap open around bottom half of panel.  Furnace operated normally until thermostat showing 24.5C then went off.When thermostat down to 23.5C furnace started up normally.  Let run for 1-2 min then put panel on, continued to run normalwith light flashing together in unison until thermostat reached 24.5C. then went off.  Left panel on until next heating cycle.  Started up at 23.5C as normal but when blower fan motor came on burners went off and then started cycling approx. 10 sec. off, 10 sec. on for about 1 1/2 minutes then burners off & did not start up again.  Red & green lights start flashing alternately. (while cycling they remained flashing rapidly in unison).  After about 1 min blower fan motor turned off.  Thermostat remained at 23.5C with "heat on" showing after all had stopped. Lights continue to alternating flash.  Thermostat cooled down to 22.5C (set at 24C) but still no start up of furnace so started up again with power off/on switch.  Hope this helps a little more. Need to keep furnace running as is -21C where I am.</p>
Customer: replied 6 years ago.
Any chance you are going to answer my repy, Jeff? Your initial answer was unsuitable as it did not apply to the model of Lennox furnace I have. Look forward to your reply.

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