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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 6022
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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My Lennox g40uh furnace will not come on. When the external

Customer Question

My Lennox g40uh furnace will not come on. When the external switch (like a light switch) is on, you can hear the hum of power. I replaced a fuse, act-3, and got the red led light on again, but the blower didn't start. Not sure if there is a trick to the order of putting stuff together? Model number is *****
JA: Did you check the furnace's filters?
Customer: put fresh ones in
JA: Do you plan on doing the work yourself?
Customer: hopefully, with the help of my dad, a contractor
JA: Anything else we should know to help you best?
Customer: not sure. Everything worked fine a month ago when we still ran the A/C. Had it turned off and recently tried to turn on the heat.
Submitted: 8 months ago.
Category: HVAC
Customer: replied 8 months ago.
The 3 amp fuse blows when turned on
Customer: replied 8 months ago.
Works fine when a/c on
Expert:  airheatman replied 8 months ago.

OK if only blows fuse in heat mode lets start by doing this. (Get a few fuses to troubleshoot). Then disconnect thermostat wires at furnace board and jump R to W on board to see if fuse blows.That will let us know if issue is in furnace or external circumstance such as wiring or thermostat.

Customer: replied 8 months ago.
unfortunately, I can't find my jumper wire. Can get a new one in the morning. I'm just glad, so far, that the blower and cooling component are okay!
Expert:  airheatman replied 8 months ago.

WE can go from here anytime.Just come back when ready

Customer: replied 8 months ago.
Jumped R to W and fuse didn't blow
Customer: replied 8 months ago.
We ended up disconnecting the a/c and now the heat works! Not sure what wire or connection to mess with now.
Expert:  airheatman replied 8 months ago.

Did you try the jumper between R and W to see if heat worked?

Customer: replied 8 months ago.
Jumper between r and w worked fine.
Expert:  airheatman replied 8 months ago.

OK sounds like short is either in thermostat, or wiring between units . Go to thermostat remove all wires. Then touch R t Y and ac should come on.Touch R to G and indoor fan should come on.Touch R to W and heat shoud come on.Let me know if fuse blows any time.