I'd check the other end of the thermostat cable where it attaches to the
control board in the furnace. I did recently encounter a case with a high
efficiency (condensing) gas furnace where a pressure sensor was out of
tolerance; that fan would not run, even jumping red-green at the control
board, but that furnace would not heat either. These new furnaces have a
more complex start sequence and a fault can lock out the fan. There is
probably a diagnostic light on the control board that might indicate a fault
but that's not 100% either (the diagnostics failed too on my furnace with
the bad sensor). In any case, recommend you find/download the installation
manual if you don't have it.
-Original Message-
From: Mercedes On Behalf Of Allan Streib via Mercedes
Sent: Sunday, December 5, 2021 11:56 PM
To: Mercedes Discussion List
Cc: Allan Streib
Subject: [MBZ] OT: Home HVAC question
Anyone know much about thermostat wiring?
I got a new furnace last winter. Gas heat, with central AC. Heat and AC work
fine. I wanted to use the "FAN" mode the other day and it does not work.
There is a switch marked FAN / AUTO and with FAN selected the blower fan is
supposed to be running but it does not. In AUTO mode everything works as it
should.n
Installer is a friend and he talked me through some troubleshooting via text
messages. Wires appear to be properly connected at thermostat and furnace. I
have 24VAC at the red wire. Green wire is supposed to bring the fan on.
Tried jumping Red to Green at the thermostat and nothing happens.
Discontinuity in the Green wire? But FAN mode worked on the old
furnace/thermostat.
Other ideas?
Allan
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