Blower Runs Continuous Thermostat Set Auto Decision Tree

Why this matters

Customer set the thermostat to Auto. Fan won't shut off. Outdoor unit not running. This is a 10-minute call if you know the four common causes: G wire shorted to common, relay stuck, smart thermostat in circulate mode, or control board fan-on logic locked. Walk it in order and it sorts itself.

Symptom presentation

• Thermostat displays "Auto" fan setting.
• Indoor blower runs continuously, regardless of heat or cool call.
• Outdoor unit cycles normally or doesn't run at all.
• Cabin temperature drifts (no heat / cool being delivered, just air movement).

Quick checks (under 2 min)

Step 1: Pull the thermostat from the sub-base. Disconnect the G wire from the sub-base. Does the blower stop?

• Yes = G signal is being driven from the thermostat or from a short on the G wire. Move to Branch 1.
• No = problem is at the control board or relay, not the thermostat. Move to Branch 2.

Step 2: If the blower stopped in Step 1, jumper G back to R briefly. Blower should start. Remove jumper - blower should stop. If it stops cleanly, the wiring and relay are fine; the issue is in the thermostat configuration. Move to Branch 3.

Step 3: At the air handler, identify the blower control: fan relay (older systems), control board with onboard fan-on logic (modern systems), variable-speed ECM with its own board (high-end systems).

Isolation tree

Branch 1: G wire shorted somewhere in the run. The thermostat is calling Auto, but the G conductor is shorted to R (or to the C / 24 VAC common with reversed polarity). Causes:

• Pinched cable in the attic or wall, often shorted at a staple.
• Water damage at the outdoor unit terminal block carrying water into the thermostat cable.
• Rodent chew on the cable.
• Cable damaged during a recent unrelated trade visit (cable contractor, painter, drywaller). Trace the cable from the thermostat to the air handler. With everything off, ring out G to R and G to C; should read open. Repair or replace the cable end-to-end - no splices in inaccessible spaces.

Branch 2: Fan relay stuck closed (older systems). On systems with a discrete fan relay (typical 1980s and 1990s air handlers), the relay can weld closed from a stuck contact or a coil that's stuck powered.

• Remove power, manually try to free the relay armature with an insulated tool. If it moves, the contacts may be welded; replace.
• With G signal removed, measure across the relay contacts; should read open. Closed = welded relay. Replace.

Branch 3: Smart thermostat in circulate or comfort mode. Modern thermostats (Nest, Ecobee, Honeywell Lyric, Carrier Infinity, etc.) have a "Fan Circulate" feature that runs the fan a percentage of each hour. Customer set Auto but the underlying mode is "Circulate."

• Check thermostat settings menu: fan = Auto, Circulate %, On.
• Switch to true Auto, exit menu, watch fan behavior.
• If the customer wants the air filtered or distributed (the reason they set circulate), explain the function and how to adjust the percentage.

Branch 4: Control board fan-on logic stuck. Modern integrated furnace and air handler boards drive the blower based on a logic call (heat call routes power through HEAT FAN tap with delay, cool call through COOL FAN tap with delay). A stuck triac or relay on the board energizes the blower constantly.

• Power off, isolate the BLOWER terminals on the board.
• Power on, measure voltage at the BLOWER terminal during a no-call state. 120 VAC present = stuck board.
• Replace board per OEM model.

Branch 5: Ecobee or Nest connected to G and W simultaneously due to miswiring or a furnace with a humidifier connection. Some smart thermostats use an unused wire for accessory humidifier control or as a secondary signal. Miswire results in G energized when it shouldn't be.

• Verify thermostat wiring against the OEM diagram for the exact equipment model.
• Pay particular attention to dual-fuel, multi-stage, and humidifier accessory connections.

Branch 6: 24 VAC common (C) shorted to G at the outdoor unit terminal block. Less common but real. Water in the outdoor terminal block creates an inadvertent jumper from C to G across the moisture path. G is energized continuously even with thermostat in Auto.

• Inspect outdoor terminal block; clean, dry, seal the cable entry with electricians' putty.
• Verify ring-out after cleanup.

Branch 7: Humidifier wired to G instead of HUM terminal. Older retrofit humidifier installs sometimes tap G as the call signal. The thermostat senses humidity, calls G, fan runs, "humidify" happens. Customer experiences as "fan won't stop." Re-wire humidifier to the HUM terminal or to its own control circuit.

Confirming the diagnosis

After repair:

• Thermostat in Auto - fan runs only during heat or cool call.
• Heat call - blower turns on after the OEM heat-fan delay (typically 30 to 60 seconds), runs during burn, continues for the off-delay (typically 90 to 180 seconds), stops.
• Cool call - blower turns on with or just before the contactor (typically 5 to 10 seconds), stops with the call.
• Fan switch on the thermostat manually toggled to On - blower runs continuously, returns to Auto behavior when restored.

Repair / remediation

• Thermostat configuration: 2-minute fix at the installer menu.
• G wire short: trace and replace the cable section; pull a new cable end-to-end if the damage is in inaccessible space.
• Stuck fan relay: 15-minute swap, OEM or aftermarket; verify coil voltage.
• Stuck control board: full board replacement per OEM model; recover all installer settings before swap.
• Outdoor terminal block cleanup: clean, dry, electricians' putty seal at the cable entry.

Customer-facing explanation

Auto fan = fan runs during heat or cool calls. Smart thermostats often add a Circulate option that runs the fan a percentage of each hour for air distribution and filtration. If customer wants Circulate behavior, set it explicitly; if not, set strict Auto. Either way, document on the invoice so the customer doesn't call back next week asking why the fan ran for 20 minutes during the night.

References

• ASHRAE Handbook - HVAC Systems and Equipment
• ACCA Standard 5 - HVAC Quality Installation Specification
• NEC Article 725 - Class 1, Class 2, and Class 3 Remote-Control Circuits
• OEM thermostat installation guides (Honeywell, Ecobee, Nest, Carrier Infinity, Lennox iComfort)