why does my heat keep blowing cold air for a minute before it gets hot? it never did this with my old thermostat.

Diagnosis

The issue is caused by a conflict between the Honeywell RTH9585WF's fan control logic and your furnace's internal blower delay. In a standard heating cycle, the furnace burners must reach a specific temperature before the blower fan kicks in to push that heat into the rooms. If the thermostat is configured to control the fan independently (G-wire activation), it triggers the blower immediately upon requesting heat, regardless of whether the burners have ignited. This results in "cold blowing," where the fan pushes ambient basement or attic air through your vents for 30 to 90 seconds until the heat exchanger warms up. This often happens after upgrading to a WiFi-enabled thermostat because the default "Installer Setup" settings may be configured for a heat pump or a specific furnace type that differs from your actual HVAC hardware. You can confirm this by listening to your furnace; if you hear the blower motor start instantly when the thermostat clicks, but don't hear the "whoosh" of the burners for another minute, the thermostat is overriding the furnace's internal logic.

How to Fix It

To resolve this, you must move the fan control from the thermostat to the furnace control board. This ensures the furnace only activates the blower once the air is actually hot.

1. Press and hold the Menu button for exactly 10 seconds to enter the Installer Setup mode.
2. Use the arrow keys to navigate through the numbered settings. You are looking for the System Setup section.
3. Navigate to the setting for Fan Control in Heat Mode (typically found in the 200 or 300 series of the setup menu depending on the firmware version).
4. Change the setting from "Thermostat" or "ON" to "Furnace" or "AUTO." By selecting "Furnace," the RTH9585WF stops sending a signal to the G-wire during the heat call and lets the furnace's internal sequence handle the blower.
5. Press Done or Menu to save the settings and exit the installer menu.
6. Test the system by raising the temperature by 3 degrees. The blower should now remain silent for 30-60 seconds while the burners warm up, then kick in only when the air is hot.

If That Didn't Work

• Check for a "C-Wire" Power Issue: If you are using a power extender kit or lack a common wire (C-wire), the thermostat may be "power stealing" from the G-wire. This can cause the fan to cycle on and off erratically or trigger a premature start. Ensure the C-wire is securely landed on the C terminal.
• Inspect the Flame Sensor: If the fan blows cold air for more than 2 minutes, your furnace may be failing to ignite. The thermostat is calling for heat, the blower starts, but the burners never light because a dirty flame sensor (covered in carbon buildup) is preventing the gas valve from staying open. Clean the sensor with a fine scouring pad.
• Verify Heat Pump/Conventional Settings: If the thermostat is accidentally set to "Heat Pump" instead of "Conventional" in the Installer Setup, it will activate the auxiliary heat and fan simultaneously. Ensure the system type is set to Conventional Forced Air.
• Blower Delay Adjustment: If the cold air lasts only 10-15 seconds, the furnace's internal blower delay (on the control board) may be set too short. This is a physical dip-switch or potentiometer setting on the furnace motherboard, not a thermostat setting.

Heads Up

• Avoid "Fan ON" Mode: Never set your fan to "ON" during the winter months. This will circulate cold air continuously through your home, lowering your overall efficiency and making the house feel drafty.
• Burner Noise: Once you switch to "Furnace" control, you will hear the burners ignite before you hear the fan. This is the correct and most efficient operation for a gas furnace.
• Filter Maintenance: If the fan starts late but the air feels lukewarm, check your air filter. A clogged filter reduces airflow, which can trip the high-limit switch and shut down the burners prematurely while the fan is still running.