Author Topic: AC Fan Delay not working with new furnace board  (Read 3955 times)

Offline sparkhvac

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AC Fan Delay not working with new furnace board
« on: October 01, 2011, 09:30:50 AM »
ISSUE: Thermostat seems to be sending fan and cool on AC request and is overiding the furnace board settings of cool delay.

EQUIP: Furnace board - Honeywell ST9120B,U; Thermostat - Honeywell RTH8500

ASK: Can I configure this to have the board control the fan and not the thermostat without removing the G wire.

I recently had my furnace board replaced and have an issue with the COOL ON DELAY. Before this new board there was a 30 second delay. The new board supports this and I have set the DIP settings to 30 seconds, however the fan now comes on instantly when it calls for cool.

If I remove the G wire the process works fine (so it appears that the thermostat is sending for the fan on AC). I am not sure why this worked on the old board, but I don't want to leave the G off as I like to run the fan all the time.


Offline Admin

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Re: AC Fan Delay not working with new furnace board
« Reply #1 on: October 01, 2011, 10:17:39 AM »
Is this thermostat new?  The installer config may be set for electric, which causes the fan to run instantly.
Also make sure the Y wire from the thermostat and ac unit both connect to Y on the furnace board, and are not just maretted together.

Read pg 16 of the thermostat manual to adjust system options - Honeywell RTH8500 Manual

Check 170, 180 and 240 function settings in the installer menu.

If dip switch #4, on control board, is on, and Y wires are connected to the board, then it should provide a 30 sec fan on delay for cooling.

The dip switch settings can be found on pg 12 - Honeywell ST9120B, U Manual

You mentioned the cooling delay works if you remove G, so I assume the Y wires are connected to the furnace board.  If the thermostat is new, then it might be working differently than the old thermostat.  On a call for cooling, it could be sending 24V to G at the furnace.