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.
DETAILS:
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.
Thanks!
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 (http://www.hvactechgroup.com/files/rth8500.pdf)
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 (http://www.hvactechgroup.com/files/st9120BU.pdf)
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.