Honeywell RTH8500 Installation Manual page 21

System setup About your new thermostat
Heating fan control
Function
Press
0180
Options: 0 Gas or oil heat: Use this setting if you have a gas or oil heating system
(system controls fan operation).
1 Electric heat: Use this setting if you have an electric heating system
(thermostat controls fan operation).
Heat pump changeover valve (for heat pumps only)
Function
Press
0190
Options: 0 Cooling changeover valve: Use this setting if you connected a wire labeled
"O" to the O/B terminal (see page 11).
1 Heating changeover valve: Use this setting if you connected a wire labeled
"B" to the O/B terminal (see page 11).
to select your heating system & fan control:
s/t
to select whether your changeover valve is used in heating or cooling:
s/t

RTH8500