Upgrading a 30+ year old GE heat/cool thermostat, running an oil fired hot air plus central A/C system to a programmable Honeywell thermostat. There are five thermostat wires. White, yellow, red and green are attached to W, Y, R and G terminals on the old thermostat and transfer easily to the new thermostat. But there is a fifth wire, color blue, that is connected to a terminal marked "2" on the old thermostat. Not clear how to transfer this to the new thermostat. Spoke with Honeywell for a half hour, no help from that quarter. GE sold its heating+cooling business to Trane in 1982. Placed info request with Trane, no reply yet. The old thermostat has a mechanical three position heat-off-cool switch and a two position fan auto-on switch. The PC board trace for the fifth/blue/2 wire clearly runs to a soldered connection on the heat position of the three position switch, so fifth/blue/2 wire is heat related, but exactly what? The guess is to control a humidifier, which the system does not have. What is the function of the fifth/blue/2 wire, and how might it map to the new thermostat? Thanks.

Your old system might have had two transformers. One for the furnace and one for the AC outside.

Does the old GE have a two stage heat system?

Click on this link to see what else it might be. You might also try to locate the blue wire at the furnace and see what it is connected to.

https://www.askmehelpdesk.com/heating-air-conditioning/heating-cooling-maintenance-repair-troubleshooting-frequently-asked-questions-58313.html#post268211

Traced mystery blue wire to furnace. No connect! Blue wire at furnace is coiled around the sheath. Only red, green, white and yellow make connects.

Good,, you should be done, forget the blue wire,

30 yr ac &fan only no heat

Has it been this way for 30 yr?