another wiring/control question

y from the condenser connects directly to y from the thermostat one way or another. c from the air handler connects to the other side of the contactor in the condenser.

when there is a call for cooling the thermostat closes g and y

y turns on the condenser by energizing its contactor

g turns on the blower in the air handler

this is the way it worked before sometime in the 90's when furnaces started to be set up to run low speed blower on a fan only call and high speed blower on a cooling call, when this change happened the air handler needed a y connection from the thermostat to bump the blower up to high speed on a cooling call.

w is for heat

not sure about the 3, look at the schematic inside one of the doors of the air handler. might just be an unconnected terminal that is used as a splice point for y out to the condenser.

@PeteA… Way back in time, just after the dinosaurs became extinct, there was this brief period in human history when there was no air conditioning… and then suddenly there was air conditioning. Historians believe this period was either called the Protestant Reformation or the Age of Antiquated Technology. The records are unclear.

During those ancient times, HVAC control systems were mostly just wires and relays. Circuit boards and transistors were rare magical artifacts. You would not believe the size of radios back then. They were basically an entire piece of living room furniture with vacuum tubes and wires going everywhere with AM reception. We would actually sit and watch the radio.

Those old analog thermostats and control systems used weird numbering and lettering schemes that made perfect sense to somebody in 1963 and absolutely nobody afterward. As you can see from many of the previous posts, Lennox was going through a transition phase where they were slowly moving away from their own thermostat labeling system while Carrier and other manufacturers were beginning to standardize terminal labels across the HVAC industry.

That is why on your ancient relic of a system “3” = Common (C) because apparently using the letter “C” directly would have been too easy.

For a cooling-only system, the W terminal was not needed, which is why it is blank on your AHU. Back then, there was also this exciting new concept called the heat pump, where early engineers basically tried putting a window air conditioner in backward so the hot air blew inside and the cold air blew outside. Unfortunately, this groundbreaking technology had a slight flaw: the outdoor coil turned into a giant block of ice every winter.

Then one glorious day, somebody invented the defrost cycle and civilization advanced once again.

Eventually, manufacturers added reversing valves to the outdoor units so homeowners no longer had to physically remove and rotate their window air conditioner every season like some kind of HVAC Amish ritual. Amazing, right?

But to answer your actual question:

On your unit, W1 was simply there for adding an optional heating section to the air handler. If your system was cooling-only, then W1 existed solely to confuse installers and future homeowners for generations to come.

So on your system:

• R = 24V hot from the transformer
• G = Fan relay
• 3 = Common from the transformer
• W1 = Emotional support terminal / installer confusion terminal

sometimes that's good, sometimes that's bad. sometimes it is both.

the 2 reds are probably because of 2 wire red and white cable to the condenser