My simple question is: When a thermostat has four wires: white, red, green, and yellow, the red is for 24V (AC?) and the white/green/yellow are for heat/fan/cooling. The role of the thermostat is to close red/24V to one or more of the other wires. The thermostat has 2 AA batteries, but given that the closing when "active" seems to be through relays, I don't see how those batteries could have any reasonable lifetime if they were powering the relays. How does the thermostat derive power from the available input wirings? Does it sink "milliamps" from the 24V and contact wires, but at a low enough current level to not trigger the relays in the furnace control? Or is something else going on there? If I have an answer to this question, I'm happy!
Long background, relevant to the underlying problem I'm trying to debug:
I have a three-zone, single-stage, forced-air heating-and-cooling system installed in 2007. Zone 1 and 2 dowstairs. Zone 3 upstairs. For the longest time, I had problems with zone 3 (upstairs) heating when it's not supposed to, and not cooling efficiently when it was supposed to. The original installer wasn't really available for later service, but was low cost (ahem.) We had a more reputable service company come out and service the system, but they didn't find a problem. Finally, trouble-shooting this myself, I found that the damper for Zone 3 was wired backwards -- it would close when activating cooling for Zone 3, thus the cooling wouldn't be effective. Looking at the damper (three-wire, red/green/white,) the "red" wire went to the terminal labeled "G" and the "green" wire went to the terminal labeled "R." I switched them, and it seemed to work fine after that.
Two days ago, I noticed that the system would open the damper to zone 3 (the upstairs) if any zone called for heat. Because the downstairs is a lot cooler, it now calls for heat in the winter (we're in California,) and the upstairs ended up with 85 degrees heat, even though the thermostat was set to 65. When both downstairs zones called for nothing, the blower turned off and the damper closed for zone 3. I disconnected the thermostat wiring entirely for zone 3, and this behavior still holds today. My guess is that something has gone wrong in the programming for the microcontroller on the controller board, but I'm going to measure this system in full and verify everything before I start looking for replacements.
The control board is labeled "2002 Research Systems Corporation," and matches a very old manual scan I found online for an Aprilaire Model 6203. The current manual for that same model number does NOT look the same. The furnace is a Tempstar SmartComfort (no discernible model number.) The thermostats are White-Rodgers "Classic 80 series."
Researching how heat/cool works here, I'm surprised how much of the smarts are in the thermostats. With a microprocessor board controlling the furnace / blower / cooler, I'd expect the thermostats to just be a temperature sensor and level setter, but apparently the wiring is plug-compatible with systems built in the '50s... The thermostats even do timing of purge and such, even though that's all re-done by the control board.
Final question: If I end up having to replace this system, is there some more modern system, where the thermostats speak something like RS-485+power over 4 wires, and the central unit is responsible for timing? And, ideally, perhaps the central unit has some wired IP-based read-outs? (but no cloud bullshit?)