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) to diode pack at thermostat conductor 5: use for thermostat C. … Connect the loose wire to thermostat C. …

Connect the new thermostat to the white, yellow, and green wires also. The last step is to bring the shared/common R and the local C from the new transformer to the thermostat. … The second terminal of the new transformer connects to the thermostat C terminal. …

The wiring diagram indicates that there's a yellow wire coming off the transformer, going into a wire nut with a blue wire. That's your C. I've highlighted it in the image below.

But the original text asks twice "how can I tell whether the furnace is hitting a temperature limit" or, said differently, "how can I tell whether the furnace stops heating before the thermostat said to … Disconnect the thermostat and connect the R and W wires directly. …

A power stealing thermostat can let some nominal current flow -- not enough to activate the furnace relays -- and use this power for part or all of its own operation. …

A two-stage thermostat might normally make its decision about calling for the second stage of heat based on how far the present temperature is from the setpoint, or based on a timeout if it has been calling … An ordinary non-programmable thermostat mounted outdoors could even fit the bill (if it'll allow you to program a set point this low!). Two operating modes come to mind. …

So modifying the thermostat is easy -- you can set the thermostat aside and use a jumper wire or wire nut to start experimenting right away. … There's more to it than just modifying the thermostat though. …

A white "W" wire came back from the thermostat and connected to one side of a relay's coil. … A thermostat will open-circuit or tri-state its W/Y/G outputs when these are de-asserted. …

Yes that blue wire sticking out of the cabinet looks like it's intended to be used that way. To verify use a volt meter to measure AC volts between the blue and the red wires. It should be somewhere i …

Humidifier must run while the furnace runs Aprilaire model 600 operates on the principle of evaporation. It's an evaporative cooler ("swamp cooler") installed in the heating system. It's installed as …

When the thermostat turns off its signal the water valve closes and the humidifier dries up. … Or, if the damper should operate only when the thermostat calls for heat, you could add a relay to make it so. …

The thermostat is actually switching off early: While the thermostat should be calling for heat, check the output voltage at the thermostat. … If you ever find that the thermostat output has switched off then you can focus your efforts on the thermostat. …

It seems that such a thermostat could control a millivolt system, though, if you wire things so that the thermostat operates a relay and the relay interfaces to the millivolt system. …

How thermostat signaling works (and how to test manually) First, a little background about the thermostat control: It's dumb. Really dumb. The signaling is done by simply shorting wires together. … What probably went wrong To call for fan the thermostat will short R to G. But you had C on the G terminal, so the thermostat shorted R to C. …

It appears your thermostat is a "communicating" type, which means that it uses a two-way digital communication between the thermostat and the air handler. … If you still have trouble with this arrangement then you can be confident there's a fault in the thermostat or in the air handler -- but it won't be clear which has the problem. …