Thermostat schematic diagram

[iancshay]

Where can I find a schematic diagram of a standard thermostat, like a simple honeywell heat/ac one? Something that will show me what the five different color wires do, what are the voltage levels, etc. All home heating systems must comply to some standard when it comes to how they are controlled; where can i get this? Thanks!

Ian

[suemarkp]

There are some common wire colors, but there are exceptions, and there are people that just grab some wire and put whatever color they want on a given terminal. Your thermostat is a bunch of switches. A 24 VAC wire feeds the thermostat switches, and different colored wires go to the other side of each switch so that 24V can be applied to the appropriate contactor. There is usually a switch for heat, cool, and fan, and the thermostat decides which one(s) should be energized and when. So 4 wires would be required (24 VAC source, heat, cool, fan) for the typical modern thermostat. These terminals on the thermostat are typically named by the color of wire that goes on that terminal (e.g. R = Red). The other side of the 24 VAC source rarely goes to the thermostat, but does go to the controlled equipment. Usually, the only time the other side of the 24V is brought to the thermostat is if it has lights or other things in it that consume power that is more than batteries can supply for a significant time. Most LCD thermostats can run off their batteries for a long time.

Common wire colors are:
R: Red Wire, 24 volts hot from transformer (usually in the air handler)
Y, Y1: Yellow wire, AC Compressor first stage
Y2: Blue or Pink, AC Compressor second stage
W, W1: White wire, heating or 1st stage of backup heat in a heat pump
G: Green wire, fan blower relay.
C: Brown or sometimes black, 24 volts common from transformer

Heat pumps need more wires, and the color usage is not universal, so be careful if you have one of those. You can be a manual thermostat by connecting the red and white wires together. This should turn the furnace on until you break that connection. Likewise, if you connect the red and yellow together, the AC should come on. The air handler usually determines for itself when the fan should come on. The green wire is usually used only when you want the fan on when there is no call for heat or cool. Connect red to green to make that happen.

If you have a Carrier Infinity or Bryant Evolution system, then all of the above is wrong, as they use a digital control method over 4 wires that works for even a complicated multi-staged heat pump setup. It is 24 VAC source, 24 VAC return, Digital +, Digital - (and I'm not sure of the colors).

[larryfis]

We have a vacation home that I would like to, via telephone, turn on our forced air furnace a few hours ahead of time to warm the place up. I bought a controller and matching thermostat but in the vacation mode, it does not shut off the FAU. It sets to 40 degrees minimum. We drain the pipes so we just manually turn off the thermostat now.

I returned the thermostat but kept the controller in hopes that the controller's SPDT relay switch might work to interrupt one of the 3 wires on my existing thermostat. If I open the RED wire thru the relay, is that the same as going from HEAT to OFF (and vice-versa)? I could preset a temperature in the HEAT position manually before turning OFF the thermostat? The thermostat does have batteries so I'm hoping it would retain the desired temperature, like 68 degrees.
Thanks in advance for your help!
Larry

[suemarkp]

I'm not sure if a thermostat trickles current through it to power itself, so removing power from the red wire may cause the batteries to quickly drain. So you could give it a try and see if it works. Interrupting the red is equivalent to OFF (but removes any possibility of powering the thermostat should it be sucking a trickle of current from that red wire).

[larryfis]

Thanks Mark....I'll try it.
Larry

[larryfis]

I spoke to an HVAC technician yesterday and he thought it best to leave the 24V Red wire intact and switch the White wire instead to turn Heat on & Off via the relay. I would always leave the HEAT-OFF-COOL switch in the HEAT position, set the desired temperature, then turn OFF the heat manually thru the Telephone controller's "Vacation" button. That would open the relay. Should a power failure occur, the relay would return to the same setting prior to power failure.

A telephone call to the controller would then close the relay via keypad tones when we want to pre-heat the cabin.