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    5 Wire 24v AC/Heat System

    Asked Nov 28, 2005, 06:34 PM 3 Answers
    Hi, I bought a home. Had no thermostat. Putting in Honeywell CT3500 Programmable. Have Red, Green, Yellow, Blue, White. AC works but no heat.
    Thermostat came with R jumpered to Rc. How do I wire this to get AC and heat? Thank you.

    I need to also explain that I did put the appropriate wires to their respective
    Color coding. My impression is that I am missing a jumper some place.

    Last edited by Naturallog2; Nov 28, 2005 at 06:43 PM.
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    labman's Avatar
    labman Posts: 10,580, Reputation: 551
    Uber Member

    Nov 28, 2005, 06:43 PM
    Usually there is a 24 volt AC transformer in the furnace with the secondary winding connected to a red wire running to the thermostat and a blue wire, common, to the gas valve, A/C relay, and fan relay. From the thermostat there will be white wire to the gas valve, yellow to the A/C, and green to the fan. The thermostat is wired to switch the power from the red to the white, yellow, and green as needed with the blue completing the circuit. Most thermostats and furnaces have the contacts labeled R, B or C, W, Y, and G for the corresponding wire colors. It may be wired to have the A/C control wires return to the furnace and its controls and then a second wire goes to the A/C unit. Internal wiring may replace the green wire if the thermostat does not give you the option of fan only or continuous fan. Digital or programmable thermostats may need the blue wire connected to them.

    Leave R and RC jumpered. You may want to verify the wires follow the same system at the furnace.
    Naturallog2's Avatar
    Naturallog2 Posts: 2, Reputation: 1
    New Member

    Nov 28, 2005, 06:53 PM
    I will verify the wires on the system. You mentioned Red and Blue in the same line. Do I need to connect my blue to red. I read the instructions with the thermostat and it seems that the blue should be connected to blue and/or Orange (This obviously confused me.) Would you happen to know where the blue wire is connected.

    FYI, I do have continuous fan and my system is electric with the cooling and heating coming from the air conditioner on the roof.
    labman's Avatar
    labman Posts: 10,580, Reputation: 551
    Uber Member

    Nov 28, 2005, 07:19 PM
    DO NOT CONNECT RED AND BLUE. They are opposite sides of the transformer. The red lead from the transformer is switched on and off to the different relays by the thermostat. The blue connects the common side of all the coils back to the transformer, completing the circuit.

    The thermostat may not have all the functions needed to switch your system from cooling to heat. I have no experience with heat pumps to draw from. Working from an installation manual or even a schematic of the system, you may be able to figure out what it needs to switch modes. It might be easier to buy a thermostat meant for heat pumps if that is what you have. They may require a 7 wire cable.

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