Is there a way to install a 3 wire thermostat on a 2 wire system ??

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Bigal41

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I was given a gift this Christmas of two programmable thermostats ... problem is this is an older home and there is only two wires behind the the old stats. I do not have A/C just heat.

I hope there is a simple (LOL ) solution

Thank You
 

WorthFlorida

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What is the model of the new thermostat?
The old thermostat, is it 24v ac or millivolt? Model if known? A picture of the old thermostat with the cover on and off would help.
Does the new thermostat use batteries? If it does it is possible that only two wires are needed.
What type of heating system? Being CT I'm assuming a boiler with hot water radiators (hydronic).

With three wires I'm assuming one is power, 24v, one is call for heat, the third for fan control if used on a force warm air furnace. The thermostat should have three terminals, W for heat, G for fan and R for power. For heat only it will be the R and W terminals for the two wires. This should be explained in the installation instructions. Do check if the manual mentions millivolt.
 
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Jadnashua

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A thermostat is basically an on/off switch, so the smarter ones need to have power to them. Those will either use internal batteries, or require power from the HVAC system. The basic thermostat just connects the R terminal to the W terminal like a switch...there is NO return to complete the circuit for power AT the thermostat...all it does it apply power to the HVAC system, the return to complete the circuit is IN the HVAC system.

IF your thermostat requires power to run FROM the HVAC system, then, you'd need the return (called common) run up to the thermostat. There are ways to make it work without running that extra wire or cable (like using a remote 24-vac transformer and an isolation relay), and some thermostats can parasitically pull power off of the line, but not all HVAC systems will like it! I had one that I tried years ago, and it fried my HVAC system. That system used solid state internal controls, and didn't like what the thermostat did to power itself.

If that third wire is to control the fan manually or for control of the A/C system, and you don't have or want to do that, then it should work with just the two wires for simple heating control as an on/off switch.
 
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