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I am thinking about upgrading my Honeywell thermostat. Every thing I have seen describes / shows a single wire for each color. My current thermostat has two wires connected to each terminal (two green wires, two yellow wires, two black wires, and two red wires).

The new thermostat I want to install (a Nest) says to only insert one wire per connector. Is this a problem? Do I need to get my furnace rewired? Or, should I just keep the wires twisted together?

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It's certainly not a normal thing to have this, so I'd want to make sure I know what's connected prior to touching it. You need to trace the wires. How is it connected in the furnace? Do you have another thermostat in the house? –  gregmac Jul 13 at 5:19
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Gregmac is right. This is an atypical installation, and you'll want a better understanding of the system before you mess with it. Electrically speaking, you can splice the wires together with a pigtail to go from one wire to two. However, without knowing why it's wired this way, I wouldn't want to connect my new hundreds of dollars thermostat to a system I don't understand. –  Tester101 Jul 13 at 12:06
    
Yup. That's what I was afraid of. I think I will contact the company that installed it. They are usually pretty helpful. Thanks guys. –  markle976 Jul 13 at 14:13

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up vote 2 down vote accepted

The answer is yes; it was a problem. The wiring wasn't done correctly. I had an HVAC guy come out. He suspected that whoever installed my thermostat cheated by using some lower grade wire and doubling them up. Sure enough when he untwisted the wires from the old thermostat and checked the voltage with a multimeter we saw that the pair would give us the necessary 24 volts but an individual wire would only give us about 17. Nice.

Anyway, I shelled out about $100 to have him run a real HVAC cable. (I had him use 10 wire just incase I want to make use of more of the new thermostat's features in the future).

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