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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
    Commented Jul 13, 2014 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
    Commented Jul 13, 2014 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
    Commented Jul 13, 2014 at 14:13

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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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    see my answer as an FYI , you might like to have it in the future.
    – Ken
    Commented Mar 2, 2017 at 19:14
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The thermostat does not have a huge current draw. So perhaps the way the wire was on the other end , resistance or otherwise - distance and wire type can also being a factor (voltage drop due to higher resistance.). Doubling the wires is not such a big deal as you might think - current travels in parallel.

That said : You had the same people who installed the unit come out .. hmm ok you're not happy with the wiring what to expect $100 to pull single wire.

Your reasoning behind upgrading the thermostat is more to the point - Why did you want to change the thermostat ? You did say it was a Honeywell - so that leads me to thinking you are experiencing some sort of cycling problem and it might be costing you a few dollars on your electric bill. The wiring would not be the issue - the Thermostat would though - my experience with Honeywell is not to buy them - they cycle way too much.

I prefer to use LUX - much better and usually has adjustable cycling range 1 to 3 degrees from temp for switching on and off - 2 degrees not a big deal but saves lots of $$.

My experience: Honeywell is set for .5 Degree generally un-adjustable. T-Stat 74, Thermometer above 74.0 Honeywell switching on as fast as it switched off (with in one minute). Thermometer never changed from 74.0 degrees nor did the thermostat yet my unit was cycling back on. Changed to LUX problem solved.

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