My existing 4-wire thermostat cable comes out of the furnace, and each of the wires is wire-nutted to another wire. That then goes to the thermostat.


I'm going to pull new cable to support a smart thermostat. Should I keep the splices, or should I run one continuous cable from the furnace all the way to the thermostat? If I should leave splices, should I put them all into a box?

The furnace was replaced a couple years ago, and I'm unsure if the splices were a result of that or a result of updated codes or best practices.


It is always best to have unspliced conductors. I would make sure to leave a small service loop (extra wire length) for other possible changes in the future.

  • This is absolutely the correct way to do it - UN-SPLICED. If you want it to work right all the time, no issues with possible breaks , resistive failures or mismatched colors end to end use a single run from unit to thermostat, you will be glad yo did.I placed a box in my wall where the thermostat comes in - so my wires are looped and restrained in the box. My thermostat mounts to the box.
    – Ken
    Mar 2 '17 at 19:02
  • @Ken I thought about putting a box in, but the ecobee3 apparently doesn't mount (nicely) to a box
    – mmathis
    Mar 3 '17 at 13:46

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