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SW Missouri's Trusted Repairman

DIY Wiring A New Thermostat. Simple? Sometimes. Complex Wiring? Always.

Thermostat Wiring

“Honey, anyone can DIY wire a new thermostat! This new one even shows me the wiring. It’s a piece of cake!”

Maybe. Maybe not.

The photo above shows you the basic wiring of your typical heat pump thermostat. It looks simple right? Just take those wires and transfer over to the new thermostat, right?

Sometimes it truly is that simple. However, what if you are switching from an old thermostat that doesn’t have a C wire? Or your new thermostat’s instructions say not to hook up the C wire? C stands for “common” and it’s the low-voltage pathway returning electricity back the transformer. Some furnaces require one, some do not. Some thermostats require a C, some do not.

You know that “simple DIY Tstat install” just got a lot more complicated when your new Tstat and your electrical wiring isn’t matching up. Now you need real electrical knowledge. Because there’s more to this than just hooking up the thermostat. Both the outdoor condenser and the indoor air handler/furnace have wiring diagrams and requirements that all have to match up with the Thermostat’s wiring. Get it wrong anywhere and your day is toast.

To complicate this you need to know how to read wiring diagrams. HVAC technicians spend a couple months devoted to learning the various nomenclature, studying diagrams, making diagrams, and trouble shooting wiring circuitry. We had instructors who took great joy in trying to trip us up by wiring things wrong in very obscure ways.

While every HVAC tech deals with electricity, not every tech is a good electrician. Most companies find one guy who’s really sharp at diagnosing electrical issues, and he runs around all day finding the problems. It’s anything but simple.

On any given day on r/hvacadvice you will find 3-5 homeowners seeking advice on hooking up their thermostats. Most of the people helping them are other homeowners. It’s the blind leading the blind, because nobody there understands electrical circuits. This can lead to major problems.

Dangers of Low Voltage Thermostat Wiring

  • Shorting voltage between wires will kill your 3amp fuse on the board, shutting down your entire system.
  • If you don’t have a fuse, or somehow bypass it, you can destroy the Indoor Furnace Control (IFC) board, or fry the transformer.
  • Low voltage can kill.

Wiring from the thermostat controls the high voltage motors of your heating and cooling system. We call it the “control voltage” and outside computer chip microcircutry it’s some of the most complex wiring in your home. Behind every thermostat are yet more wiring connections going to your indoor and outdoor units. It’s anything but a cake-walk.

Ever heard “it’s not the volts that kills you, it’s the amps?” Nobody on reddit really addresses this point, but Ohm’s law (the law about electricity) states “the lower the voltage the higher the amps.” Your thermostat runs on 12-24 volts, and your amps are normally between 3-5. Just Google “how many amps to stop the heart” and you should see results like the screenshots below:

There is really good reason HVAC technicians receive lots of electrical training. 80% of all heating and cooling problems are electrical in nature, and 100% of us want to live to go home. Professionals develop safety habits to prevent accidental shock. This is ingrained training the average homeowner isn’t aware of, let alone conditioned to adhere to.

Please, don’t think the voltage in a thermostat is harmless. Nobody can tell you what voltage will kill you and what won’t. Electricity isn’t safe, or predictable in this sense. It might only shock you, or it might stop your heart.

Safest option is to call a pro.


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