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How to Fix Nest Thermostat No Power to RC Wire Detected Problem

I've had my own handyman business as a side job for the past few years and encounter a wide variety of home repair issues.

The Nest display reporting the "no power to RC wire" issue.

The Nest display reporting the "no power to RC wire" issue.

What Causes the Nest "No Power to RC Wire Detected" Issue?

Nest thermostats are amazing devices. They enable us to adjust our home heating and cooling from our smartphones, and help us save money on our utility bills by learning our habits and adjusting home temperatures accordingly.

Once in a blue moon, however, a Nest thermostat may stop working and display a "lost power" message or another message which reads "no power to RC wire detected". After an AC technician visited our home on two separate occasions—still not finding the cause of this intermittent problem—we contacted a local HVAC technician who specializes in smart home technology. Without even having to pay a visit to our home, he was able to help us fix the issue.

The Most Common Cause of Power Loss to the Nest Thermostat RC Wire

We discovered that our Lennox HVAC's air handler unit, which is located in our attic, has a condensation drain line that leads to the outside of the home. Our line was plugged up with mold and dust, causing water to back up into the HVAC unit's drain pan and setting off a high water switch or condensation overflow switch—which caused the entire system to shut down.

Luckily, this is an easy thing to fix—in most cases—and should resolve your Nest thermostat's RC terminal power loss problem.

Please note that for power loss problems not related to a condensation overflow switch, there may be a solution for your problem on the Nest troubleshooting page.

Nest RC wire has lost power and is shown in red.

Nest RC wire has lost power and is shown in red.

How to Fix a Nest Thermostat's RC Power Issue

Check the RC Wire Status

Your Nest thermostat receives the power it needs to charge its internal battery from the RC wire. To determine the status of this wire and if it has power or not, press on the dial and then rotate it until you select the small gear icon, and press the dial once more.

Next, turn the dial until you reach "equipment", where you'll see the status of the RC thermostat wire. If the letters "RC" are red, you've lost power to this wire. Here is how to fix the most common cause of this error code.

Step 1: Turn Off the Power to Your AC Unit

Start by turning off the power to your HVAC unit. If you are unfamiliar with how to do this, consult an electrician.

Step 2: Locate Your HVAC Air Handler's Condensation Drain Line

Depending on where your air handler is located, such as in the attic or a closet, the drain line may have been placed in various locations. It will typically be a 3/4" white PVC pipe that exits somewhere on the side of the air handler, as seen in the photo below.

Step 3: Locate the Condensation Overflow Switch

The condensation overflow switch may be located next to the HVAC's air handler, near where the condensation drain exits the unit. A small low-voltage wire leads to the switch, which may also help you track it down.

Step 4: Remove the Switch, Inspect and Clean It

Once you've located the condensation overflow switch, lift up on it and remove it from the PVC pipe, then thoroughly wipe it clean using a paper towel. You can use a few drops of soap in water if necessary.

The condensation line overflow switch should have a small float at its base, which you should be able to easily move up and down. If it does not operate in an easy manner, you may need to have a technician come and replace the switch.

Place it aside and, using a flashlight, look inside the drain pipe. Inspect the pipe for any buildup of mold or dirt that could be causing the drain line to back up.

Step 5: Clean the Drain Line

If there is any serious buildup of mold and dirt, try to remove it using a pipe cleaner, cotton swab or small bottle brush. Next, using a small funnel, pour about a cup of cleaning vinegar into the pipe. This should break up any hard water deposits that may have formed, allowing condensation to flow freely. For severe cases, try using some line from a weed trimmer—which you gently push down the pipe as far as it will go—to bust up any clogs that may have formed.

Step 6: Flush and Test the Condensation Drain Line

Finally, using your funnel, try pouring a cup or two of water down the line and see if it flows smoothly. If not, you may have more serious blockage, or a broken drain pipe, which will necessitate calling a repairman.

Step 7: Replace the Switch and Turn the HVAC Unit's Power Back On

Finally, once you're sure that the condensation drain line is clean and water is flowing smoothly down it, replace the overflow switch to its original location and turn your HVAC unit's power back on.

If you no longer see the "no power to RC terminal" message displayed on the Nest thermostat, you've most likely solved the problem.

A condensation overflow switch.

A condensation overflow switch.

Vinegar can help remove hard water buildup from your condensation drain line.

Vinegar can help remove hard water buildup from your condensation drain line.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2021 Nolen Hart