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How to repair “No power to Rh wire detected.” error due to a condensate drain line clog

The Problem

My Nest thermostat was displaying the following error message when my Air Conditioner stopped working.  The odd thing was that the breaker wasn’t tripped and if I reset it the AC would run again for a short time.

 

E24

WIRING REPORT

No power to Rh wire detected.  Learn more at nest.com/e24.

The Symptoms of a Clogged Condensate Drain Line

  • No power to the Rh wire.
  • Air conditioner turns off for several hours and then starts back on, only to turn off again.
  • The circuit breaker was not tripped.
  • Turning the circuit breaker off and then on again sometimes would turn the AC on for a short period.
  • Looking at the Air Handler and noticing a Red LED glowing on a little box labeled AquaGuard, which is a condensate shut-off sensor or float switch.
AG-30000E AquaGuard Condensate Sensor with red LED glowing

How to Fix the Clogged Condensate Drain Line

Use a wet/dry shop vac to remove the clog by placing it in wet mode (remove the filter) and attaching it to the end of the condensate drain line.  Leave the vacuum on for about 3 minutes to clear the clog.

The condensate drain line is a PVC pipe that is dripping water when the AC runs.  It leads from the air handler to the outside of the house, usually located near the Condensing Unit.

Condensate Drain Line

How to attach a wet/dry vac to a condensate drain line

  • You can simply put the end of the wet vac hose over the PVC pipe and use your hand to cover the remaining opening to create a seal.
  • You could wrap some tape around it
  • You could buy a product made by DIYVAC that acts as an adapter.
  • You could make your own adapter for about $1 with a one inch to 3⁄4 inch PVC adapter and some duct tape (measure your vac hose and the condensate line as yours may differ).

Clogs could be caused by dirt dobbers or wasps making their nests or a build up of algae.

After I cleared the clog, the AC unit didn’t start-up right away and the float switch remained lit up.  So I followed the Note on the sticker affixed to the side of the air handler and removed the AquaGuard probe and dried it off.  When I reinstalled it, the A/C unit started up right away.

ATTENTION.  This unit is equipped with an AquaGuard electronic condensate shut-off sensor.  If unit is not operating properly, check for condensate in drain pan or tray.  NOTICE: If AquaGuard sensor is still activated after removing condensate clog; remove probe, clean and dry before re-installing.