Symptoms
- Furnace stops running and shows a pressure or condensate error
- Water pooling near the furnace base
- Small drain tube visibly wet or backed up
Likely causes
- CommonAlgae or sediment buildup inside the drain lineCondensate is slightly acidic and can grow slime that narrows the tube over time.
- PossibleKinked or improperly sloped drain tubingA line that doesn't slope downward properly can trap water and clog easily.
- PossibleClogged condensate pump (if the furnace uses one)Pumps can jam or fail, backing water up into the furnace's safety switch.
- PossibleFrozen condensate line in an unheated spaceA line running through a cold garage or attic can freeze and block completely in winter.
What to do
- Turn the furnace off before working on the drain line.
- Locate the condensate drain line, usually clear or white plastic tubing running from the furnace to a floor drain or outside.
- Disconnect the line at the furnace end over a bucket, and check for a clog by looking or gently blowing through it.
- Flush the line with a mix of water and a small amount of white vinegar, or a wet/dry shop vac to pull the clog free.
- If there's a condensate pump, check its float switch moves freely and the pump runs when the reservoir fills.
- Reconnect the line, restart the furnace, and watch for the error to clear.
- If the line runs through an unheated space and froze, add insulation to prevent a repeat freeze.
Tools and parts
- Bucket
- Wet/dry shop vac
- White vinegar
When to call a pro
Call an HVAC technician if the line clogs repeatedly, the condensate pump has failed, or you're not comfortable disconnecting furnace tubing.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.