Most Likely Causes (Ranked)
High-efficiency condensing furnaces (90%+ AFUE) produce significant condensation as a byproduct of the secondary heat exchanger. This water drains through a condensate line to a floor drain or pump. Dirt, algae, mold, and mineral buildup clog this line over time, causing water to back up and pool around the furnace. You can identify a high-efficiency furnace by its PVC (white plastic) exhaust pipe. Standard-efficiency furnaces have metal exhaust pipes and do NOT produce condensation — if yours has metal pipes, the water is coming from elsewhere.
If your furnace can't gravity-drain to a floor drain, it uses a condensate pump to move water to a drain or exterior. These pumps have a lifespan of 3-5 years and fail from clogged intakes, burned-out motors, or stuck float switches. When the pump fails, water fills the reservoir and overflows onto the floor. Some furnaces have a safety switch that shuts the furnace off when the pump fails — you may notice the furnace stopped working AND water on the floor.
The flexible hose connecting the furnace to the condensate pump or floor drain can crack from age, get knocked loose during maintenance, or disconnect from vibration. Even a small gap drips steadily during furnace operation.
Whole-house humidifiers mounted on the furnace plenum use water to add moisture to heated air. A stuck solenoid valve, cracked water panel, or clogged drain can cause water to leak into or around the furnace. This is especially common at the start and end of heating season when humidifiers are turned on/off.
If your furnace leaks water in summer, the likely culprit is the AC evaporator coil mounted above or inside the furnace air handler. A clogged condensate drain, frozen coil (from dirty filter or low refrigerant), or cracked drain pan causes AC condensation to drip onto or around the furnace. This is the #1 cause of 'furnace leaking water' calls during cooling season.
How to Fix It
Clear Condensate Drain Line
$0-30 DIY- 1Locate the condensate drain line — a small PVC or clear tube running from the furnace to a floor drain, utility sink, or condensate pump.
- 2Disconnect the line at the furnace end. Use a wet/dry vacuum on the opposite end to suction out the clog.
- 3Pour a 50/50 mix of white vinegar and warm water through the line. Let sit 30 minutes, then flush with clean water.
- 4Reconnect and test. Water should flow freely through the line when the furnace runs.
- 5For stubborn clogs, use a flexible drain brush or compressed air gently.
Replace Condensate Pump
$50-80 DIY / $150-250 Pro- 1Turn off the furnace at the thermostat and breaker.
- 2Disconnect the pump from power and drain lines. Remove the old pump.
- 3Install new pump in same location. Connect inlet line from furnace and outlet line to drain.
- 4Plug in pump and test by pouring water into reservoir — it should activate and pump water out.
- 5Condensate pumps are universal — a $50-80 replacement from a hardware store fits most furnaces.
Fix Humidifier Leak
$10-50 DIY / $100-200 Pro- 1Turn off the humidifier and water supply to it.
- 2Inspect the solenoid valve — if it doesn't close fully, replace it ($15-30).
- 3Check and replace the water panel/pad if saturated or mineral-clogged ($10-20, replace annually).
- 4Ensure the humidifier drain line is clear and draining properly.
When to Call a Professional
Water + electrical components = shock hazard. Turn off furnace power before investigating.
Persistent moisture causes mold growth in 24-48 hours — especially in finished basements.
A leaking furnace that also stops running may have a safety switch triggered by pump failure.
Never ignore water near electronic control boards — corrosion can destroy a $300-800 board.
If you smell gas AND see water, shut off gas immediately and call your gas company.
How to Prevent It
Pour 1 cup of white vinegar down the condensate drain every 3 months during heating season.
Replace the furnace air filter every 1-3 months — dirty filters cause frozen AC coils and overflow.
Replace the condensate pump every 3-5 years proactively.
Have HVAC technician inspect condensate system during annual tune-up ($80-150).
Replace humidifier water panel at the start of each heating season.
Frequently Asked Questions
The water itself isn't dangerous, but it can cause electrical damage, mold growth, and corrosion. The main risk is water reaching control boards or creating mold in a finished basement.
Condensing furnaces produce water as a byproduct. More running time in winter = more condensation. If the drain system can't keep up, water pools around the furnace.
It's almost certainly the AC evaporator coil, not the furnace. A clogged AC condensate drain or frozen coil drips water near the furnace. Check the air filter first.
Clearing a clogged drain: $0-30 DIY, $75-200 pro. Replacing condensate pump: $50-80 DIY, $150-250 pro. Annual tune-up to prevent issues: $80-150.