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How to fix a furnace condensate leak

We'll help you diagnose and fix a furnace condensate leak—clear clogs, fix the drain pan or slope—or know when to call an HVAC technician.

Troubleshooting · Home plumbing
30–60 min
What you'll need
  • Wet/dry vacuum (for clearing drain)
  • Bleach or vinegar (for drain flush)
  • Replacement drain pan (if cracked)
Step 1 of 8

Quick triage — pick your path

Quick triage — pick your path

Choose the option that matches what you see. You can jump straight to that section.

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Steps

Goal: Confirm the leak source, shut off power and gas, then clear the condensate drain or fix the drain pan and slope.

  • Confirm the leak is from the condensate drain, not gas or the heat exchanger. If you smell gas, evacuate and call the gas company from outside.
  • Good: Water at the drain path—condensate leak. Proceed to Shut off power and gas.
  • Bad: Gas smell or water from the burner—call an HVAC technician. See When to get help.

Shut off power and gas

Goal: Work safely with the furnace off.

  • Turn off the furnace at the thermostat. Shut off the gas supply valve at the furnace. Turn off the circuit breaker for the furnace.
  • Confirm the unit is off and cool before inspecting. If the breaker is unknown, turn off the main furnace breaker.
  • Good: No pilot or burner operation. Proceed to Locate the condensate drain.
  • Bad: Unsure which breaker—turn off the main furnace breaker. If you smell gas, leave and call the gas company.

Locate drain

Goal: Find the condensate drain path and drain pan.

Clear drain

Goal: Clear a clogged condensate drain.

  • Look for water backing up at the drain outlet or in the drain pan. Use a wet/dry vacuum on the drain line outlet to pull the clog.
  • Or pour a cup of bleach or vinegar into the drain line access and flush with water.
  • Good: Drain flows freely. Proceed to Verification.
  • Bad: Drain is still clogged—try a drain snake or call an HVAC technician.

Drain pan

Goal: Fix or replace a cracked or overflowing drain pan.

  • Inspect the drain pan for cracks. A cracked pan cannot be repaired—replace it with a matching part.
  • If the pan overflows, the drain is likely clogged—clear it first. Confirm the drain slopes downhill.
  • Good: Pan is intact and drain is clear. Proceed to Verification.
  • Bad: Pan is cracked and not accessible—call an HVAC technician.

Drain slope

Goal: Confirm the condensate drain slopes downhill.

  • The condensate drain must slope toward the exit. A flat or uphill section causes water to back up.
  • Reposition the drain line if possible—support it so it slopes at least 1/4 inch per foot.
  • Good: Slope is correct; water drains without pooling. Proceed to Verification.
  • Bad: Cannot reposition or still leaks—call an HVAC technician.

When to get help

Call an HVAC technician if:

  • You smell gas (evacuate first, call the gas company from outside).
  • The heat exchanger is cracked (carbon monoxide risk).
  • The drain is inaccessible.
  • The leak persists after clearing the drain and checking the pan and slope.
  • You are not comfortable with the repair.

Do not attempt to repair gas or heat exchanger issues. If you smell gas, leave and call the gas company.

Verification

  • Condensate drain flows freely when the furnace runs.
  • No water pooling in the drain pan.
  • No drips from the furnace or drain line.
  • Furnace operates normally without leaking.
  • Run the furnace for 30 minutes and confirm no water appears at the drain path or pan.

Escalation ladder

Work from the device outward. Stop when the problem is fixed.

  1. Confirm leak source Verify the leak is from condensate, not gas or heat exchanger.
  2. Shut off power and gas Turn off thermostat, gas valve, and circuit breaker.
  3. Clear condensate drain Use vacuum or bleach flush to clear clogs.
  4. Fix drain pan or slope Replace cracked pan or reposition drain line for proper slope.
  5. Call HVAC technician Gas smell—evacuate, call gas company. Cracked heat exchanger or inaccessible drain—call technician.

What to capture if you need help

Before calling support or posting for help, have these ready. It speeds everything up.

  • Furnace make and model
  • Where the leak appears (drain outlet, pan, line)
  • Whether the condensate drain is clogged
  • Drain pan condition (cracked, overflowing)
  • Drain line slope
  • Steps already tried

Is the leak from the condensate drain, not gas or heat exchanger?

Water at the drain line or pan is condensate. Gas smell or water from the burner area needs a technician.

Check where the water appears—at the condensate drain outlet, drain pan, or PVC line. If you smell gas, evacuate immediately and call 911 or your gas utility from outside. Good: water at drain path—condensate leak. Bad: gas smell or water from burner—call technician.

Yes No (gas or heat exchanger)

You can change your answer later.

Have you shut off power and gas?

Turn off thermostat, gas valve, and circuit breaker before working.

Turn off the thermostat and gas supply valve. Turn off the circuit breaker for the furnace. Confirm the unit is off and cool. Good: no pilot or burner. Bad: unit still on—find the correct shutoffs.

Yes No

You can change your answer later.

Shut off power and gas first

You must shut off power and gas before working on the furnace. Turn off the thermostat, gas supply valve, and circuit breaker. Confirm the unit is off and cool. Then return to the question "Have you shut off power and gas?" and answer Yes to continue.

Done

Is the condensate drain clogged?

Water backing up at the outlet or in the pan suggests a clog.

Use a wet/dry vacuum on the drain outlet to pull the clog. Or pour bleach or vinegar into the drain access and flush with water. Good: drain flows freely. Bad: still clogged—try drain snake or call technician.

Yes (cleared) No (still clogged or different cause)

You can change your answer later.

Is the drain pan cracked or overflowing?

A cracked pan must be replaced. Overflow suggests the drain is clogged.

Inspect the drain pan for cracks. Replace if cracked. If overflowing, clear the drain first. Confirm the drain slopes downhill. Good: pan intact and drain clear. Bad: pan cracked and not accessible—call technician.

Yes No

You can change your answer later.

Does the drain line slope downhill?

Flat or uphill sections cause water to back up.

Reposition the drain line so it slopes at least 1/4 inch per foot toward the exit. Support the line so it does not sag. Good: slope correct, water drains. Bad: cannot reposition or still leaks—call technician.

Yes No

You can change your answer later.

Leak fixed

Condensate drain flows freely. Run the furnace for 30 minutes and confirm no drips. Install a condensate drain tablet annually to reduce algae buildup.

Done

Call an HVAC technician

Call an HVAC technician if: you smell gas (evacuate first, call gas company from outside); the heat exchanger is cracked; the drain is inaccessible; or the leak persists after clearing. Do not work on gas or heat exchanger issues yourself.

Done

Reviewed by Blackbox Atlas

Frequently asked questions

Why does my furnace condensate drain leak?
High-efficiency furnaces produce condensate that drains through a PVC or rubber line. The drain clogs with algae or sediment, the drain pan cracks, or the line slopes flat or uphill—all cause water to back up and leak.
Can I fix a furnace condensate leak myself?
Yes, for condensate drain clogs and some drain pan issues. Clear the drain with a vacuum or bleach flush. Replace a cracked drain pan if you can access it. Gas or heat exchanger issues require an HVAC technician.
When should I call an HVAC technician for a furnace condensate leak?
Call an HVAC technician if you smell gas, see a cracked heat exchanger, or the drain is inaccessible. Shut off gas and power first. Do not attempt to repair gas or heat exchanger issues yourself.

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