How to fix a furnace leak
We'll help you locate the leak and clear the condensate drain—or know when to call an HVAC technician.
What you'll need
- Wet/dry vacuum (for clearing drain)
- Bleach or vinegar (for drain flush)
- Replacement drain pan (if cracked)
At a glance
- Shut off power and gas to the furnace before working on it.
- Check the condensate drain line and drain pan for clogs or cracks.
- Clear a clogged drain with a vacuum or bleach flush; replace a cracked pan.
- High-efficiency furnaces produce condensate—the drain must be clear.
Quick triage — pick your path
Get started
Choose the option that matches what you see. You can jump straight to that section.
- Follow this guide Work through location and repair steps.
- Clear the condensate drain Water is backing up from the drain; the drain is likely clogged.
- Check the drain pan Water is overflowing from the pan or dripping from the furnace.
- When to call an HVAC technician Gas smell, cracked heat exchanger, or drain is inaccessible.
Steps
Goal: Locate the leak and clear the condensate drain—or call an HVAC technician.
- Shut off power and gas. See Shut off furnace.
- Locate the condensate drain and pan. See Locate drain.
- Clear the drain or fix the pan. See Clear drain and Drain pan.
- Call an HVAC technician for gas or heat exchanger issues. See When to get help.
Shut off furnace
Goal: Work safely with the unit off.
- Turn off the thermostat and gas supply valve. Turn off the circuit breaker. Confirm the unit is off and cool.
- Good: No pilot or burner operation. Proceed to Locate 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.
- Trace the condensate drain from the furnace to the exit. Find the drain pan under the heat exchanger.
- Good: You identify the drain path and pan. Proceed to Clear drain.
- Bad: Cannot access—call an HVAC technician.
Clear drain
Goal: Clear a clogged condensate drain.
- 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. 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. Replace if cracked. If overflowing, clear the drain. 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.
When to get help
Call an HVAC technician if:
- You smell gas.
- The heat exchanger is cracked (carbon monoxide risk).
- The drain is inaccessible.
- 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.
- Furnace operates normally without leaking.
Reviewed by Blackbox Atlas
Frequently asked questions
- Where do furnaces usually leak?
- Common spots: the condensate drain line (clogged with algae or sediment), the drain pan under the heat exchanger, and the humidifier drain if present. High-efficiency furnaces produce condensate and must drain properly.
- Can I fix a furnace 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 electrical issues require an HVAC technician.
- When should I call a plumber for a furnace leak?
- Call an HVAC technician for gas leaks, a cracked heat exchanger, or if the drain is inaccessible. For a clogged condensate drain that you can reach, DIY is usually fine. Shut off gas and power first.
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