Skip to Content

Lennox Furnace Leaking Water? 4 Common Causes + What To Do

If your Lennox furnace is leaking water, you must identify the problem fast as water can drip onto electrical components. So, what are the most common causes of water leaking from a furnace, and how can you fix them?

Your Lennox furnace is leaking water because of insufficient insulation, a blocked condensate trap, a clogged furnace filter, or a damaged condensate pump. Examine the condensate line to ensure there are no leaks, clean or replace your air filter, and insulate leaky parts of the ductwork.

This article will help you pinpoint what’s causing the leak in your Lennox furnace and how to fix it.

Insufficient Insulation

Your furnace and ductwork need proper insulation for several reasons. If there isn’t enough insulation, you may find that air leaks through the ducts. When this happens, you’ll notice a temperature drop throughout your house. This is because heated air leaks at a point before the vents.

Sometimes, problems with the insulation in your HVAC system may also be the reason your Lennox furnace is leaking water. The heated air causes condensation inside the ductwork. The liquids produced from condensation are then transported toward the condensate assembly. However, if you have a hole or tear in the insulation, the condensate will leak through it.

In this scenario, you’ll likely see the water dripping from specific points of the ductwork. This particular issue isn’t likely to cause a build-up of water in the furnace’s main compartments but instead throughout the ductwork and vents. 

How To Fix

When you have a water leak in the ductwork, it shouldn’t be too hard to pinpoint where it’s coming from. When poor insulation causes a Lennox furnace to leak water, you’ll see water dripping from the ducts.

Here are the steps you need to take to fix a leaky duct:

  1. Walk along the duct line and see where water is dripping out.
  2. Mark all areas where water is leaking.
  3. Get insulation materials to implement this fix.
  4. Turn off the furnace.
  5. Remove the part of the duct line that’s leaking.
  6. Fix the leak.
  7. Reinstall the duct line.

Here’s a video that’ll help you wrap the duct:

Blocked Condensate Trap

The condensate trap on a Lennox furnace does exactly what its name suggests. It traps the condensation liquids generated inside the furnace. The trapped fluids are then discarded outside your house. The condensate assembly plays a significant safety role in your furnace. 

The condensate trap removes condensate from your furnace that’d otherwise drip onto the electrical circuitry.

Debris and grime accumulate in the condensate trap over the years. As grime continues to collect in the area, the drain pipe attached to the trap can get clogged. This stops liquids from geting out of your HVAC system. The result is a build-up of condensation inside your furnace. 

How To Fix

The condensate trap needs to be flushed and cleaned regularly. While thorough cleaning of the condensate assembly usually forms part of general maintenance on the furnace, you may have neglected to check this part or forgot to get someone to service it. 

Here’s how to clean the condensate trap:

  1. Turn the furnace off. Wait for 30-40 minutes to ensure the furnace has had time to cool down.
  2. Find the condensate trap and look for a black PVC pipe. The pipe connects to a drain that goes out of your house. 
  3. Disconnect this pipe from the condensate block and run warm water through it. The idea is to flush out any debris accumulated on the inside of this drain pipe. 
  4. Check the condensate trap for debris. Flush the part out with some warm water. 
  5. Reconnect the pipes. Make sure you return any hardware used to fasten the condensate trap. 

Clogged Furnace Filter

A Lennox furnace leaking water generally makes us turn to the condensate line. After all, this is where condensation should be removed from the furnace.

The air filter in the furnace is a good example. When the air filter becomes clogged, it’ll cause a blockage in air distribution throughout your ducts. In this situation, heated air starts to accumulate inside the furnace.Q

How To Fix

Grime and other particles collect on the filter while the furnace is running. Even when turned off, dust can still accumulate on the surface of the air filter. If this is the case, cleaning the filter might be a simple solution to stop the water leak:

  1. Turn off the furnace.
  2. Open the main compartment.
  3. Remove the filter.
  4. Clean the air filter thoroughly. Run the filter under warm water. Avoid using detergents during the cleaning process. 
  5. Wait for the filter to air dry.
  6. Reinstall it.

If you’re having trouble cleaning the filter, watch this video:

Damaged Condensate Pump

If you’ve taken a look at all three possibilities mentioned so far and didn’t have any luck, it’s time to check the condensate pump to see if it’s damaged or faulty. 

The condensate pump is responsible for pumping condensate out of the furnace. When the pump isn’t working, condensate will be left inside the furnace and the rest of the HVAC system. 

How To Fix

You first want to determine if the condensation pump is damaged. There’s no use in replacing this part if it works correctly. 

The best way to test it is to use a spare pump. Your local HVAC technician can help you with that. You can also try removing the pump and powering it externally. See if it comes on and if it works as it should. Replace the pump with an identical one if it’s faulty.


When there are problems with the condensate assembly in your furnace or the air filter is dirty, you may start seeing water leaking from the system. A Lennox furnace leaking water is a dangerous hazard, which is why I recommend getting a professional to fix the problem.


  • Alanna Greene

    Alanna is an avid traveler who lives in Michigan. In addition to writing for Temperature Master, he also sells crafts on Etsy and takes long walks through the forests near her home.

As an Amazon Associate, we earn from qualifying purchases. We may also earn commissions if you purchase products from other retailers after clicking on a link from our site.