How to fix a gas furnace that’s leaking water

A gas furnace leaking water is a common issue, and while it may seem alarming, many causes are relatively simple to fix. Water leakage can result from different parts of the furnace malfunctioning or becoming clogged over time. If left untreated, these leaks can damage the furnace or surrounding areas in your home, so it’s important to address them promptly. The good news is that many of these problems can be solved with some basic DIY maintenance. With the right steps, you can identify the cause and fix it without needing professional help.

Please note: The information provided here may not align perfectly with your specific appliance model. While we strive to offer helpful tips for a wide range of DIY appliance repairs, different models and setups may require unique troubleshooting steps, error codes, or part replacements. For the most accurate guidance, we recommend consulting the user manual for your appliance or contacting us with any questions.

Safety precautions   

Safety should always be a priority when working on any appliance. Here are a few practical tips to help you stay safe:

• Turn off the power to the appliance by unplugging it or switching off the circuit breaker before starting any maintenance or repairs. This protects you from the risk of electric shock.
• Wear insulated gloves to shield your hands from sharp edges, pinching hazards, or debris.
• Work at a steady pace and take your time to avoid accidents or injuries.
• Make sure your workspace is well-lit so you can clearly see and access the parts you’re working on.
• Keep the area clear of clutter, and ensure children and pets stay out of the workspace.
• Never handle internal parts with wet hands. Check that both your hands and the work area are completely dry.
• Refer to your appliance’s user manual for any specific instructions or safety guidelines related to the part or repair.
• Handle parts gently to avoid damage to the appliance or potential injuries from applying too much force.
• Use safety glasses when dealing with chemicals, dust, or large debris to protect your eyes from harm.
• If the appliance was recently used, give it enough time to cool down before working on any heated components.
• Take photos or notes of wiring connections and terminals before disconnecting anything to make reassembly easier.
• Avoid touching exposed wires or terminals. If you must handle wires, use non-conductive tools or wear insulated gloves to minimize the risk of electrical shock.

Read more: 12 simple space heater safety tips for this winter

Clogged or disconnected condensate drain

The condensate drain in your gas furnace is responsible for carrying away water created by the combustion process. If this drain becomes clogged or disconnected, water can accumulate and cause leaks. This is a fairly common issue that’s easy to address. Here’s how to fix it:

1. Always start by cutting the power to the furnace for safety.
2. The drain is typically a plastic pipe or hose connected to the furnace. It usually runs to a floor drain, sump pump, or outside.
3. Look for any visible blockages in the drain line. Dirt, algae, or debris can accumulate and clog the pipe, leading to a water backup.
4. If you see a clog, use a wet/dry vacuum to clear it from the pipe. Alternatively, you can use a small plumber’s snake or flexible cleaning brush to dislodge the debris.
5. Ensure that the drain line is securely connected and that there are no leaks or gaps. If the pipe is disconnected, simply reconnect it and secure it with clamps or other fasteners.
6. Once clear, flush the condensate drain with a mixture of water and vinegar (about a 50/50 mix). This helps clear out any remaining buildup and also prevents mold or algae growth.

Tip: Clean your condensate drain at least once a year as part of your furnace maintenance routine to prevent future clogs.

Dirty or clogged air filter

When airflow is restricted, the furnace may overheat and cause condensation to form, which can then leak. Cleaning or replacing the air filter is one of the simplest and most effective ways to prevent this issue. Here’s how to fix it:

1. First, turn off your furnace to avoid any electrical problems.
2. Remove the air filter from its compartment (typically found near the blower or where the return air duct enters the furnace).
3. If the filter is dirty, replace it with a new one. Some filters are reusable, so if yours is, give it a good cleaning (you can wash it under warm water and let it dry completely before reinstalling).
4. Before putting the filter back, check to make sure the airflow direction (often indicated by an arrow on the filter) is correct.
5. Turn the furnace back on and check if the issue is resolved.

Tip: Change or clean your furnace filter every 1–3 months depending on usage. 

Broken or leaky heat exchanger

The heat exchanger’s job is to separate the combustion gases from the air in your home. If there’s a crack or hole, water can leak into the furnace or home. While this is a more serious issue, it’s still something that can be identified and addressed with the right tools and approach. Here’s how to fix it:

1. Always start by cutting power to the furnace to avoid any electrical risks.
2. Accessing the heat exchanger can be tricky, depending on your furnace model, so refer to your user manual for guidance. Check for visible cracks, rust, or signs of wear. If you notice any holes or cracks, the heat exchanger may need to be replaced.
3. If the leak is small, you may be able to seal it temporarily with a high-temperature sealant, but it’s important to keep in mind that a cracked heat exchanger usually requires a full replacement, which can be more involved.
4. If you detect a serious crack or damage in the heat exchanger, it’s often more cost-effective to replace the unit instead of attempting a temporary fix.

Condensation from the furnace’s flue pipe

The flue pipe of your furnace is responsible for venting exhaust gases outside. If it is improperly installed or has become damaged, it can allow moisture to collect and leak, which can then cause water to pool around the furnace. This issue typically arises from condensation that builds up due to temperature differences between the exhaust gases and the outside air. Here’s how to fix it:

1. Make sure the furnace is powered down before inspecting the flue pipe.
2. Look for signs of damage or improper alignment. The flue pipe should be securely connected, with no gaps or cracks. Moisture buildup around the pipe connections may indicate a leak.
3. Make sure the pipe is angled correctly to allow any condensation to drain away. The flue should be positioned so that water flows towards the drain and not back into the furnace.
4. If the flue pipe has come loose or is misaligned, carefully adjust it to ensure proper drainage. Reattach any loose connections and make sure there are no gaps where moisture could leak through.
5. If you notice significant damage to the flue pipe (like corrosion or large cracks), it’s best to replace the pipe to prevent further moisture buildup and ensure proper ventilation.

Read more: 12 things you should know about portable space heaters

Faulty humidifier connection

Some gas furnaces are equipped with a built-in humidifier, which can add moisture to the air to prevent it from becoming too dry. If the humidifier or its connections are faulty, it can cause water to leak into the furnace, leading to the appearance of a water leak. This is especially common in homes where the furnace is used alongside a whole-house humidifier system. Here’s how to fix it:

1. Always make sure the furnace is powered off before you begin working on the humidifier.
2. The humidifier is typically located on the side or top of the furnace. It’s usually connected to the water supply line and the furnace’s air ducts.
3. Check the water line running to the humidifier for leaks or clogs. Tighten any loose connections or replace damaged hoses. You can also check the valve to make sure it’s fully open or working correctly.
4. Look at the unit itself for any obvious signs of damage, such as rust, mold, or excessive moisture buildup. If you have an electronic model, use a multimeter to check for any electrical issues.

Water accumulation in the furnace pan

Many gas furnaces have a condensate pan to collect any excess water. If this pan becomes overfilled or isn’t draining, it can result in water leaking onto the floor or other parts of the furnace. Here’s how to fix it:

1. Start by cutting power to the furnace to avoid any electrical hazards while working.
2. The pan is usually found underneath the furnace or near the condensate drain line. You might need to remove a panel to access it.
3. Check the pan for any visible debris, dust, or mold buildup. These can clog the drain and prevent the pan from draining.
4. If you notice dirt or mold in the pan, clean it thoroughly using a cloth or sponge. If mold is present, use a mixture of water and white vinegar to disinfect the pan. Make sure the area is completely dry before reassembling the furnace.
5. Check that the condensate line that leads from the pan is clear and draining properly. If it’s clogged, use a wet/dry vacuum or a plumber’s snake to clear it.
6. If the pan is rusted, cracked, or damaged beyond repair, it may need to be replaced to prevent future leaks.

Read more: Your guide to choosing a space heater

Additional information

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