Tools for Repairing Furnace
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It might seem more logical for a basement leak to be coming from the water heater or washer, but modern furnaces can also sprout leaks. Thus, finding a puddle of water beneath your furnace may indicate a problem with the furnace or adjacent portion of your HVAC system. By performing some basic checks, you can determine not only the source of the problem, but whether you will need a professional to affect repairs.

    Condensate Issues

    These are usually serious issues which require a professional to repair in the event of damage. The condensate consists of a pump, pan, drain, and trap. As condensation occurs along the evaporation coils, it runs down into a drip pan into a drain. From there, it will either flow outside directly or through a condensate pump.


    The condensate system is prone to blockages if the coils become dirty. As water mixes with the dirt, it drips onto the floor and any other surface below the runoff. This dirty water can clog the system anywhere along the line. Cleaning your coils annually and making sure you have a properly sized air filter installed will not only help prevent clogs, but will also improve the overall efficiency of your system.

    Perhaps the most common source of blockage is the drain trap. The bottom of  the trap clogs easily from even a small amount of dirt buildup. Fortunately you can keep the trap clean using a long, flexible cleaning brush to clear any potential blockage. Afterwards, flush the trap out by pouring water down the cleanout cap.

    Cracked Drip Pan

    The drip pan is one of the most common sources of leaking. This thin, cheap plastic container can be easily cracked from impacts or pressure from the PVC pipe. While the part is readily available, it is generally best to have a professional replace it due to its frailty.

    The Condensate Pump

    In some cases, the pump may have simply become unplugged, in which case you may quickly remedy the leak. However, the pump may be faulty or damaged. In such a case, you should contact a professional to replace the unit.

    External Issues

    In many cases, the flooding will be caused by an attached nearby unit. This is especially true in more moderate climates where multiple HVAC devices may be running in the course of a day. These problems are less likely to require a professional to affect repairs, so it is best to check them first.

    Air Conditioner

    Many modern HVAC units combine the air conditioner and furnace units for greater efficiency. In many cases, they are nearby or in sequence. Sometimes, a recently used air conditioner will overflow its own drip pan and leak onto the furnace, creating the illusion that it is the furnace leaking. This is a very minor issue and easily solved by emptying your air conditioner’s drip pan regularly.


    The primary purpose of a dehumidifier is to remove moisture from the air. Older models contain a drip pan which must be emptied regularly, while newer models may be connected to the drain. If your furnace and dehumidifier are close or share a common drain, overflows or clogs related to the dehumidifier may be mistaken for a furnace issue.

    Humidifiers have the opposite function, and are used to increase humidity. To accomplish this, the humidifier unit is connected to your home’s water supply. Much like other water-dependent units, humidifiers may develop leaks, clogs, or cracks. As a result, water can drip down onto your furnace or the nearby floor, again making it appear that the furnace is actually leaking.

    Plumbing and Appliances

    While not a likely factor in an attic-based HVAC system, furnaces located in the basement are sitting amid a literal waterworks. Plumbing leaks can occur along the ceiling pipes, causing water to drip down onto the furnace or the floor nearby. This is especially significant if your furnace is connected to a drain, since the floor will be sloped gently to encourage runoff. As a result, water may appear under your furnace even if the leak isn’t nearby.

    Basements which include a laundry facility create two more potential causes for water to appear beneath your furnace. The most common culprit is your washer, which may develop leaks or have a loose drain hose. Depending on its proximity to the furnace and whether your basement has multiple drains, a leaky washer can easily flood the area around your furnace, often without being the obvious source.

    Washtubs are another potential source. Unlike a partial bath where there is separate plumbing, a washtub may share a common drain with your washer. Some washers drain directly into the washtub, and debris from an unfiltered hose can clog the sinks drain and cause it to overflow. Likewise leaks in the sink’s drain may cause some flooding which will pool around the nearest drain. If the washtub is close to your furnace, this flooding may occur underneath the latter.

    Additional Resources

    Hannabery HVAC provides an article with a detailed chart of which damages will require professional repair.

    Standard Heating and Air Conditioning provides a diagram illustrating the components of a basic modern HVAC unit.

    Editorial Contributors
    avatar for Henry Parker

    Henry Parker

    Henry Parker is a home improvement enthusiast who loves to share his passion and expertise with others. He writes on a variety of topics, such as painting, flooring, windows, and lawn care, to help homeowners make informed decisions and achieve their desired results. Henry strives to write high quality guides and reviews that are easy to understand and practical to follow. Whether you are looking for the best electric riding lawn mower, the easiest way to remove paint from flooring, or the signs of a bad tile job, Henry has you covered with his insightful and honest articles. Henry lives in Florida with his wife and two kids, and enjoys spending his free time on DIY projects around the house. You can find some of his work on Today’s Homeowner, where he is a regular contributor.

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