Why Is Your Furnace Leaking Water When The AC Is On?

A leaking furnace is and should be a cause for alarm for any homeowner. The truth is water that is not drained from your HVAC system can cause various issues, including mildew and mold. This can cause rust damage to your unit and health issues such as allergies for the residents of your household.

Even worse, this water pooling can reduce the lifespan of your HVAC unit, which will likely incur costly HVAC replacements and home repairs. While there are several reasons why your furnace might be leaking, it’s most alarming to see your furnace leaking water when the AC is on. 

The cause of this is often connected to the drain pipe. Usually, if you see your furnace leaking when the AC is on, it’s likely because both the furnace and the AC use the same drain pipe. 

Does your furnace leak water when you switch your air conditioner on? We’ve created a list of some of the most common causes of this and how you can prevent it.

Furnace Leaking Water when AC is On: 5 Top Reasons

A leaky furnace isn’t fun, especially if you don’t know what is causing it. Here are some potential reasons why there is water leaking from the furnace when the AC is on.

1. Malfunctioning Evaporator Coil

The evaporator coil is one of the parts of the HVAC unit responsible for cooling the air. As the air passes over the coils, the coils absorb heat and moisture, enabling the cooling process. The moisture from the air is condensed by the evaporator coil, settles on the drain pan, and is drained through the drain pipe.

A bad evaporator coil can lead to excessive condensation. In this case, more water will settle on the condensate pan. If the pan or the drain pipe has any issues, such as a crack in the pan or a pipe blockage, you will certainly notice water leaking from the furnace.

If this is the case, call an HVAC professional to assist you. 

2. Broken Humidifier

Maybe you, like many other homeowners, have been asking yourself this common question – why is my furnace leaking water when the AC is on? The leakage could be coming from an internal component of the furnace, particularly a humidifier. Most furnaces include a humidifier, which brings moisture to the heated air produced by the furnace.

The humidifier uses water and will leak onto the floor if something goes wrong with it. To prevent this, make sure to service your furnace regularly. A technician will easily detect a leaky humidifier before it becomes a larger problem.

However, if you do not maintain your furnace regularly, water leaking from the furnace could cause significant harm to the internal system. It also could damage the floor and the walls of the house. And lastly, it can cause mold growth, which poses a real danger to your household.

3. Clogged Drain System

Condensation is a normal process of your HVAC system. Usually, during the cooling process, the AC will draw moisture from the air, which then condenses on the condensate pan and is drained from the system through the drain pipe. 

The problem occurs when water cannot drain effectively. Water drainage can be affected if the drain is full of dirt, mold, or debris. These materials clog the drain, rendering it ineffective, and lead to your furnace leaking water when the AC is on.

If this is the situation, the water will overflow from the drain pan, and you will notice water under the furnace when the AC runs. The best way to handle this scenario is by hiring an HVAC repair service to unblock the drain and restore normal water flow.

Some service providers can install a safety mechanism that will switch the unit off automatically if the drain has a clog in the future. Turning off the system when a clog occurs prevents further damage to your system.

Also, consider scheduling a yearly furnace maintenance check. A professional team will assess your unit during maintenance checks. They can look for clogged drain lines or other problems to ensure the drain is open and clear, which minimizes potential issues down the road.

4. Dirt in the Air Filter

The air filter inside the HVAC unit works jointly with the cooling and heating units to provide airflow. If the furnace’s air filter is clogged, it won’t be able to filter the air entering the system. The dirt also restricts both air and power flow to the unit’s evaporator coil.

Insufficient power and airflow to the coils can cause the temperature to decrease and the evaporator coil to freeze. Subsequently, this might cause damage to the coil or other components of your HVAC unit.   

In addition, when the ice melts, it increases the volume of water disposed of via the drain pipe. And if all the water doesn’t drain correctly, the excess water ends up in the furnace, where you will recognize water leaking from the furnace.

5. Clogged Condensate Pump

Suppose your HVAC system is among the models with a condensate pump, which aids in pumping the condensate from your system to the outdoors. This is common in homes with a basement.

In this case, a clogged, blocked, or malfunctioning condensate pump can lead to water leakage. The clogging can result in water appearing under the furnace when the AC runs. In such situations, you should inspect the drain hose and the drain pan to confirm it is not affected.

If the drain pan does not appear to be the source of the problem, then the next most probable culprit is the condensate pump. The most straightforward solution to this problem is to replace or repair your condensate pump.

With a number of tutorials available online, anyone with little mechanical expertise should be able to fix the pump themselves, especially if the problem is a simple blockage. However, if you intend to change the pump, you should involve the services of a professional.

Conclusion – Is water leaking from your furnace when AC is on? 

If you’re concerned about water leaking from your furnace when your air conditioner is on, don’t worry, you’re not alone. Thankfully the issue is fixable, and with the help of a qualified technician, it can be resolved in no time. 

If you need help with a leaky furnace in your home, Anderson Air has what you need. Book an appointment today, and one of our HVAC specialists will fix it for you. 

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