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Furnace Leaking Water When AC Is On? 7 Common Causes

Most homeowners want to fix a leaking furnace as soon as they notice the puddle. What’s more, we tend to look for the causes behind a furnace leak in and around the furnace itself. But maybe you should stop to think that the leak could be coming from the AC.

Your furnace is leaking water when the AC is on because the HVAC condensate drain line is blocked or has cracks. If not, the air filter is dirty, or there isn’t enough refrigerant. Easy fixes include cleaning the condensate drain line, cleaning or replacing the filter, and adding refrigerant. 

Because diagnosing an AC-related furnace leak may not be as easy as it sounds, this article is meant to help you understand why your furnace may appear to be leaking when the AC is on and what you can do to correct it. But first, let’s see how a running AC affects the furnace. 

How the Running of an AC Is Connected to the Furnace

Your furnace and AC are part of a larger, more complex system — the HVAC (Heating, Ventilation, and Air Conditioning). This system is responsible for the air and temperature comfort in your home. 

Because the units in your HVAC system are interconnected, they rely on each other for optimal functioning. As such, understanding how the system works is key in helping you diagnose AC-related furnace leaks and determine if troubleshooting is enough or you need to call an HVAC pro.

Here are a few key points you should know about your HVAC to understand how the furnace and AC are related:

  • Your furnace and AC have the same ductwork, which distributes cool air from the AC in summer and warm air from the furnace in winter.
  • The AC and furnace send out the moisture produced during the cooling and heating processes through the same condensate drain line. 
  • The furnace and AC are controlled by the thermostat. Cooling turns the AC on, and heating turns the furnace on. A wrong setting in the wrong season will compromise the heating and air conditioning processes.
  • Air filters in your HVAC system work with both the heating and the air conditioning units. Both send air through the filter to eliminate debris and dust and, if the filter is clogged, air quality is compromised, and you get less of it.
  • An AC doesn’t use the humidifier attached to your furnace since part of an AC’s job is to dehumidify the air. But leaving your furnace’s humidifier on in summer will set back the dehumidification process, so it should be turned off. 

The interconnectedness of the AC and furnace gives you an idea of why a leak from one unit would affect the other or make it seem like it’s the other one leaking.

Reasons Why Your Furnace Leaks When the AC Is On

So what AC issues might cause a leak in your furnace? 

Well, it can be a lot of things, but the problem is almost always related to the condensate drain system. Here are 7 of the most common causes:

1. A Clogged or Cracked Condensate Drain

Water is a normal byproduct of your home’s heating and cooling. While cooling air, your AC draws moisture from the air, thanks to the cooling process.

If your AC is working properly, water will flow into the condensate pan and be sent down the drain outside your home. If the contrary is true and your condensate drain is blocked by dirt and dust, the water will overflow out of your AC and land on or under your furnace, making it look like your heating system is leaking.

Similarly, a crack in the condensate drain line will lead to a leak that could land under your furnace and the surrounding areas and appear to be coming from the furnace.

2. The Condensate Pump of a Basement AC is Broken

If your furnace and AC are in the basement, that means your HVAC system has a condensate pump. It’s responsible for pumping the collected water up the drain so that it gets out of your home.

As with the blocked condensate drain, a dysfunctional condensate pump will leave condensate water from your AC lying in the pump tank. Once it’s full, it’ll overflow at the pump and stagnate in the condensate pan and draining pipe, causing a leak. 

This causes water to leak and pool underneath your furnace.

3. A Cracked Condensate Pan

If your furnace has an old metal condensate pan, it can rust and get filled with holes over time. A plastic condensate pan is even less durable. 

In both cases, the holes or cracks will cause the condensate pan to leak and send water to your furnace below. 

While this is the easiest of the furnace leak causes to fix — by replacing the worn-out pan or using a sealant to repair a cracked plastic one — the leaking water can be a real danger to your HVAC and home flooring if not noticed in time.

4. A Super Dirty Air Filter

We mentioned earlier that the air filter in the HVAC system works with both the heating and cooling unit to supply airflow. If your furnace’s air filter is dirty, it can’t filter the air going into the system. This also limits the flow of power to the evaporator coil.

When the evaporator coil doesn’t receive enough airflow and the power flow is compromised due to a dirty filter, temperatures drop, and the evaporator coil can freeze.

Of course, a frozen evaporator coil can be easily damaged or cause damage to other parts of the unit. But the melting ice will also increase the amount of water being disposed of through the condensate drain. 

If the water doesn’t get drained as it should, the extra water will end up on your furnace — that’s when you’ll notice a leak and associate it with the furnace.

5. Bad Evaporator Coil

The evaporator coil is a series of pipes situated above your HVAC system. I’ve mentioned that poor airflow causes a bad evaporator coil, mainly because your furnace’s air filter isn’t working properly. 

As such, the evaporator coil can’t remove heat from your home’s air to lower its temperature and send it as cool air through the ducts back into your home. Incidentally, this is what dehumidifies the air.

If your condensate pan and drain are clogged, or the pan is riddled with holes, the condensate that drips from the evaporator coil will end up on the furnace, the blower, or the surrounding floor. 

Again, if the evaporator coil isn’t working properly, excess moisture may be formed and won’t get drained as fast as it’s collected, leading to a leak in the furnace.

6. Your AC Has Insufficient Refrigerant

Refrigerant is a fluid or gas compound found in AC systems. It absorbs heat from the environment and cools the air when combined with evaporators and compressors.

Because the refrigerant line in your AC is a closed-loop, it shouldn’t cause any problems. However, if the line has a leak and you lose some of the refrigerant, your AC won’t have enough refrigerant. The consequences of a leaking refrigerant line include:

  • Low airflow.
  • Warm air coming from the vents.
  • Higher electricity bill. Your AC system is working harder than it should to provide cool air.
  • A bubbling or gurgling noise from your AC system.
  • Refrigerant intoxication. Feelings of dizziness and nausea, or shortness of breath are all common symptoms.
  • A compromised cooling process that causes lower temperatures in your AC system, resulting in a frozen evaporator coil

If the evaporator coil is frozen, the collected moisture can’t vaporize. Once the temperatures rise again, the frozen moisture will melt and create excess water that could leak onto your furnace or to the floor around your HVAC system. 

7. You’re Running the AC When the Outside Temperatures Are Too Low

Cooling air systems are created to work when the air outside your home is warm. If you leave your AC running long after temperatures have gone down, you risk causing the evaporator coil to freeze. The frozen moisture melts and gets to the furnace when temperatures go up.

So, you should avoid running your AC when temperatures outside your home are below 60°F (15°C).

Now that you know all the different causes behind your leak, keep reading to learn what steps you need to take to fix it.

It’s not going to be easy to tell which of the above issues with the AC is causing or creating an apparent leak from your furnace.

So, if you intend to troubleshoot the cause of the leak before seeking the help of an expert, you’ll need to assess your HVAC system for the issues discussed above. You can then decide if you can fix the problem yourself or need to call an expert.

Follow these 5 steps to troubleshoot and fix your furnace leaking water when the AC is on.

1. Turn Off the AC and Dry the Water From the Leak

This is a preparatory step since you should never attempt to touch or open a part of an electrical appliance while the power is on. Since we’re dealing with issues that could affect your entire HVAC system, turn off the switch that supplies power from the circuit breaker to the system. 

Also, wipe your floor dry, along with any other wet surfaces. If the water seeps into your floors, it’ll leave irreparable damage.

2. Assess the Condensate Drain Line for Blockage and Cracks

A clogged condensate drain line is the most common cause of leaks in your HVAC system. Luckily, it’s also the easiest to troubleshoot. 

Begin by checking for blockage at the drain hose connected to the drain pan. Clean any dirt blocking the waterway. Watch this video to learn how to do it:

If that resolves the issue, you’re good to go.

If not, inspect the drain pipe that sends water outside your home. Look for dirt and debris that may be blocking the pipe. Next, use a fine wire or a plumbers snake to clean any dirt inside the pipe. 

You should then rinse the pipe with hot water and a bit of kitchen soap or vinegar down the drain to get rid of any leftover dirt in the drain line. 

Remember to check for any cracks, holes, or broken joints on the condensate drain line as well.

If the leak is still there after you cleaned the drain, check for the next possible cause.

3. Check if the Condensate Pump Is Working

If your HVAC condensate drain is clear and the leak still happens, check if the condensate pump is working. The following video will guide you through the process:

Here’s a quick step-by-step guide on how to check the condensate pump:

  1. First, ensure that it’s running by checking the power button and circuit breaker. 
  2. Pour water into the pump tank if there isn’t any already.
  3. Run the pump. 

If the pump runs, but water doesn’t seem to be going up the drain hose, then you need to clean the hose.

If it still isn’t working, it could be that the microswitch or pump motor has failed. In that case, call an expert to confirm that the condensate pump is damaged before having it replaced. 

When replacing a condensate pump, opt for a high-efficiency model like the Little Giant VCMA-20ULS Condensate Removal Pump (Amazon.com). It can pump 80 gallons (363.68 l) of water per hour and is fitted with a safety switch.

Should you discover that your condensate pump has no issues, check the air filter.

4. Inspect the Air Filter

The air filter in heating and cooling systems should be inspected every month to ensure it’s clean and functional. If it’s dirty but less than 3 months old and in good shape, you can clean it and put it back into place. But if it’s over 3 months old or picks up plenty of dirt from the environment, replace it. 

Watch this video to learn how to clean your furnace filter using compressed air:

Cleaning or replacing the air filter should also resolve any issues with the condensate coil. A frozen evaporator coil is caused by a dirty filter that doesn’t send sufficient air to the coil.

If the leak is resolved after cleaning the filter, you’re good to go. If not, there’s not much else you can do on your own.

5. Call a Pro To Fix a Refrigerant Leak

If none of the steps above has revealed the source of the leak, your last card is to check for a refrigerant leak. 

Assessing and addressing refrigerant leaks is best done by an HVAC expert. So, you should call one ASAP. Don’t try to resolve the issue yourself. Exposure to refrigerant chemicals can be extremely dangerous.

There’s one last thing you can do: confirm that the temperature outside your home doesn’t drop below 60°F (15°C), as it would mean you’re running the AC when it’s already cold.

Final Thoughts

The HVAC is a system of interconnected units. This means that issues in your AC also affect your furnace.

When your AC doesn’t eliminate the water produced from the cooling processes as it should, your furnace may appear as if it’s leaking. 

You can troubleshoot most of these issues yourself. But a leak in the refrigerant or a condensate pump with electrical issues should be left for an HVAC expert. 

As a rule of thumb, if you don’t know what to do when your furnace appears to be leaking, call a professional ASAP.