6 Reasons Why Your Furnace is Leaking When AC is On

Your furnace leaking water when your air conditioner is on can be worrisome. Thankfully, you can troubleshoot your furnace leaking water right at home.

Your furnace is likely leaving when your AC is on because:

  • Evaporator coil is frozen
  • Drain pan is cracked
  • Drain pipe is clogged
  • Drain line needs to be flushed
  • Heat exchanger is overheating/broken
  • Condensate pump is broken

Below you’ll find the 6 reasons why your ac is leaking water and how to fix it.

Let’s get started.

6 Reasons Why Your Furnace is Leaking When Your AC is On

Before you troubleshoot your furnace, it’s always a good idea to turn off your HVAC system before you do any fixes.

You should turn off your HVAC system at the circuit breaker level to ensure that your furnace and AC are totally off.

1. Evaporator coil

Your evaporator coil can freeze in your air conditioner, which can then lead to dripping water.

Low airflow is normally the cause of a frozen evaporator coil, so check the air filter and ductwork to ensure that air is moving freely.

If your air filter is dirty, either replace it or give it a rinse.

2. Drain pan is cracked

Air conditioners drain water into a drain pan.

If you have a damaged drain pan, it can lead to water dripping down into your furnace.

Check if the drain pan is cracked, and if so, it’s the cause of your furnace leaking water.

3. Drain is clogged

The condensation that’s created by your AC unit is typically removed through a drain line.

If however this drain line becomes clogged, your furnace may seem like it’s leaking water.

The best way to unclog your drain line is to use a wet vac to get any debris that may be stuck inside.

You can also buy a metal snake that will be able to remove any debris out of the clogged drain line.

Another option is to purchase tablets that will unclog your drain line at your local hardware store.

A clogged drain can also be the cause of your furnace blowing cold air.

4. Furnace drain line needs to be flush

Your furnace leaking water may be caused by a loose drain line.

Check if your drain line is positioned properly and is in line with the drain from your unit.

5. Broken heat exchanger

If you have a high efficiency furnace, then it is possible that your secondary heat exchanger is causing a leaking water.

If a cracked heat exchanger the cause of your furnace leak, then you’ll have to replace the entire furnace.

A broken heat exchanger can also cause carbon monoxide leaks, so do not continue using your furnace if this is the problem.

6. Condensate pump is malfunctioning

If you have a drain line coming from the basement, then you’re using a condensate pump.

Check around the condensate pump to see if there are any obvious leaks.

If you don’t spot any, try to pour water down the drain line that goes to your condensate pump.

If the pump doesn’t start up, then that’s the reason why your furnace leaks.

Can a Leaky Furnace Be Dangerous?

A leaky furnace can eventually rust your furnace and cause your furnace to stop working properly.

It can also lead to mold and mildew problems in the room your furnace is in and even cause structural damage to your home.

When to Call an HVAC Technician?

If you’ve tried to fix your leaking furnace and nothing is working, then you should call an HVAC technician for furnace repair.

If you’re dealing with high efficiency furnaces, you may need to replace the entire thing if you have an issue with the secondary heat exchanger.

Final Thoughts

If your furnace is leaking water when the ac is on, you should look into having it fixed as soon as possible.

While it’s normal for your air conditioner to produce condensation, it’s not normal for your furnace to be leaking water.

Your furnace leaking water when the AC is off is another problem that needs to be investigated independently.

About Roy Cohen

Roy Cohen has a burning passion for heating, cooling, and ventilation. He aims to help you save money on expensive repairs and bring you the best HVAC products. He has years of experience behind him in HVAC repair and garage maintenance.

You can find Roy at his LinkedIn or his email: [email protected]

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