Furnace leaking water? Find out why and how to fix it

Stepping into a puddle of water near your furnace is often the most obvious symptom of a leaking unit. There are several possible reasons behind your furnace leaking water, a few you may address yourself and others that will require a technician to remedy the problem.

To better troubleshoot why your furnace is leaking water, you need to know the type of unit you have in your home. We’ll walk through the two primary types of furnaces and how to tell which you own.

Once you’ve narrowed the field, we’ll dive into the possible causes of a leaking furnace, highlighting the fixes you can try at home and noting when it’s best to call in an HVAC specialist.

What to do about a leaking furnace

Even if you eventually call in a furnace repair company, understanding why your furnace is leaking water empowers you to choose the best solution and ask questions to understand how the repair will be made.

Below are our recommended steps to take after discovering a furnace leak but before you contact a repair company:

1. Determine the type of furnace in your home.
2. Consider the most likely leak sources based on your ownership of either a high-efficiency condensing furnace, or a conventional furnace.
3. Establish whether the furnace leaks when the air conditioner is on.
4. Inspect the unit to find the source of the leak.
5. Consider the ease and your comfortability in performing small DIY tests.
6. Call a repair technician if the leak persists.

Our guide is meant to walk you through possible causes and solutions but certainly doesn’t replace the expertise of a reliable industry technician. Avoid further damage by calling on a local company if water leaking from furnace components is one of multiple issues you’re experiencing with your unit.

What kind of furnace do I have?

A high-efficiency condensing furnace and a conventional furnace have a lot in common. However, there are three main differentiators to help you determine the type of furnace in your home.

Furnace exhaust pipe

The furnace exhaust pipe material is the biggest indicator of the type of unit you have. The exhaust pipe, often referred to as the flue pipe, is responsible for venting the exhaust from your furnace out of your home. If the pipe is polyvinyl chloride (PVC), meaning it looks like white plastic, then you likely have a high-efficiency furnace. A metal pipe suggests you have a conventional standard efficiency furnace. 

Furnace AFUE rating

Another way to determine the furnace type is to locate the Annual Fuel Utilization Efficiency (AFUE) rating. The rating should be printed on a yellow EnergyGuide label on the unit. Conventional standard efficiency furnaces have an 80% efficiency rating, meaning they convert 80% of the energy created to heat your home. The other 20% is lost through the chimney, air leaks, and other openings. High-efficiency furnaces have an AFUE rating of 90% or above. Your AFUE rating should help you determine the furnace type.

Age of home

Most newer homes have a high-efficiency furnace. If you’ve recently moved into a new-construction home or a home where the heating and cooling systems were replaced, you likely enjoy the benefits of a high-efficiency model. Older homes that have not been updated often have a conventional standard efficiency furnace. 

High efficiency condensing furnace is leaking water

Now that you’ve successfully determined you’re dealing with a high-efficiency unit, let’s discuss the most common reasons for why your furnace is leaking water. 

Condensation is created when the two heat exchangers inside the furnace absorb so much heat that the gas that’s typically expelled from your unit through the exhaust pipe turns to liquid. That water is usually carried out of your home by the condensate drain.

If the condensate drain or line becomes clogged, the water has no choice but to leak from the furnace. Clearing the line may be as simple as pouring an equal ratio mixture of hot water and white vinegar through the line.

Flush the line several times with the mixture if needed. This condensate line cleaning method typically removes small clogs. To avoid a clogged furnace condensate drain or line, have the furnace inspected and cleaned at least once a year.

Even if the condensate drain and line are clear, a problem with your condensate pump can create a furnace leak. The pump is responsible for pushing the condensation (water) out of the unit and into the pipe.

The average lifespan of a condensate pump is 3 to 5 years. If your pump has aged past that timeline, it may not perform optimally. The pump will cost about $50, and to have it professionally replaced can range between $100 to $200.

Conventional furnace is leaking water

Water leaking from a conventional furnace is typically a symptom of either a malfunction with the built-in humidifier or an ill-fitted exhaust pipe. The humidifier empties excess water through the furnace drain line. If the drain line is clogged, the water will build up, leaking from the humidifier into the furnace. It will appear that water is leaking from furnace components when the humidifier is the challenged part.

Inspect the furnace humidifier water lines for cracks or pinhole leaks. Double-check the fit of the lines to the humidifier. Ill-fitted lines can easily create leaks.

If the humidifier appears to work correctly, inspect the exhaust pipe. If the pipe is loose or improperly fitted to the furnace, water that’s meant to be expelled will leak from the unit.

Be cautious as you’re examining the pipe. Do not touch it. A conventional furnace exhaust pipe can become dangerously hot. To avoid the possibility of injury, contact a furnace expert to repair or replace the pipe.

Furnace leaking water when AC is on

If the puddle of water around your furnace only seems to grow when the air conditioner is on, the problem is likely with your AC, not the furnace. When the AC runs, it’s not just cooling your home, it’s also dehumidifying the air.

The air conditioner must release the moisture it has pulled from the air, and typically does so through the unit’s condensate drain line. If that drain line becomes clogged or forms a leak or there’s a leak in the drain pan, the water will accumulate on the ground.

In many cases, the AC drain line is located near or even on top of the furnace. So, it’s very easy to question, “why is my furnace leaking water?” when, in fact, it’s the AC drain line.

If, however, your AC hasn’t operated lately, the drain line isn’t the source of your leak. Call in an HVAC professional to help you determine the next best step.

Repair or replace leaking furnace?

Many of the challenges that cause a furnace to leak can be repaired for a much smaller investment than replacing the entire unit. The average furnace repair cost, according to Home Advisor, is around $300. The cost to replace a furnace is $4500 on average.

While the difference between the two investments in large, it may be time to replace the entire system if:

• The furnace is 16+ years old.
• You’ve recently made several repairs or upgrades to your furnace.
• Energy bills continue to increase.
• Unit continuously makes strange noises

Don’t ignore the warning signs of a failing furnace. It’s better to replace an aged unit while it’s still operable rather than wait until the winter months and have to battle dropping indoor temperatures while you wait for an available company.

If you do decide to replace your furnace, HVAC.com can deliver comprehensive HVAC quotes within 24 hours, giving you peace of mind to move forward with confidence.

Tips to hire furnace repair company

When it’s time to repair or replace your furnace, do your company-review homework. Read online reviews about previous customer experiences.

Ask for a quote in writing. It’s best to get at least three companies to estimate your furnace repair or replacement cost. This way, you can immediately eliminate any estimate that stands in stark contrast to the others.

Also, be sure to request a copy of the company’s warranty on parts and labor. Most furnaces last 16 to 20 years, but it’s nice to know you have access to professional assistance when needed.

If a leaking furnace isn’t the problem you face, explore our furnace troubleshooting guide to help identify why your unit isn’t performing properly.