Furnace Leaking Water? Causes, Fixes & Prevention

Square Footage	Furnace BTU
1,000-1,200 sq. ft. Home	40,000 - 60,000 BTU
1,200-1,500 sq. ft. Home	60,000 BTU
1,500-1,800 sq. ft. Home	60,000 - 80,000 BTU
1,800-2,500 sq. ft. Home	80,000 - 100,000 BTU
2,500-3,500 sq. ft. Home	100,000 to 120,000 BTU

Hearing the sound of water moving inside your furnace is usually nothing to worry about. Seeing water around the unit, however, is a clear sign that something isn’t working as it should. A furnace leaking water isn’t just inconvenient — if ignored, it can lead to costly repairs, safety risks, and unnecessary downtime during Ontario’s cold months.

This guide walks you through the most common causes of furnace water leaks, how to troubleshoot the issue, what a professional will look for, and how to prevent future leaks. All explanations are grounded in real-world HVAC experience to help Canadian homeowners make confident decisions.

1Why Furnaces Leak Water

2High-Efficiency Furnace Leaks

3Standard Efficiency Furnace Leaks

4Is a Furnace Leaking Water Dangerous?

5How HVAC Professionals Diagnose a Leaking Furnace

6How to Prevent Furnace Water Leaks

7Tips for Choosing a Reliable Furnace Repair Company

8Conclusion

9Frequently Asked Questions

10📌 When Are You Planning to Install a New Furnace?

Why Furnaces Leak Water

Before you can diagnose a leak, it’s essential to know what type of furnace you’re dealing with. High-efficiency and standard-efficiency furnaces operate differently, and the reasons they leak can vary significantly.

Let’s break down the differences and the most likely causes.

High-Efficiency Furnace Leaks

A high-efficiency furnace — generally one with an AFUE rating of 90% or higher — extracts more heat from the combustion process. This results in more condensation, which the furnace is designed to drain safely.

When any part of that drainage system is blocked, damaged, or worn out, water can escape and collect around the furnace.

Below are the most common reasons high-efficiency furnaces leak water.

1. Blocked Condensate Drain System

Condensation buildup is the most frequent cause of leaks in high-efficiency units. These systems rely on a network of drain hoses, a drain trap, and often a condensate pump to remove water created during heating. When anything in this system becomes obstructed, water quickly backs up.

Drain Trap Blockages

The drain trap acts much like the trap under a sink. It collects debris and prevents gases from escaping. Over time, dust, dirt, and organic material can accumulate inside the trap. If the trap becomes clogged, the water has nowhere to go and begins to leak around the furnace.

A long cleaning brush or gentle flushing through the cleanout cap usually clears the obstruction, but if this isn’t done regularly, the trap can continually clog.

Drain Hose Issues

The condensate drain hose connects the furnace to the drainage line or pump. These hoses can become blocked with dirt or mould, or they may shift out of place if bumped during maintenance. Cracks in the hose or loose connections also allow water to escape.

A quick inspection may reveal kinks, blockages, or damage, all of which can cause leaks.

2. Cracked or Damaged Drain Pan

Beneath the evaporator coil sits a drain pan that catches condensation before it enters the drainage system. Over time, this pan can crack due to age, vibration, or pressure from the condensate line.

A damaged drain pan lets water spill out instead of directing it to the drain. Plastic pans become brittle with age, while metal pans may rust and eventually perforate.

Once cracked, the pan usually needs replacement — it’s not a component that can be reliably repaired.

3. Faulty Condensate Pump

In many homes, especially where the furnace isn’t located near a floor drain, a condensate pump moves water from the furnace to the appropriate drainage point. These pumps typically last three to six years.

A failing pump may:

stop running altogether
run without pumping
become clogged with debris
have a malfunctioning float switch

When the pump can no longer remove water, it overflows and causes pooling around the furnace. Because a pump interacts with electrical components, it’s wise to have a professional inspect or replace it.

In rare severe cases, if water damage affects internal furnace parts, replacement of the furnace may become necessary.

Standard Efficiency Furnace Leaks

A standard-efficiency furnace — usually around 80% AFUE — produces less condensation. For this reason, leaks in these systems often point to issues outside the furnace’s combustion process.

Here are the most common reasons an 80% furnace may leak water.

1. Whole-Home Humidifier Problems

Many Canadian homes use a furnace-mounted humidifier to maintain comfortable humidity levels in winter. These systems rely on a steady water supply, which means they can leak when the water line, pad, or internal components clog or fail.

Because the humidifier often drains through the same condensate line as the furnace, any leak near the furnace may appear to be coming from the heating system itself.

Routine cleaning and seasonal maintenance help prevent these issues.

2. Clogged Furnace Filter

A severely restricted air filter reduces airflow through the furnace, increasing internal temperatures and affecting how moisture forms and drains. In some cases, the evaporator coil can freeze and thaw irregularly, leading to unexpected water buildup.

Replacing your furnace filter every one to three months (depending on usage and indoor air quality) helps maintain proper airflow and prevents moisture-related problems.

3. Damaged Heat Exchanger

The heat exchanger separates combustion gases from the air that heats your home. If it becomes cracked or corroded, water may leak from the furnace — though this issue is more serious than a simple condensate blockage.

A failing heat exchanger can also allow carbon monoxide to escape, making it a critical safety concern. If you suspect the heat exchanger is damaged, shut off the furnace and contact a licensed HVAC professional immediately.

In most cases, a cracked heat exchanger requires a full furnace replacement due to safety regulations.

Is a Furnace Leaking Water Dangerous?

A furnace leak may start small, but it can turn into a serious problem if not addressed promptly. Here are the main risks:

1. Electrical Hazards

Water near wiring or internal electrical components can lead to shorts, equipment failure, or shocks. The longer the water sits, the higher the risk.

2. Potential for Fire

Short-circuited electrical systems can spark or overheat, creating a fire hazard — especially in older systems.

3. Mold and Poor Indoor Air Quality

Moisture around HVAC equipment promotes mould and mildew growth, which can spread through ductwork and aggravate allergies or respiratory issues.

4. Structural Damage

Persistent leaks can damage flooring, drywall, insulation, and support structures. Repairing water-damaged building materials can become much more expensive than repairing the furnace itself.

For these reasons, leaks should never be ignored — even if they seem minor.

How HVAC Professionals Diagnose a Leaking Furnace

A trained technician follows a systematic process to find the root cause. Here’s what professionals typically inspect:

1. Condensate Drain System

They start by checking the drain trap, hoses, and any connected PVC piping. If buildup or debris is present, they’ll clear it using specialized brushes or flush tools. They may also test the flow to confirm water can travel unobstructed through the system.

2. Condensate Pump Testing

If your furnace uses a pump, the technician will manually fill the reservoir to activate the float switch. If the pump doesn’t run or cannot move water effectively, they’ll determine whether cleaning, repair, or replacement is required.

3. Drain Pan Inspection

The technician will look for cracks, rust, or corrosion. If the pan is damaged, replacing it is the only reliable solution.

4. Heat Exchanger Evaluation

Professionals often use inspection cameras or mirrors to view the heat exchanger from the inside. They’ll also look for signs of overheating, improper airflow, or blockages that may have contributed to the damage.

5. Air Filter and Humidifier Check

They’ll confirm the filter allows proper airflow and that your humidifier isn’t leaking into the furnace. Any damaged parts are cleaned, repaired, or replaced as needed.

How to Prevent Furnace Water Leaks

Many furnace leaks are preventable with routine checks and proper maintenance. Here’s how homeowners can reduce the likelihood of moisture problems:

1. Turn Off the Furnace If You Spot Water

Locate the furnace switch (typically mounted nearby) and shut it off. If you can’t find it, turn the unit off at the breaker panel. This prevents electrical hazards while you investigate.

2. Clean Up Standing Water

Wipe up any water around the furnace right away. This stops moisture from spreading and reduces the risk of mould or electrical damage.

3. Check and Replace the Air Filter

A clean filter allows proper airflow and minimizes the risk of icing, overheating, or condensation buildup.

4. Determine Whether the AC Is Part of the Problem

If the furnace only leaks while your air conditioner is running, the issue may be:

a clogged AC condensate line
a frozen evaporator coil thawing
a disconnected drain hose

Your furnace may simply be the area where the water collects, even though the source is your AC.

5. Schedule Regular HVAC Maintenance

In Canada, most professionals recommend servicing your heating system before winter and your cooling system before summer. Routine inspections help identify worn-out parts or developing problems before they cause leaks.

6. Call a Professional for Persistent or Unknown Leaks

If simple checks don’t uncover the issue, or if the leak comes back, a qualified technician can diagnose the problem accurately and safely.

Tips for Choosing a Reliable Furnace Repair Company

Hiring the right HVAC professional can make a significant difference in the quality and longevity of your repair. Here’s what to look for:

1. Confirm the Technician’s License

In Canada, HVAC technicians are required to hold proper training and certification. A licensed professional ensures the work meets safety standards.

2. Verify Liability Insurance

Always ask whether the contractor carries liability coverage. This protects homeowners from costs arising from accidental damage during repairs.

3. Request References

Reputable companies can provide references from previous customers. Speaking to past clients helps you understand the technician’s workmanship, reliability, and professionalism.

4. Ask for Their Assessment

A knowledgeable contractor should be able to explain the issue clearly and outline your options for repair or replacement without pressuring you into unnecessary upgrades.

5. Discuss Furnace Brands Neutrally

If replacement is recommended, ask about different furnace options and what might suit your home. The goal is not to push specific brands but to understand efficiency, performance, and budget considerations.

6. Establish a Clear Timeline

For non-emergency repairs, get a written estimate and timeline so you know when the work will be completed.

Conclusion

A furnace leaking water is something every homeowner should take seriously. While the problem may be as simple as a clogged drain or dirty filter, it can also signal a more significant issue like a failing pump or cracked heat exchanger. Addressing leaks promptly helps protect your home, maintain indoor air quality, and keep your heating system running reliably throughout the Canadian winter.

If you’re uncertain about the cause or don’t feel comfortable troubleshooting it yourself, contacting a qualified HVAC professional is always the safest choice.

Frequently Asked Questions

1. How much water should come from a furnace?

High-efficiency furnaces naturally produce condensation, so a slow, steady drip through the drain line is normal. Excessive water or pooling around the furnace typically indicates a blockage or malfunction.

2. Why does my furnace leak only when the AC is on?

If the leak appears only during cooling season, the problem likely stems from the air conditioner — commonly a clogged condensate drain, frozen coil, or disconnected drain tubing. The water often collects near the furnace, making it seem like the furnace is the source.

3. What happens if I ignore a furnace water leak?

Ignoring a leak can lead to electrical hazards, mould growth, structural damage, or costly repairs. It may also shorten the lifespan of your furnace.

4. How do I know if the leak is serious?

Large or continuous leaks, water pooling around the furnace, or repeated issues even after cleaning the drain line all warrant a professional inspection.

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💰 Key Factors That Influence Furnace Installation Pricing

Furnace replacement costs in Ontario vary based on several important considerations:

Home size & heating load – Larger homes need higher-BTU furnaces
Installation complexity – Venting changes, gas-line upgrades, or electrical work can increase costs
Brand & efficiency rating – High-efficiency furnaces typically cost more upfront but save long-term
Your Ontario location – Labour rates differ between major cities and smaller communities
Repairs vs. full replacement – Full system replacements cost more than standalone component fixes

Understanding these elements helps you set a realistic budget for your installation.