How Do I Know If My Furnace Heat Exchanger Is Bad?

How Do I Know If My Furnace Heat Exchanger Is Bad?

Anthony Barnes

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Your gas furnace has a heat exchanger. It is located between the combustion chambers and blower.

When the combustion chamber heats the heat exchanger, heat is exchanged with the air being blown across its surface.

Free of the flue gasses, this air is then sent into the ducts to provide heat within your home.

If you’ve noticed some problems with your furnace, it may be related to the heat exchanger.

This guide will help you to learn about finding a bad heat exchanger, and what you can do about it!

What Is A Heat Exchanger?

Heat exchangers are devices that allow heat from a fluid (liquid or gas) to pass to another fluid (another liquid or a gas).

Heat exchangers transfer heat without mixing the fluids.

To do this, the heat exchanger needs to have an airtight sealing system to prevent the flue gas from escaping.

How Do I Know If My Furnace Heat Exchanger Is Bad?

Carbon monoxide is a poisonous gas that can be fatal if inhaled.

Shell and tube heat exchangers are used to transfer heat energy from a source to a sink.

In this case, the source is a hot fluid and the sink is a cooler fluid.

The hot fluid runs through a coil of tubes, while the cold fluid passes over the outside of the coils.

The heat energy is transferred from the hot fluid to the cold fluid, without any physical contact or mixing of the two fluids.

How A Gas Furnace Uses A Heat Exchanger

Gas furnaces use heat exchangers to raise temperatures of air before dispersing them throughout a building.

A hole at the bottom allows air into the unit while a hole at the top allows hot gasses out of the unit.

A burner generates combustion gasses and sends them through the first opening.

At the same time a blower channels indoor or outdoor air over the exterior of a heat exchanger.

The exhaust gasses from the combustion process pass through a flue to the outside.

Do Electric Furnaces Have Heat Exchangers?

Electric furnaces don’t use a heat exchanger because it doesn’t burn any fuel. Instead, electric furnaces heat the air directly as it flows over hot coils.

What Else Are Heat Exchangers Used For?

Heat exchangers are used in many ways to transfer heat from one place to another.

Refrigerators and air conditioners are examples of heat exchangers.

They take heat energy from a compartment or room where there isn’t enough heat, and pump it away in a liquid (usually water) to an area where it can be dumped.

This process is called cooling.

Air conditioners also make sure that the air stays cooler by removing heat from the air.

Heat exchangers are used to keep buildings warm in winter and cool in summer.

Heat pumps extract heat from the exhaust gasses, and then transfer the heat to the building using a heat exchanger.

This allows the exhaust gas to go straight to the building instead of going out into the atmosphere.

The heat from the exhaust gas can be transferred to the cold air, making the air warmer, and thus reducing the amount of electricity needed to warm the building.

The heat from the hot air can also be used to heat the office near the smokestacks, saving energy.

A shower waste-water heat-exchanger is an apparatus that transfers heat from a liquid to another medium.

It uses this process to transfer heat from a hot liquid (usually water) to a cooler liquid (usually air).

This allows the hot liquid to be used as a source of heat or power.

A shower waste-water heater is a device that heats water using waste heat from a building’s plumbing system.

Waste heat is generated by the flow of hot water through pipes, heating systems, and other components of buildings.

Waste heat is usually released into the environment through exhaust stacks, radiators, or other means.

A heat exchanger is used in showers to save energy. Cold water flows through the pipes while warm water passes over them.

This allows us to conserve energy by using less hot water.

Do All Heat Exchangers Fail?

All heat exchangers eventually break down. Heat exchangers are used in furnaces to transfer heat from hot gasses to cool air.

When heat exchangers are exposed to high temperatures, they expand and contract. This causes them to bend over time.

Eventually, these bends cause the heat exchanger to break. Contamination occurs when the heat exchanger is no longer safe.

Poor maintenance, poor initial system designs, and poor equipment designs by the manufacturers can cause heat exchangers to fail within a few years

A gas furnace heat exchanger lasts about 10-20 years.

It’s made out of metal. Metal expands when it heats up and contracts when it cools down.

Over time, the cycles of expansion and contraction can damage the metal in the heat exchange.

This can cause cracks or breaks in the metal.

When this happens, the seal separating the flue gasses from the air flow can be damaged, allowing the flue gasses to leak into the house.

This creates a dangerous situation that requires service from a qualified professional!

Cracked Heat Exchangers And Their Causes

Cracked heat exchangers are caused by the constant expansion and contraction of metal over time.

Cracked Heat Exchangers And Their Causes

Heat exchangers are cycled through heating and cooling, causing them to expand and contract.

Eventually, the metal cracks due to stress and fatigue. Even though your furnace is in good shape, cracks are inevitable after many years of use.

A cracked heat exchanger can be caused by improper installation or poor maintenance. Cracks can appear quickly if the furnace isn’t properly installed.

A heat exchanger must be visibly inspected at installation and on a regular basis to ensure that there are no cracks or holes in the unit.

Cracked heat exchangers are a problem because they cause problems with the furnace.

A cracked heat exchanger means that there is something wrong with the furnace. You need to get new parts for the furnace.

Unwanted Water Ingress

If water leaks into the evaporator coil it means there is something wrong with the system.

You need to contact an HVAC technician as soon as possible. Leaking water could cause serious problems if left untreated.

A cracked heat exchanger could mean more expensive repairs down the road.


Rust and corrosion eat away at the heat exchanger. This causes the heat exchanger to break down and fail.

Dirt and deposits block the air filters, blowers, and coils. Air cannot pass through to be warmed up. Heat is emitted but nothing can absorb it.

The heat exchanger cracks and breaks apart.

Poor Air Flow

Airflow is vital to the health and safety of your heating and cooling system. Cleaning out your air filters regularly will help ensure you get the most efficient airflow possible. 

Dirty air filters also cause your furnace to work harder than necessary, causing unnecessary wear and tear on your equipment.

Changing your air filters every three months is essential to keeping your home safe and comfortable.

Undersized Furnace

Furnaces should be sized according to the size of your house. Too-small furnaces will cause airflow problems, leading to overheating.

Overheated furnaces will cause your heat exchangers to crack more quickly than they would if your furnace were sized correctly.

Oversized Furnace

Furnace systems must be sized properly. Overly large furnaces cause the heat exchangers to wear out prematurely.

Condensation forms inside your heat exchanger. You need to turn off your furnace before it gets too cold or too hot.

When you do turn it back on, make sure it doesn’t get too hot or too cold.

This will prevent the formation of condensation inside your heat exchanger, and it will also prevent rusting and corrosion.

How Can I Prevent A Cracked Heat Exchanger?

Having an annual inspection and service performed by a qualified professional helps you avoid problems with your heat exchange or other HVAC parts.

Cracks in your system can be detected early on, preventing them from becoming deadly.

Clogged filters and blowers are common reasons why heat exchangers crack.

A furnace tune-up is the best time to inspect your furnace. You can prevent expensive repairs by getting an annual tune-up.

You can also learn about your furnace and how to maintain it yourself.

Inspectors aren’t experts in heating or cooling systems, but they will know if there are any cracks in the heat exchangers.

A cracked heat exchanger could mean that your whole system needs replacing.

Bad Heat Exchanger Symptoms

Heat exchangers need to be inspected regularly.

Infrared video inspections can help technicians see if there are any problems with the heat exchanger.

Warning signs include rust, cracks, leaks, or other damage.

Changes In Flame Color

Gas furnaces produce a blue flame when they work correctly.

However, if the flame changes to yellow, this could mean that there is an issue with the heat exchanger.

Discolored Metal Or Soot Buildup

Heat exchangers are used to transfer heat from one medium to another. When a heat exchanger cracks it lets soot build up inside.

Soot builds up inside the furnace because the burner isn’t working correctly.

The cracks break the seals from the combustion chamber and allow soot to leak out into the other side.

Carbon Monoxide Detected

Carbon Monoxide is a dangerous gas that kills people when it accumulates in their homes.

A cracked heat exchanger allows carbon monoxide to enter the house.

When this happens, you should immediately open the windows to let in air and turn off the furnace.

Then leave your home until help arrives. If you feel sick or dizzy, call 911.

Strong Odors

Carbon monoxide is odorless. But if your heating system emits an unpleasant smell, it could mean your heat exchanger is broken.

Formaldehyde smells like rotten eggs. It’s toxic too. Call a qualified technician right away if you smell this gas.

Excess Condensation Or Water On The Floor

Your furnace may need repair if there is excessive condensation on the inside of the unit.

You should check the drain pan (see also ‘Is A Drain Pan Required Under A Water Heater?‘) before calling a plumber.

Clogged Heat Exchanger

Dirt and dust can accumulate in the furnace and cause problems. Cleaning out the ducts regularly helps prevent this problem.

Poor Heating

A damaged heat exchanger means you need to replace your furnace. The furnace will be working harder to provide warmth, and you’ll pay more for energy.

What Should I Do If I Think My Heat Exchanger Has Failed?

You shouldn’t attempt to repair the furnace yourself if you notice any of these symptoms.

Contact a professional HVAC company to check out your furnace.

Heat exchangers are often expensive to fix or replace. You should always check your owner’s manual before buying a new one.

Warranty coverage usually covers the cost of the part, but not the labor costs involved in replacing it.

Heat exchangers should last about 10 years, but you shouldn’t replace them until they’re 20 years old or more.

Cracked heat exchangers usually indicate other problems in your furnace.

How Do I Prevent My Heat Exchanger From Failing?

Regular maintenance checks can help your furnace run more efficiently. A cracked heat exchanger could cause problems if left unattended.

Air filters should be replaced regularly to ensure the proper functioning of the furnace. Dirty filters restrict airflow and could cause overheating of elements.

What Tools Are Needed To Determine If A Heat Exchanger Is Bad?

A technician uses an infrared camera to inspect a heat exchanger.

He or she sees the inside of the heat exchanger and confirms whether it is bad or not.

What About A Carbon Monoxide Test?

Carbon Monoxide is a gas produced when fuel burns. A test for carbon monoxide can reveal whether a furnace is producing carbon monoxide.

A furnace making carbon monoxide is a symptom of bad burning in a furnace because, unlike cars, carbon monoxide isn’t a regular by-product.

A heat exchange system can be breached and if there is no carbon monoxide coming out of the furnace, then the breach will remain undetectable.

A CO test or alarm is  therefore not an appropriate solution. Call a professional HVAC tech to fix this problem.


There are only two options if the heat exchanger is bad: Replace it or replace the furnace.

But you should consider replacing the furnace first because it is more expensive than the heat exchanger.

Energy efficiency and cost of service are important factors when choosing a new furnace.

A furnace that is under warranty may be more expensive than an older model. However, a newer furnace may be less efficient than an older unit.

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By Anthony Barnes

Anthony Barnes is the founder of Water Heater Hub and a second-generation plumber by profession. Before developing Water Heater Hub, Anthony Barnes was a full-time plumber, and he has undertaken a wide variety of projects over the decades. As a second-generation plumber, it was easy for Anthony to get used to the technicalities of all from a tender age