Gas & Oil Furnace Chimneys: What You Need to Know

Most of us grew up with HVAC systems, so it’s not surprising that we often take the comfort they provide for granted. Typically, we don’t even think about these systems until there’s a problem, right? But the truth is, just like any other area of your home, your furnace and the venting system it relies on require routine care. And when you neglect the chimney venting your gas or oil furnace, boiler, or water heater, you can run into trouble…

Not only will your appliance itself suffer in terms of performance and efficiency, but you may also see an increase in fire and carbon monoxide risks, as well as bigger repair bills down the line.

We want to help you avoid expensive repairs and unnecessary risk, and we believe education is the key. So, let’s dive into what you need to know about gas and oil furnace flue maintenance.

Ready? Let’s get started.

Do Gas Furnace Chimneys Need to Be Maintained? Why?

Gas is one of the most popular choices for home heating appliances in many areas of the country, and for good reason. Natural gas is an inexpensive fuel, and new, higher-efficiency appliances help keep fuel consumption down. With new natural gas pipelines being put in across the country, this economical option is becoming available to more and more people.

But while most homeowners are aware of the need to have their fireplace or woodstove chimneys cleaned and inspected, many don’t realize that a gas heating appliance – be it a furnace, boiler, or even a hot water heater – also relies on a chimney for proper venting of the exhaust.

…Which means that even gas furnace chimneys need to be maintained.

Like wood-burning fireplace chimneys, gas furnace chimneys can develop damage or blockages and become a danger. And while appliances fueled by natural gas or propane may not produce the visible soot that appliances burning other fuels do, they can deposit corrosive substances in your chimney.

These acidic byproducts could wreak havoc on your chimney without producing any external symptoms initially. Which means you may not know there’s an issue until the problem has become dangerous or expensive to repair.

Ignoring your gas furnace, boiler, or water heater can be especially dangerous because carbon monoxide is involved. So, don’t ignore these venting systems!

Here at the CSIA, we adhere to the recommendations of the National Fire Protection Association (NFPA) and encourage homeowners to have their gas furnace chimneys inspected every single year and cleaned and maintained as needed.

Does My Gas or Oil Furnace Chimney Need to Be Cleaned?

The answer is yes. Appliances fueled by natural gas or propane may not produce the visible soot that appliances burning other fuels like wood do, but they can still deposit corrosive substances in your chimney. Likewise, oil furnaces and boilers can produce a heavy soot-like byproduct if they’re not working efficiently or venting properly.

Obviously, you want to avoid unnecessary threats and expenses, so it’s important to have your oil or gas furnace chimney cleaned when needed.

Note: Many homeowners think that the technicians servicing their gas appliances or oil appliances are cleaning, or at least looking at, the attached venting systems. Unfortunately, that’s not the case. The reality is that the oil burner company may shovel out the base of a brick chimney and they may brush out the connector pipes, but they will most likely not clean the chimney system. Similarly, the gas appliance technician is going to tune and service the gas appliance, but the chimney isn’t their main concern. So, when you have your fireplace chimney inspected and cleaned each year, have a CSIA-Certified Chimney Sweep® look at your gas furnace flue at the same time.

If Creosote Isn’t a Concern, What Do I Have to Worry About with My Gas Appliance Flue?

Gas appliances don’t produce creosote, so what is there to worry about? Well, while gas is widely marketed as a clean-burning fuel (and under optimum conditions, it is a clean-burning fuel), it can still leave behind corrosive deposits that can damage the flue.

Here’s how it happens…

1. To burn the gas, your heating appliance needs intake air for combustion. The same acids in the air that cause acid rain may also be present in the intake air your heating appliance uses. This air may also carry chlorides, which are often picked up from household cleaning products or other pollution.

2. Water vapor is always produced during combustion. In fact, allow us to wear our nerd hats for a moment and share this fun fact with you: One cubic foot of gas yields two cubic feet of water vapor. When the chlorides in the intake air combine with this water vapor, hydrochloric acid is formed. Other acids may also form when water mixes with residues in the flue or with other airborne pollutants.

3. These acidic deposits eat away at the liner. Older, unlined chimneys are the most susceptible to corrosion partly because of their age – use and general wear and tear over the years. If the flue was used to vent appliances using different fuels in the past (like wood) and soot deposits were left behind by those fuels, that can also speed up the corrosion of the chimney’s interior. But regardless, eventually, corrosion caused by water condensation and acidic deposits inside of the flue could cause the liner to crumble.

4. Debris from the liner can then create a blockage in the chimney that can prevent carbon monoxide from leaving the system, and instead allow it to enter your home. And of course, if the chimney is unlined or in poor condition, left behind carbon monoxide is especially dangerous.

That’s why all chimneys – even those venting a ‘clean’ fuel like gas – should be inspected annually and cleaned as needed.

I Have a High-Efficiency Gas Furnace – Do I Have to Worry About Problems Like Flue Corrosion or Condensation?

Many homeowners think that if they invest in a modern high-efficiency gas appliance, they won’t have to worry about having problems with flue corrosion and condensation. Unfortunately, that’s not true.

Here’s why: Modern high-efficiency appliances gain their higher efficiency by extracting the heat that would otherwise be sent up the chimney and delivering it to your home instead. While this is good in the sense that no heat is ‘wasted’, a certain amount of heat is necessary in the chimney to provide adequate draft and prevent condensation of flue gases on the chimney walls.

If the flue temperature becomes too low because the appliance is preventing heat from going up the chimney (as is often the case with modern appliances), you could experience issues with corrosion and condensation.

As odd as it may seem, the combustion of any hydrocarbon (like gas) results mainly in carbon dioxide and water vapor. In fact, the average furnace puts out about 1.5 gallons of water in the chimney every hour it’s used.

The high stack temperatures of older inefficient furnaces kept this moisture from condensing inside the chimney, and it was often visible as steam escaping the chimney top. But because newer high-efficiency furnaces now steal this extra heat from the chimney, all this water now often condenses inside the cooler flue.

The problem becomes even more complicated because the water in the flue is usually highly acidic and corrosive (as we talked about above). Now, you don’t just have a gallon or more of water in your chimney every hour, you have a gallon or more of dilute hydrochloric or sulfuric acid eating its way through the mortar and brick of your chimney – from the inside out.

So, contrary to what you might think, high-efficiency furnaces don’t eliminate condensation and corrosion issues – they can, in fact, sometimes exacerbate them.

How Do You Know if Your Venting System Is Properly Matched to Your Gas Furnace?

To work as safely and as efficiently as possible, gas appliances must meet specific venting requirements. When the heating appliance and venting system are properly matched in size, the water vapor produced by burning gas is carried out of the system quickly and at a warm enough temperature that it doesn’t condense inside the flue.

If, however, the heating appliance and venting system aren’t properly matched, there won’t be enough heat to swiftly move that water vapor out, and the moisture will turn to liquid inside the flue. It’s that acidic condensation that can deteriorate the inside of the flue.

So, it’s vital that your venting system be properly matched to your gas furnace. How do you know if they’re properly matched?

One of the best ways to know if your gas heating appliance is properly matched to your venting system and will operate correctly is to have a CSIA-Certified Chimney Sweep® evaluate the chimney when the appliance is installed.

Evaluations are especially important when older chimneys are paired with high-efficiency appliances and boilers (those with efficiency ranges of 80%+), but it’s also important when new chimneys are paired with older heating appliances.

An evaluation will provide helpful information, result in a better match between the heating appliance and chimney, and give you and the chimney pro a baseline to refer to when looking for changes at subsequent yearly inspections.

If you didn’t hire a CSIA-Certified Chimney Sweep® when the appliance was installed, it’s important that you have the compatibility of the flue liner and furnace checked ASAP. There are also a few signs and symptoms of an improper venting match that you can be on the lookout for:

  • Damp patches on interior walls or exterior walls
  • Peeling wallpaper
  • Blistered paint
  • Stains on the ceiling
  • White stains (efflorescence) on the outside of the masonry chimney
  • Eroded mortar joints or crumbling bricks on the chimney

Does Draft Matter in a Gas Venting System? Why?

Draft matters in a gas venting system for a couple of big reasons:

  • Draft is needed for complete combustion. The chimney is not only responsible for letting the combustion byproducts passively escape up the flue, but also for generating enough draft to actively pull combustion air into the appliance. Complete combustion requires sufficient oxygen – to burn one cubic foot of natural gas, the appliance needs more than 10 cubic feet of air. If combustion is incomplete because of inadequate draft, more carbon monoxide will be produced, and less heat will be delivered to the home. Not only that, but a poorly drafting chimney is less likely to effectively vent the carbon monoxide produced by the system.
  • Proper draft reduces condensation. Inadequate draft can lead to more residual moisture in the flue. Sometimes the chimney flue is too large for the appliance’s venting requirements, giving the gases more opportunity to condense inside the flue. An exterior chimney, because it’s not surrounded by the warmth of the home on every side, will make the problem worse. The same problems can arise if the connector pipe that joins the appliance and the chimney is too long, or if there are too many bends or elbows in the venting system. Basically, anything that interferes with draft can allow moisture to condense inside of the flue instead of being carried out of the venting system – and that moisture can cause corrosion in the flue.

Note: If your gas heating appliance is properly matched to the venting system, that will help ensure adequate draft in the system.

Do You Need a Chimney Liner for an Oil/Gas Furnace Flue?

Yes. Every gas- or oil-fired appliance requires a chimney liner. Not all liners are compatible with gas or oil appliances because of the material from which they’re made, so make sure that the lining system you purchase – or the one you currently have in your chimney – is appropriate for use with your gas or oil appliance. A CSIA-Certified Chimney Sweep® can help if you have questions or are unsure. Find one in your area right here.

What Causes Damage to Chimneys That Vent Oil-Fired Appliances?

There are three main causes of damage to chimney flues venting oil-fired furnaces and boilers:

#1 The acidic nature of the byproducts of combustion.
Oil-fired appliances produce corrosive residue that can settle on the chimney walls. When these byproducts mix with moisture (often as a result of the next two issues), it can cause the liner walls to soften and flake over time and create dangerous openings in the liner for carbon monoxide.

#2 A mismatch in flue/appliance size.
High-efficiency oil-fired appliances require smaller flue sizes than older units did. For example, where an old unit may require an eight-inch flue, the new replacement unit’s installation instructions may call for a five-inch flue. This is a 60% reduction in flue size. If the flue isn’t relined to accommodate the new appliance, you’ll likely experience issues with draft, performance, efficiency, and carbon monoxide. Additionally, flue gases will hang around longer, settle and cool on the flue walls, and erode the liner.

#3 Lower flue gas temperatures.
Water vapor is a normal byproduct of combustion, but the lower flue temperatures of newer, highly efficient units can cause more water vapor to cool and condense on the flue surface, rather than swiftly exiting the chimney. When this happens, you’ll get spalling and flaking brick, eroded mortar joints, moisture transfer to walls, and possibly even carbon monoxide in your home.

How Do I Know If I Need to Have My Oil Furnace Flue Relined?

When the oil burner technician services your oil-fired appliance, they’ll remove the connector pipe and open the cleanout door. If they find any of the following, you may need to have the chimney relined:

  • Small particles of the chimney liner at the base of the chimney
  • No chimney lining
  • Cracked or broken flue tiles
  • A wet chimney

All of the above can cause performance and safety problems with your appliance, so it’s crucial that you have the chimney relined if any of these things are present.

If you need to have your oil furnace flue relined, a UL-listed stainless steel flue liner or UL-listed cast-in-place chimney lining system designed to vent the flue gases of oil-fired appliances should be installed.

Not only will a new chimney lining system for your oil-fired appliance improve its performance, boost its safety, and prolong the life of your chimney, but it will also lead to increased efficiency.

What does it mean at the end of the day? It means you’ll spend less money on oil to heat your home, less money on repairs over the years, and less time stressing over your safety.

What if I Need to Have My Gas Furnace Flue Relined?

If you need to have your gas furnace flue relined, the good news is that there are UL-listed lining systems approved for gas appliances. These can typically be fitted in a masonry chimney with relative ease.

A CSIA-Certified Chimney Sweep® can properly evaluate, size, recommend, and install the appropriate chimney liner for your home heating equipment.

What Will a Chimney Sweep Look at During Annual Service of My Oil-Fired or Gas-Fired Appliance Chimney?

Annual service of your oil-fired or gas-fired appliance chimney will include going on the roof and inspecting the exterior masonry (if applicable), the flashing, the chimney cap, and any other readily accessible portions of the chimney exterior. It will also include an inspection of the chimney interior with the use of video scanning technology.

Here are a few of the things the tech will be looking for:

  • Deposits/buildup
  • Flue blockages
  • Cracked mortar, spalling brick, and other water damage
  • Improper flue size, material, or connection

Ultimately, the tech will be evaluating the system for safe continued use. At the conclusion of the inspection, you should know exactly what condition your chimney is in and if anything needs to be done to boost safety, efficiency, or longevity.

And remember, finding a qualified chimney professional to evaluate your chimney is easy – search your area for a CSIA-Certified Chimney Sweep® right here.

We hope we’ve given you a good understanding of why gas and oil furnace chimneys need to be maintained and what kind of issues you should keep an eye out for. If you still have questions about gas- or oil-fired furnace flues, please reach out to us at We’re happy to help!


Among many other things you’ll find in our Homeowner Resources are answers to What To Do When…anything happens with, to, or around your chimney.