Does A Furnace Vent Need To Be Sloped?

The furnace vent is a small yet necessary part of a furnace's operation. We've put together this comprehensive article based on research on the importance of proper furnace vent positioning to ensure efficient condensate drainage.

The furnace vent should always be sloped so the water will be returned to the furnace for proper drainage. This helps to prevent any water from building up in the venting system. Water accumulating in the vents may eventually freeze and narrow the vents, which could cause severe problems with your furnace's operation. 

A properly installed furnace can provide your family years of service and comfort. By following the recommended slope angle for a furnace vent, you will avoid any surprises or potential headaches down the road. Read on to find out more.

Home high efficiency furnace, Does A Furnace Vent Need To Be Sloped?

What Is A Furnace Vent?

Furnaces and other heating systems work by burning fuel, like natural gas or propane to generate heat. When the fuel burns, it releases carbon dioxide and other harmful combustion gases.

Gases need to escape and be removed from the room where the furnace is located. Most of the time, they are released through an exhaust or furnace vent.

A furnace vent is responsible for the exhaust of combustion gases from your furnace. Furnace vents come in various types and styles.

The first type of furnace vent is a flue. A flue is a pipe that goes up through the roof and out the roof's chimney. The flue acts as a path for the furnace exhaust gases to travel through.

The second type of furnace vent is the draft fan. A draft fan is a fan found inside the furnace close to the heat exchanger. The draft fan draws the hot air from the furnace and pushes it out of the furnace.

Furnace Vents: To Slope Or Not To Slope?

For the venting system to work, there must be a slope. This slope is called the vent angle. Vent angles may vary depending on how the vent is positioned, but the standard is 1.25 inches per foot leaning down to the location of the furnace.

This vent angle provides a good drainage angle and creates a strong updraft that can pull in hot air from the bottom of the furnace.

Further, water in the venting system can cause several problems, but narrowing the vent system is the most common. 

If the vent system becomes narrowed, the exhaust flow will slow down. As a result, the pressure inside the furnace will decrease. When the pressure gets too low, the furnace safety pressure switch will shut the system off. 

How To Know If Furnace Ventilation Is Working Properly?

The gas furnace's primary job is to heat your home. However, because of how the furnace is built, it releases a lot of heat out the exhaust pipe. The furnace needs to be vented outside to prevent moisture from entering the home.

Moisture will condense on your windows if the furnace is not properly vented. This will make your home feel damp, cold, and musty. If your windows have a lot of condensation, it could be because the furnace is not venting properly. 

Window with water drops

What Is The Maximum Length For Furnace Vent Pipe?

To maintain a comfortable temperature and efficient vent system, you should ensure that you don't go beyond the maximum of 15 feet vertical distance for a furnace vent. You don't want to have too long a pipe, as it will take the air too long to move.

Can You Use PVC For Furnace Vents?

Vent pipes connect the furnace to the outside, and they are needed to prevent air from stagnating in the furnace and heating it. However, PVC is not rated for heat transfer and can easily melt, causing leaks.

While some HVAC contractors have been known to use PVC pipes for this purpose, it is not recommended because of the potential risks involved.

In addition, the biggest problem with PVC is that it contains phthalates. Phthalates are a class of chemical compounds used to make plastics flexible.

In a furnace, these chemicals can leach into the air because of the high temperature and are thought to be a potential cancer-causing agent. 

PVC is generally not considered as safe as other materials like galvanized steel, stainless steel, and aluminum.

Is It Necessary To Insulate Furnace Exhaust Pipes?

A leaky vent is a big problem when it comes to heating and cooling systems. You may not notice it for a while, but it'll eventually affect your comfort level.

Vents are an essential part of the HVAC system, allowing warm air to escape and allowing cold air to enter. Leaky vents will let out a lot of warm air and a little bit of cool air. As a result, you'll have to keep the thermostat adjusted to compensate.

If you don't know where the vents are, you'll have to tear up your walls to find them. Your furnace vents are most likely where your furnace is. 

Once you find the vents, you'll want to make sure that they are sealed properly. You'll want to check for gaps and seal them tight with a high-temperature caulk.

We have a separate article that discusses this topic in detail: Does Furnace Vent Need To Be Insulated?

Check out this high-temperature caulk on Amazon.

Furnace Behaves Like An Air Conditioner Blowing Cold Air - What's Going On?

A home high energy efficient furnace in a basement

When your furnace is running fine and performing at its best, the air filter is the last thing that needs attention. But when the filter is dirty, you'll notice some odd things happening.

If the filter is clogged, the heat exchanger will overheat. Consequently, your furnace will shut down, resulting in a decrease in heat output; worse, it may think it's an air conditioner. You need to clean out your air filter regularly to avoid this problem.

If you see clumps of dust on the filter, shake the filter out. Be careful not to use excessive force, or you'll tear the filter. If you can feel slight resistance when shaking the filter, try gently pulling it out.

Furnace Too Loud When Running - What's Wrong?

When the air and gas have a balanced mix, the furnace works optimally and produces the most effective heating.

You can tell when your furnace is out of balance by its noise. If your furnace has too much air and insufficient gas, it will produce a sound that becomes louder and louder until it reaches the point where it's too loud to be acceptable.

The wider the discrepancy of the air-to-fuel ratio, the louder the sound gets.

Is It Possible For Gas Furnaces To Explode?

Gas furnace

One of the most common questions homeowners ask is whether or not a furnace can explode. This is a serious question because it can cause not only property damage but physical injury.

However, modern furnaces are equipped with safety devices that will shut them down if they detect anything that might lead to an explosion. 

To keep it in perspective, the most common causes of house fires are kitchen stoves and space heaters. It is much less likely that a furnace will explode.

How Many Years Do Furnaces Last?

A home Goodman high efficiency furnace with Bradford White Residential gas water heater & an Generalaire humidifier.

The home furnace can be your home's backbone, especially in the winter. In addition to keeping your home warm and comfortable, furnaces also can help to save you money by reducing energy consumption.

A properly maintained furnace will last at least 15 to 20 years. Regularly inspect the unit and perform simple repairs can keep your system running for years.

When it comes to furnace maintenance, you should check to see if the filter needs to be replaced.

In addition, the air filter is responsible for removing dust and other contaminants from the air. A clogged air filter will lead to the furnace producing more heat than is necessary.

Always be on the lookout for signs of a potential problem. If you start noticing that your furnace is making unusual noises or it's not heating up as much as usual, it could be a sign of trouble.

Common symptoms include loud banging or clanging sounds, high or low noises, whistling sounds, loud rattling or shaking, or other noises related to a malfunctioning unit.

Regarding furnace repair, you want to ensure that you get a quality technician to perform the work. A furnace repair specialist should be licensed and certified to do the job correctly.

He should be able to give you an accurate estimate of the project's cost, along with the warranty and the manufacturer's guarantee.

Furnace Sounds Like A Pounding Jackhammer - What Could Be Wrong?

Furnaces work by burning fuel and circulating hot air into the room. Burners heat the air, which in turn heats the house. It is essential to keep the burners clean because they are directly exposed to combustion residues which can clog the burners over time.

Further, when a furnace burns clean, it usually makes a steady, comfortable humming noise. However, when it is dirty, it will often make a loud, jarring noise like that of a pounding jackhammer


A properly installed furnace will last longer and provide greater comfort than an improperly installed furnace. There are some factors that go into a proper installation, such as slope angle, length of the flue, and vent size. If you can get these factors right, then you should be able to avoid some common problems that may arise during the life of your furnace. 

You might also like:

Low Humming Noise In Furnace [Even When Off]: What To Do?

Furnace Working But Not All Burners Lighting: What To Do?

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