Wood Furnace Chimneys: Dos and Don'ts - HY-C

The fire is undoubtedly the most important part of a wood-burning furnace. It’s what provides the heat that warms the home. But, to paraphrase an old adage, “Where there’s fire, there’s smoke.” All of that smoke needs to be vented safely — and that’s where a chimney comes in.

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Chimneys seem simple. After all, they’re just a pipe that carries smoke out of the home. But between temperature variables, pressure differences, length requirements, metal types, elbows, and more, there’s a lot more to chimneys than meets the eye.

Because of all these complex factors, it can be hard to know how to build a safe chimney for a wood heating appliance. But we want to help you unravel the mysteries.

At HY-C, our Fire Chief furnace division has been manufacturing forced-air wood-burning furnaces for over 40 years. From the chimney connection point all the way to the chimney cap, we know what it takes to build a proper furnace chimney.

In this guide, we’re going to cover the dos and don’ts of building a chimney for your wood-burning furnace. We’ll outline important considerations for:

1. The chimney connection from the furnace
2. The chimney itself
3. The chimney relative to the roof

By the time you’re finished here, you’ll have an informed understanding of wood furnace chimneys to ensure you can design your system for safety and efficiency.

The Chimney Connection from the Furnace

The furnace and its chimney are two separate systems that work together to vent smoke out of your house. Bridging the gap between those two systems is the chimney connection, a length of pipe connected to a port on the back of the furnace (which usually has a 6-inch diameter).

This pipe must follow several rules and standards. First and foremost, it should never be connected to a chimney that’s servicing another appliance. Aside from that, there are several other rules it must follow, and we’ll cover them one by one here.

Chimney Connection Pipe: Metal Types

When selecting the type of metal for your connector pipe, you have two options:

1. 24-gauge single-wall black stove pipe
2. Stainless steel pipe

It’s important to note that you should NEVER use galvanized steel pipe for your furnace chimney connection. This is because galvanized steel is coated with zinc. When the furnace causes this zinc to heat up, it burns away, turning into a gas.

This zinc gas can potentially spread throughout your home, causing what’s called metal fume fever. Breathing these fumes can be very dangerous, causing a myriad of symptoms from a fever and chills to nausea, vomiting, and fatigue.

Chimney Connection Pipe: Clearances to Combustibles

All wood heating appliances have what are called clearances to combustibles — distances from which parts of the furnace must be kept from flammable materials. These differ from furnace to furnace and even from certain sides and parts of the same furnace.

As an example, on our Fire Chief FCE furnace, the connector pipe’s clearances to combustibles are 18 inches. This means that you should keep combustible material (like paper, wood fuel, wooden structure supports, drywall, etc.) at least 18 inches from the pipe at all times.

Chimney Connection Pipe: Rise and Run

The length of the connection pipe itself and the angle at which it’s connected to the chimney are important installation factors. Getting them right ensures both efficient operation of your furnace and safety from smoke and heat.

As far as the length of the pipe (i.e., its “run”), it must not exceed 5 feet. If the connector pipe is longer than this, it can cause flue gasses to move too slowly, resulting in smoke rollback into the home. A long connector pipe can also cause flue gasses to cool too quickly, resulting in creosote.

As for the angle of the pipe (i.e., its “rise”), the pipe must maintain 2 inches of rise for every 1 foot of run. Imagine your connector pipe ends up being 3 feet long. This means that the end of the pipe that connects to the chimney should sit 6 inches higher than the end that connects to the furnace.

Maintaining a proper rise ensures a good draft. If the pipe were straight and had no rise, the gasses from the fire would quickly stagnate in the pipe and roll back into the home. Rise helps facilitate a natural draft, ensuring all flue gasses flow forward into the chimney at an acceptable rate.

Chimney Connection Pipe: Elbows

When connecting a furnace to a chimney, the orientation of the furnace and the layout of your home may necessitate the use of elbows to allow everything to fit properly. If you do need to install elbows, though, it’s important never to install more than 2.

Elbows mean that smoke and flue gasses need to change direction. If they change direction too often (i.e., if you use too many elbows), the gasses will slow, potentially creating a backdraft. Best practice indicates that two elbows — but no more — are fine to use.

Also, if you do need to use elbows to connect your furnace to your chimney, using 2 45-degree elbows is preferable to using 1 90-degree elbow.

The Wood Furnace Chimney

Now that we’ve covered the connection pipe from the furnace to the chimney, let’s talk about the chimney itself. When construction a chimney, there are three options from which to choose:

1. A clay-lined masonry chimney
2. A masonry chimney with a 6-inch stainless steel liner
3. Class A chimney pipe (rated to 2,100 °F)

Clay-lined masonry flues are common in older homes. Provided that they’re cleaned, inspected, and okayed by a chimney professional, they are suitable to use as a chimney for a wood-burning furnace.

As a wood furnace manufacturer, though, we’d recommend having a masonry chimney lined with a 6-inch stainless steel liner for use with a furnace. Lined chimneys are better suited to furnaces. They’re less likely to experience damage or develop creosote.

If you don’t have a preexisting masonry chimney in your home, you can always opt for Class A chimney pipe. This type of chimney pipe is manufactured specifically with wood heating appliances in mind, so it’s always a good bet.

Even with these three options available, there are a couple of important things to keep in mind regarding a furnace chimney, namely flue diameter and flue draft.

Wood Furnace Chimney Diameter

Whichever type of chimney you connect your furnace to, it’s vital that its diameter be no narrower than 6 inches and no wider than 12 inches.

If the chimney pipe is too narrow, it will inhibit a proper draft. This can lead to backdrafts and smoke spillage back into the furnace (and out into the home). A chimney with a poor draft will also develop creosote more quickly, increasing the likelihood of a chimney fire.

On the other hand, if the chimney pipe is too wide, it can cause overdrafts. This phenomenon can potentially supply too much oxygen to the fire, causing it to burn very hot and potentially warp or damage the furnace.

For these reasons, it’s important to use a properly sized chimney flue. Also, to prevent exacerbating any of the problems listed above, the diameter of the chimney stack should stay the same throughout the length of the chimney.

Wood Furnace Chimney Draft

By now, it should be clear that a furnace chimney should maintain a good draft. That means that air, smoke, and flue gasses need to move through the chimney at an acceptable rate. Specifically, though, the flue draft should not exceed 0.08 water column inches (WCI) of static pressure.

Static pressure is essentially a measurement of air resistance in a system. Static pressure is measured in water column inches. For reference, 1 water column inch is the amount of pressure required to raise a column of water by 1 inch. The higher the measurement, the more static pressure (i.e., airflow resistance) there is in the chimney.

For this reason, it’s important to keep the static pressure in the chimney as low as possible. You can do so by following all of the recommendations listed in this guide. Also, you can ascertain the static pressure measurement (in water column inches) using a draft gauge.

Wood Furnace Chimney: Through the Roof

Finally, when building a wood-burning furnace’s chimney through your home’s roof, there are some vital specifications that need to be followed to ensure a proper draft.

For starters, the chimney must extend at least three feet higher than the highest point of the roof through which it passes. This not only helps to improve the efficiency of the chimney’s draft, but it also prevents downdrafts (i.e., when wind pushes smoke and other gasses back down the chimney).

The chimney must also be at least 2 feet higher than any part of the building within a horizontal distance of 10 feet. Maintaining this distance also helps to improve draft and prevent downdrafts. It assists in preventing blockages caused by ice, snow, and debris, too.

Lastly, the furnace chimney’s cap should be properly sized to fit the chimney flue. It should be installed in such a way that keeps out rain and nuisance wildlife while also maintaining proper airflow and preventing downdrafts.

How Do You Maintain Your Wood Furnace’s Chimney?

There they are: the dos and don’ts of installing a chimney for your wood-burning furnace. After the chimney is built, though, how do you maintain it to ensure it stays properly operational and creosote-free?

One option, of course, is hiring a professional chimney sweep. Cleaning and maintaining chimneys is their job. They’ll be able to point out any problems and make any repairs that need to be made.

On the other hand, you can also use a DIY chimney cleaning kit. These drill-powered kits allow you to do the job of a chimney sweep yourself, cleaning and maintaining your own chimney on your own schedule.

If you are looking for more details, kindly visit Fuming Furnace.

What is a fume hood? A chemical fume hood is an enclosure that safely contains and ventilates hazardous fumes, vapors, gases, and dust generated by chemical processes performed in the fume hood. Sometimes called a chemical hood or a lab hood, a fume hood protects workers from inhalation of hazardous substances. In a fume hood, proper maintenance and airflow monitoring are crucial for ensuring safety and compliance.

The clear sliding window on a fume hood, called the fume hood sash, also shields workers from spills and splashes that may occur in the chemical fume hood.

Fume hoods are the workhorse of laboratory exhaust systems and are the most widely used approach for local ventilation. A well-equipped fume hood or lab hood provides critical protection when working with volatile and hazardous substances. The importance of regularly inspecting and maintaining a fume hood or lab hood cannot be overstated, as it ensures optimal performance and worker safety. A properly functioning fume hood is essential for any laboratory environment to manage risks effectively.

The term “chemical hood” is often used interchangeably with fume hood, particularly in laboratory and industrial settings where chemical processes pose inhalation or splash hazards. A chemical hood is designed to isolate volatile substances, drawing contaminated air away from the user through a ventilation system that protects both personnel and the work environment.

In educational and research laboratories, a chemical hood plays a vital role in day-to-day experiments involving acids, solvents, or particulates. Choosing the right chemical hood depends on the type of chemicals being used, airflow requirements, and space constraints within the facility. Whether for routine handling of reagents or emergency containment of unexpected reactions, a well-maintained chemical hood is an essential safeguard.

Whether you’re installing a new fume hood, upgrading existing lab equipment, or training staff on best practices, understanding the capabilities and limitations of your chemical fume hood is key. A certified fume hood not only supports compliance with safety regulations but also reinforces a culture of safety in your lab. Always consult fume hood performance data and maintenance schedules to ensure continuous protection.

Chemical Fume Hood

How does a fume or lab hood hood work?

A fume hood works by pulling air away from the user into the lab hood enclosure with a blower. The chemical fume hood then filters and vents the air to the outdoors through a facility exhaust system. Alternatively, a fume hood may filter the air to remove dangerous fumes and then return the air to the room. Most fume hoods, or lab hoods, are equipped with gauges or alarms that warn the user of low airflow and potential exposure to hazardous fumes. For those designing or managing a fume hood lab, ensuring proper exhaust functionality and regular inspections is essential.

Proper use of a fume hood requires keeping the sash at the recommended height, avoiding sudden movements that disrupt airflow, and ensuring the interior is free from clutter that could block ventilation. A well-maintained fume hood is a critical safety feature in any laboratory environment where volatile chemicals are used.

What is a fume hood used for?

Fume hoods, also referred to as lab hoods, allow lab employees to work with potentially dangerous chemicals while minimizing the risk of exposure to toxic fumes. The sash (window) on a chemical hood allows the worker to view and manipulate objects within the enclosure while keeping fumes from toxic or volatile chemicals away from the worker’s face.

Industries that use fume hoods include:

• Semiconductor manufacturing
• Aerospace surface finishing such as passivation with nitric acid
• Research departments at colleges and universities.

A fume hood lab in these industries ensures safe handling of harmful chemicals and complies with safety standards.

What types of hazards can a fume hood protect the user against?

When do you use a fume hood? In general, use of a laboratory fume hood is advised whenever working with hazardous chemicals. If the Safety Data Sheet (SDS) for the chemical you plan to use in the lab hood has warnings like “Toxic by inhalation” or “Do not breathe dust, fumes or vapors,” then a fume hood or other ventilation system is needed.

Fume hood use is also recommended when working with compounds that have a low boiling point, or chemicals that emit noxious odors.

Types of materials that should be used inside a chemistry fume hood include:

• Any volatile materials
• Corrosive acids and bases
• Irritating vapors and dust
• Asphyxiating gases
• Open sources of volatile radionuclides

Examples of chemicals used with a fume hood include nitric acid and hydrofluoric acid.

Fume hood vs. Laminar flow hood: What’s the difference?

Chemical fume hoods are often confused with laminar flow hoods, but they are not the same. Both use airflow as a means of protection, but the object of protection differs: Chemical fume hoods protect personnel, while laminar flow hoods protect the product.

In a fume hood, air is pulled away from the worker to protect the worker from hazardous fumes. By contrast, a laminar flow hood (also called a clean bench) blows filtered air outward. The smooth, non-turbulent airflow prevents contamination of the product such as a semiconductor wafer or biological sample from particulate matter.

Biosafety cabinet vs. Fume hood: What’s the difference?

Biosafety cabinets are another category of laboratory equipment that is frequently confused with chemical fume hoods. Both use airflow to protect, but the focus of protection is on different hazards: Fume hoods protect against chemical fumes and vapors, while biosafety cabinets protect against pathogens and biological agents.

A biosafety cabinet (also called a biological safety cabinet or BSC) uses HEPA filters to remove infectious organisms from exhaust air. Depending on the class and type, a biosafety cabinet may also use HEPA filters on intake air to protect the product from contamination. By contrast, a fume hood does not usually use HEPA filtering on exhaust air vented outdoors.

Laboratory Enclosure Comparison

Fume Hood Laminar Flow Hood Biosafety Cabinet Primary Function Protect the user from hazardous chemical fumes and vapors Protect the product from contamination with particulate matter Protect the user and the environment from pathogens Used With Chemicals that generate harmful fumes, volatile vapors and gases Particulate-sensitive materials such as semiconductor wafers or biological samples Infectious biological agents or hazardous particulates Airflow Away from the user Non-turbulent, may be toward the user Away from or around the user, varies by class HEPA filters Optional on exhaust On intake air Required on exhaust, optional on intake air

Best Technology does not offer biosafety cabinets, as they are an entirely different product category from fume hoods, and outside our range of chemical process equipment.

Fume hood buying tips: What to look for in a fume hood

Size. A basic starting point when shopping for and specifying a fume hood is size. What size is the working area required for your application? What size is the available space on the factory floor or the laboratory for a fume hood? Best Technology builds custom fume hoods, so even if your size requirements are unusual, we can build a fume hood to fit your space.

Materials. Some chemistries require specialized materials for containment. Highly corrosive acids at high concentrations, for example, often require a fume hood constructed from polypropylene rather than stainless steel for better corrosion resistance. For hydrofluoric acid, polycarbonate is the preferred material for the sash, to avoid etching of a glass window. Knowing the expected chemistry that will be put in the fume hood is crucial to ensuring years of reliable functioning.

Standards. Does your fume hood need to meet certain industry standards, such as FM ? Does your chemical process need to meet other specialized standards or requirements? Sometimes a “one-size-fits-all” chemical fume hood just won’t work. In those cases, look to the applications engineers at Best Technology for a customized fume hood.

Window orientation. Some fume hoods use a horizontal sliding window, which limits the access of the operator to the working area inside the fume hood. Look for a vertically sliding window (sash), so that users have more available working area in the interior of the fume hood. Best Technology offers fume hoods with counterbalanced, vertically sliding windows.

Fume Hood for Semiconductor Wafer Chemical Processing

Ducted fume hoods

A ducted fume hood relies on the facility’s ventilation system for venting exhaust air outdoors using ducts. The fan or blower is typically located on the roof of the building, allowing for quiet operation of the fume hood. To prevent recirculation of contaminated air, the ductwork for a fume hood should be separated from the rest of the facility’s ventilation ducts.

Avoid ductless fume hoods. A ductless fume hood, also called a recirculating fume hood, uses a blower on the fume hood to pull contaminated air through a HEPA filter, and then recirculates the air back into the room. The type of filter required varies depending on the chemistry, so the operator must ensure use of the proper filter for safety. Filters also must be changed regularly for safety.

Though ductless fume hoods avoid the upfront cost of ductwork installation, they add to noise, maintenance requirements, and a greater risk of chemical exposure to workers and other equipment. For these reasons, many research universities in the U.S. have banned the use of ductless fume hoods.

Best Technology offers only ducted fume hoods, and not ductless fume hoods.

Fume hood safety

Keep your head out of the fume hood. User behavior is critical to ensuring that the safety features of a fume hood are not defeated. Only hands and arms should be inserted into a fume hood, and never the head.

Sash down. To obtain the safety benefit of a fume hood, the user needs to keep the sash down. The lower the sash position, the safer the user is from chemical exposure. Many fume hoods specify a maximum fume hood sash height to maintain safe exposure levels.

Fume hood safety instruments

Most fume hoods are equipped with a means of verifying airflow within the enclosure, either a magnehelic gauge or an airflow meter. These instruments verify airflow in slightly different ways.

• A magnehelic gauge measures the differential in air pressure, inside vs. outside the fume hood.
• An airflow meter measures the actual flow rate of air within the fume hood.

These instruments allow the operator to confirm that the exhaust system is functioning. If the air isn’t exhausting as expected, many organizations have emergency procedures for containing potential hazardous vapors.

Magnehelic Gauge

Fume hoods also use a measurement called face velocity to evaluate exhaust power. The face of a fume hood is the opening where air enters. Face velocity, then, is the velocity of air when it enters the fume hood, measured in feet per minute (fpm). A face velocity of 80 – 125 fpm is generally considered acceptable.

Fume hood operation tips

Keeping the sash as low as possible, and closed when not in operation, reduces energy costs. Since the fume hood exhausts conditioned air (heated or cooled, depending on local climate), reducing the amount of exhaust reduces the load on the facility’s HVAC system.

Be aware that sash position can affect air velocity within the enclosure such that a closed sash may have enough increased airflow to disturb delicate parts or instruments. Some fine-tuning of the blower speed may be required to find the balance that works for you.

Fume hood design options

Proper construction materials for a fume hood depend on the intended application. Options for construction materials for fume hoods include:

• Stainless steel
• Polypropylene
• PVDF / Kynar®
• Teflon® / PFA
• ECTFE (HALAR®).

Fume hoods are a flexible engineering solution for a variety of chemical handling needs. Here are a few of the many possible options and features that can be included in a fume hood:

• Rinse sinks and tanks with connection to a DI water (deionized water) source
• Tank heaters
• Strainers for handling small parts
• Test tube and beaker holders
• Space for customer-supplied laboratory hot plates and magnetic stirrers
• Compressed, clean dry air (CDA) spray guns for drying parts
• Nitrogen spray guns (N2 spray guns) for ultra-clean drying of dust-sensitive materials such as semiconductors.

Fume Hood for Wet Chemical Processing

To learn more about fume hoods offered by Best Technology, visit our Fume Hoods page.

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