Choosing an Offset Smoker

Understanding how a wood fire flavors our food requires some background knowledge on combustion - the chemical reaction that takes place when wood burns in oxygen. If wood were burned under conditions of complete combustion, meaning that there is enough oxygen in the fire to completely react with every compound that makes up the wood, the reaction would produce only heat, water, and carbon dioxide - much like the fire from a propane grill. Luckily for us, combustion within a wood fire is always partially incomplete and will also produce a variety of flavorful compounds often referred to as ‘organic wood vapor’, as well as very small particles that we see as smoke. The types of flavor compounds and the size of the particles within the combustion gases that cook our food will depend on how clean or smoky a particular fire is burning. A fire that is starved for oxygen will produce off-tasting flavors and very large particles that we see as thick, billowing smoke; these large particles can deposit onto the food and produce the bitter ‘bad smoke’ flavor that we want to avoid. A properly managed fire with plenty of oxygen and an open flame will burn close to complete combustion and produce excellent flavor; the smoke from this fire will appear very thin and almost grey/blue in color (thin blue smoke) owing to much smaller particulate. Many pitmasters choose to run a smokier fire during the early stages of the cook to impart as much flavor as possible, and finish the cook with a cleaner, more efficient fire once the meat is wrapped or above the temperature will it will take on smoke.

The 'Stack' Effect
A near-clean burning fire is the key to great barbecue flavor, and a clean burning fire requires a constant and ample supply of fresh oxygen. In an offset smoker, fresh, oxygen-rich air is pulled into the firebox by a smokestack - typically a tall, vertical tube or pipe - using a principle known as the ‘stack effect’. As hot combustion gases from the firebox make their way to the base of the stack, they will rise and create a region of pressure inside of the stack that is lower than the pressure outside of the pit. This pressure differential essentially acts as a vacuum and draws more fresh air into the firebox to be heated, combusted, and pulled across the grates to cook and flavor the meat. We often refer to the airflow from the firebox to the stack as ‘draft’.

Smokestack Size
The amount of draft generated by a smokestack is proportional to its height and interior volume. If the stack is too short, there will not be enough air drawn into the firebox to fuel a clean fire; if the stack is too tall, the hot combustion gases may cool down too much before they exhaust, creating a backpressure and hindering the draft. The dimensions of the stack will be specific to the size and flow characteristics of each individual offset smoker, though there are general guidelines and calculators available to provide a good starting point.

In our experience, a stack that is 40 - 50 in. tall will work best for most horizontal offset smokers and the inner diameter of the pipe can be sized proportionally to the draft requirements of the pit. Square or rectangular smokestacks have corners that will create drag and restrict airflow more than their round counterparts, so the volume will need to be increased accordingly. A note for vertical offset smokers - in this design, the tall cook chamber itself creates significant amounts of draft and the stack can be much smaller compared to what would be needed for a horizontal offset of the same size.

Smokestack Location
The location of the opening to the stack serves as the exhaust point for the pit and will influence how the combustion gases move out of the firebox and across the cooking grates. On a traditional style offset smoker, the stack is located opposite the firebox and the exhaust point from the cook chamber is ideally located at grate level to create uniform flow across the entire length of the cooking surface. Some offset smokers also have the stack offset from the cook chamber using a tapered ‘smoke collector’ or reducing elbow to ensure heat and smoke flow uniformly across the full width of the end of the cooking grate prior to exiting the cook chamber. The stack position on a reverse flow offset smoker is on the same side as the firebox and commonly on the top of the cook chamber; the top position is often chosen to minimize any flow resistance beyond what may have already been caused by the reverse-flow baffle plate (more on reverse flow offset smokers later).

The firebox is the heart and soul of an offset smoker. Mastering the craft of fire management for great barbecue requires many hours spent in front of a firebox, so it should not come as a surprise that every pitmaster eventually develops their own unique methods and style. There are books, videos, forums, and in-person courses entirely dedicated to teaching the art of fire management on an offset smoker; a summary of some key aspects are described here to provide context for our review of the firebox below.

Firebox Design
The design of the firebox for an offset smoker significantly impacts the ability of the pitmaster to manage a clean burning fire and the ease with which they can maintain consistent, uniform cooking temperatures within the cook chamber. There are many design variables for a firebox including shape, size, material thickness, insulation, door style and air intake, as well as how the firebox integrates to the cook chamber. There is no one-size-fits all firebox, and each should be uniquely designed to the size and style of offset smoker it will be powering. We have summarized our thoughts below on how each firebox design variable impacts the performance and user experience of the pit, though it is important to understand that much of this will come down to personal preference and what matters to you most when burning your fire.

Round vs Square Firebox
The most obvious (and polarizing) decision when designing a firebox is whether it should be round or square. Square fireboxes provide a modern look/feel and have become increasingly popular in recent years, while round fireboxes are more traditional and provide a timeless aesthetic to the pit.  Apart from looks, each design comes with advantages and disadvantages that need to be weighed when deciding which is best for you and your cooking style.

A square profile provides great accessibility to the firebox. There is a reason we don’t make our refrigerators out of a long cylinder with a round door - it is really nice to open the door and have full, open access to everything inside. The improved access provides plenty of real-estate for working inside the firebox and the flat bottom makes ash cleanout a breeze. The flat top of a square firebox provides a great surface for preheating splits or for integrating other features like a warming oven or grill. The square profile also creates corner spaces that allow you to preheat your splits right inside the firebox without touching the coal bed; in a round firebox, it can be more challenging to keep the splits away from the coal bed unless your firebox is particularly long (more than 24 in.)

As discussed in our article on fire management, it is critical when building a fire to maintain a hot, concentrated coal bed while maintaining the ability to create an airflow gap underneath the burning split. This is challenging to do within a square firebox because of the flat bottom surface. A solution to this problem is to add an elevated fire grate and/or a ‘V’ shaped basket to concentrate the coal bed and create the appropriate airflow. This works well, though we find that it quickly starts to look like a lot of hardware to recreate the function of a round firebox.

Round fireboxes have historically been the most popular choice to pair with an offset smoker, and for good reason. First, they are economical to make from a variety of materials - round fireboxes can be fabricated from new/repurposed pipe, new/repurposed propane tanks, or rolled and welded steel plate; often, the round firebox is simply made from the same material as the cook chamber and the length is adjusted to suit. Second, many people prefer the vintage aesthetic of having the firebox profile match the cook chamber profile. Looks and cost aside, the round firebox is also perfectly suited for managing a wood fire; laying splits across an arc of the round profile allows you to easily set and manage the air flow gap, and the falling coals will naturally concentrate into a hot bed at the bottom of the firebox.

While round fireboxes are excellent at their primary job - building and managing a fire - they lack some of the utility and convenience of a square firebox. The doors on a round firebox are smaller than their square counterparts, leaving less room to access the fire. Also, the round top does not provide a large, flat surface area for pre-heating splits (particularly on smaller cookers). Finally, the round shape makes it more difficult to integrate other firebox features like a warming oven.

The choice of a round or square firebox really depends on what you value most in the cooking experience. If you prefer a modern look with great accessibility and a wide variety of features/options, the square firebox is a great choice. If you prioritize fire management and performance over all else, and prefer a vintage look/feel to your pit, stick to the round firebox.

Firebox Size
The dimensions of an offset smoker’s firebox will have a significant impact on both the performance and the usability of the pit. While there are calculators and ‘rules of thumb’ to help, every firebox should be uniquely designed to the style and size of the cook chamber for that particular pit. There are four critical parameters when designing a firebox: volume, length, width, and height; in the case of a round firebox, width and height will both be represented by the diameter of the circle.

The volume of a firebox can have a significant influence on the overall performance of the pit. Large fireboxes provide more room for the hot gases created by the burning fire to expand and cool before they are pulled into the cook chamber. A very small firebox will result in a small volume of very hot gas moving quickly over the cooking grates, while a large firebox will create a larger volume of cooler gas moving gently over the cooking grates; while both may show up as the same temperature on the gauge, the larger firebox will produce a much gentler cook with less drying out of the meat being cooked.

A deep firebox is much easier to use than a shallow one. Being able to move the fire closer to, or further away from the throat provides an additional ‘knob’ to turn for managing the heat within the cook chamber without having to change the size of the fire itself. While a longer firebox increases the footprint of the pit, it is a well-advised tradeoff to improve the overall operation. In our experience, it is really nice if the firebox is at least twice as long as the splits you use in the pit. For a large backyard offset that uses 12 in. long splits, a firebox that is at least 24 - 28 in. long is a perfect fit.

The width of the firebox is important for two reasons; first, it is helpful when building and managing a fire if a full sized split can be laid horizontally across the firebox (this makes it easy to pre-heat the split right inside the firebox without getting too close to the coal bed); second, the width of the firebox determines the maximum width of the ‘throat’ - the opening between the firebox and the cook chamber. Ideally, the width of the throat is at least 75% of the width of the cook chamber to avoid any cold spots at the corners of the cooking grates closest to the firebox. In practice, this means that the firebox should be at least as wide (or the same diameter) as the cook chamber.

While the height of the firebox isn’t as critical as some of the other variables, it is important that there is enough space to access and manage the fire and to prevent the flames from constantly torching the top of the firebox. In general, a firebox that is at least 22 - 24 in. tall will provide plenty of space and volume for burning a fire.

Sizing the perfect firebox requires a balance of maintaining each of the dimensions listed above. While larger fireboxes add cost to the build and increase the overall footprint of the pit, there is no better place to invest your money or space when it comes to the performance of an offset smoker.

Firebox Steel Thickness
The firebox takes more abuse than any other component of an offset smoker, and it must be built to withstand constant thermal cycling over decades of use. For nearly all scenarios, a properly cared-for firebox constructed from 1/4 in. thick steel should last a lifetime. Fireboxes are also available with 3/8 in. and 1/2 in. thick steel which may provide additional peace of mind for offsets that are used continuously within a commercial/restaurant environment.

Insulated vs. Non-Insulated Firebox
The question, ‘should I choose an insulated firebox?’ is a polarizing one, and the answer really is, ‘it depends’. Insulated fireboxes use a double-walled design with either air or a thermally insulating material between the two layers. The inner layer which sees the most heat from the fire is typically the same 1/4 in. thick steel as a non-insulated firebox, and the outer shell can either be 1/4 in. thick steel or a slightly thinner gauge depending on the type and effectiveness of the insulating material. The insulated design serves one primary purpose - to keep more of the heat from the fire inside the pit; while the goal is simple, the consequences must be understood in order to make the right decision.

The primary consequence of an insulated firebox is that the pit will become much more efficient - a smaller fire is needed to maintain the same cook chamber temperature compared to a non-insulated firebox. This also means that splits are added to the firebox at a much lower rate. On a backyard offset, neither of these changes are necessarily a good thing. While burning fewer splits may seem like an advantage, we must keep in mind that wood is what flavors the meat and using less of it will impact the taste of the final product. If we didn’t want to burn wood, we could simply cook with propane. The temperature of a backyard offset is also more responsive to the heat from the firebox; the additional heat retention of an insulated firebox can make it difficult to hold cook chamber temperatures of 225 - 275 ℉ without limiting the air flow to the fire. A smaller, smoldering fire will produce more bad smoke which can further affect the taste of the food. While larger, commercial offsets in a restaurant environment can benefit from the fuel cost savings brought by an insulated firebox without any impact on flavor, we recommend using a non-insulated or semi-insulated firebox for most all other offset smokers.

Firebox Door and Air Intake
The door of an offset smoker firebox serves two primary purposes. First, the door is required to seal up the firebox and the pit when it is not in use. Second - and more importantly - nearly all offset smokers use the firebox door as the primary air intake for the pit. There are three common ways to design air intake into the firebox door. The simplest method is to use a solid door that is opened or closed to allow or restrict airflow into the firebox. The door can be left wide open throughout the entirety of the cook to ensure a sufficient supply of oxygen to the fire, or - in order to prevent too much heat from escaping through the open door - it can be ‘cracked’ open just enough to ensure a clean burning fire as evidenced by very light/thin smoke coming out of the stack. The second style of door uses a built-in adjustable damper to control the amount of air entering the firebox while keeping the door closed. These dampers provide very fine control over the airflow, though it is common to leave the damper fully open while cooking to ensure plenty of air to the fire and fully closed when the pit is not in use. The third style of door uses a permanent cut-out which allows ample airflow into the pit without any possibility of choking the fire - even with the door closed. In our experience, the simplest approach here is also the best approach - a solid door provides adjustable airflow into the pit, has no moving parts or cutouts (unlike door style # 2) and allows you to fully seal the pit when not in use (unlike door style #3). Like many features of an offset smoker, all three approaches will work well and the choice will be a matter of personal preference.

Firebox Baffles
There are many styles of offset smoker, most of which can be roughly categorized as ‘traditional flow offset smokers’ or ‘reverse flow offset smokers’. Apart from the location of the smokestack, the other defining feature of these different styles is the presence and design of a baffle plate at the ‘throat’ where the firebox connects to the cook chamber.

For traditional offset smokers with the smoke stack located opposite the firebox, there are two common baffle designs used to guide the combustion gases from the firebox through the cook chamber. The first design uses a short (~ 6 - 8 in.) metal baffle plate that extends across the width of the throat which is angled downward to direct the heat and smoke from the firebox under a series of ‘tuning plates’. Tuning plates are floating metal baffles that create adjustable air flow gaps located ~ 4 - 6 in. below the cooking surface that selectively guide the heat distribution along the length of the cook chamber and can be used to convert the pit from one with a uniform temperature distribution across the cooking grate to one with gradients or ‘hot zones’. The flexibility of tuning plates allows you to set your offset for whatever you may be cooking that day; a uniform temperature is nice if you plan to cook a large batch of a single cut, while hotter and colder areas can be nice for a mix of different cuts with different temperature requirements. It can also be beneficial to use the cooler zone during the early parts of the cook to absorb as much smoke flavor as possible, and then move to the hotter zone to finish up the cook. One consideration of the tuning plate design is that it forces heat to go down beneath the cooking grates which goes against its natural tendency to rise; this, along with the friction from additional baffles along the length of the pit can reduce the amount of smoke flowing across the meat.

The second style of traditional offset smoker uses a minimalist baffle to allow the combustion gases from the firebox to free-flow unrestricted through the cook chamber and out the smoke stack. The baffle itself is normally a very small metal plate located at the throat to create some degree of turbulence within the cook chamber to help even out heat and smoke distribution. In this design, the heat and smoke will exit the firebox and rise to the top of the cook chamber; as the hot gases flow across the top of the cook chamber towards the stack, they will cool and fall in a rolling/tumbling-like manner across the cooking grates. Unlike the tuning plate design, the free-flow design tends to cook from the top down rather than the bottom up. Without tuning plates, the side of the cook chamber closest to the firebox will naturally be slightly hotter than the center of the grates. A variety of baffle systems, water pans, etc. have been used to help solve this problem, though generally the first 6 - 8 in. of the firebox side of a cook chamber on a traditional offset is not used for meat.

A reverse flow offset smoker has the smokestack located on the same side of the pit as the firebox and uses a full-width, welded baffle plate located above the throat that runs nearly the entire length of the cook chamber. The heat and smoke from the firebox will flow underneath this plate until it reaches an opening at the opposite end of the pit where it will ‘turn the corner’ and then flow back across the cooking grates (in ‘reverse’) before exhausting out the stack. The primary advantage of a reverse flow offset smoker is an extremely uniform temperature and cooking environment across the entire length of the cooking grate. This allows you to use every square inch of the cooking grate without needing to rotate or shuffle meat around during the cook. The disadvantage of the reverse flow design is much slower airflow through the pit due to the friction of the baffle plate and the requirement of the gases to essentially flow through the pit twice before exiting the stack. One common feature of the reverse flow offset smoker design is to locate the smoke stack on top of the cook chamber rather than at grate level; forcing the heat/gases to ‘bend’ one more time down to grate level would only further slow down the pit. One other consideration with the reverse flow design is that the baffle plate will get very hot and can carbonize the drippings from the meat; the vapour from this process will impart somewhat of a grilled flavor rather than a barbecue flavor. Whether or not you like this is a matter of personal preference.

So, which style of firebox baffle / offset smoker is right for you? In our experience, the free-flow design produces the absolute best barbecue flavor that is unmatched by any other cooker or cooking style. While this style also requires the most time, attention, and care throughout the cook, we feel this style is the perfect fit for anyone who wants to truly master the craft of wood-fired barbecue. The reverse flow design also produces excellent barbecue and allows you to evenly cook very large quantities of food with less interaction required with the pit. This style is great for catering or restaurant environments where economics and ease-of-use may be more important than having artistic freedom with the cook. The tuning plate design seeks to strike a compromise between a free-flow and a reverse flow cooker, and produces great barbecue flavor while adding flexibility to adjust the temperature zones for every cook. It should be noted that removing the tuning plates does not directly convert this style of pit to the free-flow design, as they are typically designed for less airflow (often from a smaller stack) through the pit to allow the heat to rise up through the gaps and create the uniform temperature zones. Consequently, an offset designed for tuning plates operating with the plates removed will typically have a much higher temperature gradient across the grates than a free-flow style cooker, and without some of the benefits (and challenges) of the high air velocity.

We hope the information presented above provides some insight into our experiences with the design and build of wood-fired offset smokers. As we’ve discussed, there are many ways to design and build a pit and every decision will come with advantages and disadvantages. In the end, nearly all of these designs will produce great barbecue and everyone should choose the style that is most appealing to them. The right offset smoker is a significant investment and will last a lifetime - make sure you love the one you choose.