Propane air mixers are an important part of any fire feature that uses propane gas. If you’re building a DIY propane fire pit or fire table, you want to be sure to include the right-size mixer in your project. But if you’ve never even heard of an air mixer, where do you begin?
In this article, we’ll explain what an air mixer is, why it’s needed and how it functions. We’ll take a look at a cross-section of one to see how it works, provide sizing and installation information and share some technical tips from our own experience. This may be more info than you need to install a mixer and ensure top-notch performance of your fire feature. But, if you love to learn about the mechanics of things (like most of us here!), you’re in for an interesting read.
What is a propane air mixer?
In a nutshell, a propane air mixer is a mechanical component that adds air to propane gas just prior to combustion (burning). This results in a cleaner and hotter flame, which reduces the likelihood of a black soot deposit being left behind. Air mixers are a required component for all decorative fire features that use propane. This is especially important with propane fire pits and fire tables, since you definitely don’t want ugly black residue on your sparkling fire glass or around the edges of your burner pan.
Air mixers are not needed for natural gas, since natural gas burns clean without air being added to it (more on that later on).
ONE cubic foot of natural gas provides about 1,000 BTUs of heat; whereas, the same amount of propane provides about 2,500 BTUs of heat. This is why natural gas fire pits require more than twice the gas flow of propane.
Why is an air mixer needed with propane fire pits?
Unlike natural gas, propane has a high carbon content (C3H8 versus CH4). When propane gas is burned, not all of the carbon in the gas is burned off – and this unburned carbon stays behind in the form of black soot. Propane gas fire pits are prone to sooting because in order to achieve the desirable yellow-colored flame, the fire must get its oxygen from the ambient air around it. The air mixer adds just enough air to the gas to help it burn more completely without adding too much, which would result in blue flames.
The truth is that you can run your propane fire pit without an air mixer. But you’ll get a lot of soot around the burner and on your fire glass or lava rocks. And who wants that?
How does an air mixer work?
An air mixer works by creating a vacuum and drawing air into the propane gas. Those of us old enough to know what a carburetor is will likely be familiar with this mechanism. Basically, the air mixer has a narrow section inside a section of pipe that speeds the flow of gas. This increased speed creates a vacuum that then draws air in through a set of holes around the air mixer. This process is called the Venturi effect.
DID YOU KNOW that the Venturi effect is also used to measure the speed of a fluid in a pipe? By measuring the vacuum a Venturi creates the speed of the fluid can be calculated.
Anatomy of an air mixer
An air mixer is made up of three components:
- The air mixer itself
- A socket (FNPT x FNPT)
- A nipple (MNPT x MNPT)
*Note: FNPT stands for Female NPT (National Pipe Thread), MNPT stands for Male NPT.
The main part of the air mixer is composed of a section of pipe with a narrow section in the center and a series of holes around the perimeter. This component accelerates the flow of the gas through the narrow section, drawing air in through the perimeter holes. The socket extends the length of the air mixer to ensure proper internal gas flow.
The air mixer should never be installed without the socket attached to it. The socket ensures there are no bends or “Ts” too close to the air mixer. The nipple offers compatibility with many types of burners, since it can be removed if not needed. To attach to a burner pan, you’ll usually need to remove the nipple; whereas, to attach to a burner ring, you’ll leave it attached.
What is the right way to install a propane air mixer?
It’s important that the air mixer be correctly installed to ensure it works properly – and safely. The first thing you want to check is the direction of the gas flow in the air mixer. The air mixer has a small arrow stamped into it showing the direction the gas should flow. Keep in mind that the air intake holes should face away from your burner.
When installing, you want to use yellow Teflon pipe tape on all tapered fittings. The yellow pipe tape is designed for use with gas fittings. The Teflon tape should only be applied to tapered fittings (NPT) and should never be applied to flared fittings. With tapered fittings, the threads of the fitting provide the seal against leaks. Since these fittings are tapered, the more you tighten it, the tighter the seal around the threads.
With flared fittings, the seal is not provided by the threads, but by a flared tip that fits into an inverted flare. The threads provide the pressure to keep the flared tip pressed tight against the inverted flare to ensure the seal.
SPIDERS can be a problem with air mixers. For some reason they like to make their home inside the air mixer in fire pits which go unused for a long period of time. If you’re having a problem, check that the inside of the air mixer is clear of webs and debris.
Most air mixers are 1/2” NPT (National Pipe Thread/Taper). Note that NPT sizes are based loosely off the inside diameter of the pipe. So, 1/2″ NPT threads actually measure slightly more than 3/4″ when measured at the threads.
The air mixer should be installed directly under your burner, keeping the following in mind:
- If your burner has female threads, you can simply thread your air mixer directly into the burner. Be sure to use yellow pipe tape for these connections.
- If you have a burner pan with male threads, you’ll need to first remove the nipple from the end of the air mixer (use a pair of pliers, since the threads are sharp).
- Be sure to leave the socket (FNPT x FNPT) attached to the air mixer, otherwise the air mixer won’t work properly and can potentially leak gas out of the air intake holes. The other end of the air mixer attaches to the hose from your key valve or adjustable regulator.
An easy way to test that it’s working
We call this the “one-handed eyeball test” (ok not really, but it sounds catchy). While your fire pit is on, keep an eye on the flame and carefully wrap your hand around the air intake holes on the air mixer. If you see the flame drop slightly when you block the air flow, your air mixer is working properly and drawing air in. Congrats!
What size air mixer do I need for my fire pit?
Air mixer sizes are measured in BTUs. The burner you’re installing in your fire pit should have a maximum BTU rating. In general, you want the air mixer to be the closest size to your burner. However, air mixers only come in three sizes: 90,000 BTU, 150,000 BTU and 300,000 BTU.
See the following chart to determine the appropriate size of air mixer to use with your burner.
|Air Mixer Rating||Burner BTU Rating|
|90k BTU||45k BTU to 120k BTU|
|150k BTU||120k BTU to 250k BTU|
|300k BTU||250k BTU and above|
Will a higher BTU-rated air mixer give me a hotter flame?
No, the BTU rating of the air mixer does not affect the heat output of your burner. Rather, the BTU rating of an air mixer is a measurement of the gas flow it’s designed for. It should be matched to the BTU rating of your burner. This ensures the narrow section of the air mixer is large enough to allow sufficient gas flow, while also being narrow enough to speed the gas and create a vacuum effect.
If the BTU rating is significantly higher than the burner BTU rating, a vacuum effect won’t be created – and you could potentially leak propane out through the air intake holes in the air mixer. While an air mixer will seldom match the exact BTU rating of your fire pit, you want to pick the size closest to it.
What happens if I add too much air to my propane gas?
Adding air to your propane affects the heat the flame puts out and the color of the flame. Think of a small propane torch used to solder water pipes. These torches have a very hot blue flame that’s achieved by adding a high amount of air to the gas. While you’ll never get your fire pit to burn flames like this, having too much air mixed with your propane will make the base of your flames turn blue. Having too much air mixed with the propane will not affect the operation of your fire feature otherwise – it’s strictly cosmetic.
Is an air mixer required for natural gas?
As mentioned previously, an air mixer is neither required nor recommended for a natural gas fire pit. Natural gas is a clean-burning gas with low carbon content, so it isn’t prone to sooting. However, if you run your fire pit in windy conditions, you can get some sooting from natural gas. That’s where a flame guard can come in handy.
I hope you enjoyed “geeking out” with us on the topic of propane air mixers! Building your own propane fire feature is fun, but it can also be intimidating at times. With articles like this one, we aim to help you get a basic understanding of fire pit mechanics and how everything works together. This will empower you to build a fire pit showpiece that not only looks beautiful, but also operates safely. Then you can have complete peace of mind when you’re relaxing by that warm, glowing fire with your favorite folks.
As a final note, we’d like to remind you that all gas connections should be made by a professional gas installer. Our best advice is to work with a certified NFI Gas Specialist (search for one in your area here: https://www.nficertified.org/public).