Oxygen is one of the three key ingredients, along with fuel and heat, that make your barbecue tick. Pit takes you through techniques for harnessing the power of air
It’s easy to forget in the world of complex rubs, injections and marinades that the most important part of any barbecue is the fire. If you can’t control it, you will just have to hope your guests have recently developed a taste for raw meat or freshly-formed carbon.
Just like a recipe, to start a fire you need ingredients in the right proportions. Heat, fuel and oxygen must all be present and, if you take one away, the fire will go out.
Once you understand these three ingredients, the mechanics behind controlling your barbecue start to make sense. If the fire is too hot, you need to remove heat (by spraying in water for instance), remove fuel (if you can access your fire basket or box) or restrict airflow (by using the vents). Likewise, to increase temperature you can add extra fuel and heat in the form of a freshly lit chimney starter, or add air, again by adjusting the vents.
A barbecue smoker at its simplest will have vents at the bottom to allow air in and vents at the top to allow smoke out; closing either for long enough will smother the fire. When charcoal or logs burn, they rapidly combine with oxygen in the air, producing more heat and smoke. The smoke has to
go somewhere otherwise it will smother the fuel and separate
it from the oxygen it needs to continue burning. Fresh air contains about 21 per cent oxygen, and fires require at least 16 per cent oxygen to burn.
To keep fresh oxygen coming in to feed the fire, barbecues take advantage of what is called the stack effect. The fire creates hot waste gases (delicious smoke to you and me) with a lot of energy. These gases spread out as the molecules whizz around and the gas becomes less dense. As there are now physically fewer molecules in the same area of space than there would be with cold air, the gas weighs less which causes it to rise, creating pressure differences between the inside and the outside. Around the fire you have an area of low pressure as warm gases are leaving, and at the top, you have an area of high pressure as the warm gases squash in.
With the bottom vents open, cool, dense, oxygen-rich air is sucked into the area of low pressure of its own accord. Hot smoke and waste gases billow out the top of the barbecue as they are at a higher pressure than the outside air. That’s why you can’t close the top vents to keep the heat in and create a hotter cooking temperature; with nowhere for the waste gases to go, there is no pressure difference to draw in fresh air to feed the fire.
This is also why you need to be careful not to overload your grill. Not only will it stop the smoke from wafting around your food and flavouring every part of it, it will block the airflow needed to keep the fire burning. So, leave the top vents open and use the bottom vents to control the fire. Over time you’ll learn the vent positions needed for different common cooking temperatures.
All barbecue designs try to use the stack effect to control the heat in some way. If you are planning on designing or building your own smoker, take a look at feldoncentral.com before you start chopping up metal – their handy build calculator is invaluable. You will want just the right size firebox, cooking chamber and chimney to make sure you get the optimal draught.
If you are lucky enough to have a barbecue with a chimney, it increases the stack effect in a number of ways. The higher the chimney, the bigger the difference in air pressure between the inside of the barbecue and outside, meaning more smoke gets pulled out the top. A good breeze over the chimney also increases the airflow, the faster the air goes the less pressure it exerts below it. This pressure difference assists in pulling exhaust
gases from the chimney. Don’t go welding a drainpipe to your barbecue though, too long a chimney and the air will cool before it exits, blocking the flow.
Vertical smokers, like kettles and bullet smokers, have a fire
at the bottom with air inlets. The food sits above the fire and the outlets are in the lid. Adding a chimney to the top of the outlet will increase the draught, as will installing a fan at the inlet to blow in extra air when it is needed.
The world of automatic temperature controllers has grown from simple fans that turn on and off to wifi-enabled, app-powered mini Skynets which can monitor the temperature inside your pit and cover multiple cuts of meat.
Offset smokers have a fire box attached to one side of a horizontal drum where the food is cooked, with a chimney attached to the other end. Simply putting the chimney end up on a few bricks will increase its height, increasing the draught through the firebox and increasing the heat. But cheap offset smokers are cursed by air leaks, as the exhaust gases sneak
out the hinged door of the cooking chamber.
Controlling your barbecue can seem like a game of trial and error sometimes, part science, part art, but over time you’ll learn to tame the fire and channel its power into some incredible cooking.
Pit’s golden rules
How to please the gods of fire
1. Don’t overload your grill
2. When building a BBQ it’s better to have a big vent you can adjust, than a small one that won’t get any bigger
3. It’s easier to increase the temperature by opening the vents then drop it by closing them
4. There is a lag between vent adjustments and temperature changes
5. Arrange the vents so that smoke has to pass the meat before exiting!
This article was originally published in Pit issue 03