What gas grill flashback is, and why it starts at the tube
Each burner on a gas grill is a hollow tube. At its front end the valve sprays gas from a small brass orifice into the mouth of the tube, and the venturi, the narrow first stretch of it, pulls in air through an opening called the air shutter. Gas and air mix on their way down the tube and leave through a row of ports, where they burn as a steady blue flame under the heat shields.
Gas grill flashback happens when that mixture cannot travel down the tube. Blaze's use and care guide explains that spiders and small insects spin webs or build nests in burner tubes, that the obstruction can cause a fire in and around the tubes known as flash-back, and that while it is not the only cause it is the most common one. The gas that cannot get past the blockage spills out at the front, beside the valve, and burns there.
Why spiders and wasps pick the burner tubes
A venturi tube is dry, dark and narrow, and on a grill that sits under a cover for weeks it is the quietest shelter in the yard. Lynx names the burners, venturis, valves and orifices as places spiders and insects like to nest and says a grill that has stood unused for a long time needs those parts inspected and cleaned before it is lit. Mud-building wasps and spiders do not care whether the grill is a cart model or a built-in head.
Source: Blaze — Professional Series Use and Care Guide — Webs in burner tubes cause flash-back and are its most common cause; flashback has a whooshing, jet-like sound, turn off the burners; symptoms of nests are gas smell with yellow flames, low temperature, uneven heat and popping; inspect burner tubes before each use; burners must be seated over the valve orifice
Signs that flame has moved behind the knobs
The first sign is usually sound. Blaze describes flashback by its whooshing, jet-like noise, quite unlike the soft rush of a healthy burner. Its insect warning lists the other clues that a nest is choking a burner: a gas smell together with yellow burner flames, a burner that never reaches temperature, uneven heat across the grate, and popping noises.
- A roar or whoosh from the front of the grill, below the lid line
- Flame or glow visible in the gap under the control panel or around a knob
- One knob, or the panel beside it, too hot to touch
- Lazy yellow flame at the ports of one burner while the others burn blue
- Melted or blistered knobs, or a scorch mark on the panel after cooking
Wind can put heat behind the panel too
Not every hot panel is a nest. Lynx explains that its grills draw air in at the front and vent hot gases out the back, and that wind blowing into the rear can stop those gases leaving, until the control panel and knobs become extremely hot. It suggests a wind break behind the grill and turning the grill so the prevailing wind does not blow into its back. Napoleon's manual likewise tells owners not to place a grill where strong wind comes from directly behind it.
Source: Blaze — Professional Series Use and Care Guide — Webs in burner tubes cause flash-back and are its most common cause; flashback has a whooshing, jet-like sound, turn off the burners; symptoms of nests are gas smell with yellow flames, low temperature, uneven heat and popping; inspect burner tubes before each use; burners must be seated over the valve orifice
The moment it happens: gas off from a safe spot, then step back
The flame is fed by gas leaking past the blockage, so closing the gas starves it. On a cart grill that means the handwheel on top of the propane tank; on a built-in or natural gas grill it is the shutoff valve on the supply line. Close it only if you can reach it without leaning over or past the control panel. Blaze's instruction is to turn off the burners immediately, and where the knobs sit right in the flame, the tank valve or the shutoff is the safer way to do the same thing.
Then step back and keep everyone away from the front of the grill. Do not pull the knobs, open the lower cabinet to look, or throw water at the panel. If the flame is out once the gas is closed, leave everything off. If fire keeps burning behind the panel, spreads to the hose or the cart, or you cannot reach the gas, move away and call 911.
Why nobody reaches through the heat
The space behind the knobs holds the manifold, the valve bodies and, on many grills, the hose and the igniter wires. Flame there is heating the very parts that carry gas. Reaching past it to turn a knob that may already be soft puts a hand and arm in the hottest spot on the grill for no gain, when the valve at the tank does the same job from a step away.
Source: Blaze — Professional Series Use and Care Guide — Webs in burner tubes cause flash-back and are its most common cause; flashback has a whooshing, jet-like sound, turn off the burners; symptoms of nests are gas smell with yellow flames, low temperature, uneven heat and popping; inspect burner tubes before each use; burners must be seated over the valve orifice
Keeping the venturi tubes clear through the year
CPSC's grill safety fact sheet asks owners to check the venturi tube for blockage by insects, spiders or food drippings at least once a year, clearing it with a pipe cleaner or wire that pushes the blockage on through into the main body of the burner. Blaze goes further and says burner tubes must be inspected and cleaned before each use. Many makers now fit a fine screen over the air shutter to keep insects out: Napoleon puts one on each burner, and Weber's US owner's manual shows stainless spider and insect screens and asks for them to be kept clean, because lint on the outside can starve the burner of air.
A grill coming out of its winter cover is the one most likely to have tenants. A few minutes with the grates and shields lifted, a flashlight and a flexible venturi brush, all with the gas closed and the grill cold, is the whole job for most owners.
What stays in place during that cleaning
- The valve bodies and the manifold behind the panel stay where they are.
- The brass orifices are not unscrewed, drilled or swapped.
- Air shutters that the maker set at the factory are not turned by guesswork.
- A burner that is lifted out follows the manual's steps and goes back seated over its orifice.
Source: Blaze — Professional Series Use and Care Guide — Webs in burner tubes cause flash-back and are its most common cause; flashback has a whooshing, jet-like sound, turn off the burners; symptoms of nests are gas smell with yellow flames, low temperature, uneven heat and popping; inspect burner tubes before each use; burners must be seated over the valve orifice; CPSC — Safety Checks for the Grill (fact sheet, Publication 467) — At least once a year check the venturi tube for insects, spiders or food drippings and clear it with a pipe cleaner or wire, pushing the blockage through into the burner; Napoleon — Built-In Series owner's manual (N415-0601) — Do not locate the grill where wind can come from directly behind it; burners carry an insect screen on the air shutter; keep venturi tubes free from spider webs; Weber — Spirit natural gas grill owner's manual (8653810, US) — Burners are fitted with stainless spider/insect screens; debris on the outside of a screen can obstruct airflow, keep the screens clean
Before the grill is lit again
Heat behind the panel can do damage that does not show from the front: a valve seal baked hard, an igniter wire with cracked insulation, a hose softened where it passes behind the manifold. A burner that flashed back may also have been knocked slightly off its orifice, which Blaze notes must be avoided for proper operation. Each of those can turn the next lighting into a leak or a second flashback, so the burners, valves, hose and wiring are looked at before the gas is opened.
Source: Blaze — Professional Series Use and Care Guide — Webs in burner tubes cause flash-back and are its most common cause; flashback has a whooshing, jet-like sound, turn off the burners; symptoms of nests are gas smell with yellow flames, low temperature, uneven heat and popping; inspect burner tubes before each use; burners must be seated over the valve orifice
