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Gas Fireplace Explodes When Starting? Here’s Why

If your gas fireplace explodes when starting, shut the main supply valve immediately. Although modern gas fireplaces are reasonably safe, there is always a possibility of a critical component malfunctioning. So, why does a gas fireplace explode when starting, and can you prevent it?

A gas fireplace explodes when starting if there is a delay in the ignition sequence while the air-fuel mixture builds up in the firebox or insert. The actual cause could be ignitor flame issues, clogged burner ports, excess primary air, low gas pressure, stuck safety valve, and other defects. 

Many brands make different types of gas fireplaces, with or without vents and other features. It is thus necessary to detect the cause based on the type of gas fireplace you use. In this article, I cover every possible problem you may have if your gas fireplace explodes when starting. 

Here’s Why Gas Fireplace Explodes When Starting

A few years ago, Hussong Manufacturing Co. Inc. recalled ~16,000 gas fireplaces due to explosions, reportedly caused by their faulty control module. This recall covered 22 models of the brands Kozy Heat Fireplaces, Stellar Hearth Products, and Ambiance Fireplaces.

Similarly, Security Fireplace had an issue with their relief dampers, increasing the risks of an explosion. Therefore, a gas fireplace can explode due to manufacturing defects and deteriorating components. 

However, mild explosions or unusual ignitions may happen due to a few manageable issues. A typical problem is a significant delay in the ignition sequence. Now, a delayed ignition is the symptom, not the cause, if a gas fireplace explodes when starting. 

Here are the underlying causes.  

The Ignitor Flame Is Weak or Misaligned

Gas fireplaces have a pilot light or ignitor flame. Your model may have a standing pilot flame or intermittent pilot light. Both types of pilot lights should function properly to ignite the gas. 

So, the pilot light should have a strong flame, and the ignitor must be accurately aligned over the burner. Otherwise, your gas fireplace may explode. Here’s why:

  1. Suppose the pilot light has a weak flame, or it is misaligned. 
  2. The thermocouple or thermopile next to the pilot light or flame senses heat and sends an electrical signal to the control module to open the gas supply valve for the burner. 
  3. The gas then fills up the burner and the firebox, but the weak or misaligned flame fails to ignite the air-fuel mixture immediately. 
  4. The ignition sequence is then delayed while natural gas or propane builds up inside the fireplace. 
  5. When the pilot light eventually ignites the air-fuel mixture, too much gas is inside the fireplace, ready to combust, thus causing an explosion.

The solutions are simple if the pilot light flame is weak or misaligned. 

  • Clean the pilot light and thermopile or thermocouple with a hard-bristled brush. 
  • You may use sandpaper, but a steel or brass brush with a handle might be more convenient.
  • Fix the alignment or orientation of the pilot ignitor so that it is near the burner’s ports.

A Few or Many Burner Ports Are Clogged

A lack of maintenance can lead the burner ports in your gas fireplace to clog. 

Most gas fireplaces have several of these holes or ports, some of which are near or around the pilot light. If these ports are clogged with dust and carbon, the pilot light or flame may not ignite the gas instantly. However, the other burner ports will continue to supply gas to the fireplace. 

As a result, excess gas will build up in the fireplace, ready to explode when it contacts the pilot flame.

You can watch this video to clean the burner ports of your gas fireplace quickly:

The Venturi Has Excessive Primary Air

Suppose your gas fireplace burner does not have clogged pores, and the pilot light or flame is working fine. In that case, a gas fireplace can explode if there is excessive primary air.

Gas fireplaces have a venturi that mixes air with fuel so that the propane or natural gas is ready for combustion. The venturi’s inlet may allow more air to mix with the gas than combustion requires. Thus, your gas fireplace will have what is known as excessive primary air.

The pilot light will not ignite the fuel mixture quickly if there is too much air and little gas. Hence, gas may build up inside the firebox, and you will have a delayed ignition. Both factors can lead to an explosion. 

The solution to this problem is adjusting the venturi. 

Many gas fireplaces, like those of Heat & Glo, have an air regulator valve on the control box. This valve is essentially a set screw that you can turn clockwise to reduce the airflow. Refer to the manual to locate this set screw or similar adjustable feature to regulate the airflow. 

Some gas fireplaces have a venturi slider for the same purpose, like in this video:

The Fireplace Has Low Gas Pressure

The air-fuel mixture is not dependent solely on the airflow into the venturi; the amount of gas flowing into the burner is also a determining factor. A fireplace may have excess primary air due to low gas pressure. 

You can use a manometer to test the gas pressure in your fireplace. Generally, the manometer should read 5.5” to 7” WC (0.2 to 0.25 PSI) for natural gas and 10” to 14” WC (0.36 to 0.5 PSI) for propane. You can check the manufacturer’s recommended gas pressure in the manual.

The Burner’s Gas Valve Is Stuck Open

A gas fireplace has multiple valves, like the one that regulates the gas supply to the pilot light. This valve allows very little gas to flow to the pilot light, enough to sustain the ignitor flame. 

In contrast, the burner’s gas valve regulates a much greater supply and is thus a safety feature. If this valve is stuck or fails and thus remains open, propane or natural gas will continue to flow into the fireplace.

Normally, the pilot light heats the thermocouple or thermopile, which then signals the control module to open the burner’s gas valve. However, an open valve will supply gas regardless of the pilot light, ignitor flame, and heat sensed by the thermocouple or thermopile.

There are reported cases of this safety valve failing to close, thus causing an explosion. Additionally, and even more worryingly, you may not know if this valve is open or defective as you ignite the pilot light. To mitigate this risk, you should get the burner’s gas supply valve inspected by a technician. 

The Main Control Module Is Defective

Last but not least, a gas fireplace may explode due to a defective control module. Modern gas fireplaces route every signal and action through the onboard control module. The safety valves and other features may malfunction if this essential component does not operate flawlessly. 

Like a burner’s gas supply valve, a defective control module exponentially increases the risks of an explosion. You should consult a certified technician and get the control module inspected, or contact the manufacturer if you sense this integral component is faulty. 

Final Thoughts

In rare cases, the problems discussed here may not be why your gas fireplace explodes when starting. If so, you should inspect the venting system you have. 

Closed relief dampers, strong drafts, and unusual combustion in a gas fireplace can cause or facilitate an explosion of varying intensity. You should have a professional inspect the gas fireplace and its ventilation system. 


  • Chris Hewitt

    Chris is a Texas-based freelance writer who loves the outdoors and working in his garage. When he's not enjoying the Texas sun, he can be found tinkering with all sorts of things in his workshop.

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