Dropping flame height is typically a cold weather problem for gas fire pits that use propane as their fuel source. It is caused by the inability of the gas in the tank to vaporize at sufficient speed to fuel the flames.
Another, but less likely, the cause could be a malfunctioning gas regulator in both propane and natural gas installations.
Propane Vaporization Rate
We’ll take a look at the most likely cause first. A flame that gradually drops over time is typically the result of a low vaporization rate.
In other words, the ability of the liquid to change to gas is lowered by several factors including the outside temperature, the volume of liquid gas remaining in the tank, and the design of the tank itself.
When the weather gets very cold, it often causes a problem for fire pits utilizing a higher BTU/hr. burner running on a standard 20 lb. propane tank.
Propane is stored under pressure in a liquid state in the tank. To be used as a fuel source, this liquid must change to gas. The chemical process of going from a liquid to a gas requires heat in order for the liquid propane to vaporize. The warmer it is outside the tank, the easier it is for the gas to make this change. Conversely, the colder it is, the less efficient this process is.
When the weather is cold, the vaporization rate of liquid propane is reduced due to the lack of ambient heat. This is more noticeable if your gas fire pit is rated for more than 95,000 BTU/hr.
The 95,000 BTU/hr. measurement is the textbook rate at which a standard 20 lb. propane tank can convert liquid to gas based on the surface area of the liquid. We say textbook because, in practical use, these tanks can output more than twice that amount if the ambient temperature is not too low.
If you see frost on the outside of your propane tank, this is a sure sign that you’ve reached the vaporization limit for your tank.
Tank Design and Volume
While heat is one factor, the surface area of the liquid also affects the vaporization rate. This becomes more of an issue the colder it gets outside.
Low levels of liquid propane in the tank mean there is less liquid to come in contact with the sides of the tank. And that means there is less heat, reducing the vaporization rate.
Additionally, liquid propane in a standard (upright) tank is a lot like water in a bucket. That water will evaporate much more slowly than the same amount of water poured out over the driveway. The increased surface area facilitates more rapid evaporation.
How to Improve the Propane Vaporization Rate
To help guard against this issue, locate the tank in a place where it won’t be exposed to extremely cold conditions. If it’s located under the tabletop in a fire pit table, it’ll be more protected from dropping temps. Try to avoid placing your propane tank (fire feature) in the areas of your outdoor space that are most exposed to the cold.
Also, when it’s really cold outside, use full propane tanks. Liquid propane gets the heat it needs to vaporize from the absorption of heat through the sides of the tank. The more contact the liquid has with the sides of the tank, the more heat can be absorbed.
Another option to consider if you often want to use your fire pit in cold conditions—consider changing the tank style to increase the surface area of the liquid propane. Using a specially designed horizontal propane tank will do that, thereby increasing the vaporization rate.
However, a horizontal tank lacks the convenience of a standard (upright tank). Those you can pick up at many locations, but a horizontal tank will have to be refilled at a gas supplier.
Never lay a standard propane tank on its side. Horizontal tanks are designed to be placed on their side. The pressure relief valve is located in a different place than on a standard tank so that it can safely release excess gas.
If you turn a standard tank on its side, the release valve is now located underneath the surface of the liquid propane. When it releases pressure, it will release liquid propane instead of gas. Liquid propane is far more volatile and even small amounts can cause an explosion.
Propane regulators aren’t often the cause of a flame slowly dropping over time. Typically, if the propane regulator is defective or not working properly, you’ll see a sudden drop in flame height rather than a gradual dropping of flame height.
Replacing your regulator will often fix this type of problem. For more details on regulators and how to test them, see our article, “Why Is My Fire Pit Flame so Low?”.
Natural Gas Regulator
If you have natural gas and the flame height drops over time, this is most likely a problem with your gas regulator.
Outdoor temperatures can greatly affect how the regulator functions. There’s a spring and rubber membrane in it, which are affected by temperature. When it’s hot outside, they’re more pliable. When it’s cold, they’re stiffer. Both of these things can affect the output pressure.
If your natural gas regulator is located outside where it’s exposed to extreme temperature changes, it can result in the output pressure dropping over time. This can cause the flame height to slowly drop.
It can also be caused by a malfunctioning or improperly sized natural gas regulator. Check that your input and output pressure specifications on your regulator are correct.
Output pressure for natural gas should be between 3” WC and 6” WC (or in the range of 3.5” WC to 7” WC).
Input pressure should be rated as 1/2 PSI, which is typical for most homes. In some instances, you may have higher pressure coming into your home (2 PSI). In this case, you want to have the appropriately sized natural gas regulator for a higher input pressure.
If you determine that your natural gas regulator has the correct specifications, but you continue to see a declining flame height at normal outdoor temperatures (above 40 degrees Fahrenheit), you may want to switch out your regulator with a new one. Celestial Fire Glass has a natural gas regulator designed for outdoor gas fire pits.
If you have a propane fire pit and you notice the flames slowly declining when it’s super cold outside, a low vaporization rate is a likely problem. However, if it’s happening under other circumstances, it’s time to take a closer look at the health of your regulator.
The Celestial Fire Glass Technical Team is always happy to help you troubleshoot your gas fire pit projects. Feel free to contact us with your questions.