Top Causes of Propane Sooting

Anyone who works on propane fired equipment has run across a few sooted heat exchangers, and they probably have a few chronic sooters to boot.

Chronic sooters are easy to identity; every panel on the unit is covered with permanent soot smudges and hand prints and they smell nasty when they’re running. You can also tell when a technician’s been working on one because they’re just as soot stained as the offending unit.

What causes sooting?  Why are some units’ chronic sooters? More importantly, how do you stop the @#$%&* sooting?

The root cause is one (or more) of the three problems below, and the root-cause/s of each described in the second list.


  • Incorrect fuel / air mixture (1-2-3-6)
  • Incomplete combustion (1-2-3-4-5-8)
  • Poor quality fuel source (7)

Root Cause/s

  1. Incorrect or improperly installed conversion kit
  2. Low manifold pressure
  3. High manifold pressure
  4. Flame impingement
  5. Misaligned burner crossovers
  6. Improper combustion air
  7. Excessive odorant concentration
  8. Flame disturbance

Now for the “why’s”.

  • Incorrect fuel / air mixture (1-2-3-6)

The fuel / air mixture is critical in propane applications and installing the correct conversion kit is a must. Some units require a simple gas-valve  regulator spring and burner orifice change. Some require burner spoiler screws. Some require the installation (or removal) of air baffles or air regulator plates. And some require a manifold pressure change. How do you know what modifications are required? Follow the manufacturer’s instructions. Period.

  • Incomplete combustion (1-2-3-4-5-8)

First, the wrong fuel / air mixture will cause incomplete combustion….that’s a no brainer, and covers 1, 2, and 3.  Number 4 can be sneaky. Flame impingement is just what it sounds like; instead of the flame going straight into the burner tube its hitting something, like the side of the heat exchanger tube. Number 5 is another sneaky,but easy to identify problem. Misaligned crossovers cause a gas-rich area in the flame that messes up combustion. Number 8 is caused by air from the blower compartment disturbing the burner flames and causes the flames to ‘dance around’ in the burner compartment and may also cause nuisance roll-out safety trips.

  • Excessive odorant concentration (7)

Odorant is the stuff (usually ethyl mercaptan) they add to gas to make it smell awful to make gas leaks noticeable. Odorant poses no problems with combustion….unless there’s too much of it. Excessive odorant is normally caused by frequent propane tank top-offs. If the tank level is never allowed to run down, it can lead to excessive odorant in the tank…which changes the combustion properties of the gas, which can cause sooting.

Misaligned burner crossovers             Flame disturbance / impingement


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Patrick is Zen HVAC’s diagnostic and training guy. Patrick started in the trade the day he left technical school and never looked back. He's served in various technical and training roles in the HVAC industry but specializes in system troubleshooting and diagnostics, retro commissioning, and technical training. His moto: If I can understand it, anybody can. Patrick uses the Zen common sense approach to teach Patrick’s Likes- His Wife, kids and dog. Old pickup trucks. Hiking. The industrial Revolution. Patrick’s Dislikes- Taking work too seriously. Anything unintuitive. Emoticons :( Patrick’s Favorite famous person- Theodore Roosevelt “I am only an average man, But I work harder at it than the average man" Famous Patrick Quote- “Well, that was stupid of me”

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