Incomplete Combustion

Combustion is a rapid chemical reaction when oxygen combines with the fuel to produce heat. By modifying any of the three, oxygen, heat, or fuel, you create an imbalance and can produce incomplete combustion.

Carbon monoxide (CO) and aldehydes are products of incomplete combustion.

Carbon monoxide is the most harmful product of incomplete combustion. It’s colorless, odorless, tasteless, and toxic. In large quantities, it can be combustible.

Aldehydes are irritating to the eyes, throat, and nose. They smell terrible. Aldehydes taste somewhat metallic and are likely accompanied by carbon monoxide.

Completing a combustion test is the only way to be sure that you’re providing a safe appliance for the homeowner with the best possible efficiency possible.

There are many causes for incomplete combustion, and we’ll cover several reasons here.

Low oxygen (O2) readings in the flue

Low oxygen (O2) reading is probably the most common issue to have and will usually have high carbon monoxide (CO) readings with it.

Low oxygen (O2) readings could be caused by too little combustion air, weak draft in the flue, or an overfired system.

  • Check for blockages in combustion air supply, combustion air sizing, exhaust equipment removing too much air from the structure, dirty burners or obstruction at the heat exchanger inlet. Adjust burner air shutters as needed
  • Check for blockages in the venting/chimney/heat exchanger. Check for any barometric damper setup incorrectly or not opening.
  • Check the length of venting or too many elbows used. Check for proper sizing of venting and chimney
  • Check gas pressure and adjust if needed

High O2 readings in the flue

High O2 readings will be caused by excessive combustion air, high draft in the stack, or an underfired system.

  • Check gas pressure and adjust as needed
  • Check for missing plugs on sides of the appliance, missing door, or burner adjustment shutters
  • Check venting size or barometric damper

Low Stack Temperature

Low stack temperature can be caused by an underfired system or too much air across a heat exchanger.

Check gas pressure and adjust if needed

Check temperature rise and adjust if needed. When you’re moving too much air across the heat exchanger with too high of fan speed, you will not allow enough heat to flow through the venting system and could cause condensation to form on the heat exchanger or venting and force it to rust out.

High Stack Temperature

High stack temperature can be caused by an overfired system or not enough air across a heat exchanger.

Check gas pressure and adjust if needed

Check temperature rise and adjust if needed. When you’re moving too little air across the heat exchanger, you will not provide enough air to absorb heat from the heat exchanger so the heat will escape out the flue. A dirty filter, dirty blower wheel, and undersized or blocked return or supply duct will cause this

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