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
What this means on a real service call
Incomplete combustion is not a single fault. It is the result of an imbalance: not enough oxygen, the wrong gas pressure, a dirty burner, a blocked vent, or heat being pulled away from the flame too quickly. The danger is that the appliance can still produce heat while it produces carbon monoxide, so a customer may not notice anything wrong except a headache, a strange smell, soot, or a burner that does not look right. Aldehydes, which irritate the eyes, throat, and nose and smell terrible, often appear alongside carbon monoxide, but carbon monoxide itself has no smell, color, or taste.
That is why the original post stresses a combustion test. A test does not just confirm the appliance lights. It shows whether the fuel is burning cleanly after warm-up, whether oxygen and carbon monoxide are in a sensible range, and whether stack temperature fits the firing rate and airflow. Without that test, you are guessing about safety.
Numbers and measurements a technician actually sees
On a combustion test, watch the pattern, not just one number. Low oxygen in the flue usually goes with high carbon monoxide and points to too little combustion air, weak draft, or overfiring. High oxygen usually points to excess air, high draft, or underfiring. Low stack temperature can point to underfiring or too much air across the heat exchanger. High stack temperature can point to overfiring or too little air across the heat exchanger. Temperature rise across the furnace, gas pressure, and draft should all be checked together, because changing one without checking the others can move the problem rather than fix it.
Also use the limit already given on this site: on the About Combustion post, the maximum carbon monoxide allowed in the vent is 400 ppm on an air-free basis. A reading that stays high after the first few minutes, rather than settling after light-off, should be treated as a fault to find, not as normal operation.
A common mistake
A common mistake is to replace a flame sensor, igniter, or control board for a sooting or carbon monoxide problem, when the real cause is air, venting, or gas pressure. A new part may let the burner light more reliably, but it will not fix incomplete combustion. Another mistake is to test only at light-off, when carbon monoxide is normally higher for a short time, and then either panic over a normal spike or, worse, test only briefly and miss a reading that stays high once the unit is warm.
A practical checking step
If you suspect incomplete combustion, do not just clean a burner and leave. First, look for soot, a yellow or lazy flame, and any blocked combustion air opening or vent. Second, check gas pressure and temperature rise against the rating plate. Third, run a full combustion test after warm-up, if you are trained and equipped, and write down oxygen, carbon monoxide, and stack temperature. Fourth, check the room and the vent for carbon monoxide with a proper detector, because the appliance is not the only place carbon monoxide can show up.
Safety caution: if a carbon monoxide alarm sounds, occupants feel ill, or you find high carbon monoxide that does not settle, shut the appliance down if it is safe to do so, get people out, ventilate, and do not restart it until a qualified technician has found and fixed the cause. Never disable a safety control, and never use a gas appliance as a space heater for a room it was not designed to heat.
Related posts
For the full picture, see About Combustion and Combustion Air Supply, and for the fuels involved, see Characteristics of Gas.