HVAC Repair Tips


Only light smoke should come out of the chimney during the operation of an oil furnace. However, if you notice black, white, or gray smoke releasing from the chimney, vents, or the combustion chamber, there is something wrong with the furnace. You must not take it lightly because it’s a serious problem and it can lead to an explosion or a house fire. Try the below fixes if you’ve repaired your oil burner before. If not, consult a furnace repair service. Turn off the furnace and fuel supply in any case until the issue is fixed.

Fixes For Black Or Thick Smoke Coming From An Oil Furnace

Use these methods to fix this problem with your oil furnace.

Clean The Oil Furnace

A common cause of black, gray, or thick smoke from oil furnaces is dirty components. So, you should clean your oil furnace to fix this issue. A thorough cleaning of a furnace is a difficult job as it includes cleaning the internal parts of the unit like the air intake, oil nozzle, burner assembly, draft regulator, and others. These parts need an expert who can carefully clean them without damaging them.

However, here is a basic method of cleaning your oil furnace:

  1. Turn off the furnace, remove the power cord, and also shut off the oil supply valve.
  2. Start by cleaning the combustion chamber because it’s the dirtiest as the process of combustion occurs there. You will see a layer of soot on the walls of the chamber. If you haven’t cleaned it for long, the layer will be very thick.
  3. Use a small wire brush to scrape off the soot and then employ a vacuum to remove the soot you scrapped off.
  4. Check the chamber for any damages, cracks, or holes.
  5. Now, move to the flue pipe and clean it. Inspect it for damages as well.
  6. Replace the oil filter. Close the oil valve, remove the old filter, and place the new one. Many times, the oil nozzle is clogged and causes black smoke from the furnace. This step is helpful to avoid this.
  7. Then, change the air filter of your furnace. It traps impurities from the air similar to how the oil filter keeps impurities of the oil from reaching the furnace.
  8. If you think that the oil nozzle is clogged, clean it or replace it with a new one.

Prevent Puff Backs

A puff back is something that causes black smoke into the exhaust and/or your house. But what are puff backs?

Usually, oil is supplied to the ignition chamber where it’s burned and a flame is produced. However, sometimes, a furnace doesn’t ignite even when it’s receiving oil. It ignites with a delay which causes a minor or noticeable explosion producing a puff of black smoke. These are called puff backs. They are an uglier form of what you witness when you turn on your gas stove top but don’t ignite it for a few seconds. When you do show it fire, it turns on with a minor boom.

The black smoke that’s produced with puff backs contains soot, debris, and other harmful gasses. It goes from the exhaust into the vents. This means this toxic smoke reaches all the rooms of your house that have the vents open.

Why Are Puff Backs Caused?

To prevent puff backs, you should know why they are caused. The cause of puff backs is unburned oil left in the combustion chamber of the furnace. This happens due to various reasons:

  • Leakages in the oil supply line or any other part.
  • Bubbles formation in the oil supply line. These bubbles of oil are pushed through the oil nozzle.
  • A faulty oil valve.
  • Clogged oil nozzle.

Signs Of Puff Backs

Puffs back may be visible, but when this problem is in the earlier stage, puff backs are not noticeable even when they are occurring and pushing toxic gasses into your house. So, look for signs of puff backs:

  • Soot on the furnace and the walls near the furnace.
  • Too much soot production in the combustion chamber in a short amount of time.
  • When the furnace is turned off, the combustion chamber still makes noises that it does when it’s working and igniting fire.
  • A puff of black smoke or banging noise when you turn on the furnace and the ignition chamber starts functioning.

Avoiding Puff Backs

If you think your furnace is puffing back, you must contact an expert to inspect your furnace. They will check all the components like the oil filter, oil lines, oil tank, flue pipe, oil burner nozzle, and others and clean, repair, or replace them as required.

Even when puff backs are eliminated, you can only prevent this problem in the future if you have your furnace maintained by a professional at least once a year. Moreover, look for the signs mentioned above like soot or black spots on the furnace, walls, and ceiling.

Look For The Signs Of An After Fire

After fire is the burning of the oil even when the furnace is turned off. This happens due to improper burning of the oil. Because of this, some oil accumulates at the bottom of the ignition chamber. This leftover oil keeps burning when the actual burning process is finished. It then keeps burning freely till the entire leftover oil in the chamber is burned.

Another cause of this is an oil leak. This also leads to oil accumulation at the bottom of the combustion chamber.

As the burning of this oil is not regulated by the furnace, a thick smoke is produced which can be black or white.

So, if you see black smoke or the flame burning even after turning off the furnace, you will need an expert to determine the cause of the after fire in your furnace. As mentioned, it can be either due to improper combustion or an oil leak.

Check The Flue Pipe For Blockages

The flue pipe is a passageway for the gasses to vent out of the furnace and your house. Any blockages in the flue pipe or abandoned maintenance of the flue pipe, can keep the gasses from escaping the furnace through the flue pipe and produce black or thick smoke.

Make Sure The Fuel Supply Is Not Contaminated

The oil supply to your furnace should be consistent or it won’t combust oil properly. This will cause black smoke. So, you should first check the shutoff valve of the oil line. Then, check the oil line and the oil tank. Is it damaged or are there any leaks?

Another thing you should ensure is that the oil is not contaminated. It can be due to leaks, debris in the oil line, cracks in the oil tank, etc.

Lastly, using low-quality fuel can also cause black or gray smoke. So, make sure the oil you use is of high quality. If not sure, find an expert to carry out oil testing or suggest the right oil.


Black or thick smoke coming from your oil furnace, the vents, or the chimney is an alarming situation. Turn off your furnace immediately and look for problems by following the above-mentioned tips. However, it’s recommended to consult an HVAC repair service Arlington.



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