The Most Common Reason For Back Puffing
Blocked or Dirty Chimney
If your fireplace has been working fine and then suddenly starts to smoke, it is likely the chimney has built up enough creosote on the flue walls to become restricted. If the chimney is restricted or if there is a blockage in the flue it will cause the fireplace to have poor draft and smoke. Keeping your chimney clean and free of restrictions is the easiest way to prevent a fireplace from smoking. You can clean the chimney with a rotary cleaning system or a standard chimney brush. It is suggested to clean the chimney flue at least once a year. If you use the fireplace regularly, you may have to clean it more frequently.
Wind Driven Downdrafts
If the chimney flue is clean but every now and then you are getting a little back puff of smoke, it could be wind driven downdraft. To troubleshoot this, you will have to pay attention to the weather and if it is windy outside when the fireplace has a back puff. There are chimney rain caps that are designed to prevent and stop wind driven downdraft. The Vacu-Stack chimney cap is a unique design that cyclones the wind and creates an updraft when wind hits the cap. This will prevent any wind from entering the chimney. There is also a Storm Shield Weather Shield chimney cap that does a great job at preventing the wind from entering the chimney. The Storm Shield cap works like a chimney shroud creating a barrier around the cap to allow the chimney cap to vent while blocking the wind from entering the flue. This will also help prevent wind driven rain from entering the chimney in extreme weather situations. If you are noticing a wind related downdraft, it might be time to change out your chimney rain cap.
The Fireplace and Chimney Are Originally Constructed Incorrectly
Incorrect construction is by far the most frustrating and expensive reason why your fireplace is causing smoke to roll out of the front and fill your home with smoke. If the fireplace has never worked correctly or has been temperamental, it is likely incorrect construction. This issue is most commonly found with larger fireplaces, the biggest culprit being an insufficient flue size. The area of the fireplace must be in scale with the face area of the fireplace. If the flue is not in scale with the fireplace, it will not have enough area for the volume of smoke and will cause the excess smoke to spill out of the front of the fireplace.
Square and rectangle flues sizes should be 1/10th the area of the face of the fireplace. Rectangle flues with an aspect ratio of greater than 2 to 1 should have a flue size of 1/8th the area of the fireplace face. For round flues, the area of the flue should be 1/12th the area of the fireplace face. If the flue to fireplace ratio is just a little off, you can install a smoke guard to prevent the fireplace from smoking. A smoke guard is a steel plate that spans the top of the fireplace opening, reducing the area of the fireplace to make the ratio more equal. In situations where the flue is significantly undersized for the fireplace, you can install a top exhaust ventilator to create more draft and pull the smoke up and out of the fireplace. The Enervex top ventilator is a high temperature fan at the top of the chimney to pull the smoke out. The fan requires electrical and has a variable speed control that can be adjusted based on how the fire is burning.
Negative pressure is when there is a force inside the home that is stronger than a fireplace venting system can produce, causing insufficient draft and smoke to be pulled in the house. Sources of negative pressure can be caused by everyday appliances such as a bathroom fan, a kitchen range hood, clothing dryers and other combustion appliances such as a gas furnace or hot water heater. Negative pressure can often be overlooked as the source of the smoky fireplace, homeowners often think it is wind related downdraft. If a fireplace is on the same level of the home as the appliances listed above or if a fireplace is in a basement, it is likely to experience negative pressure.
Negative pressure can also be caused by lack of fresh air in the home which is sometimes due to tightly sealed doors and windows. Once a fireplace runs out of fresh air it can cause smoke to spill out of the front of the fireplace.
To test for negative pressure, you can hold a match at the face of the fireplace. If the flame is pulling into the room it has negative pressure, if it is pulling into the fireplace the pressure is fine in the home. Fresh air is the only way to solve negative pressure. Installing a fresh air intake near the fireplace or cracking a window when you are lighting the fireplace can even out the pressure and prevent the fireplace from smoking. You can also solve negative pressure by installing the powered Enervex top exhaust ventilator at the top of your chimney to pull the smoke out. When troubleshooting a smoky fireplace, always start by looking to see if negative pressure is the culprit.