For those who enjoy fireplaces, wintertime presents the opportunity to put another log on the fire. There’s nothing like the crackling sound, the ambiance of the glowing flame and the soothing warmth of a fireplace. However, the minute the flue is opened and the match is struck, the laws of nature can bring unintended consequences.
Since air is invisible, it’s nearly impossible to see how it’s behaving in a home unless you have test equipment, smoke generator devices and an infrared camera. Air in the home is always on the move. It enters through opened windows and doors, as well as worn-out door and window seals. Unsealed areas where electric wiring and plumbing have been installed are other airways, as well as parts of building frames that were not caulked and sealed properly during construction.
In homes, it’s natural for air to try and escape from upper areas while it’s being supplied from lower parts of the home. This is the nature of warm air rising, or the stack effect. This effect will cause the unwanted air infiltration to escape from non-airtight-recessed light fixtures, attic access points, whole house attic fans, knee-wall access locations and any non-sealed cracks. In addition, mechanical devices like kitchen and bath exhaust fans, clothes dryers and air handlers from the central heating cooling system will exacerbate air movement within the home.
Now, imagine your home as a layered cake. A house generally has an upper, lower and neutral layer, or plane. The lower plane normally has negative pressure, which means the air is being drawn into the space. The upper part usually has positive pressure and air is being pushed, or forced out. Somewhere in the middle is a neutral plane. This plane is constantly changing depending on circumstances within the home and when the wind is applying pressure to the outside of the home.
Now, let’s open the fireplace damper, which is generally located in the lower plane. This action has created another hole or open airway in the home. Even before the fire has been lit, air will naturally flow up the flue pipe to the outside. This movement of air is the stack effect at work and why it’s important to make sure the fireplace damper is always closed when not in use. If you’ve ever had a home energy audit, you’ve watched the energy professional on their hands and knees shining a flashlight up into the fireplace while making that inspection. Unfortunately, it’s not uncommon for them to learn the damper had been open year-round.
For the firewood to burn, it will require an ignition source and air. Once the stack of firewood begins to burn, the chimney or flue pipe begins to heat up. This causes the required air to pressurize, or in other words, draft. The bigger the chimney or flue pipe, the more air drafts to outside. The fireplace has become a negative pressure source, pulling air out of the home. This action creates a need for more air to replace the air that is moving up through the chimney. The replacement air generally comes from the air leaks within the lower plane. On a cold winter night, the replacement air coming into the home will be the same temperature as outside. Therefore, various parts of the home that are located away from the fireplace, especially if they have lot of air leaks, will drop in temperature. This could be why the rooms in the back of the house are often colder when the fireplace is in use.
Furthermore, some of you may have experienced smoke coming back into the home. This unwanted scenario is called backdrafting and can occur with energy-efficient homes that have been properly caulked and sealed. The large opening for the chimney or flue makes it easy for a mechanical air device such as an HVAC air handler to pull air back into the home. This is why many fireplaces have a combustion air source vent installed in the unit that enables outside air to be pulled into the fireplace box to help the fire burn. If you have this feature on your fireplace, be sure to open it when enjoying a fire. However, when the fire is out, be sure to close both the damper and combustion air vent.
For those who have a fireplace without a damper or a damper that will not close properly, there are several retrofit options available at box stores and online. A simple solution is the pillow or balloon type which are easy to install and remove. There are also thermal covers with magnetic attachments that cover the front opening of the fireplace. These are also easy to install and remove.
Please email firstname.lastname@example.org with energy efficiency questions. Also, follow www.facebook.com/smartenergytips.org, visit www.smartenergytips.org, as well as listen to the Smart Energy Tips podcasts.
Bret Curry is the residential energy marketing manager for Arkansas Electric Cooperative Corporation.