Every fireplace season, we get calls from homeowners reporting that they can’t get their wood fireplace burning right, or that smoke spills back out into their house when the fire is going. So let’s take a look at what goes into your fireplace system, and what might be interfering with it:
Draft is the force pulling air from inside your house up through the chimney. Draft is primarily a function of the temperature differential between your fireplace and the outside air, as warm air from inside is drawn up the chimney to meet the cold air outside the chimney stack. The first implication of this is that it will be harder to start a fire on a warm day, since there won’t be a significant temperature difference to kick-start the draw of the chimney. It also means that when you first use your chimney after it’s been idle for awhile, you might have trouble getting the entry to the flue warm enough to get the smoke moving. If this happens, try holding a lit piece of newspaper or kindling up to the flue entrance right by the damper for a minute or two to quickly warm the flue and establish a draft. This can also be helpful on cold, damp days; humid air is heavier than dry air, so your chimney has to work harder to overcome that resistance.
If the flue is warm enough, the outside air cold enough, but you’re still getting smoke spilling back out into the house, the next culprit is likely to be a dirty flue or chimney cap. Creosote deposits will interfere with the airflow through your chimney, slowing it down and kicking smoke back into the house. Having your chimney cleaned and inspected every year or so helps keep this from ever becoming a problem, as well as heading off other potential chimney issues.
Sometimes, even a warm, squeaky-clean flue will refuse to draw properly, depending on how your home and chimney are constructed. A short chimney will often have problems drawing properly; this can be addressed by adding a flue extender or chimney pot to the top of your flue. This flue extender can help address another cause of smoke spillage, negative pressure. We often have homeowners report that when they use their upstairs fireplace, smoke spills out of the fireplace directly below it. This is because basements, which are partially or completely below grade, are negative pressure zones, which try to pull air in from outside the house (think of it as a very weak vacuum cleaner); positive pressure zones, in upper levels of the home, are where the warm air is trying to escape to the outside. Since the two flues are directly next to each other, extending one flue may be necessary in order to get the smoke away from that negative pressure zone.
A flue extender can also combat the problem of stack effect, which occurs when your chimney termination is lower than a nearby rooftop. Hot air wants to go to the highest point in your house; if this point isn’t your chimney, the smoke will be drawn back into your home as it tries to reach the highest point in your building envelope.
Additionally, adequate replacement air can be a challenge for wood-burning appliances in newer, more energy-efficient homes. In an older, leakier house, cold air is pulled in around windows, doors, outlets, and even through the walls. While you normally want to keep that cold air outside, your wood stove or fireplace needs the replacement air to maintain combustion. In tighter modern houses, an outdoor air kit may be needed. This kit is simply a 4″ diameter duct that runs from the outside directly into your wood appliance, providing enough air to keep the fire from choking out and filling your house with smoke. An alternative to the outdoor air kit is to crack a window or door while the fire is going. For obvious reasons, this isn’t a great long-term solution, but it’s a wonderful diagnostic tool. If you consistently get smoke spillage from your fireplace, try cracking a window during your next fire; if that fixes the problem, it’s quite likely that either negative pressure or insufficient combustion air is your culprit, and an outside air kit may be the best solution.
Not every chimney problem has a simple solution, but there are often steps you can take to make it easier for your chimney to do its job. If your chimney isn’t working as it’s supposed to, it’s important to remember that it’s part of a whole-house system, rather than an isolated appliance, and the root of the problem may lie somewhere else in the house, rather than in the chimney itself.