One evening early last winter, I caught a distinct whiff of ashes as I relaxed in my living room. The scent was coming from the fireplace, although no fire was burning and both the damper in the chimney and the glass doors were closed.
The rumble of the clothes dryer downstairs supplied the clue we needed to explain the mystery. The dryer, because it vents to the outside, was sucking the air out of the house. Some of the replacement air was coming in down the chimney and leaking around the damper and glass doors, carrying the telltale scent of ashes with it.
This was a good example of “backdrafting,” a reversal of the normal airflow in a chimney. Usually, air flows upward, drawn by the wind outdoors or by the natural buoyancy of warm air.
In fact, it’s critical that your home can depend on this natural updraft in the chimney flues to carry out smoke from the fireplace as well as fumes from furnaces and water heaters that burn natural gas, oil or propane. (Electric appliances do not create fumes.)
But sometimes, like that evening in my home, something disturbs the natural updraft and causes it to reverse and flow downward. You’ll likely sense this backdraft immediately if you have a fire in the fireplace; the room will quickly fill with smoke.
Usually, you can restore the updraft by opening a window or door a crack. This allows outside air to rush in and restore the natural updraft in the chimney.
When exhaust fans operate in a tightly built house (one with little air leakage around doors, windows and other areas), replacement air can come down the chimney and cause the combustion fumes from a furnace, water heater or fireplace to spill inside. These fumes might contain deadly carbon monoxide.
But if it’s a furnace or water heater that’s backdrafting, you can be in serious — even deadly — trouble. That’s because chances are that you won’t see or smell the fumes spilling into the house.
While oil, natural gas, and propane burn cleaner than wood, their exhaust fumes might contain carbon monoxide, an odorless, colorless gas that can kill you if it reaches high enough levels. Furthermore, backdrafting can lead to moisture buildup in your home and the growth of mold, mildew, and rot.
Backdrafting is a threat to the health of your family and your house. In this article, I’ll tell you what causes it, how to detect it, and how to prevent it.
What is backdrafting? Why is it a problem?
Backdrafting became an increasing problem after the energy crisis of 1973 when gas lines and rising fuel prices made energy conservation a high priority.
Homes in those days had lots of air leaking in and out through cracks around doors, windows, and foundations and through the ceiling and attic. Builders didn’t plan the leakage; it simply happened in the course of construction. You couldn’t complain.
Leaks supplied plenty of replacement air to keep the natural chimney drafts flowing upward. Of course, leaky houses wasted a lot of energy — heat in winter and cooled air in the summer — but that didn’t matter much until fuel costs rose so dramatically.
The push for energy-efficient homes is gradually making the leaky home obsolete. Better insulation, tight-fitting windows, weathertight doors, caulking and weather-stripping, and other energy-conserving improvements sharply reduced air leakage.
To our surprise, they cut down on drafts and made homes more comfortable too!
As a result, most homes now have less replacement air available to keep those natural drafts flowing up the chimney flues. This was the case in my home, where I had recently insulated the walls and caulked around the foundation to reduce air leakage.
Besides this trend toward tighter homes, a second key factor also contributes to backdrafting — an increase in both the number and size of ventilation fans:
You probably now have an exhaust fan in every bathroom and in the kitchen to flush out odors and moisture. Those downdraft fans connected to ranges are particularly powerful.
Every time you switch on the kitchen or bathroom fan or turn on the clothes dryer for that matter, you suck air out of your house, air that the chimney might need to sustain its natural updraft.
If the updraft fails, or for that matter, if the fans actually pull air down the chimney (Fig. A), your furnace, water heater, and fireplace might backdraft and dump their dangerous fumes into your house.
How do I test my house for backdrafting?
Fortunately, you can easily test your furnace and water heater for backdrafting.
Set up a worst-case scenario by closing up your house as you would during cold weather or during hot weather when the air conditioner is running full time:
- Shut and fasten all your windows and doors tightly,
- Close the fireplace damper (if you have one),
- Then turn on the kitchen and bathroom fans and clothes dryer.
In real life, you probably won’t have all these on at once, but you want to set up the worst possible case.
Now start the furnace (by turning up the thermostat) or water heater (by turning on a hot (by turning on a hot water faucet until you hear the burner go on).
After a minute, locate the draft diverter above the water heater or furnace (Fig. B) and hold a lighted or smoking match near its edge.
Hold a lighted match by the edge of the draft diverter on your furnace or water heater. The flame and smoke should be drawn up the flue if it’s venting properly.
Newer furnaces have rectangular draft diverters on the front, as shown in Fig. C, rather than on top. The flame and smoke should be drawn into and up the draft diverter if the flue is drawing properly.
An open duct placed near the furnace and water heater ensures that adequate air is available for combustion and venting. The “.1” at the end of the duct slows unwanted drafts.
If the flame or smoke is not drawn to the flue, or is blown away from it, you have a backdrafting problem.
Turn off your furnace and water heater or open a nearby window and then call your local utility or a heating specialist for a professional evaluation. Sometimes backdrafting is caused by chimney blockages or factors other than inadequate replacement air. A professional evaluation will get to the root of the problem.
Do the smoking match test routinely whenever you improve the energy efficiency of your house. Even replacing a couple of old, leaky windows with new, tight ones can upset the sometimes delicate balance of airflow through your house and cause backdrafting.
How to stop and eliminate backdrafting in my home?
The simplest way to prevent backdrafting is to provide a source of replacement air (also called combustion air) for your furnace, water heater and fireplace to replace the air that flows up the chimney with the exhaust fumes (Fig. C).
In fact, building codes in many regions require the addition of a special replacement air duct when you replace a furnace.
You can purchase both a screened wall vent (to keep critters out but not block the air flow) and insulated flexible duct for a fresh air supply at a heating and cooling supplies dealer.
Generally, codes require that you size the incoming air duct to match the combined heating output of your furnace and water heater, so check with your local code official for exact size requirements.
If it’s time to replace your furnace or water heater, you can choose two other strategies that builders sometimes use in new, energy-efficient homes.
One is to install an “induced draft” furnace or water heater, which simply means that the exhaust flue includes a built-in fan that blows the fumes up the chimney. In this way, it can successfully compete with other exhaust fans in the house.
Another solution is to install a “closed” or “sealed” combustion furnace or water heater, which draws its combustion air directly from outside through a pipe and expels the fumes outside as well. It neither relies on a natural draft chimney nor draws on air from inside your home.
These sealed combustion appliances may well be the systems of the future, as home builders and buyers push for higher energy efficiency in new homes.
As you might expect, both the induced draft and closed combustion systems cost more than those that rely on natural drafts.
How do I stop backdrafting in a chimney?
Supplying replacement air to fireplaces is a tougher chore.
Newer ones often have an outside air supply already installed that you manually open when you start a fire. But old ones, especially masonry fire-places, rarely have an outside air supply.
Since installing a supply is usually difficult and expensive, discuss your particular situation with a fireplace builder, and be sure to confirm any retrofit plans with a local building inspector before making changes.
It’s often easier to ensure a good natural draft by slightly opening a window in the same room as the fireplace while the fire is burning, and keeping it open until the embers are completely out.
Why bother with backdrafting and building new ducts?
it might strike you as odd that after tightening up your house to make it more energy efficient, you intentionally create big, new air leaks with ducts. Why bother tightening the house in the first place?
Installing air supply ducts actually wastes less energy than you might imagine. They reduce the amount of air leaking in from other sources and channel much of the incoming air to the combustion chamber, draft diverter and the flue of your furnace or water heater.
The result is less inside air that’s already either heated or cooled is lost up the chimney. The energy cost still comes out in your favor. And you’ll make your home healthier and more comfortable as well as more energy efficient.