BACKDRAFTING - Mother Earth News Nov./Dec. 1988
Why Is CO Staying in Houses?
One of the first questions that comes to mind when one learns of the increase in the incidence of carbon monoxide poisoning is: Why now? Houses have had furnaces for more than a century and fireplaces for most of a millennium. What has changed?
As you'll soon learn, there are many factors that can conspire to foul indoor air, but the root of the problem - the basic design rule that has changed - is that modem houses are prone to operate at lower air pressure than older ones. Often, the air pressure indoors is significantly lower than that outdoors, and if the level of depressurization exceeds the strength of the chimney draft -at best, a modest force in natural draft chimney - the flue will work backwards. The chimney will serve as the air inlet, and the deadly gas will be exhausted into the house - a phenomenon called backdrafting. What sorts of influences can bring depression to a crucial point? Here's a partial list :
1. Weatherization : Because of efforts to reduce air leakage, it takes less to depressurize a modern house than a turn of the century residence. Homes built between about 1940 and 1975 in northern climates are about one third as leaky as the previous housing stock. According to Gary Nelson of the Minneapolis Blower Door Company (which builds equipment for testing house leakage), the average new Minneapolis home can reach serious levels of depressurization under the influence of a fireplace or a large exhaust fan. And a tight, energy efficient home, may be depressurized by a single bathroom exhaust fan.
2. Fireplaces : When a fireplace is burning actively, it draws as much as 600 cubic feet per minute (cfm) of air from a home - easily enough to backdraft other combustion appliances. Even more serious, the fireplace's natural draft drops as the fire burns down. It may win the battle of depressurization early in the burn, backdrafting other appliances, and then begin to backdraft itself later, during the charcoal phase, when carbon monoxide concentrations are highest.
Fireplaces are one thing that all the experts dealing with backdrafting agree on : If a modern house is to have one (and if it is to be used), a fireplace should have an ample outside air source and should be equipped with tight-fitting doors that are closed as soon as the fire begins to die.
3. Indoor barbecues and kitchen island exhaust fans : Modern houses have many exhaust fans, some of which are very powerful. Jim Whit says that two of the most potent of house air inhalers are indoor barbecues and kitchen island exhaust fans. Typically, neither type of device is fitted with an outdoor air supply, so they suck 400 to 600 cfm of air out of the house. No natural-draft burner can overcome this level of depressurization, if even in an average home. When the exhaust fan is operating, the furnace, water heater, fireplace, and woodstove, too, will work backwards. White cites examples where the suction has bowed picture windows to the extent that images distort.
Attempts to fit indoor barbecues and island exhaust fans with their own air inlets are impractical unless the retrofit incorporates an intake fan comparable to the exhaust fan. To provide natural make-up air to the average indoor barbecue fan, you'd need a hole about 32 inches in diameter.
4. Smaller exhaust fans acting in concert : In order of the volume of air they expel from a building (after indoor barbecues and island fans), clothes dryers (about 200 cfm), furnaces (about 120 cfm), standard range hoods (about 100 cfm), bathroom exhaust fans (50 to 80 cfm), woodstoves (less than 50 cfm) and gas water heaters (about 40 cfm) all can contribute to depressurization.
5. More-efficient and smaller furnaces : Most of the improvements in furnace efficiencies achieved in the last decade have come from more-efficient heat extraction. As a result, flue gas temperatures are lower and draft is reduced proportionately. In fact, draft is so threatened by low exhaust temperatures that the most-efficient furnaces now have fans to remove the waste products. It's the middle ground where problems can arise. A chimney that was marginal with a 65%-efficient furnace - because it was too large or was leaky or was on a cold outside wall - may not work at all at 80% efficiency.
The problem can be magnified when a house is extremely weatherized, reducing the need for heat, and a smaller more-efficient furnace is installed. The flue gases will be cooler and their volume will be too little for the chimney's capacity.
To cope with lower flue temperatures and smaller furnaces, natural-draft chimneys should be built from insulated stainless steel, or even better, insulated refractory in a size appropriate to the furnace capacity. In the U.S., assuming the federally mandated minimum-efficiency levels are instituted on schedule in 1992, the majority of new furnaces will have induced-draft fans, effectively eliminating the possibility of backdrafting.
6. Leaky forced-air furnace ductwork : Furnace installers and homeowners usually devote a lot of attention to sealing up furnace supply ducts, the tubes through which hot air is pushed by the fan. Typically, though, the return lines, through which the cool air is pulled back to the furnace, get much less attention. Consequently, if the return ducts happen to be leaky in the furnace room, which isn't unusual, the furnace's own fan can depressurize the area near the furnace. In this case you get a double whammy: the chimney backdrafts because of the fan, and then the fan distributes poisons to the living areas.
Well-sealed ductwork is particularly important on gas air-conditioning systems. While in the heating mode, the supply fan won't come on until the plenum has heated to a set level, allowing the chimney time to establish good flow. In air-conditioning mode, however, the distribution fan already may be on when the burner cycles. Bear in mind, too, that natural chimney draft is low in summer because of warmer outside temperatures.
In general, it's unwise to run a furnace blower constantly for any reason, though it won't be a problem as long as neither the furnace burner nor any other combustion appliance operates while the fan is on.
7. Leaks in the wrong parts of the house: wind blowing against a house creates high pressure on the windward side and low pressure on the lee. So, even a leaky house can become seriously depressurized if the predominate leaks are on the downwind side. These leaks could be intentional. Fresh-air inlets for a fireplace or furnace our even a dryer exhaust can allow wind to pull air from the house. And any house can become depressurized if there's a window open on the downwind side.
Leaks at the wall-ceiling junction are also likely to encourage depressurization, since they help the house to act like a good chimney. This effect is more pronounced in a multi-story house, simply because it's taller. Likewise, open windows on the second story (and particularly downwind) may further increase depressurization.
Gary Nelson also noticed an interesting phenomenon in three Minnesota houses he's investigated. All three were fitted with power attic ventilators in attempts to solve moisture problems in the insulation. According to Nelson, the moisture was coming from the house through holes in the ceiling, so exhausting air from the attic only increased leakage rates. Not surprisingly, increasing the leakage rate increased depressurization, and all three houses had backdrafting problems.
8. Unsafe practices : There aren't, and aren't likely to be, regulations prohibiting a homeowner from a foolhardy practice such as heating the kitchen with a gas range turned on high. Likewise heating systems about which any scientists are skeptical, such as unvented space heaters, are quite common.
(Bruce Eugene Davis, Housing Director of the Equal Housing Opportunity Agency of Washington County, Arkansas, reports that 12% of the houses in his state have unvented heaters, and that he has found CO in the exhaust streams of 19% of the units he's tested.)
As for misusing an appliance such as a gas range to heat a space, we can adamantly say DON'T! But whether you want to have an unvented combustion appliance in you house at all (be it a [ed: unvented gas log set], range, an oven or a gas, oil or kerosene heater) is another matter entirely. You should be aware, however, that even in the absence of carbon monoxide, such an appliance could be contributing to chronic health problems your family members may have. (Canadian researchers are finding out alarming things about certain fungi that flourish in the damp environment often produced by unvented space heaters.) And should an unvented heater malfunction, it could be life-threatening.
WHAT TO DO
If you have a woodstove or an oil furnace that backdrafts, you're probably aware of it. Wood smoke is visible and fuel-oil furnaces provide a telltale whiff of sulfur to the nose. Unfortunately, gas appliances don't offer such ready hints. In any event, if backdrafting has been occurring often in fossil-fuel burner, there should be some signs on the appliance itself. On an oil-burner, look for soot smudges around the draft control (a T in the exhaust from the furnace that has a free-swinging flap). If they're extensive, you've been having at least some spillage. On a gas furnace or water heater, look at the outlet from the unit. There will be a cone-shaped hood there that admits extra air to the chimney to stabilize draft, and soot will show up around it if the appliance has been backdrafting significantly. Even if you find no signs that your furnace or water heater has been backdrafting, it's a good idea to perform the simple test described in the sidebar. [ed: located after this article]
If one or more of your combustion appliances is backdrafting for longer than about 30 seconds at start-up, you should fix the problem. As long as the eater is correctly tuned, the situation is not life threatening. But poor air quality may be already causing health problems - sore throats, irritation to the nostrils and eyes and asthmatic symptoms - and may be making the indoor environment overly humid. If you ignore the problem, the performance of the burner will deteriorate, and the situation could well become deadly.
Experts take two approaches to solving backdrafting problems. One method goes after the mechanism of backdrafting, the other eliminates sources. The potential for backdrafting can be controlled by balancing indoor and outdoor air pressure with a fan that pushes air into the house. Jim White points out that air will get into the building anyway, so it may be best to heat it and push it in intentionally.
Source control, as advocated by Lstiburek and Nelson, is done by a procedure called aerodynamic uncoupling. Fortunately, this isn't as complex as it first sounds. It simply means that anything that burns fuel gets its supply air from the outdoors and dumps its waste back out there. For the purposes of combustion, the furnace, water heater, gas dryer, and woodstove or fireplace all operate as if they were outdoors. The first three can be exhausted with a power venter, which can be plumbed to handle all of the burners at once. Fireplaces, if they're used, should have outside air inlets inside the fire chamber and well sealed glass doors. Woodstoves are difficult to retrofit with outside air supplies, but models built for mobile home come so equipped. One advantage of utilizing aerodynamic uncoupling over pressure neutralization is that major exhaust devices (like indoor barbecues) are no longer a problem, since there is no pollution to be drawn in, In addition, in colder climates, infiltration (air moving into the house through the walls) is preferred to exfiltration (air moving out through the walls) because water vapor in the indoor air can cause problems if it condenses inside a wall.
[sidebar] TESTING FOR BACKDRAFTING
Though sophisticated instruments are required to accurately measure the potential for backdrafting in a house, you can perform a worst-case test of your own with nothing more than your finger as a test instrument. There are two stages to the test, the procedures for which we've adapted from recommendations by CMHC, Gary Nelson, and Joseph Lstiburek.
FURNACE RETURN TEST
Close all outside doors and windows. Shut the door between the furnace room and the rest of the building, and turn on the furnace fan (no need for the burner). Using a portable smoke source (an incense stick or cigarette) inside the furnace room, look for air movement under or around doors. If the smoke trails toward the furnace, you've got return ductwork leaks that need repair.
BASIC DEPRESSURIZATION TEST
On a mild, still day, close all exterior doors and windows. At a time when the furnace hasn't run for several hours, turn on every exhaust device in the house (kitchen and bath fans, dryer, water heater, attic and whole-house fans) and open all possible doors between the furnace room and the rooms where the exhausts are located. Keep all other interior doors closed. Go to the furnace room and have someone else turn on the furnace at the thermostat. Feel at any draft control on an oil furnace or the draft hood on a gas furnace for the heat of a backward flow in the chimney. Redo the test on gas water heaters and gas dryers after they've had a chance to cool thoroughly. If spillage continues for longer than about 30 seconds on any combustion appliance, you could have a problem.
"BACKDRAFTING YOUR LAST GASP?", pp. 92-96, Mother Earth News, November/December 1988
