Oregon State University Extension Energy Agent, Bryan Boe uses a digital manometer to measure the air pressure near a new gas furnace.

In September 1993, the City of Chicago passed an ordinance requiring carbon monoxide, or CO, detectors in all new homes with oil- or gas-fired furnaces. Ohio Edison now requires CO detectors in all homes with fireplaces or other combustion appliances built under its Good Cents residential construction program, and other utility-sponsored programs have promised to follow suit. These rules reflect a growing concern among utilities and lawmakers.

Carbon monoxide is an odorless, tasteless, invisible, yet potentially deadly gas that's produced wherever there's incomplete combustion. Since combustion is never 100% efficient, any combustion appliance can pose a threat. Low-level carbon monoxide (CO) poisoning often goes unrecognized, since its symptoms resemble those of flu and colds. A 1995 study printed in The American Journal of Epidemiology reported that even a slight rise in outdoor CO levels sends an increased number of people, especially the elderly, to hospitals. In fact, CO related hospitalizations caused an estimated $33 million in increased medical costs for residents of seven U.S. cities.

While CO detectors are a needed safeguard the real solution is to design and build homes that minimize the likelihood of elevated CO levels. Once the house is complete, performing a simple, inexpensive test can help confirm whether it will have any problems.

The Problem

When a combustion appliance is working right, exhaust gases go safely up the chimney flue. Most of these devices rely on the natural buoyancy of the hot exhaust gases to carry them up the flue and out of the house. As the gases escape, air from the room is drawn into the flue through intake devices like draft diverters, dilution ports or barometric dampers. But since the flue is open to the house, negative pressure near the appliance can overpower the relatively weak upward force created by the hot flue gases. Combustion by-products can then be drawn into the house rather than going up the flue--a phenomenon known as backdrafting or spillage. For most naturally-vented devices, the air pressure of interior spaces should not fall below - 3 to -5 Pascals (Pa). (A Pascal is a very small unit of pressure.) This doesn't guarantee that chimneys won't backdraft under some circumstances, but it does provide a safe margin for properly constructed chimneys in the most likely situations.

While there have been no exhaustive state or national studies done of house depressurization, there is ample evidence from local surveys that between 10 percent and 50 percent of homes have potentially dangerous depressurization problems. Consider the following recent studies:

• Of 16 randomly selected, older houses in Springfield, Oregon, six (37 percent) had depressurization in the area of fireplace, wood or pellet stove worse than -5 Pa.
• Of 58 homes in Medford, Oregon with combustion appliances inspected: five (8.6 percent) had combustion zone depressurization worse than -5 Pa.
• In a 1994 sample of new homes in Oregon, Washington, Idaho and Montana, 14 out of 25 (56 percent) had combustion appliance zone pressures worse than -5 Pa.
• In a 1994-95 inspection by an industrial hygiene firm of houses for carbon monoxide problems in Chicago, Illinois 4 of 28 houses (14 percent) had depressurizaton at or worse than -5 Pa. This study also found CO levels in 12 percent of the homes above 10 ppm. (The American Society of Heating Refrigeration and Air-conditioning Engineers says that indoor CO concentration shouldn't exceed 10 ppm).

Design Precautions

You can reduce the chance of back-drafting by improving the draft characteristics of the chimney, minimizing the causes of depressurization in the house, or both. Make sure the ductwork is sealed well enough to minimize leakage. In homes with forced-air systems, "pressure relief" grilles installed between bedrooms and return registers of forced air heating system will equalize house pressures when the doors are closed. In some cases you may need additional returns (see April, 1996).

When buying a hot water heater or furnace, avoid naturally-vented models. Instead choose models with power venting or a combustion path that's sealed off from the indoor air. Of course, electric water heaters eliminate the problem entirely.

If you install a powerful exhaust fan, such as a range hood, you should also install a make-up air source of similar capacity to bring outside air into the house. Passive make up air is required by some building codes, but may not be adequate for the powerful downdraft range fans.

These pressure-related issues are discussed in the new edition of Manual D, Residential Duct Systems which is widely accepted by the HVAC industry. It costs $36.00 and is published by the Air Conditioning Contractors of America (202-483-9370).

Testing for Depressurization

Although the above precautions may reduce the likelihood of depressurization, they won't verify whether a completed house has any potentially dangerous conditions. You can find out by borrowing a technique used by advanced weatherization companies. They check that a building is safe and efficient with a series of tests, the heart of which is a simple "Worst Case Depressurization Test" that can be performed by any builder.

The only equipment you'll need for the test is a manometer that reads in Pascals with an accuracy of at least 2 Pa. The procedure is simple:

1. Attach a tube to the reference port of the manometer and run that tube to the outside. (A good place to run the hose is through a door. Find a place where the weatherstripping leaves a slight gap, place the hose and then close the door so that it doesn't pinch the tube. The door will probably not close all the way. You also can open a window slightly, run the hose through and then use masking tape to cover the opening.)
2. Close all exterior windows and doors.
3. Close interior doors to all rooms (except those with exhaust devices) and turn on the air handler fan of the furnace or heat pump.
4. Turn on all exhaust devices (bath fans, range hoods clothes dryers, etc.).
5. Use the manometer to measure the pressure difference between the area of each combustion appliance and the outside. If the manometer reads -5 Pa or further on the negative side, there's a potential problem for naturally-vented combustion devices.

You may consider the test an unnecessary burden, but it's both inexpensive and simple to perform. That makes it cheap insurance against tragedy. You can get a good manometer for around $75 (Dwyer Instrument's "Magnehelic"). Digital manometers are more expensive, but often make sense for HVAC contractors or energy specialists. Digital manometers can be purchased from Infiltec, The Energy Conservatory or Modus Instruments, Inc. Find a list of manometer suppliers in the Oikos Product Directory. With this simple 15-minute test you can increase customer safety and satisfaction, as well as reduce callbacks and exposure to liability claims.

David Brook is an extension agent with the Oregon State University Extension Energy Program.

Wind is one natural force that causes air pressure changes in a building. Even when the wind isn't blowing the building can create its own negative pressure through a process called the stack effect. When there are holes in the building envelope near the top of the structure, warm air escapes through these openings while colder air enters the building through lower openings. The best defense against the stack effect is to carefully seal your homes against air leaks.

Mechanical equipment can also cause negative pressure. Unbalanced air flows between the return and supply sides of a duct system may be caused by leaky or disconnected ducts, by blockage in the ducts or by closed interior house doors that block air flow between supply registers and returns.

In a small or tightly built house, exhaust devices often cause the most powerful depressurization. Even a small fan can create potentially dangerous depressurization. Leaky houses can have problems, too, if there are appliances with powerful fans. These include downdraft ranges, new clothes dryers, attic or whole-house cooling fans, and central vacuum cleaners.

Poor chimney design can also be a culprit. Chimneys should be sized to match the appliances they serve. In addition, interior chimneys provide better draft than exterior chimneys. (Draft is created when the temperature in the chimney is warmer than the temperature outside. So, an interior chimney doesn't cool off as much during off-cycle periods.)

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