The History of the
Blower Door by Abba Anderson
Perhaps no piece of equipment has changed the way energy professionals look at buildings more than the blower door. Over the past 15 years, entire diagnostic procedures have evolved around this relatively simple device that can make subtle, but measurable, changes in house pressures.

The blower door as we know and love it today springs from technology first used in Sweden in 1977, where it was actually a blower window. The idea migrated to the United States with Ake Blomsterberg, who came to Princeton University to do research in 1979. We started using it because we were trying to understand infiltration, says David Grimsrud, who was a researcher at Lawrence Berkeley National Laboratory (LBNL) in Berkeley, California, at the time.

According to Grimsrud, the Princeton researchers decided to mount the fan in a door because door sizes are more uniform than windows. Ken Gadsby (who was and still is at Princeton) recalls that they based the height of the lower door panel on his inseam length! With the help of the blower door, the researchers discovered that hidden leaks accounted for a much greater proportion of air leakage in a home than the more obvious culprits, such as windows, doors, and electrical outlets--a giant leap forward in our understanding of how a house operates (and malfunctions). Researchers at LBNL began to see how useful the blower door would be in weatherization and retrofitting work.

Blower door companies started springing up to serve the new market. LBNL energy researcher Max Sherman even got into the business of manufacturing blower doors for a short time. The Department of Energy put out a solicitation to buy ten blower doors, so my father and I started a company and bid on the contract and we won, Sherman remembers. These were big, heavy, clunky blower doors, made of plywood and Formica.

We were all working out of our garages, recalls Gary Anderson, cofounder of the Energy Conservatory. In 1986, Home Energy (then Energy Auditor and Retrofitter) identified 13 blower door manufacturers, with combined revenues for sales and testing nearing $10 million per year (see A Healthy Outlook for the Blower Door Industry, EA&R May/June '86, p. 6). Home Energy estimated that blower door sales alone reached $1.2 million in 1985. The focus at that time was on making more powerful fans in more manageable sizes.

The industry has consolidated today, with only three North American manufacturers--the Energy Conservatory, Infiltec, and Retrotec--now vying for sales in a growing market.

Of the three, Energy Conservatory (manufacturer of the Minneapolis Blower Door) is easily the largest, selling 800 to 1,000 blower doors per year, along with Duct Blasters, digital pressure gauges, and other diagnostic tools and procedures.

The Energy Conservatory was hatched over lunches between partners Gary Anderson, then an auditor in St. Paul, and Gary Nelson, an engineer at the Minnesota Energy Agency, during which they would discuss the latest discoveries in residential energy efficiency. The blower door was one advance that captured their imagination.

It got to be an expensive hobby, Anderson recalls. The pair retrofitted a two-story garage to use as a calibration chamber and strove continuously to create a design that would be more practical for mainstream contractors. That meant it had to be less expensive, lighter, and easier to use. They worked to make blower door testing more friendly, accurate, and efficient, and helped develop protocols for weatherization programs to prioritize air sealing efforts.

In the late 1980s, the Energy Conservatory was involved in research that led to the understanding that duct leakage is a big problem--not just for energy waste, but also because pressure imbalances caused by the duct system can result in backdrafting and indoor air quality problems. This realization led to the development of a duct leakage testing fan (the Duct Blaster) and a digital manometer for more precise pressure measurements.

Infiltec sells blower doors, in addition to developing energy software and conducting indoor air quality studies for the Environmental Protection Agency.

We sold our first blower door in 1980, says Infiltec's David Saum. Saum got into the business when his retired father was looking for ways to make his home more energy-efficient. Saum did some research and found articles on the Super Sucker, the window-mounted unit being used in Texas. We decided that we could do better, Saum says. In addition to blower doors, Infiltec now sells duct testers, mostly for testing ducts in new construction. Infiltec has recently taken its blower doors to Russia to test multifamily buildings there (see To Russia with Blower Doors, HE Sept/Oct '95, p. 8).

The Canadian firm Retrotec concentrates on selling units for testing fire protection systems, and on teaching HVAC contractors how to use the blower door to boost their business (see HVAC Contractors Discover Blower Doors). Retrotec has also been in business since 1980. The founders originally opened their factory because they needed blower doors to use in research projects for Natural Resources Canada (the equivalent of the U.S. Department of Energy).

At the time, you couldn't buy a blower door, vice president Brendan Reid says. Retrotec now offers seven models--all different configurations of the same equipment. Four of these are used in testing industrial fire protection systems. The company offers an optional panel system made of molded plastic sheets, which Reid says installs faster than the standard doors and looks better, although it is less adjustable. Retrotec also sells a sealed smoke puffer, and a duct leak testing system for new construction. With nine employees and two independent sales contractors, the company has annual revenues of just over $1 million.

Old-timers in the industry can best appreciate the evolution of blower door technology over the years. They're the ones who remember struggling with a heavy, bulky fan and door panels. We had a chance to use one of Max's blower doors, Gary Anderson recalls, and when we'd lug the blower door out there, two things happened. We'd be blown away by what we were learning, but at the same time, we were frustrated by how long it took.

Our first blower door moved 4,200 cubic feet per minute (CFM) maximum flow, weighed about 55 lb, and was about 28 inches long, says Anderson. Then we made a 10-inch long fan that moved 6,400 CFM, increasing flow by 50% with a shorter fan. Refining the frame was the next major improvement in the blower door, and the Energy Conservatory went to a cloth-covered aluminum frame.

Infiltec's design also evolved over time, switching from a DC to a much less expensive AC motor. They also introduced a flexible door panel and started using fan rpm instead of just pressure drop to measure flow, improving the accuracy of calibration.

The equipment has pretty much evolved to optimum, says Anderson. The fan weight has dropped from 60 lb to 35 lb, at the same time increasing flow. The next step is toward computerization and improved error analysis. A computerized, digital blower door will make data more accurate and repeatable at lower house pressures. It will also make it easier for researchers to track a wide variety of conditions. For example, David Grimsrud is now at the University of Minnesota, where he is conducting blower door tests by remote control, tracking measurements by computer to study backdrafting in buildings. His studies use the newest development by the Energy Conservatory: a 16-channel data acquisition system that processes input from carbon dioxide monitors and pressure and temperature sensors.

In another interesting development, a monster blower door that takes the name of the window-mounted unit used in Texas all those years ago is being used to test large residential and commercial buildings in Canada. The Super Sucker is a whopping 55,000 CFM fan that is 40 ft long and 5 ft in diameter. It is transported to the site on a flatbed trailer, and it takes a team of five people to hook it up (to a pair of double doors) and perform the test.

At the other end of the spectrum, the Canadian consulting firm CanAm Building Envelope Specialists Incorporated is marketing an individual window depressurization testing kit, called the MiniLab, which is used to identify and quantify air leaks around windows. This mini-blower door allows retrofitters to demonstrate improvements in air leakage when they replace or seal windows, and enables new home builders to specify and measure the performance of their windows.

Window leakage is tested by installing 6-mil plastic on the inside around the window frame, cutting a hole in the middle of the plastic and attaching and sealing a tube in the hole. The hose is then connected to the blower, which pressurizes the space between the plastic and the window. The device has flow meters and a Magnahelic gauge, like a blower door, to measure flow in CFM at a given pressure. New standards in Canada require windows to meet air leakage ratings at 75 Pascals (Pa) of pressure.

The market has changed from year to year, says Infiltec's Saum. Originally it was entrepreneurs during the energy crisis; then it changed to utilities and weatherization agencies; and then a few years ago, to the fire protection business. Blower doors are used to test fire protection systems that use halon gas instead of water (to prevent water damage in case of a fire). These systems used to be tested by setting them off and timing how long it took the halon to dissipate. The EPA banned this procedure when it was found that halon was the worst ozone eater by a factor of ten, says Saum. With a blower door, contractors can calculate how long it will take for the halon to leak out of a structure without actually releasing it into the atmosphere.

To some extent, we have begun to saturate the weatherization market, Anderson says. However, the blower door is also a valuable tool for HVAC contractors and builders. Everybody who goes into houses and changes the way the houses operate wants to be sure they don't do damage. HVAC contractors are becoming more aware than builders; they are much more familiar with the problems that can arise. Retrotec's Reid agrees, There's high potential because of the numbers, he says, with about 50,000 HVAC contractors in North America.

We're moving to a time in housing construction when we'll see more mechanical ventilation systems, adds Grimsrud, requiring the near elimination of infiltration, and heralding a demand for infiltration testing in the construction industry.

Anderson points out that most of the people who stopped by the Energy Conservatory's booth at a recent builders' show weren't familiar with blower doors, but just about every one of them talked about moisture problems, backdrafting, fireplaces that won't draw--all of which have to do with the airtightness of the envelope.

They definitely will benefit from this technology, Anderson continues. The option is to build the house and then deal with the problems that show up. A blower door gives builders some control over these problems, because they know how tight the house is, how to deal with duct leakage, and how to size a ventilation system.

The most important legacy of the blower door, concludes Anderson, is the evolution of the understanding of the house as a system and how you can characterize and diagnose the problems using pressure analysis.