Negative Pressure and Infiltration

The No. 1 ventilation problem plaguing residential structures nationwide is negative pressurization. When a home is under negative pressure, air is drawn into the home wherever it can get through. Often, it travels through attics, crawl spaces, and other dirty spaces before reaching living areas, which negatively impacts IAQ.

“Primarily, it’s exhaust systems that are causing homes to have negative pressures, creating a situation where the air to make up for the exhausted air enters the home completely uncontrolled through undesirable passages, such as cracks in basement floors,” said Robin Boyd, HVAC advisor and consultant at H-VAC Consulting, Willow Street, Pennsylvania. “[That brings] in underground, heavier-than-air gases, such as radon. To resolve the issue of negative pressure in houses, we can either bring in outside air through a duct tied into the return ducting of the HVAC system or bring in outside air by adjusting an HRV/ERV [heat recovery ventilator/energy recovery ventilator] system to bring in slightly more air than the structure exhausts.”

Kristof Irwin, building science consultant at Positive Energy, Austin, Texas, completely agrees, saying exhaust-only ventilation is inappropriate. “This is especially true for hot-humid climates, where mold and durability risks occur when drawing raw outside air through the enclosure,” he said.

“In humid climates, ventilating dehumidifiers from Ultra-Aire are great tools in residential applications for ventilation control/scheduling as well as dedicated latent control. They offer high-MERV filtration of the ventilation air and can be integrated into the existing air distribution system so the client does not see or hear any difference.”

Carter Stanfield, director of the air conditioning technology department at Athens Technical College and author of “Fundamentals of HVAC/R,” said the concept of makeup air isn’t a difficult one to understand.

“Once people grasp that, you simply ask the question, ‘Where is that air going to come from?’ One way or another, it has to come from outside,” he said. “So, then, the only remaining question is, do you want your ‘fresh air’ to be sucked in through your fireplace or through a filtered makeup air source?”

Stanfield said ventilation problems are often caused by the equipment installer.

“I think one reason you see so many ventilation fans with no makeup air is that they are often installed by electrical contractors who are installing them like any other electrical fixture,” he said. “In the days before Tyvek, foamed-in double-pane windows, and caulked penetrations, you generally could count on enough leakage to satisfy the needs of bathroom and kitchen vent fans. But, with tighter houses and larger vent fans, that just won’t work. Even new clothes driers present a problem because they move a lot of air out of the house.”

HRVs, ERVs, and More

While humidifiers, dehumidifiers, HRVs, and ERVs can be used to help temper the air entering the home or building, they must be specified and installed correctly in order to be effective.

“Be careful what you vent through [ERVs and HRVs],” Stanfield said. “The cores can be easily stopped up, and most specifically tell you not to vent kitchen exhaust or clothes dryers through them. Also, if you live in a cold climate, pulling more air in than you exhaust can freeze the core. That’s not a problem here in Georgia, but I know folks in colder climates who generally want their ERVs and HRVs balanced, with the same amount of air leaving as entering. This means you still need a source of makeup air. Field Controls makes an inexpensive ventilation control that solves this problem.”

While ERVs are a great way to provide ventilation and promote positive pressures, they’re often also associated with latent control issues, Irwin said. “This is because proper ERV operation requires an exhaust airstream that is both cooler and dryer than the incoming ventilation air. Without a dedicated dehumidification system, the exhaust airstream is cooler, but often not dryer than outside air, so that in the mixed-air condition, no drying occurs. This means that, especially during part-load conditions, the ERV is unable to effectively reduce latent gains from outside air brought in for ventilation.”

As enclosures improve through changing building codes and performance testing requirements, two changes will occur in the ventilation market, said Irwin: “Ventilation becomes more crucial for IAQ during occupied conditions, and the strategy of drying the indoor air as a side effect of space cooling is becoming increasingly unrealistic and risky for contractors. For both of these, a ventilating dehumidifier is the way to go.”

Other Issues

Often, ventilation issues can be easily fixed once the root cause is determined.

“Most of what we fix is very simple: rotation of the fan wheel in relationship to the discharge fitting, a lack of duct sealing, undersized ducting, no balancing, a lack of dampers to even balance, and this could go on for a while,” said Ronald Sage, vice president of Westside Mechanical Inc., Naperville, Illinois. “If the guys are trained properly, the mistakes occur less frequently. We just looked at a health care facility where the final filters had been removed on a system for a sterile area — the negative pressure part could be an article all by itself — we solved a stack effect issue on a high-rise building last month in Chicago that was out of hand. [The] solution was simple, yet there were lots of people with ideas on how to fix it.”

Most often, ventilation issues are a result of poor planning, said Rob Goodfellow, vice president of marketing, Dynamic Air Quality Solutions, Princeton, New Jersey.

“Most of the ventilation mistakes we get involved with are on the commercial side of our business and are often simply the result of poor planning,” he said. “Dynamic Air Cleaners are often used to correct situations where ventilation air intakes are located in poor locations, such as in schools where the idling buses load and unload students, near hospital ambulance bays, or around hospital helipads.”

How to Sell It

Convincing a homeowner or building owner to invest in a ventilation system isn’t always easy, but arming oneself with facts and data often goes a long way in making a sale.

Eric Kjelshus, owner of Eric Kjelshus Energy, Greenwood, Missouri, suggests contractors test, test, and test again. “Then, sometimes, the end user will pay for [the] upgrade if the pain is greater than the cost.” Often, he said, a customer wants to look at data on “mold, dirt, high energy bills, lack of air, CO, CO2, high/low relative humidity, radon, gases, VOCs [volatile organic compounds], and more.”

Many contractors’ sales teams don’t have a thorough enough understanding of overall home system performance to be able to sell ventilation products, said Boyd. “[They] are unprepared and, therefore, not confident to inform consumers of the benefits of proper IAQ practices,” Boyd said. “They fail to sell products that really are the most beneficial to consumers.”

James Bowman, national technical manager for the HVACR market, Rectorseal Corp., said the easiest way to sell any solution is to first find out if there is a perceived problem. “Simply ask the right questions,” he said. “What temperature do you set the thermostat at? Does that change during the day or night? Are you comfortable at that temp? Do you wake up at night? Are you overly cold or hot? Do you have allergies? If they don’t think they have an issue, then you don’t have a solution.

“I have seen active mold 3 inches deep in ductwork, and the homeowner doesn’t care because they aren’t allergic or having any ill effects while other people are having asthma attacks from surface mildew on grilles,” Bowman continued. “You can tell someone anything, but find out what their particular issues are, and you can sell them solutions.”

In the end, Boyd said, those in the industry must take responsibility for teaching the consumer about ventilation and IAQ.

“This is a conversation that takes most HVAC sales persons out of their comfort zones,” he said. “They must have support literature readily available to show the consumer why installing a ventilation system that will dilute the indoor air with fresh, filtered, and temperature-tempered air that will prevent the home air from going into a negative pressure is every bit as important as is cooling and heating that air.”

Publication date: 4/20/2015

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