Writing from St. Paul, Minnesota, W Ramsay wants some feedback on his plan to install whole-house ventilation fans in the house he’s building. He likes the idea, the building inspector does not.
Ramsay sees the benefits of improved indoor air quality and energy-efficient cooling, while the inspector is apparently concerned that the fans would depressurize the house and possibly lead to backdrafting of combustion appliances.
Originally, the inspector was prepared to require a 1:1 makeup air system, which would not only be expensive but also defeat the purpose of the fans, Ramsay says. The inspector backed off that original request as long as Ramsay could guarantee that a sufficient number of windows would be open while the fan was running. More recently, the inspector says he wants an engineer to guarantee there would be enough passive makeup air for the system.
“Is a whole-house fan beneficial?” Ramsay asks. “To me this is a very easy ‘yes.’ On some days we can get enough air through our house with natural breeze, but many days there is either not enough breeze or it is coming from the wrong direction.
“A whole-house fan creates the breeze and with windows open provides fresh air, reduces or eliminates VOCs and CO2, and cools the house for a lot [less] energy and dollars than running the AC,” he adds. “Am I nuts?”
Further, Ramsay wonders whether a whole-house fan could be considered dangerous. He says he found one instance in which a whole-house fan had been linked to a death; the victim was a high school student who turned on a fan after smoking marijuana but didn’t open any windows.
Are any of these concerns well-founded? That’s where we start this Q&A Spotlight.
Whole-house fans are great
“What’s not to like?” asks Zephyr7. “I love whole-house fans,” he writes. “They work great to quickly cool a house off at night. They work great to bring in fresh, clean air, and they’re cheap to operate. In northern areas, whole-house fans can often replace air conditioning during the spring and fall.”
The only potential for danger that Zephyr7 can imagine would come from the spinning fan blades themselves. As long as Ramsay keeps his hands away from the blades, Zephyr7 isn’t aware of any safety concerns. Based on his own professional experience, Zephyr7 suggests that as long as an open window is at least as big as the area of the fans, air pressure inside the house won’t be an issue.
“It sounds like your inspector is going a bit overboard with your fan project,” he says, adding that it wouldn’t be very difficult to add a sensor that would prevent the fan from operating unless a certain number of windows were open.
“I can’t see why your inspector would be trying to require anything more complicated,” he says.
Use common sense in hot, humid weather
To Dana Dorsett, the efficiency of nighttime ventilation with a whole-house fan depends on how it’s used. When outside humidity is high, a fan may bring in moisture from the outside and increase latent cooling loads. But most people can figure that out by how the air feels, he adds.
“Nighttime ventilation schemes can be very efficient in drier climates like the West Coast states,” he says, “but just make it clammy indoors in much of the Eastern states, deriving a higher indoor relative humidity than using a modest amount of air conditioning.”
With those humid conditions in mind, Dorsett adds, it’s not clear whether a whole-house fan would always use less overall power than 20+ SEER heat pump/air conditioner, such as a cold-climate minisplit.
Fears of negative air pressure
The inspector’s worries seem focused on the risks of negative air pressure in the house and the potential for backdrafting from fuel-burning appliances. The inspector seems intent on getting some type of documentation that would guarantee that Ramsay’s plans are sound from an engineering point of view, and he’s unmoved by Ramsay’s arguments.
“We also tried to explain to him that there are no engineering studies on this because they are not a problem,” Ramsay says. “Tens of thousands are installed every year and they aren’t a problem, and aren’t viewed as a problem by other building inspectors.”
Further, adds Zephyr7, the axial fans used in whole-house fans aren’t capable of generating negative air pressure.
“It really takes a centrifugal blower or positive displacement pump to generate levels of negative pressure that can damage things,” he says. “If your inspector is worried about negative pressure causing structural damage, it just isn’t possible here. If his concern is backdrafting in combustion appliances I suppose that is a valid concern, but also easy to address.”
Backdrafting, of course, isn’t an issue with sealed-combustion appliances, which have their own source of combustion air separate from the air inside the house.
“Yep,” replies Ramsay, “we’ve talked with him about that. This is a code issue, not a physics issue. And physics does whatever code and code inspectors say it will do.”
Martin Holladay’s 2012 article
GBA editor Martin Holladay notes that he provided advice on this question in his 2012 article, “Fans in the Attic,” which included this paragraph:
“Because they depressurize a home, whole-house fans can cause atmospherically vented appliances located inside a home — for example, a gas-fired water heater — to backdraft. If the homeowner remembers to open plenty of windows before turning on the fan, backdrafting probably won’t occur. But the best way to avoid backdrafting problems in a house with a whole-house fan is to make sure that the house doesn’t have any atmospherically vented combustion appliances.”
Some other points to consider
Timothy Tucker’s house in southeastern Michigan had a whole-house fan when he moved in, but he removed it a year later because the uninsulated 1990s-era fan was causing more problems than it was solving. He wonders whether Ramsay would be better off spending the money he’s set aside for fans on solar panels instead and lowering his electric bills that way.
He also raises these points:
- If Ramsay already has another system for circulating and filtering air — for ventilation or for central heating and air conditioning — why introduce another? A whole-house fan is only practical in warmer weather, so it might make more sense to focus efforts on something that works all year ’round.
- A whole-house fan is designed to flush hot air out of the house and replace it with cooler outside air. But if the outside air is tainted by industrial pollution or high pollen counts, recirculated air from a central AC system might be cleaner.
- In moderate temperatures, a ceiling fan can make the air feel cooler than a whole-house fan. Moreover, a ceiling fan uses considerably less electricity and costs far less than the Tamarack whole-house fan that Ramsay is considering.
- Installing central air conditioning plus $1,500 in solar panels will likely have an overall lower cost than central AC plus a whole-house fan. Or, investing the cost of a whole-house fan in air sealing, better windows, or more insulation offers year-round payback in comfort.
- Noise pollution may be an issue. Although Tamarack fans are much quieter than some of the old-school options, they still make more noise than most central AC systems, ceiling fans, or single-room exhaust fans.
Our expert weighs in
Peter Yost, GBA’s technical director, added these thoughts to the mix:
In just about any climate, but particularly in a cold and relatively dry one, if I were to cut a decent-sized hole in my top-floor ceiling and stick a fan in it, I would be make sure that when the fan was not running, the opening was air-sealed and well-insulated. Tamarack Technologies has been making high-performance whole-house attic fans for a very long time and it’s a great family-owned business.
Frankly, it never occurred to me that there was a safety issue with makeup air. So I called Nelson Warner, vice president of R & D for Tamarack Technologies. “In more than 20 years in this industry, I bet I have gotten only a handful of questions about ensuring makeup air for our attic fans, while I am inundated with calls regarding makeup air for kitchen range hoods,” says Warner.
I figured if makeup air for whole-house attic fans was an issue, particularly one that a building inspector chimes in on, then the issue would be covered in the building code. I searched my copy of the 2018 International Residential Code extensively, and found only this one reference: “M1501.1. Outdoor Discharge. The air removed by every mechanical exhaust system shall be discharged to the outdoors in accordance with Section 1504.4.3. Air shall not be exhausted into an attic, soffit, ridge vent, or crawlspace. Exception: Whole-house ventilation-type attic fans that discharge into the attic space of dwelling units having private attics shall be permitted.”
That’s it. There are plenty of detailed requirements for other mechanical exhaust systems, particularly ducted ones; for example, M1502 (Clothes Dryer Exhaust) and M1503 (Domestic Cooking Exhaust Equipment).
And then Warner sent me an excerpt from the California Energy Commission (CEC) Residential Compliance Manual of January 2017. While the model building codes have little information on or requirements for whole-house attic fans, there are plenty of requirements on this type of equipment in the CEC residential manual:
- 7.10 (Ventilation Cooling) has essential background information on whole-house attic fans and central fan systems.
- 7.10.1 (Whole House Fans) has detailed informantion on the different types of these fans (ones with simple barometric dampers; ones with electronic dampers that air seal and are insulated; ones with remote fans for noise reduction), including energy standards and listings in the Energy Commission’s appliance database.
- 7.10.3 (Prescriptive Requirements): Under CEC Component Package A for CA climate zones 8-14, a qualifying whole-house fan is required, and must to meet the following eligibility criteria:
- Whole-house fans must meet combustion safety requirements related to indoor gas-fired appliances (essentially an interlock for gas-fueled appliances that aren’t direct-vent appliances or sealed-combustion appliances).
- The fan must be listed in the Energy Commission’s database (as stated above).
- The installation must meet net free vent area requirements for attic ventilation (Tables 4-20 and 4-21). The worry is not about makeup air but about sufficient attic vent area to prevent pressurization of the attic, with the primary concern being disruption of attic insulation.
- The homeowner must be provided with a copy of a “How to operate your whole-house fan” informational sheet. (An example from the City of Davis, California, can be found here.)
It sure seems to me that something as simple as a cover plate for the attic fan switch having this warning would work: “Do not operate this fan without first opening windows.”
Warner cautioned me that many fans nowadays have remotes. OK, print the same warning on the remote. And for smart phone app operation? OK, add a pop-up screen on the app with the same warning.
And then I found this information resource from the U.S. Department of Energy on whole-house attic exhaust fans. It aligns pretty neatly with the position California has taken.
In any event, the state of California seems to have this figured out well enough to prescriptively require whole-house attic fans in a component package for Climate Zones 8 through 14. Qualifying whole-house attic exhaust fans can be an efficient form of cooling, particularly in the right climate, and can be easily operated safely.
Information on California climate zones: California has 16 climate zones. An example of a Climate Zone 8 location is Long Beach, with 1400 heating degree days (HDD) and 1200 cooling degree days (CDD). It’s somewhat tempered by its proximity to the ocean, experiences no frost, and gets 12 inches of rain per year, mostly in “winter.” For Climate Zone 14, let’s use the example of Barstow, California: 2500 HDD, 3000 CDD (medium to high desert), less than 12 inches of annual precipitation, and wide temperature swings, with frost common in winter. It appears as though Climate Zones 1–7 are very mild and coastal; Climate Zones 15 is low desert; and Zone 16 is a large semi-arid mountainous region.