Is Your Ventilation System Working?

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Is Your Ventilation System Working?

There’s no way to know whether your fans are working properly unless someone has measured the airflow rates

Posted on Apr 15 2016 by Martin Holladay

What’s a “faith-based ventilation system”? It’s a ventilation system installed by a contractor who never verifies the air flow rates after the equipment is installed.

So, will this type of ventilation system work? It’s hard to say — because no one measured anything.

At last month’s BuildingEnergy 16 conference in Boston, two ventilation experts — Terry Brennan and David White — gave a presentation titled Brennan and White explained why new homes need mechanical ventilation systems, and shared important design principles underlying these systems. They ended their presentation by providing advice on (and a demonstration of) different ways to measure airflow.

Terry Brennan is a building scientist and a member of the ASHRAE 62.2A standard for residential mechanical ventilation systems established by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers. Among other requirements, the standard requires a home to have a mechanical ventilation system capable of ventilating at a rate of 1 cfm for every 100 square feet of occupiable space plus 7.5 cfm per occupant. committee (the committee responsible for the residential ventilation standard). David White is a principal at Right Environments, a New York architectural firm, and an assistant professor at Parsons.

You’re not getting the airflow you think you’re getting

Most builders have no idea whether the ventilation equipment they install is working properly. Many builders assume, for example, that a bathroom exhaust fan rated at 110 cfm is moving 110 cfm of air. It almost certainly isn’t, as anyone who bothers to measure their fans’ performance soon learns.

A few years ago, blogger Allison Bailes recounted the tale of a builder who installed nine bathroom exhaust fans, each rated at 110 cfm. While the builder assumed that each of these fans would be able to move at least 50 cfm — less than half of their rated airflow — it turned out that only five of the nine of the fans achieved that fairly modest goal. (One fan barely limped over the finish line — it was tested at 51 cfm.)

When tested, four of the nine fans — the ones moving between 30 and 46 cfm — were particularly pathetic.

What about a kitchen range hood fan rated at 400 cfm? At high speed, do you think it’s actually moving 400 cfm? Think again. According to Brennan, when you test an installed 400-cfm range hood fan, you’re likely to find that it’s actually moving between 150 and 300 cfm of air.

So why aren’t these fans moving as much air as builders assume? The answer is simple: because they are connected to ducts. If the ducts have a large diameter, no elbows, and are extremely short, they won’t introduce much static pressure to the system, so the fan airflow might be fairly close to the fan rating.

Oops! This exhaust duct decided to make a U-turn...

However, most ducts have elbows; many ducts are long; some are leaky; many are convoluted; and some have the wrong diameter. Under these conditions, the fans are fighting a lot of static pressure, and the resulting airflows are low.

You have to measure airflows

So what’s the moral of the story? If you care about ventilation performance, you have to measure the airflow of your ventilation equipment.

As Brennan puts it, “It’s good to test — to figure out what you really have and to see whether the system is working. If you want to get beyond faith-based ventilation, you have to measure stuff.”

In today's tight homes, a ventilation system isn't optional

In the 1970s, progressive builders — those interested in energy efficiency — learned about the importance of airtight construction. Inspired by these insights, they began to build very tight houses. Unfortunately, many of these builders didn’t yet realize that a tight home needs a mechanical ventilation system.

Brennan recalled, “We learned that if you build a tight building in a cold climate without a mechanical ventilation system, you get high RH and condensation problems. The first phone call is, ‘Our windows are all fogged up.’ The solution is to dilute the contaminant — in this case, the water vapor. That’s what we do with unavoidable contaminants: We dilute them.”

Brennan made a few general comments on residential ventilation. “A ventilation system is more than a fan; it’s a system,” he said. He also noted that common sense doesn’t always lead to the best ventilation rates: “We are more likely to ventilate for odors that are harmless than we are to ventilate for contaminants that are dangerous but don’t smell.”

Brennan reported that skeptical builders often ask him, “You mean you want me to air-seal the heck out of this building, and then you say we have to ventilate it?” The answer is yes: as Brennan pointed out, “Leaky homes have uneven levels of air exchange through the seasons.”

Instead of a leaky home, you want a tight home equipped with a fan that has a predictable air flow rate.

Two types of ventilation systems

Brennan tells builders that most homes need two types of ventilation equipment. He calls a home’s main ventilation system the “dilution ventilation system,” since its chief purpose is to provide a measured amount of fresh outdoor air in order to dilute any contaminants. (For more information on this topic, see “Designing a Good Ventilation System.”) Typical examples of this type of ventilation system include a heat-recovery ventilator (HRV), a central-fan-integrated supply ventilation system (or, as Brennan describes this type of system, “outside air ducted to the return side of the air handling unit”), or an exhaust-only ventilationMechanical ventilation system in which one or more fans are used to exhaust air from a house and make-up air is supplied passively. Exhaust-only ventilation creates slight depressurization of the home; its impact on vented gas appliances should be considered. system (using a bathroom exhaust fan or fans operating for 24 hours a day or controlled by a timer).

Homes also require a different kind of ventilation: namely, local exhaust fans. Most homes have one exhaust fan in each bathroom as well as one in the kitchen range hood. Brennan noted that some appliances include additional examples of local exhaust systems, even though they aren’t usually thought of that way: “Clothes dryers and chimneys for combustion appliances are also local exhaust systems.”

These local exhaust fans control contaminants at the site where the contaminants are produced. Brennan noted, “Whenever possible, you want to collect contaminants near the source. Source control makes more sense than dilution.”

Range hood fan advice

Brennan emphasized the importance of using a range hood exhaust fan when cooking. “The kitchen range is the biggest stationary source of fine particulates in a residential setting,” he said. “Pyrolizing and burning of the food is what is making most of the particulates and contaminants. Breathing fine particulates is bad for you. Fine particles are three times worse than the next worst contaminant commonly found in homes.”

He continued, “Here’s my guidance on kitchen range hoods: Keep the hood low, with less than 24 inches between the cooktop and range hood. The hood should be at least 20 inches deep.”

He also urged listeners to check out the results of research on range hoods performed by Brett Singer at Lawrence Berkeley National Labs. Here are some links to more information:

Bathroom exhaust fans

Brennan provided the following tips on bathroom exhaust fans:

  • The exhaust grille should be mounted in the ceiling or high on a wall (within 6 inches of the ceiling).
  • If you want any residual moisture in your bathroom to dry quickly after a shower, remember to leave the bathroom door open after your shower.
  • To reduce the chance of mold, paint bathroom ceilings with mold-resistant paint containing zinc oxide.
  • Homeowners who want a bathroom exhaust fan that is capable of keeping their mirror clear of condensation should install a fan that moves at least 80 cfm. (Research has shown that most bathrooms need a fan that moves 80 cfm to prevent condensation on a bathroom mirror.) This type of exhaust fan exacts an energy penalty, of course, so homeowners who don’t care about foggy mirrors may be satisfied with a less powerful fan.
  • Brennan said, “I like bathroom fans that run continuously at 20 cfm. You need a fan that has a boost to high speed — say, to 80 cfm — with the boost controlled by a motion sensor or light switch.”
  • Brennan also said, “I like remote-mounted exhaust fans. If possible, put the exhaust fan in the basement and vent it through the rim joist.”

For more information on this topic, see Bathroom Exhaust Fans.

HRV advice

Brennan provided the following tips for HRVs and ERVs:

  • Choose an HRV or ERVEnergy-recovery ventilator. The part of a balanced ventilation system that captures water vapor and heat from one airstream to condition another. In cold climates, water vapor captured from the outgoing airstream by ERVs can humidify incoming air. In hot-humid climates, ERVs can help maintain (but not reduce) the interior relative humidity as outside air is conditioned by the ERV. with quiet, low-watt fans.
  • Locate the fresh air supply registers where they won’t blow on someone: for example, in bedroom closets, or aimed across a ceiling.
  • Locate the exhaust grilles in bathrooms, laundries, and in the “general kitchen area” (although not near the range), since these areas are sources of moisture or contaminants.
  • Use dedicated ductwork for at least the supply or the exhaust — ideally both. Don’t use the furnace or AC ducts for both the supply and exhaust, no matter how far apart you put the ducts.
  • Install a MERV 8 or MERV 13 filter between the outdoor air duct and the HRV or ERV.
  • Make sure that the outdoor air intake is located where the air quality isn’t contaminated by car exhaust, flue gases, or hot roofing.
  • The best duct systems use rigid (galvanized) ducts for supply, and galvanized or PVC ducts for exhaust; flex duct should only be used for short runs.

At the BuildingEnergy 16 conference, David White demonstrated a variety of techniques for measuring ventilation system air flow rates.

Measuring airflows

At their presentation in Boston, Brennan and White demonstrated several methods used to measure airflow. (Most of the methods they discussed are also explained in a useful YouTube video produced by Corbett Lunsford.)

A full treatise on airflow measurement techniques is beyond the scope of this article. Readers who are interested in learning more on the topic should read a GBA article called “Simple Methods for Measuring Air Flow.”

Another valuable online resource that GBA readers may wish to read is an article called

Brennan and White used an exhaust fan connected to ductwork and elbows to demonstrate a variety of airflow measuring techniques, including the use of a flow hood. They also demonstrated the garbage bag test method discussed in “Simple Methods for Measuring Air Flow.” Some methods are more accurate than others, and it’s not unusual to find that different test methods yield different results. (The garbage bag method, for example, is fairly loosy-goosy.) At the Boston presentation, the different testing methods yielded results ranging from 70 cfm to 130 cfm.

Using a cardboard box and a manometer. Choose a cardboard box that is a little larger than the grille or register that you’ll need to cover. Remove the cardboard flaps, and cut a rectangular hole (start with a hole measuring about 20 square inches) in the side of the box opposite the open end. When you’re ready to perform the test, place the cardboard box over the intake grille or the register that you are measuring, and turn on the fan. Set up your manometer to measure the difference in pressure between the inside of the cardboard box and the room. For the most accurate results, adjust the size of the rectangular opening in the cardboard box so that the measured pressure difference is between 4 and 5 pascals. If the opening in the cardboard box is too large, it can be easily adjusted with ordinary gray duct tape. If you have a pressure difference of 4 pascals between the interior of the box and the room, the air flow in cfm will be twice the area in square inches of the hole in the box. (For an online tool to help you translate the pascal pressure difference to air flow in cfm, visit .) According to Brennan, “The cardboard box is usually more accurate than a $2,000 flow hood.”

Using an iris damper. Brennan is a fan of iris dampers that include taps designed to be connected to a manometer. These dampers need to be permanently installed in the ductwork, so this method of measuring air flow is usually only possible if an iris damper can be installed at the time of new construction. (In other words, this measurement technique is most appropriate for the homes of energy nerds.) “I always recommend that an iris damper be installed into the ventilation system if I have the authority to specify them,” Brennan said.

Iris dampers are adjustable. “You have an iris ring that can be opened or closed with a little wrench,” Brennan said. “The adjustments are labeled with numbers — 1, 2, 3, 4, 5. These dampers have ports — little rubber tubes — that you can tap into with your manometer. They allow you to measure the pressure drop across the sharp-edged orifice. Knowing the damper adjustment number and the pressure drop, you can look up the airflow in cfm on a chart.”

Manufacturers of iris dampers include Flaktwoods (which makes the SuperLite iris damper) and . A 6-inch Fantech iris damper can be purchased for about $47.

Pitot tube traverse measurements. According to Brennan, this air flow measurement technique “requires you to make measurements at many, many locations. It is tedious and annoying.”

Using a Duct BlasterCalibrated air-flow measurement system developed to test the airtightness of forced-air duct systems. All outlets for the duct system, except for the one attached to the duct blaster, are sealed off and the system is either pressurized or depressurized; the work needed by the fan to maintain a given pressure difference provides a measure of duct leakage. as an exhaust vent flow meter. This technique is used for rooftop measurements on multi-family buildings with low-slope (flat) roofs. You need a box that is open on the bottom, and you need to be able to attach the Duct Blaster to the top of the box. The box has weatherstripping at the bottom (the open end of the box); it must be large enough to install over the exhaust duct termination that penetrates the roof. When the exhaust fan that you are trying to measure is running, measure the pressure difference between the interior of the box and the outdoors. Adjust the Duct Blaster fan until the pressure difference is zero to get an accurate measurement of the total flow through the building's exhaust system.

Sitting in the audience at the Boston conference, Kohta Ueno raised his hand and commented that this method of measuring air flow is “difficult when it is windy.” Brennan agreed, and said, “Yes, when it’s windy, roof air pressures can be all over the place. My way around that is to average several readings taken over a long time.”

Measuring air flow through kitchen range hoods

Brennan noted, “It’s not easy to measure flow rates from kitchen range hoods. In homes where the grille comes out of the wall or the roof, you may be able to make a Duct Blaster-type measurement. In some cases, you can stretch duct wrap over the bottom of the range hood if it is new. But you might not be able to do that if there is a lot of grease. To measure the pressure drop across the wrap, start with a big hole, and then cover the hole up until you get to 4 pascals. That’s a pretty good pressure for a measurement.

“In some homes, you can block off the kitchen and set up a blower door in the kitchen door. You need to seal off the range hood. Then you can determine the pressure difference that the range hood induces off the blower door frame. If you have a tight house, a house that is 150 cfm50, you can do a blower door test, and then cycle fans on and off. In that case, the house is your test box. You can also use that method to determine the air flow rate of the clothes dryer and the range hood.”

More testing tips

Here are some additional airflow measuring tips shared by Brennan and White:

  • If your ventilation fan has more than one speed, you should make airflow measurements at each speed.
  • It’s a good idea to measure the pressure drop across the fan to determine the fan curve.
  • To determine how much power the fan is using, use a watt meter to measure power draw.
  • Always perform a worst-case depressurizationSituation that occurs within a house when the indoor air pressure is lower than that outdoors. Exhaust fans, including bath and kitchen fans, or a clothes dryer can cause depressurization, and it may in turn cause back drafting as well as increased levels of radon within the home. test; it’s important to verify that there is no spillage from the home’s combustion equipment.
  • In addition to measuring airflow, you may want to measure the fan’s sound level. Quieter systems are better than noisy systems (because homeowners often disable noisy equipment).

When for testing sound levels, Brennan advised, remember that “a fan shouldn’t be louder than a refrigerator. Refrigerator manufacturers have learned that a refrigerator can’t be louder than 50 or 55 dB about 3 feet from the refrigerator. You can’t sell one if it is louder.”

Brennan advised that in some cases, it makes sense to create a pressure map of the house. He advised, “Measure the pressure drop across the enclosure to determine the impact of the ventilation equipment on the pressure relationships in the house. Then close the bedroom doors. Are there signs of crazy pressures being produced by the ventilation system?”

If you're suffering from information overload...

Many residential builders may not want to buy a digital manometer, or may lack the confidence to perform their own airflow testing. That's fine, as long as you're working with an energy rater or energy consultant who can perform the testing for you.

Martin Holladay’s previous blog: “Managing Lead Paint Hazards.”

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Image Credits:

  1. Image #1: Martin Holladay

Apr 15, 2016 8:56 AM ET

Measure air flow and then?
by Adam W

1. I like the idea of testing (though I'm sure I'll get some amount of push back from the contractors). You mention "most ducts have elbows; many ducts are long; some are leaky; many are convoluted; and some have the wrong diameter." - that makes sense. But if we go ahead and do the tests and find that we're not getting the ventilation that we expect - what practical options do we have at that point? Rip out the ducting?

2. Can an HRV exhaust in a bathroom replace the need/requirement for a dedicated bathroom fan? If so - what CFM would be required? Is it ok for the humidity of a shower to be pulled into the HRV?

Apr 15, 2016 9:43 AM ET

Response to Adam Wride
by Martin Holladay

Q. "If we go ahead and do the tests and find that we're not getting the ventilation that we expect - what practical options do we have at that point?"

A. The options are the same as for any other discovery that reveals a construction error. Basically, you can either (a) ignore the problem, or (b) fix it.

I discussed possible fixes in my 2014 article, Bathroom Exhaust Fans:

"Let’s say that your fan is hooked up to a funky duct system, and it’s only pulling 35 cfm. There are two solutions to this problem: you can swap out the 110 cfm fan for a more powerful model — say, a 200 cfm fan — or you can fix all the duct problems. (A good duct system has a large diameter, smooth walls, and few elbows.) Either approach will work, but the latter approach is preferable — because the system will use less energy and will be quieter.

"Fan makers have begun to respond to reports that builders are failing the 50 cfm airflow test by offering more powerful fans. For example, ads for Panasonic’s EcoVent fan boast that the fan includes a booster switch that a builder can flip to ramp up the fan’s speed if the fan fails its airflow test. While that solution is easy for the builder, a better solution would be to fix the funky ductwork."

Apr 15, 2016 9:46 AM ET

Second response to Adam Wride
by Martin Holladay

Q. "Can an HRV exhaust in a bathroom replace the need/requirement for a dedicated bathroom fan? If so - what CFM would be required? Is it ok for the humidity of a shower to be pulled into the HRV?"

A. Answers to your questions can be found in my article, Does a Home with an HRV Also Need Bath Fans?

Apr 19, 2016 8:55 AM ET

Martin - thanks!
by Adam W

Thank you for pointing those out. I've found the index you built and now realize there is an article for just about everything here. Pretty incredible.

Apr 19, 2016 9:05 AM ET

Thanks, Adam
by Martin Holladay

I'm glad that you like the index.

If any GBA readers are wondering what Adam is talking about, it's this page: How To Do Everything.

Apr 20, 2016 2:14 PM ET

Edited Apr 20, 2016 2:17 PM ET.

Are range hoods improving?
by Derek Roff

Following the links in this article, and some links from those links, it looks like there was no satisfactory solution to cooking-produced pollution, when some of the articles were published, four years ago. The most effective range hood discussed was described as being too loud to hold normal conversation in the kitchen. Jumping forward to this current GBA article, "Brennan advised, remember that 'a fan shouldn’t be louder than a refrigerator.'" Does the market now offer any range hood product, that can come close to meeting Brennan's advice on fan loudness, while providing reasonable effectiveness in removing the pollutants from cooking?

Apr 20, 2016 2:30 PM ET

Response to Derek Roff
by Martin Holladay

I have forwarded your question to Terry Brennan. I also invite other GBA readers to respond.

Apr 20, 2016 11:18 PM ET

Rooftop Exhaust Fan Measurement
by Kohta Ueno

Using a Duct Blaster as an exhaust vent flow meter. This technique is used for rooftop measurements on multi-family buildings with low-slope (flat) roofs. You need a box that is open on the bottom, and you need to be able to attach the Duct Blaster to the top of the box. The box has weatherstripping at the bottom (the open end of the box); it must be large enough to install over the exhaust duct termination that penetrates the roof.

This Building America report shows the method of using a "flow box" for measuring rooftop exhaust fan flows ("BA-1209: Multifamily Ventilation Retrofit Strategies," ). The downside of the "flow box" is that you have basically built a lightweight box kite that's sitting on top of a multistory building roof. Losing your equipment off the roof due to wind gusts has the potential to be really embarrassing!

But more interestingly: this CEE report ("Multifamily Ventilation Assessment and Retrofit Guide," ) does a great job of summarizing several methods for measuring rooftop exhaust fan airflows, including inserting an Energy Conservatory TrueFlow into the "throat" of the rooftop exhaust. Lots of clever stuff from Jim Fitzgerald and colleagues.

Apr 21, 2016 4:25 AM ET

Response to Kohta Ueno
by Martin Holladay

Thanks for providing the links to two excellent online resources for those who need to measure ventilation system air flows in a multifamily building.

I've taken the liberty of posting a photo from the second resource you linked to (). Jim Fitzgerald knows a lot about ventilation.


Customized capture box with three Duct Blaster fans.jpg

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