Measuring Mechanical Ventilation Airflow

January 03, 2012
January/February 2012
A version of this article appears in the January/February 2012 issue of Home Energy Magazine.
Click here to read more articles about Tools of the Trade

If the air in a house were a smoky brown or green and we could actually see particles floating around in it, and if it suddenly became crystal clear when the ventilation system was turned on, it would be a lot easier to convince people that mechanical ventilation is necessary! As the house gets tighter, the effectiveness of the ventilation system becomes increasingly critical. It's critical because people need air to breathe, and we have all sorts of chemicals in our homes that may diminish indoor air quality (IAQ). Cleaners and perfumes, cooked food, and materials that off-gas formaldehyde, such as some furniture, add things to the air that we're better off not breathing.

Paul Raymer
is chief investigator of Heyoka Solutions, a company he cofounded in 2006. He has multiple BPI certifications and is a HERS Rater.

That is why building codes and standards require ventilation systems in homes. There are numerous approaches to residential ventilation, but all of them rely on moving fresh air from the outside of the house to the inside of the house, and assume that the air outside is cleaner than the air inside. Although leaky walls and designed passive-ventilation systems can provide the needed ventilation in some cases, we are concerned here with fan-forced systems and airflow that is relatively constant.

Table 1. Devices Tested
Table 1. Devices Tested
Table 2. Test Results
Table 2. Test Results

The mechanical ventilation systems we are concerned with move a relatively small amount of air, and that makes testing their flows difficult. Small variations in flow will have big impact on the measurement. And since the fans are moving air from the outside to the inside and back to the outside, walls and doors get in the path of the air. For example, closing a bathroom door can have a significant impact on the flow through a bathroom fan. And then there are additional complexities to consider, such as the building's altitude, and the humidity level and temperature of the air (see "Airflow Variables").

The only way you can tell how much air the system is moving is by testing it. The Home Ventilating Institute (HVI) provides independent, thorough, repeatable laboratory testing of most ventilation products. During testing, HVI will generate a flow curve that indicates the fan performance at different pressures and different ducting configurations.

Airflow Variables

Airflow is influenced by temperature, humidity, and altitude. Vane anemometers like the Testo 417 are influenced less by these variables than are hot-wire anemometers or pressure-measuring devices like the Alnor LoFlo Balometer. The user manual for the Balometer explains how to adjust the unit to provide correction factors for standard flow rate to actual flow rate.

Using a powered measuring device compensates for the measuring device's restriction of the airflow. For this analysis, for example, I used a duct tester fan blowing into the fan being tested. When the pressure between the two fans equals 0 Pa, the airflow through the duct tester equals the flow through the fan under test, and the airflow can be read on the manometer attached to the duct tester.

I compared the performance of three measuring devices by testing three different types of systems: an exterior-mounted fan, a typical ceiling-mounted bath fan, and an inline fan (see Table 1). This was not a laboratory test but a practical one. The fans measured are installed fans in actual bathrooms, with duct-work running to an outdoor termination. The location was very close to sea level, with 92% relative humidity and 83°F air temperature. Results of the test are summarized in Table 2.

I believe the results of my testing indicate that the industry needs to add a tolerance component to our codes and standards. Otherwise the tendency will be to install a more powerful fan. The ASHRAE Standard 62.2 currently offers no tolerance percentage, but since it is a minimum-ventilation standard, slightly oversizing the fan would not be a bad approach. (For more on ASHRAE 62.2, see "Weatherization Ventilation," HE Mar/Apr '10, p. 44.)

Alnor 6200 Flow Hood
Alnor 6200 Flow Hood
Alnor 6200 flow hood measures the airflow exhausted by a remote mounted, inline fan. (Paul Raymer)
Minneapolis Exhaust Fan Flow Meter and DG700
Minneapolis Exhaust Fan Flow Meter and DG700Measuring the airflow with a Minneapolis Exhaust Fan Flow Meter and DG700. (Paul Raymer)
Testo 417 Vane Anemometer
Testo 417 Vane AnemometerTesto 417 vane anemometer mounted on the small funnel from the funnel kit. (Paul Raymer)
Duct Tester/Powered Fow Hood
Duct Tester/Powered Fow Hood
Duct tester as a powered flow hood. Note duct stretching up to fan. Top number is the pressure in the duct, and the bottom number is the airflow. (Paul Raymer)

Exhaust Fan Flow Meter from the Energy Conservatory

The Exhaust Fan Flow Meter (EXH) is very similar to a pressure pan, but it is deeper and includes an adjustable door or opening with a soft rubber gasket around the edge that will seal the opening of the EXH to the surface around the fan or grille. On its own it doesn't tell you much. It must be connected to a manometer. If you key into the manometer the size of the opening and the pressure difference across it, the manometer can calculate the airflow.

The EXH is calibrated to the Energy Conservatory (TEC) DG-700 digital manometer, but it will work with any digital manometer. You can use the EXH with other manometers by reading the pressure across the opening and referring to the table attached to the side of the EXH housing. The numbers on the chart correspond to the manometer's pressure readings.

The maximum exhaust flow that can be accurately measured with the EXH is 124 CFM, which is generally acceptable for most residential ventilation systems. The EXH measures only exhaust flows, not supply flows. It comes with a Velcro pad that can be attached to a painter's pole, so you can reach up and hold the EXH over a fan's ceiling grille. Accuracy with the TEC manometer described above is ±10%. The EXH costs $135?not all that bad, even if you add a dedicated DG700 manometer for another $825. If you already have a digital manometer, it's a downright bargain. The EXH is the most versatile of the three types of system that I tested.

Alnor 6200D LoFlo Balometer

The LoFlo Balometer is a dedicated flow-measuring device consisting of a formed plastic base with handles on the sides, a digital display, and a fabric flow hood that mounts on the top. Inside the base is a sampling manifold that has 16 evenly spaced sensing ports used to average the flow, which is then displayed on the digital meter as a flow reading in ft3/min (cubic feet per minute), m3/hr (cubic meters per hour), or l/s (liters per second). The Balometer can also be used without the flow hood for outlets that are less than 13 inches in diameter with flow rates of less than 150 CFM.

The Balometer can be used to measure airflow in supply and exhaust systems. The direction of flow is indicated on the graphic control panel. The flow range can be selected manually or automatically. The ranges in cubic feet per minute are 0-25, 0-50, 0-100, 0-250, 0-500, and 0-1,000. The flange of the base and the flange of the fabric hood each terminate with a ½-inch-thick, soft foam rubber gasket to seal the hood to the surface around the fan or grille. There are four different models with hoods of different heights. The 6200D has the biggest opening at the top of the fabric (23 inches x 23 inches) and the tallest hood, giving the unit an overall height of 34 ½ inches.

The display screen has a digital readout of the flow along with a bar graph that simulates an analog dial, with a pointer going up and down to indicate flow level. The screen also has indicators for the direction of flow, with squiggly arrows pointing up for return flow and down for supply flow. It also displays a low-battery indication if the Balometer is in MANUAL range mode. There is a red button on the base that can be pushed to freeze the reading.

Getting the Balometer ready to use is simple if you don't need the fabric hood. It is a bit of a struggle, however, to get the hood mounted on the Balometer, and once you have done so, the Balometer is somewhat less than portable. It should not be stored assembled; it must be broken down and put back in the bags to protect it from damage.

The Balometer should be used in the configuration that most closely matches the size of the diffuser or grille. Using a 23-inch x 23-inch hood on a 10-inch x 10-inch diffuser may cause substantial "recirculation regions on the sides of the fabric hood and cause 'odd' airflow patterns as it passes over the manifold," according to the user manual. The Balometer turns on with a slide switch on the right side of the display. Select the RET flow direction with the push button, and cover the fan opening with the Balometer. In the AUTO range mode, the system will seek the optimum scale for the flow being measured.

The Balometer has operable vents that allow the unit to measure higher flows and work with larger diffusers. The VENT CLOSED mode should be used for measurements for airflows of 10-150 CFM. Airflows of less than 8 CFM will be displayed as 0.

The Alnor LoFlo Balometer is a dedicated airflow-measuring tool. It makes accurate readings, and it can be used for testing and balancing the higher airflows in HVAC systems. If you're testing a lot of HVAC systems and/or ventilation systems, the Balometer is a good choice.

When the 417 is used with the funnel kit accessory, the unit must be set to the FACT mode. In that mode, the volume calculation is automatic. The maximum accurate flow measurement with the funnels is 75 CFM, due to turbulence caused by the funnels. When the FACT mode is turned off, the actual area of the grille or register being measured should be entered so that the 417 can calculate the volume of flow or CFM. By entering the area of a grille and selecting a mean reading, a continuous traverse can be made that will calculate a high volume of air (up to 99,999 m3/h or 58,899 CFM). This can be very useful in measuring the performance of HVAC systems. (See "Measuring Flow Traverse.")

Measuring Flow Traverse

A traverse means calculating the mean of a series of airflow readings across the face of the grille. The clear opening of the grille must also be determined by calculating the total area minus the area blocked by the vanes in the grille. Both the area and the airflow are used to calculate the volume of the airflow.

The funnel kit comes with two formed plastic funnels, each with a handle on one side and a mounting impression that the 417 snaps into, with a lock to keep it in place. The smaller funnel has a 7½-inch x 7½-inch opening and the larger funnel has a 13-inch x 13-inch opening. Snap the 417 into the appropriate funnel, turn the 417 on, make sure it is set to FACT/ON, select Vol, put it over the fan to be measured, and take a reading. If the grille is too big for the hood or the flow is too high, the 417 should be removed from the funnel, and the clear opening of the grille should be entered into the 417 and a traverse made across the grille opening, using the 417 to calculate a mean airflow. Calculating the clear opening of the grille is the most difficult part of this process. The open area will be the total area less the area blocked by the vanes in the grille.

The Testo 417 with the funnel kit provides an accurate reading of the low flows in small ventilation systems. Without the funnels, it can also be used to measure the flow volume of larger systems, as long as the clear opening is accurately measured, calculated, and entered into the 417. The 417 is most useful for ventilation systems if the flows are low and grilles are small enough to fit into one of the funnels. In those cases, it is the most accurate device of the bunch. It can also be used effectively for HVAC systems by performing traverses and calculating the area of the opening properly.

Final Thoughts

When I started working with fans and airflows, I thought this stuff was pretty simple. How hard could it be? Air is air. But air is different in different places at different times, temperatures, and humidity levels. The Canadians worked out a great process for measuring airflows using a garbage bag and a wire hanger (see "Air Flow Measurements in the Bag," HE Sept/Oct '02, p. 8). But we need accurate and repeatable numbers, for complying with codes and standards?not approximations.

learn more

"Verification of Performance and Testing," chapter 9 of my latest book on residential ventilation, is all about the verification of performance and testing. See Raymer, Paul. Residential Ventilation Handbook. New York: McGraw Hill, 2010. You can get the book at or other booksellers.

The use of a duct tester fan to measure airflow is described in "Using the Duct Blaster as a Powered Capture Hood." Minneapolis Duct Blaster Operation Manual, pp. 54-59. Minneapolis: Energy Conservatory, 2011.

There are other ventilation systems, such as heat recovery ventilators and energy recovery ventilators that have multiple supplies and returns. It may be possible to take flow readings at the exhaust and supply hoods on the exterior of the building with these systems, but the readings may be distorted by breezes and the irregular surface on the outside of the house. These systems should be balanced during installation, and if they don't have built-in air flow sensors, I recommend using Nailor Model 36FMS and Nailor Ampliflow air flow measuring devices.

There are ventilation systems that supply air to the return side of the air handler. Flows on these systems can be measured (with the air handler running) with a flow hood on the outside of the building or with a pitot tube inserted into the duct and connected to a digital manometer to convert the reading to fpm. This can then be converted to CFM by multiplying the measurement by the area of the duct.

There are many ways to deliver the required amount of ventilation airflow in a home. But without measuring the airflow, you won't know how the system is working before you start, or after you finish, fixing it.

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