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According to DOE’s Building America program data, typical, residential ductwork efficiency is about 67%. That means that if you install a 90% efficient furnace, your system efficiency would be just over 60%. Another way to look at this is that ducts lose 25–40% of the energy that moves through them. Just from a practical standpoint it makes sense to improve the delivery system. Shorter, straighter, better engineered and installed ductwork will improve delivery efficiency.
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The total leakage from the system is the total of all the leaks, those that are both inside and outside the conditioned space. The leakage to outside is the total of the leaks that are outside the conditioned space, such as leaks in ducts in an unconditioned basement, crawl space, or attic. Measuring leakage to outside can be accomplished by using the duct tester accompanied by a blower door to cause the pressure inside the conditioned space to equal the pressure in the ductwork. With equal pressure on both sides of a hole in the ductwork, air won’t move, due to the second law of thermodynamics: High pressure goes to low pressure. No pressure difference means no air movement.
There is some debate about whether the ducts should be pressurized or depressurized during testing. Tradition is on the side of pressurizing the ducts, but depressurizing the ducts puts less stress on the duct masking used to seal the grilles and registers, sucking the temporary sealant material more tightly rather than attempting to blow it off. The other advantage to depressurization testing is that if the leakage to outside must be tested, which requires the use of the blower door, the blower door is commonly set up to depressurize the house. The blower door fan won’t have to be turned around to match the depressurization of the HVAC ductwork. Table 1 shows measurements using two different duct blasters of total leakage and leakage to the outside using pressurization and depressurization. The numbers in the table reflect the performance of two different duct blasters in a 1,000-ft2 house with seven supplies and three returns that would have allowable code total leakage in Massachusetts of 119 CFM 25 and leakage to outside of 79 CFM 25.
Table 1. Depressurization versus Pressurization Testing
| Total Leakage (CFM 25) | Leakage to Outside (CFM 25) | |||
|---|---|---|---|---|
| Pressurize | Depressurize | Pressurize | Depressurize | |
| Duct Blaster | 140 | 140 | 55 | 54 |
| DU200 | 140 | 135 | 48 | 53 |
These are real house measurements and not scientific, in-the-lab measurements, but the two approaches—pressurization and depressurization—are close enough. There certainly are advantages to not blowing the tape off the grilles through pressurization. Depressurization lends itself to less costly approaches to duct masking, such as the use of food wrap.
In this column, I will review two duct-testing systems, the Energy Conservatory’s Duct Blaster and Retrotec’s DU200. Both systems are used in conjunction with dual-channel, digital manometers—the DG700 for the Duct Blaster and the DM2 for the DU200. I reviewed these two devices in an earlier article (see “Digital Manometers,” HE Jan/Feb ’11, p.12).
Both the Duct Blaster and the DU200 can be used to evaluate the leakage from HVAC ductwork. They can also be used as a powered flow hood to measure the flow through exhaust fans. For very tight houses, they can be used as blower doors. Here I will discuss their use for measuring duct leakage.
The Energy Conservatory (TEC) Duct Blaster
The Duct Blaster (whose name has been universally adopted as the label for the process of duct testing) is a conservative-appearing, black axial fan in a 14-inch diameter housing. The system comes in a black 26-inch x 14-inch x 14-inch carrying case. The fan weighs in at 7.8 lb, with one flow ring and the duct connector installed. Sitting on the floor, with the 12 feet of 10-inch flexible connector ductwork that comes with the system attached, it can move up to 1,250 CFM. The flow is reduced to 1,000 CFM with 50 Pa of back pressure.
Tip: When the flow through the HVAC ductwork exceeds approximately 300 CFM, the resistance of the air handler’s blower wheel can obscure the leakage on the supply side of the system.
Pressurizing. When the system is used to pressurize the HVAC ductwork, the fan is typically attached to a return grille, and all the other grilles and registers are blocked. The flow rings are used to adjust the flow range to match the HVAC system leakage (see Table 2).
Table 2. Adjusting Flow Range on the Duct Blaster
| Flow Ring | CFM Range |
|---|---|
| Open (No flow ring) | 600-1,500 |
| Ring 1 (A1) | 225-800 |
| Ring 2 (B2) | 90-300 |
| Ring 3 (C3) | 10-125 |
The DG700 should be set up for DB B (Duct Blaster Series B) and the Config should be set to match the flow ring that has been installed. The green hose should be inserted into the duct system and attached to the Channel A Input tap on the DG700. One end of the red hose should be attached to the Channel B Input tap and other end to the brass pressure tap on the Duct Blaster fan. The Energy Conservatory (TEC) recommends setting the Mode on the DG700 to PR/FL @25. In that mode, once the pressure gets reasonably close to 25 Pa, the flow reading will be displayed on Channel B on the manometer. If Channel B displays LO, a smaller flow ring should be installed, because not enough air is flowing past the stainless steel flow sensor on the inlet side of the fan. If the fan can’t reach 25 Pa when it is running at full speed, a larger ring should be installed.
If the fan reaches its maximum flow, it is a good idea to try to figure out why. The Duct Blaster with connector duct attached and on the floor, not connected to the HVAC system, and with no inlet rings attached, will blow approximately 1,070 CFM25. So if the flow is in that vicinity, that’s a pretty big hole! It may be that a grille or register was missed and is wide open or that the covering on a grille or register has blown off. It’s worth becoming familiar with running the Duct Blaster disconnected and wide open, so you can get a sense of when you might not have the HVAC system completely closed up. Lay the Duct Blaster out on the floor, attach the static-pressure probe to the end of the green hose, shove it up the Duct Blaster connector duct, and see what you get for flow with everything open.
When the system is connected to the HVAC ductwork and running at PR/FL @25, press the Units button three times on the DG700. The display on Channel B should read “EOA @25 in2.” This is the equivalent orifice leakage area in square inches at 25 Pa, which is an estimate of the sum of all the holes in the HVAC ductwork at that flow rate. TEC sells a calibration plate that can be attached to the end of the connector duct. The flow through the plate should be 103–109 CFM. It’s interesting to take the time to play with this known opening size and develop a feel for the EOA measurement by taping over half the hole or three-quarters of the hole and seeing how it responds to changes of flow.
The flow at the low end of the range is supposed to be 10 CFM25. I was only able to get it down to 14 CFM where it was not getting a steady reading, flashing back and forth between LO and a value. So for a very tight duct system, an alternative approach may be required. One approach would be to increase the flow and pressure to 50 Pa and divide the result in half for a 25 Pa reading. (Check with TEC on the accuracy of alternative approaches.)
Depressurizing. To use the Duct Blaster to depressurize the ductwork, the fan needs to be turned around. Since the sensor on the inlet side of the fan will then be upstream of the fan, the white foam flow straightener needs to be inserted into the duct connector. (I have witnessed people throwing the flow straighteners away, thinking they were packing material. They’re not!) One of the flow rings needs to be sandwiched between the duct connector and the Duct Blaster fan.
The red hose is attached to the brass fitting on the fan. The clear hose should be attached to the secondary flow ring on the duct fitting and connected to the Channel B Ref fitting on the DG700.
The flow rings attach to the Duct Blaster fan either with the edge connector that completely wraps around the edge of the fan, or with individual, 2½-inch sections of connector that attach to the quarter points around the edge of the fan. Either process can be awkward, especially when setting up for depressurization testing.
Although the high top end flow rate of the Duct Blaster is not really necessary for duct testing, it allows the Duct Blaster to be used as a small blower door, for some of the very tight houses that are being built these days.
Tip: When you are performing the leakage to outside test, the space that is considered “outside” needs to be truly connected to the outside. The Duct Blaster manual notes that “if ducts run through unconditioned spaces such as attics, garages or crawlspaces,” you should “open vents, access panels or doors between those spaces and the outside to eliminate pressure changes during the test procedure.”
The Duct Blaster Kit includes
Duct Blaster fan and system case
12-foot, 10-inch diameter Mylar connector duct
Grille attachment flange
Flow rings (3)
Power cord with speed control
Grille mask 8 inches wide
Door-mounting plate for speed control and manometer
White foam flow straightener
White foam fan stand
DG700 digital manometer with case
Long [How long?] clear hose
Static pressure robe
16-foot USB cable
6-inch pressure probe
10-foot red hose and 15-foot green hose
TECBlast Duct Airtightness Test software
Retrotec DU200 Duct Tester
The DU200 looks considerably different from the Duct Blaster. The housing is bright yellow and red, to match the DM2 manometer. It is a shiny, blow-molded, squarish housing. The housing with fan and instrumentation weighs 12.2 lb. The square housing keeps the fan from rolling when it is being used and lifts the opening of the fan slightly off the flat surface of the floor or table, easing the flow of air into the fan venturi.
The heart of the system is a backward-curved, motorized impeller that moves up to 600 CFM. On one side of the housing is the receptacle for the AC power connection, along with a power switch and a green power indicator light. On the other side are two Ethernet cable control receptacles, a speed control knob, green and yellow hose connections, and a green status LED. The hose connections are dual tapered, pressing on both the inside and the outside of the hose—unlike a barbed connector, which will tear up the inside of the hose.
The fan is accompanied by Retrotec’s DM2 manometer, which can be connected to the fan through the umbilical cabling that includes an Ethernet-type cable and yellow, green, and blue hoses. The red cable sleeve protects the hoses, keeping them together and limiting crushing. The green and yellow hoses can be connected to the two corresponding hose connections on the fan and the two corresponding yellow and green hose connections on the DM2 manometer (Channel B Ref and Input). The blue hose should be connected to the blue Channel A Input and inserted into the HVAC duct system.
The Device setting on DM2 should be set to DU200 and the Range Config should be set to the correct flow ring (see Table 3). The Mode can be [should be?] set to PrA/Flow. If the yellow control cable is connected to the fan, the flow can be set to 25 Pa by pressing the Set Pressure control and punching in 25 and Enter. The fan will then automatically seek to pressurize (or depressurize) the HVAC ductwork to 25 Pa. If the control cable is not connected to the fan, the Status LED on the fan will blink, and the speed can be set manually, using the knob on the fan. The knob must be rotated completely counterclockwise to activate it. (There is a slight, electronic click when it gets to the end point.) This keeps the fan from roaring to life if the power cord is plugged in with the speed control knob at an intermediate setting.
Table 3. Adjusting Flow Range on the DU200
| Flow Ring | CFM Range |
|---|---|
| Open (No flow ring) | 100-700 |
| Mid | 25-300 |
| Low | 7-48 |
Pressing the @ Pressure button on the DM2 during the test will display a calculated flow at 25 Pa, even if the pressure is not exactly 25 Pa. Pressing the Mode button can set the Channel B display to EqLA, which will correspond to the approximate size of the total of the holes in the duct system. This is quite close to the DG700’s EOA. These readings can give you a good idea of the holes that you would be looking to seal.
(Note: I was able to use Retrotec’s calibration plate to observe the EqLA readings. The full-size opening is 18.6 square inches. I taped over half of it and got an EqLA reading of 9 square inches. Reducing it to one-quarter, I got a reading of 4.7 square inches. Half again read 2.3 square inches; and half again, 1.2 square inches. So the system was reading a composite total hole that was only 1.2 square inches. That would be a pretty tight duct system. By taping over half of the inside of the fan’s Low flow ring opening, I could have approximated an even lower flow by dividing the manometer reading by 2.)
Using the DU200 for either pressurizing or depressurizing the HVAC ductwork is a simple matter of turning the fan around. The flow rings are changed by releasing the Velcro straps on either side of the flow ring, or by pivoting the two white clips on the smallest ring to release it.
The Retrotec DU200 Kit includes
Model 200 fan with control and power panel and system case
Mid- and Low-range flow rings
12½-foot, 10-inch diameter plasticized fabric connector ductwork
Grille attachment flange
Power cord
Umbilical connector including hoses (3) and Ethernet-type control cable
Grille mask 12 inches x 160 feet, high stick, white
DM-2 digital manometer with case
Rechargeable batteries NiMH (4)
9V power supply (to charge batteries)
Tubing accessory kit, including straight tube, static pressure probe, T-connectors (2), splices (2), assembled T, and 35-foot lengths of blue, green, yellow, and red tubing
Conclusions
The Duct Blaster and the Retrotec DU200 are both excellent products. (For a summary of features for both devices, see Table 4.) Both have available accessories that can expand the functionality of the device, from Retrotec’s flow hood to TEC’s filter grille attachment. If your kit of analysis tools already includes Energy Conservatory gear, the Duct Blaster will round it out nicely. It works well with the DG700, and the controls and setup will be familiar. I am not fond of the process for changing flow rings or setting the system up for depressurization testing, but that is a small annoyance. TEC’s documentation is very informative and worth reading, even if you invest in a Retrotec DU200.
On the other hand, if you are already working with a Retrotec blower door, the DU200 will be a good complement to your gear for duct testing. The DM2 is an extremely versatile manometer that can provide lots of information, but it takes some work to learn it. (The DM2 will also work with the Duct Blaster.) The DU200 is simple to use for either pressurization or depressurization of the HVAC ductwork. The lower top end flow is not a significant limitation for testing residential ductwork. I enjoy reading a clear written description, so I sometimes find the dominantly graphic style of the Retrotec documentation confusing.
Table 4. Duct Blaster and DU200 Features
| Duct Blaster w/DG700 Manometer | DU200 w/DM2 Manometer | |
|---|---|---|
| Fan weight w/1 flow ring | 8.8 lb | 12.2 lb |
| Sound level at 4ft at 700 CFM | 74 dBa | 70.4 dBa |
| Minimum airflow | 10 CFM | 7 CFM |
| Maximum airflow (w/ duct connector) | 1,000 CFM | 700 CFM |
| Price | $1,875.00 | $1,850.00 |
Contact the author at paul.raymer@heysol.com.
Reprinted by permission of Home Energy magazine.
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