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Duct Leakage Testing

For an energy-efficient forced-air system, seal seams with mastic and test with a Duct Blaster

Posted on Feb 19 2010 by Martin Holladay, GBA Advisor

For years, Americans who would never put up with leaky plumbing pipes have been willing to accept leaky ducts. While water damage is hard to ignore, the damage caused by leaky ducts is more subtle. Yet leaky ducts not only waste huge amounts of energy — they can also lead to comfort complaints, moisture problems, mold, and rot.

Most green certification programs require builders to pay attention to duct tightness. Now that duct testing requirements are starting to appear in some local building codes, more and more builders are asking questions about the ins and outs of duct leakage testing.

Duct basics

If you’ve never had to worry about duct tightness before, you may want to read the “Forced Air” section of the Encyclopedia.

Most green builders already know their duct basics:

  • Duct leaks are very common; in many homes, duct leaks are responsible for significant energy losses.
  • For ducts located in an unconditioned attic, any leaks in the supply system tend to depressurize a house, while return-system leaks tend to pressurize a house. Either condition can cause problems.
  • Duct leaks outside of a home’s thermal envelope waste more energy than duct leaks inside a home’s thermal envelope.
  • Even if ducts are located inside of a home’s thermal envelope, duct leaks can still connect to the outdoors. For example, supply system leaks in a ceiling between the first and second floors of a two-story home can pressurize the joist bay, forcing conditioned air outdoors through cracks in the rim joist area.
  • It’s much easier to seal duct seams during new construction than in an existing house.

Characteristics of a good duct system

A good duct system:

  • Has been designed to meet ACCA Manual D requirements, with each duct carefully sized to provide the airflow needed to meet room-by-room heat loss and heat gainIncrease in the amount of heat in a space, including heat transferred from outside (in the form of solar radiation) and heat generated within by people, lights, mechanical systems, and other sources. See heat loss. calculations;
  • Has been designed so that duct runs are as short and straight as possible;
  • Does not use building cavities (for example, panned joists or stud bays) as ducts;
  • Locates all ducts within the home’s thermal envelope;
  • Includes ducts or air paths that allow return air to flow back to the air handler from every room with a supply register;
  • Has all seams sealed with duct mastic; and
  • Has been tested for duct leakage.

Code requirements for duct sealing

Although model codes have included duct-sealing requirements for years, enforcement has been spotty or nonexistent. For example, a 2001 study of 80 new homes in Fort Collins, Colorado, found that the number of homes that complied with code duct-tightness requirements was zero. Astonishingly, the average duct leakage in the studied homes was 75% of total system airflow.

Another 2001 study found that Massachusetts Energy Code requirements for duct sealing were widely ignored. Researchers who inspected 186 new Massachusetts homes reported that “serious problems were found in the quality of duct sealing in about 80% of these houses.”

The 2006 International Residential Code (IRCInternational Residential Code. The one- and two-family dwelling model building code copyrighted by the International Code Council. The IRC is meant to be a stand-alone code compatible with the three national building codes—the Building Officials and Code Administrators (BOCA) National code, the Southern Building Code Congress International (SBCCI) code and the International Conference of Building Officials (ICBO) code.) requires (in section N1103.2.2) that “Ducts, air handlers, filter boxes and building cavities used as ducts shall be sealed.” Elsewhere, in section M1601.3.1, the IRC requires that “Joints of duct systems shall be made substantially airtight by means of tapes, mastics, gasketing or other approved closure systems.” Hardware-store duct tape is not an approved tape.

Mandatory testing

Builders will soon need to get up to speed on duct testing, since recent code changes will require that all residential duct systems except those that are located entirely within a home’s thermal envelope will need to be tested for leakage.

If some ducts are outside of the thermal envelope, the 2009 IRC will require duct tightness to be verified by either a “rough-in test” or a “post-construction test.” Either test requires all register boots to be taped or otherwise sealed during the test.

The threshold for the rough-in test is total duct system leakage of 6 cfm per 100 square feet of conditioned floor area (when tested at 25 Pascals). If the air handler is not installed, the total leakage must be less than or equal to 4 cfm per 100 square feet of conditioned floor area.

The threshold for the post-construction test is duct system leakage to outdoors of 8 cubic feet per minute (cfm) per 100 square feet of conditioned floor area when tested at 25 Pascals. Alternatively, total duct system leakage must be less than or equal to 12 cfm per 100 square feet of conditioned floor area.

Testing for duct leakage

Energy auditors have developed several methods for testing duct tightness. These methods vary from fast and dirty to time-consuming and accurate. Builders interested in tight duct systems should familiarize themselves with the range of available duct testing options:

  • Using only a blower door;
  • Using a blower door and a pressure pan;
  • 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.;
  • Using a Duct Blaster and a blower door; and
  • Using a theatrical fog machine.

A fast, rough test

In Residential Energy, authors John Krigger and Chris Dorsi describe a quick (but not particularly accurate) method for estimating duct leakage: “The simplest way to estimate duct leakage in cfm50 is to perform two blower-door tests: one with the home’s registers sealed with paper and tape, and one without. Subtracting the two readings provides a very rough estimate of total duct leakage.”

Because of the inherent inaccuracies of this method, it is rarely used.

The pressure-pan method

A pressure pan is a diagnostic tool consisting of a metal pan (similar to a cake pan) connected by a tube to a manometer (that is, a pressure gauge). The device is used to temporarily cover a forced-air register to measure the pressure exerted on the pan by a blower door.

To conduct a pressure-pan test, you need a pressure pan and a blower door. Here are the basic steps:

  • A blower door is used to depressurize the home to 50 Pascals.
  • The air handler fan is turned off.
  • The tester then blocks each register (one at a time) with the pressure pan and records the reading of the pressure-pan manometer. (The manometer shows the pressure created by air leaking into the duct system.) Typical readings of the duct system pressure (with respect to the house pressure) range from 1 Pascal to 45 Pascals.
  • The higher the reading, the leakier the duct run.

In a Home Energy magazine article, “Pressure Pans: New Uses and Old Fundamentals” (January/February 1998), Jeffrey Siegel and Bruce Manclark explain, “A duct system at 0 Pa is entirely within the pressure envelope of the house and has no leaks to the outdoors. A system approaching 50 Pa is essentially outside the pressure envelope, meaning that it has catastrophic leakage to the outdoors.”

It’s important to note that the pressure pan readings don’t really provide measurements of duct leakage; rather, they provide a method for comparing the relative leakiness of several duct runs in the same home.

That hasn’t prevented some energy experts from recommending thresholds for pressure-pan measurements. According to Krigger and Dorsi, “Registers of newly installed ducts should read less than 0.5 Pascals and existing duct registers should read less than 1 Pascal after being sealed.”

In “Duct Improvement in the Northwest,” (Home Energy magazine, January/February 1996), author Ted Haskell provides this advice for existing homes: “Houses with fewer than three pressure-pan readings above 2 Pa are unlikely to be cost-effective to seal.”

The pressure-pan test has several virtues. First, it is fast — especially when an auditor is already conducting a blower-door test. Second, it identifies which of a home’s duct runs are the leakiest, so that a contractor knows where to focus duct-sealing efforts.

However, many experts warn contractors not to jump to conclusions based only on the pressure readings recorded during a pressure-pan test.

As with many other diagnostic tests — infrared scanners come to mind — it takes an experienced auditor to interpret the results of a pressure-pan test. “Pressure-pan readings are difficult to interpret, and the same number can reflect quite different leakage rates in different houses,” wrote Siegel and Manclark. “The disadvantage of the pressure-pan test is that it is more art than science. … The one exception is when homes have very similar duct geometry and installation, as is the case with manufactured homes or identical homes in a subdivision.”

The Duct Blaster test

The most common method for testing the tightness of a duct system is the duct-blower test (also known as the Duct Blaster test). A duct blower resembles a miniature blower door; the most common brand is the Duct Blaster, manufactured by the Energy Conservatory in Minneapolis.

Here are the steps:

  • All supply registers and return grilles are sealed with polyethylene and tape.
  • The air handler fan is turned off.
  • The Duct Blaster is set up and attached to the duct system (near the furnace or at a large return-air grille).
  • The manometer’s reference probe is inserted into the air handler plenum.
  • The Duct Blaster is turned on to pressurize the duct system to 25 Pascals (a pressure which represents typical operating pressures for forced-air systems). The airflow through the Duct Blaster fan (which is displayed in cfm on the Duct Blaster’s manometer) equals the flow escaping through leaks in the duct system. The results are reported as “cfm @ 25 Pascals” or “cfm25.”

Although this test reveals the leakiness of the duct system, it doesn’t tell the tester where the leaks are located; nor does it quantify what percentage of the leakage is leakage to the outdoors. Moreover, it doesn’t tell us how much a duct system leaks under normal operating conditions — conditions which may differ from Duct Blaster pressurization to 25 Pascals.

According to Krigger and Dorsi, “Leakage ranges from less than 50 cfm25 for a fairly tight duct system to more than 500 cfm25 for a very leaky duct system.”

Builders hoping to comply with the 2009 IRC duct-testing requirements will need Duct Blaster test results showing total duct leakage equal to or less than either 6 or 12 cfm (depending on whether it is a rough-in test or a post-construction test) per 100 square feet of conditioned floor area.

Using a Duct Blaster to test duct leakage to the outdoors

Energy auditors often want to know how much of a duct system’s leakage is leakage to the outdoors. Leakage to the outdoors can occur when air escapes through a leak in a duct installed in an unconditioned attic. It is also possible for a portion of the leakage through a duct that seems to be within a home’s thermal envelope to turn out to be leakage to the outdoors, since such leaks can pressurize joist bays, forcing conditioned air through rim-joist cracks.

One quick-and-dirty indication that a duct system has sizable leaks to the outdoors occurs when an auditor notices obvious air flow from forced-air registers during a blower-door test. Since the house is strongly depressurized, the airflow represents exterior air; and since it’s coming from the duct system, the airflow is a sign that the duct system has leaks that connect with the outdoors.

To determine how much duct leakage is leakage to the outdoors, a tester needs a blower door and a Duct Blaster. Here are the steps:

  • A blower door is set up in an entry door.
  • All supply registers and return grilles are sealed with polyethylene and tape.
  • The air handler fan is turned off.
  • A pressure tap is temporarily installed in the duct system to measure the pressure of the duct system with respect to the house.
  • Another manometer or tube is set up to measure the outside pressure with respect to the ducts.
  • The Duct Blaster is set up and attached to the duct system (usually near the furnace).
  • The blower door is turned on and the house is pressurized to 25 Pascals.
  • The Duct Blaster is turned on to pressurize the duct system; the Duct Blaster fan is adjusted until there is no pressure difference between the ducts and the house. At that point, all of the air going through the Duct Blaster is going outdoors through duct leaks. The airflow indicated on the Duct Blaster’s manometer (in cfm) quantifies that duct leakage to the outdoors.

Obviously, duct leaks to the outdoors represent heating or cooling energy that is lost.

Testing ducts with a fog machine

To find the location of duct leaks, nothing beats a theatrical fog machine.

Gary Nelson, the founder of the Energy Conservatory, describes the method: “You tape up all the registers and you pressurize the ducts. Then you introduce fog into the Duct Blaster — you aim the fog nozzle at the fan blades, without letting the fog get drawn into the vent holes in the motor, and you watch where the fog pours out. Sometimes you may be working with an HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. contractor who says, ‘This is a good duct system. This is the way we have always done it. This is normal.’ Well, when you show them the fog coming out of the leaks, they shut up really fast.”

For more information on this technique, see “Pinpointing Leaks With a Fog Machine.”

Sealing the leaks that matter most

The mechanics of duct sealing are beyond the scope of this article, but it’s worth noting:

  • Duct seams need to be mechanically fastened (using sheet-metal screws for galvanized ducts and compression straps for flex duct) before being sealed.
  • For sealing most duct leaks, mastic works better than any tape. (Bruce Manclark calls tape “the band-aid of the HVAC industry.”)
  • Mastic is messy, so wear old clothes when you apply it.
  • Install mastic “as thick as a nickel.”
  • Cracks or seams wider than 1/8 inch need to be repaired with fiberglass mesh as well as mastic.

It’s important to prioritize duct-sealing efforts so that the most important leaks are addressed first. As Philip Fairey, the deputy director of the Florida Solar Energy Center, likes to say, “Duct leakage is like real estate — it’s all about location, location, location.”

  • In existing homes, it’s surprisingly common to find disconnected duct components — takeoffs that are coming loose from ducts or ducts disconnected from register boots — in attics or basements. Such disconnected ducts can waste tremendous amounts of energy.
  • Leaks connected to the outdoors are more important than leaks inside the home’s thermal envelope.
  • Holes that see high pressures — in other words, holes near the air handler — are more important than distant holes that see relatively low pressures. Bruce Manclark’s mantra is, “Follow the pressure: boots for show, plenums for dough.”
  • Most furnaces have many bad leaks close to the blower fan, including leaks in the furnace jacket seams, leaks between the furnace and the plenums, and leaks between the duct takeoffs and the plenums.
  • Supply system leaks waste more energy than return system leaks.

For more information on sealing duct leaks, see Duct Tape and Mastic.

Measuring air flow

Anyone who commissions a duct system needs to learn how to measure airflow at registers and grilles. Manufacturers offer an array of accurate (and expensive) instruments to measure airflow. However, builders who need to troubleshoot problems may be interested in several low-cost methods of measuring airflow:

  • The August 2002 issue of Energy Design Update describes how to build a homemade flow hood using a cardboard box and a $90 digital anemometer.
  • Two Lawrence Berkeley National Laboratory engineers, Iain Walker and Craig Wray, have written a paper describing a method of measuring airflow with a “calibrated” laundry basket and a manometer.
  • Terry Brennan promotes a method of measuring bath exhaust fan airflow with a cardboard box and a credit card.
  • The easiest way to measure airflow at a supply register is the garbage bag technique developed by Don Fugler of the Canada Mortgage and Housing Corporation.

Duct testing is coming to your job site — soon

For decades, plumbers have routinely tested newly installed supply and drain pipes. Meanwhile, most HVAC contractors have gotten away with leaky, untested duct systems. However, the tide is now turning. In the future, testing residential duct systems for leaks will become a routine part of residential construction.

Last week’s blog: “Energy-Efficient Garage Doors.”

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Image Credits:

  1. Residential Energy Builders Guide, University of Kentucky
  2. Florida Solar Energy Center
Fri, 02/19/2010 - 16:03

Wall Stud Cavity Returns--Retrofit
by Jan Juran

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Hi Martin: many thanks for your thorough summary of the important issue of duct leakage. I've sealed (with mastic) and insulated the ducts in my circa-1988 (cold climate location) house. However the returns utilize interior wall stud cavities from the (high up on the walls) intake grills down into the basement where they connect to the return trunk duct. Result=leakage in the returns; worse, we periodically may be inhaling day old spider breath. Are you aware of any product/solution which can seal or retrofit a flexible "sleeve" or "lining" duct-equivalent into a vertical stud cavity inside an interior wall? I'm reluctant to tear out large sections of my house's interior walls to this end. Many thanks.

Fri, 02/19/2010 - 17:43

Wall stud cavities for returns
by Martin Holladay, GBA Advisor

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For new construction (at least here in Vermont, where galvanized ductwork is the norm), we use 3 1/4 inch by 12 inch or 3 1/4 inch by 14 inch "wall stack" duct for 2x4 wall cavities. It's roughly equivalent in cross-sectional area to a 6-inch round duct, so it has limited capacity — equivalent to a single residential register.

I don't know of a retrofit solution. GBA readers -- anyone done this? Is there a solution?

You should calculate the necessary airflows for your return ducts -- it's certainly possible that your returns are undersized. If they're undersized, the solution is to bite the bullet and build new chases, properly sized, that enclose new ducts with sealed seams. That will make your rooms a little smaller, of course.

Sun, 02/21/2010 - 12:27

Another perspective
by Gavin Healy

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Hi Martin,

Thanks for putting this out there. In some of your past articles you discus standards beyond code which as I am sure you would agree is a bare minimum standard. I agree that most are falling significantly short of meeting reasonable duct leakage standards. I think it's important to emphasize a couple of performance factors that don't recieve enough attention when we are talking about ducts and duct sealing. Some areas measure against a nominal air flow of 400 CFM per ton which rarely exists (in our test typical is 56% of nominal airflow), and like you sugest with the Colarado study the resulting ductleakage is significalty higher that what is reported. I think the IRC again shoots for too loose a standard by allowing additional leakage as square footage increases, are those larger buildings allowed to leak more from thier plumbing systems as well? We are trainging build performance contractors on the West Coast that the ducts are better off in the envelope, if they are outside the envelope the sytems should leak less than 5% measured air flow not to exceed 50 CFM25 regardless of system size. Most of our systems come in at less than 20 CFM25. Since our ducts are located in the attic this a target we achieve on all of our retrofit systems. I realize that ducts located in the conditioned space are bit of a differnent story, but there are still consequences to the pressure loss assocaited with those ducts being more leaky. The other piece that is too often ignored is that when we seal poorly designed ducts the result is an increas in static pressure, often to the point where variable speed fans are experiencing premature failure. I think that when ever duct test is completed a coresponding static pressure test should occur so that we are looking at the whole system. In the early adoptor states like California the duct sealing of existing systems has with airhanders designed to push 2000CFM and ducts only capable of distributing 1000CFM has led to static pressures in exess of 150% of the manufactures limits. On this forum I think we should describe where and how the code is a bare minimum, what it's shortcomings are, and some ideas for truely golds standards. What is cost effective tommorrow will not be the same as what is cost effective today. If this changes when will we go back to further tighten and inaccessible duct system. You did touch on these things, and I am not writting to correct you, instead merely adding a regional point of view. Keep up the exceptional work!

Sun, 02/21/2010 - 14:34

duct leakage based on sf conditioned area in weak
by Whetstone Green

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Great article, Martin. I don't know how many times I've heard HVAC contractors claim they build tight ducts, yet they don't own a Duct Blaster. I ask how do they know? Even the best crews sometimes make mistakes. You can't see air leakage. Of course, the only way to know for sure is to QA your installation crews by conducting leakage tests.

All mechanical contractors should conduct leakage and static pressure test-outs on every system they install. Unfortunately, it’s easier to sell high-SEER equipment than high efficiency *systems*, a distinction invariably lost in a competitive marketplace. I try to convince every builder I meet that they (and their clients) are better off paying a little extra for a quality duct system than paying a lot extra for high SEER equipment.

Sun, 02/21/2010 - 15:22

opps! wrong subject
by Whetstone Green

Helpful? 1

The subject line of my previous comment obviously has nothing to do with the comment. I was going to comment on the validity of using percentage of floor area as the duct leakage standard. I guess I should finish that thought...

As the article points out, Energy Star and most other efficiency programs base duct leakage standards on conditioned floor area (Title 24 being the notable exception). That's a weak method. Leakage standards should be based on nominal fan flow. Otherwise the standard becomes a very low hurdle for efficient homes with right-sized HVAC equipment. It's not unusual to see homes exceeding 1500 sf per ton. At 6%, this translates to 90 CFM(25) per ton, or 23% of nominal fan flow -- a low hurdle indeed!

According to Mike Barcik, an early advocate for the floor area method, this method was selected for ease of calculation, as raters don't necessarily know tonnage of AC. If this rationale was ever valid, it certainly holds no water today. Since 2006, raters have been required to verify a proper load was done and equipment is sized to that load. For the purposes of this calculation, one can assume nominal fan flow is 400 CFM per ton. I've also heard the argument that using nominal fan flow encourages HVAC contractors to oversize the equipment. I won't even dignify that argument with a response.

Energy Star 2011's answer is to reduce requirement to 4% of floor area for outside leakage, but still allow 6% total leakage. I tried to get them to change, but I'm afraid the floor area method is here to stay.

David Butler

Mon, 02/22/2010 - 00:34

cfm25 to floor area
by Allison A. Bailes III, PhD

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David, you're right that 6% duct leakage based on floor area represents a low bar for high performance homes with right-sized equipment. That doesn't mean, however, that using air handler flow as the measure is the way to go.

I work with Mike Barcik a lot, having taught 17 HERS classes with him over the past two years, and he's never said using floor area as the measure was for ease of calculation, as you say. The real reason is that when you use floor area, you have a valid comparison from one house to another. The amount of ductwork--and thus the amount of potential leakage sites--depends more on the size of the house than on the system.

Also, if a house has a 2 ton cooling load, for example, and a 4 ton AC, it ends up with half the duct leakage (when expressed as a ratio of cfm25 to air handler flow) that it should have, merely because of the oversized AC. I don't know that any HVAC contractors out there are oversizing just for that reason, but the fact that the ratio changes just by changing equipment--even when the ducts aren't touched at all--means that it's not the best way to talk about duct leakage.

I think ENERGY STAR has it right in going with the cfm25 to conditioned floor area ratio. Getting that number as low as possible is the goal. Really, though, the best way to eliminate duct leakage is to bring them into the conditioned space.

Allison A. Bailes III, PhD

Sat, 02/27/2010 - 17:57

I don't buy it
by Whetstone Green

Helpful? 1

...the fact that the ratio changes just by changing equipment--even when the ducts aren't touched at all--means that it's not the best way to talk about duct leakage. The amount of ductwork--and thus the amount of potential leakage sites--depends more on the size of the house than on the system.

Many factors affect leakage - in particular, the number of duct connections and terminations. However, if the run-outs are flex, the overall duct length is almost irrelevant from a leakage perspective. All other things being equal, a 4 ton system can be expected to leak roughly twice as much as a 2 ton system. For a given house, an oversized 4 ton system compared to a right-sized 2 ton system will have larger cabinet and plenums, and larger (and/or more) collars, boots and other fittings. This means a reasonably tight standard pegged to floor area will require little or no effort for high performance homes with right-sized HVAC.

For a mastic-sealed system, 10% of fan flow is considered a low hurdle (see below). Now consider a well insulated 3000 SF home with a 2 ton heat pump. Using the floor area method, even 2% is a low hurdle in terms of the impact on overall HVAC performance (7.5% of system capacity lost to leakage, depending on location of leaks). But if we apply that same standard to a 3000 SF home with a typically oversized system, say 4 tons, even a conscientious HVAC crew would have trouble complying. And we haven't even talked about how the floor area method necessarily creates a climate zone bias.

In the commercial market, duct leakage standards (established by SMACNA) are based on duct surface area rather than fan flow. But here there are two important differences: 1) the SMACNA standard was developed for fabricated ducts, which are far more prevalent in the commercial sector, and 2) with commercial, there's a separate leakage standard for cabinets.

I submitted a comment on the duct leakage metric to Energy Star (e: 2011 draft program requirements). In its response, Energy Star said it previously followed the fan flow method (prior to 2006) and argued that it "...promoted increases in fan size and associated airflow, which minimized savings." Whoever wrote this obviously doesn't understand that "nominal" fan flow is based on condenser tonnage, not blower capacity. No contractor would ever select a larger fan to 'game the system' since 1) it would not change the prescribed leakage limit, and 2) a larger fan would actually *increase* duct leakage. This response was nothing more than 'arm waving'. Considering this response, and the "ease of calculation" explanation (conveyed to me by Barcik in 2006), I've yet to see a rational justification for the floor area method.

Consider this: Prior to RESNET's about-face (in the lead-up to Energy Star 2006), virtually every residential duct leakage standard was based on nominal or design fan flow. Other notable examples include:
* Energy Star's Advanced New Homes Construction BOP
(3% of fan flow, ducts inside conditioned space)
* California Title 24
(6% of fan flow, performance method for new construction)
* Building America - Performance Analysis Procedures
(referencing 1998 IECC definition of tight duct system as < 5% of rated fan flow)
* ACCA Standard 5 - HVAC Quality Installation Specification
(10% of fan flow, total leakage)
* Texas Residential Standard Offer Program (electric utilities)
(10% of fan flow, total leakage)

Finally, I have been involved in or have reviewed a number of utility-driven residential "AC tune up" programs over the years and all of those that included duct testing used nominal fan flow as the reference. When selecting a quality assurance metric, it's important that it correlate with the level of effort or skill required to achieve the desired result. Residential duct leakage correlates far better with fan flow than with floor area. This makes it a more appropriate metric for doing "QA" on HVAC installation crews.

Energy Star's decision to switch to the floor area method has led to many other standards and programs adopting following suit (including the 2009 IECC), so it's apparently here to stay. Nevertheless, I recommend we hold installers to the fan-flow metric whenever possible, especially for homes with exceptionally small HVAC systems. A leakage target of 5% (to the outside) of nominal fan flow would be reasonable.

Regarding your last point -- just because ducts are inside the envelope doesn't eliminate concerns over duct leakage. But I know that you know that. In any case, I certainly agree with the sentiment of your comment.

David Butler

Fri, 03/12/2010 - 16:19

Do you need transfer grilles in bathrooms or laundry rooms?
by John Walls

Helpful? 0

Martin, your blog says that a good duct system "includes ducts or air paths that allow return air to flow back to the air handler from every room with a supply register"---sounds right to me. However, my HVAC sub says he never puts transfer grilles in bathrooms or laundry rooms (even though they have supply ducts) because they have vent fans. I think his philosophy is that any overpressure would flow through the vent fan duct to the outside---doesn't sound very efficient to me. My plan is to insist on transfer grilles. I would like your opinion. Note that this is in a HH climate (central Texas) with a GSHP and foam insulation (including roof rafters). The AHU, heat pump, and most of the ducts are in the semi-conditoned attic. This brings up another question---should I have transfer grilles to the attic (no supply or return currently planned for that space)?

Fri, 03/12/2010 - 16:43

Response to John Walls
by Martin Holladay, GBA Advisor

Helpful? -1

1. Many people omit the transfer grilles for bathrooms and laundry rooms because these small rooms have lower cfm requirements than bedrooms. Usually a door undercut is sufficient to avoid problems. However, if you want to install a transfer grille — assuming you have considered the noise transmission issue — go right ahead.

2. A semi-conditioned attic that is within the home's thermal envelope does not require a supply register or a return grille.

Sun, 03/14/2010 - 16:06

no transfer grills for attics!
by David Butler

Helpful? 1

All great questions, John!

I don't recommend a transfer grill for encapsulated attics, as this would be wasteful. In fact, I recommend reasonable efforts to air seal the ceiling (although many foam installers recommend not to air seal). Several years ago, I asked Ed Reeves at Icynene about this issue. He said when they sought recognition in the IRC for encapsulated attics (Sect 806.4), their models only considered *conducted* heat flow through the ceiling. My philosophy is that if direct conditioning or infiltration is needed to maintain non-condensing temperature, then there's not enough insulation on the roof. In any case, in a HH climate, winter condensation is a non-issue.

I agree with Martin regarding baths and laundries. Your contractor may be extrapolating from a rule that no direct returns be installed in baths, kitchens or laundries. You are correct in your efficiency concern. Here's some useful information that can help you decide how to handle:

According to Manual D 3rd Edition (Table A1-2), door undercuts can handle 2.1 CFM per square inch. So a 24" door with 1" undercut can handle 50 CFM, more than enough for a standard sized bath in a high performance home. The design load for a high performance home in central Texas is likely less than one ton per 1000 square feet, which equates to 0.4 CFM per square foot. Many of the loads I've done for high performance homes in similar climates are closer to 0.25 CFM per SF. A bath or laundry may be more or less than the house average, depending mostly on windows.

A transfer grill (or jumper duct) may be required in the following circumstances:
1) a large master bath and/or a bath with large window(s)
2) the HVAC system is oversized, pushing more air to room than undercut can handle
3) the Bath or laundry ducts are oversized relative to their load (but this can be fixed with damper or adjustable grill)

You can tell if you need a transfer grill by observing whether the vent flapper opens when HVAC blower is operating. If no flapper, as in the case of a down-facing outlet, attach a piece of confetti or similar on the grill.

David Butler

Sun, 03/28/2010 - 10:54

Attic moisture control with jump ducts---foamed house
by John Walls

Helpful? 0

Martin and Dave, thanks for the good advice. I do have one further concern regarding HVAC air flow to the attic. For foamed houses with semi-conditioned attics in a HH climate, don't you need some type of HVAC circulation in the attic to maintain a reasonable relative humidity (say 50%). If you have a leaky ceiling or put in a jump duct, you would get some circulation. If you have a well-sealed ceiling and only conducted heat transfer occurs between ceiling and attic (see Ed Reeves comment), then the attic temperature may be ok, but what about moisture control? It seems that in the hot and very humid summer, the attic would reach a steady state of high humdity if it doesn't get a little help from the HVAC system. Note that I have 2x6 rafters and knee walls with full thickness open cell foam insulation.

Sun, 03/28/2010 - 15:21

Semi-conditioned attics
by Martin Holladay, GBA Advisor

Helpful? 0

Researchers have looked into the question of whether there are any advantages to supplying conditioned air to (or having return air grilles in) semi-conditioned attics. The answer is no.

There are no advantages to supplying conditioned air. There are no downsides or problems in semi-conditioned attics without supply registers or return grilles. If you have a good air barrier that prevents exterior air from leaking into your attic, you won't have any humidity problems. Remember, the humidity is outside -- so seal it out.

Moreover, energy costs are higher in attics with supply registers.

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