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Building Science

Not all duct design manuals are created green

Manual D is about comfort, not energy efficiency. Its requirement in LEED-H makes green certification too expensive to justify the benefit.

I’ve been building solar and green for 30 years. I have built homes that Energy Star certify at 76% more efficient than code and score gold on our North Carolina Green Building Program as well as NAHB’s green building program, but I have never built a house that would qualify for even basic LEED-H certification. It doesn’t seem likely that I will unless I get a client who specifically requests the LEED-H program over the alternatives. “LEED-H” stands for the United States Green Building Council’s Leadership in Energy and Environmental Design for Homes program. The reason why my homes don’t typically qualify is because they use oversize ductwork with airflow controlled by butterfly dampers. The LEED-H program requires that airflow be controlled through implementation of the Air Conditioning Contractors of America (ACCA) Manual D duct design.

What’s the difference between Manual D and Manual J?

There is some confusion in the market about the difference between Manual D, which sizes ducts to best match the equipment and needs of the rooms served, and Manual J, which sizes the equipment to match the actual projected load of the home (and is a basic minimum requirement of Energy Star and most green building standards, including the NAHB/ICC National Green Building Standard.)

The Manual D duct-design standard forces HVAC installers to use 4″ insulated flex for smaller rooms, 6″ for medium-size rooms, and 8″ for larger rooms. In my market its calculation and implementation adds significant cost to the HVAC system, especially on smaller, one-of-a-kind homes that LEED-H is hoping to encourage (the “top 25% of the most environmentally conscious builders” and all that). The Manual D standard is a good system and certainly worth rewarding but doesn’t really fit with the “mandatory minimum for green” in that it is more oriented toward optimizing comfort than saving energy, enhancing durability, or improving indoor air quality in the types of homes that would be reaching for LEED-H certification. It’s a comfort standard, not a green building standard.

The green home I’m building now won’t pass LEED-H

We’re building an aging-in-place home with a hybrid solar-propane radiant-floor heating & domestic hot-water system with a 15 SEER heat pump for AC and back-up heat that is better than 30% more efficient than code. The house scores gold in NAHB’s Model Green Home Building Guidelines and North Carolina’s Healthy Built Homes, but it will not qualify for basic LEED-H due to a “lack of comfort” in the AC design that will be used at most two months out of the year. If not for this requirement I think the house would likely be LEED-H Silver but I’m not going to pay to have the house scored when I know that it will fail because of this single requirement.

Seems like a missed opportunity to me.

NAHB is taking advantage of that opportunity

Last summer as we worked on the new NAHB-ICC National Green Building Standard the group discussed following LEED’s footsteps on this issue and decided that we shouldn’t disqualify a house for a green rating because the bathrooms and bedrooms might occasionally be slightly less comfortable than the living room. So we awarded points for Manual D implementation but didn’t make it mandatory. The goal is to step lightly on the planet, not to assure that everybody is optimally comfortable at all times regardless of the additional cost.

—Michael Chandler is a home builder and master plumber in Mebane, North Carolina. His website is


  1. Jay Walsh | | #1

    Manual D LEED Requirement
    Hi Michael,

    The LEED requirement for Manual D calculations were included in the pilot version of the LEED-H Checklist and were dropped in the current (post pilot) version. While using Manual D for commercial projects is standard practice, its application to residential, especially small residential projects was really overkill.

    I'm not sure I follow your logic about sizing duct work with Manual D "that it is more oriented toward optimizing comfort than saving energy". If a person is not comfortable in a room (hot or cold) due to the under sizing or over sizing of the duct work they will turn up (or down) the thermostat - how is that being energy efficient. The proper sizing of the duct work should provide both comfort and energy efficiency and should always be designed to supply the proper amount of conditioned air to (as well return this air to the air handler) the rooms they serve. The proper sizing of supplies and returns also ensures performance and life of the air handler (goes to durability of the system).

    In the next paragraph you speak about the heating and cooling system of your project and that it "will not qualify for basic LEED-H due to a "lack of comfort" ". Could you explain this further how your systems "will not qualify", because there is no criteria in LEED for "lack of comfort". Comfort is a subjective term and not a specific measurement, nor is this term referenced in any of the LEED-H criteria I am aware of.

    Jay Walsh
    Energy Star and LEED-H Rater


    Definition of "lack of comfort"
    What I mean by "more oriented toward optimizing comfort than saving energy" is most that definitions of "comfort" per Environments for Living and manual D equate it with a situation where all areas of the home are within 3-5 degrees of the set point on the thermostat.

    My house is designed so the core of the house meets this criteria, living, dining, kitchen, kids rooms and office. But the master bedroom, guest bedroom, sun room and workshop are not held to the same standard and are often just left unconditioned or under conditioned.

    I'm willing to deliberately "sacrifice" comfort for energy efficiency. My customers have asked fairly often to have less heating and cooling directed to their bedrooms so they don't have to feel bad about sleeping with the windows open.

    I'm perhaps a bit extreme as I have no AC at all in my bedroom and work shop, radiant heat only, and I live in North Carolina which some consider to be a hot and humid place. I do have passive cooling and I tend to leave all twelve windows open from early spring to mid fall. Not that radical actually.

    I was not aware that LEED-h had dropped the Man-D requirement. Glad to hear it.

    All the best

  3. Expert Member
    CARL SEVILLE | | #3

    Manual D
    As far as I can tell from the LEED for Homes program, Manual D or its equivalent is a prerequisite for certification. Here is the exact wording: "Room by Room Load Calculations. Perform design calculations (using ACCA Manuals J and D, the ASHRAE Handbook of Fundamentals, or an equivalent computation procedure) and install ducts accordingly". While there is some choice, the requirement is still there.

  4. Rodney Graham | | #4

    LEED-H Manual D
    We built a home under LEED-H. The manual D requirement is a bit unclear. Yes, EQ6.1 does say that a room-by-room load sizing using Manual J and D is required, but subsequent paragraphs state that only a Manual J is required (one of the outputs of a manual J is a room-by-room sizing), and the reference guide goes on to say that while a Manual D is strongly recommended, it may be more than in necessary in some situations. So, the USGBC is itself a bit unclear on this topic.

  5. Scott Nichols | | #5

    LEEDH manual D
    I would agree with Carl where you have some flexibility on using the complete manual D program or an equivalent procedure (abridged version ) the requirement still exist in the current LEEDH version.

  6. Whetstone Green | | #6

    Manual D is basic
    I can't believe what I'm reading here. Unfortunately, Michael's opinion is apparently shared by most residential mechanical contractors, who prefer to design ducts by rules of thumb, without regard to friction loss calculations. Moreover, few take the time (15 minutes) to verify their systems operate at an acceptable external static pressure.

    Every home's duct system should be designed according to Manual D procedures, or equivalent. It's basic. The reason why LEED (and Energy Star 2011) require compliance is because in NEW construction, poorly designed and installed duct systems arguably account for more energy waste and certainly more comfort complaints than any other single factor.

    I'm sorry, but I don't buy the argument that following Manual D is too expensive. Are you talking about the time it takes to run a D calculation, or the additional cost to build a duct system that works?

    As for your specific issues with Manual D, you obviously are unfamiliar with the procedure. The fact is, it allows great flexibility in how one achieves a good result. Furthermore, it makes no such requirements as the ones you cite. (I say this with some authority. You'll find my name in the front of the book.)

    The latest edition of Manual D even clears up a common misconception about oversized ducts. And I quote: "There are scores of things to worry about when designing and installing a comfort system. Low velocity through a duct airway is not one of them" (Manual D 3rd Edition, pg 258)

    David Butler


    A clarification
    David, Thanks for writing.

    When I wrote this a couple of years ago I didn't mean to disparage Manual D so much as to say that it should be possible to build a green certified house without it. When I have asked my HVAC contractor what it would cost to add manual D he responds that a $12,000 system with J alone would be $15,000 with D. That may be his way of saying that he doesn't know how to do it but the fact remains that this is a very common situation with residential HVAC contractors who can barely implement manual J. My point is that it would be better to give extra points for implementing manual D than to mandate it and save the mandates for Manual J and other items that are more about efficiency and indoor air quality than comfort.

    But lets take a different approach.

    I live in a house (in North Carolina) with air conditioning in the living area and radiant heat only in the bedroom wing. How would I implement manual D in a house with whole sections missing duct work? We often have clients who, like me, are willing to sacrifice comfort for energy efficiency. And what of those who are building to the passive house standard where their heating load is small enough to be met by an electric coil in their Energy Recovery Ventilator? Do we want to exclude them from green certification because their homes don't have even heat distribution and comfort?

    My point is that manual D (and thanks for clearing up my misconception about over-sized ducts) should be a strategy that is recognized with additional points but that the omission of which should not be grounds for disqualification.

  8. Whetstone Green | | #8

    Manual D is basic
    Michael, you may be right about your hvac contractor not being familiar. In any case, I would never hire someone who quotes different prices for 'with D' and 'without D'. That's like saying, here's my price for doing it right, and here's my price for doing it the way I always do. I make a point to tell builders they (and their clients) are better off paying extra for someone who knows how to design and build good duct systems than to pay extra for high-SEER equipment (or most any other efficiency upgrade).

    BTW, if I understood your situation, your heat pump only supplies part of your home? No problem. Manual D could care less which rooms are conditioned with forced air and which are not. I can't speak for LEED, but I can't imagine the provider would have made an issue of that.

    You may be less concerned than others about getting air distribution right, but D is just as much about designing a duct system that won't restrict (reduce) airflow. Undersized and otherwise restricted duct systems create havoc with the refrigeration system. One problem I often see in higher end homes is a 5" media filter installed right up against the return opening. If contractors were running D's, they'd realize these new blowers don't have the 'overhead' to handle that much static drop. In most cases, it's necessary to flare out to a larger filter or side-by-side filters to increase the surface area of the filter. And this is before we even start talking about getting the right amount of air to the room.

    I offer this advice to builders who may be reading this thread: Contact the technical services manager at a couple of the major equipment distributors and ask each to provide the names of a couple of residential HVAC contractors who they believe are best qualified to design and build duct systems 'by the book'. Interview them and ask to see their manual D and Manual J paperwork for a couple of completed homes. Then have them bid on your next home. I recommend having a 3rd party specialist do the Manual J and Manual S (equipment selection). By stipulating the equipment, your bids will be apples-to-apples, and you'll likely save far more than the design fee.

    As for LEED and Energy Star 2011... I agree that making D mandatory may be a problem in small markets. On the other hand, this may be just what's needed to get more residential guys to get with the program. A good LEED provider who is proactive should be willing to help organize a Manual D class, perhaps in cooperation with one of the distributors. At a bare minimum, the LEED provider should make it crystal clear from the outset whether a D is required.

    There's so much more I could say about HVAC for high performance homes... If you're really interested, I recommend my "Elephant" paper, posted on my home page.

    David Butler

  9. GBA Editor
    Martin Holladay | | #9

    An interesting dialog
    Michael and David,
    Your interesting dialog raises several questions. Michael's experience shows that in many regions of the country, HVAC contractors are a long ways away from implementing very basic sections of the building code.

    In the 2006 IRC, section M1601.1 requires that "Duct systems serving heating, cooling, and ventilation equipment shall be fabricated in accordance with the provisions of this section and ACCA Manual D or other approved methods." Of course building inspectors can (but rarely do) request documentation of Manual D compliance.

    Michael: what "other approved method" does your HVAC designer use if he disdains Manual D? Since Manual D compliance isn't that complicated, I wonder what "other approved method" he uses that can possibly save him $3,000 in his bid.

    The intent of the code is for HVAC contractors to perform the required design calculations, not to avoid them. He can choose other approved software, or a pencil and paper and calculator, if he wishes, but he can't legally avoid the calculations. Using a paper and pencil and a calculator is not $3,000 cheaper than using Manual D.

  10. MICHAEL CHANDLER | | #10

    Great dialogue - so long as I'm okay with being proven wrong
    The inspectors here in NC aren't even requiring Manual J documentation, let alone D and S, and I'd say that less than 10% of the installations are properly sized and sealed and that the duct design is virtually non-existant. Finding a mechanical contractor who would be willing to "do it by the book" would limit me to two or three of the very best and most expensive guys in the area

    Our "other approved method" is to provide jumper ducts in the bedrooms and adequate returns to assure good air flow back to the air handler and to use a third party engineer to do our manual J separately from the HVAC contractor and use a flow hood and butterfly dampers to balance things out as close to the room by room cfm specs as possible from the J. This works so long as you have access to the trunk line take-offs in the crawlspaces to adjust the dampers but generally doesn't work for slab houses, which is a lot of what I do and where I eat crow.

    As far as the $3,000 cost, that's just his way of saying no, but if I switch to the contractors who are sophisticated enough to implement manual D then the price goes up about that amount anyway. (one of the local guys who is smart enough to implement manual D is the brother of the guy I've been using and is stunningly expensive in comparison.)

    We focus on envelope design and passive solar w/ exterior shading so it seems that a high priced heating and cooling system is not as good investment of limited money as more foam in the roof. I do like putting in multiple 4" filters in the returns but have stopped doing that recently as we've moved to zoned bypass systems for better dehumidification. I'll likely pick that up again as well as raise the manual D question on the next house.

    By the way, I was in Austin Texas a couple years ago and saw two side by side Manual Js for a 3,500 sf house documenting need for 13 tons of cooling! This for an Energy Star project, spray foam insulated on a shady lot.

  11. GBA Editor
    Martin Holladay | | #11

    About that "other approved method"
    About that "other approved method" — which you describe as,"to provide jumper ducts in the bedrooms and adequate returns to assure good air flow back to the air handler and to use a third party engineer to do our manual J separately from the HVAC contractor and use a flow hood and butterfly dampers to balance things out as close to the room by room cfm specs as possible from the J."

    Your description does not include a method of duct sizing. So, how big is your main trunk? What size takeoffs are used? What duct sizes are run to each register? How does your contractor figure out how many registers per room? How does the trunk size change as each takeoff reduces airflow in the trunk?

    Obviously, there are a lot of ways to do this -- including "I've always used one 6-inch-round duct to each register, and this bedroom looks like it needs two registers" -- but I wonder what method your HVAC contractor uses.

  12. GBA Editor
    Martin Holladay | | #12

    How we did it in the 1970s
    In 1976 I got a job at Shepard Corporation, a plumbing and heating wholesale outfit in St. Johnsbury, Vermont. I worked behind the counter, and occasionally drove the truck, delivering boilers and furnaces and water heaters and copper tubing all over the north country.

    I also did all of the heat-loss calculations for all of our customers. I designed the hydronic and forced air systems, and gave each customer a complete materials takeoff. I worked off of a crude sketch made by one of the salesman. The sketch showed the sizes of the rooms and the size of the windows.

    I used a paper-and-pencil system, using the "IBR calculation sheet." Actually, considering the fact that none of us had computers, the calculations were fairly sophisticated. I did a room-by-room heat loss calculation (this is northern Vermont, so there is no cooling). I entered different U-factors for the windows, walls, ceilings, and floors. I used ASHRAE Fundamentals to figure the R-values and U-factors of different wall assemblies. There was a crude default assumption for air infiltration. I ended up with a BTUH figure for each room, and a total for the whole building.

    For hot air systems, I used an "air duct caluclator" — basically a slide rule — that showed recommended duct sizes for different cfm requirements. The slide rule calculated friction per 100 ft. of duct, and proposed duct sizes.

  13. Whetstone Green | | #13

    T&B verification is best
    Yes, it is possible to achieve good results without the formalities of Manual D. Some old-timers who really understand air distribution have produced outstanding duct systems with no more than a laminated 'ductulator'. But how do you replicate that knowledge and experience? Sadly, the concept of apprentice, journey, master is a relic of a bygone era.

    Manual D is a relatively simple procedure that anyone can learn. Building a quality duct system is another skill entirely, and requires an experienced and disciplined crew. Contractors who want to build good duct systems have to pay their workers more. Of course, this means they get beat up in new construction. Most of the 'good guys' move to the service side of the business. That would all change if builders understood the value of quality HVAC. The problem is, a builder can't look at at a duct system and tell if it's good, or not. I encourage my builder clients to have at least one of their hvac sub's duct systems evaluated by a 3rd party T&B specialist.

    Speaking of which, you mentioned using flow hoods and dampers to balance the duct system. If I were a LEED provider, I would accept a *third party* T&B report in lieu of a Manual D report every day of the week. In fact, I'd much prefer to see LEED and Energy Star require independent T&B verification than to require a piece a D report. It really doesn't matter *how* you achieve the result. On the other hand, I doubt many residential guys can install a duct system that would pass muster without having done a D.

  14. Mike Bergen | | #14

    Send the hate mail to Dave Butler, he sent me the link
    I called Dave about an unrelated item and he mentioned this discussion. I must say everyone is really polite and Isee no evidence of name calling, so I thought I would add my 2 cents.
    Dave and I both served on the recent review and revision of Man D. Don't be impressed, we are still as qualified or unqualified to offer some thoughts as before this "honor".The value in the D committee was hearing some of the deliberation on some of these topics.
    D is the place where several tables of Equivelent Fitting values are published for typical duct,and fittings are foud. The person who mentioned his expierence with hydronic system did the same process for designing water (yes air is a fluid) that we use in D. One difference is our air systems are expected to leak (somewhat) where it is tougher to explain that to the customer when the hydronic systems leaks even a little. What we have with an airhandler is a pump called a fan that moves that other fluid, air, through pipes we call ducts and cut holes in the pipes and let the air squirt out. It is a pressure thing that make it work! Once we picked the unit we can find out how much ESP (External Static Pressure we have to make the air move form the most difficult return grill to the most difficult supply register. We size the duct using a friction rate that will automaticly give us the round or rectangular size that will not exceed the pressure we have availible. If we make a mistake or we make some heroic field modification that adds resistance to that "DESIGN LEG then we don't have enough pressure to force the air out and therefore don't have the quanity of air that the desgn requires ( on that most extreme design day)
    This simple process is not unique to D or hydronics and IBR, it is what every contractor should understand so you get the quanity and temperature of the air you need to do the job is delievered
    Oh, by the way, every other supply "hit" needs to be dampered because they are closer to the high pressure fan than that most difficult register. If someone wanted to be really scientific, I quess they could install smaller/ higher resitacen duct to allow "self balancing" but I don't think anyone in their right mind would want to paint themselves into a corner if their calculations proved to be less than helpful.
    Notice that the returns are not dampered which leads to another thorny subject that we can talk about later, if you eyes haven't already glazed over.
    So that is the procedure, D only gives us the official design values for fittings so that there will be only one value and no confusion.
    Does that help the conversation? If not direct the hate mail to Dave Butler

    Regards, Mike Bergen, You have a friend in Philadelphia PA

  15. Wayne Grebe | | #15

    Manual D Realities
    Yes, this all has been a very interesting read. I'm a builder outside of Richmond, VA. Last year we completed the first LEED for Homes project in the Greater Richmond area. I can tell you from direct experience that getting a residential HVAC system designed using Manual D is no easy feat... in fact it's more akin to pulling a rabbit out of your hat.

    I literally spent months looking for a residential HVAC subcontractor that could perform Manual D for us. The reality is that that there is virtually nobody, at least on the residential side, doing it. Most of the HVAC subcontractors I have talked to are still using "rule of thumb" calcs of 1 ton of capacity for each 600-800 SF! I found one company up in Northern VA that peforms Manual D, but he would only perform the work for systems he installed, and I couldn't convince him to travel down. In the end I had to purchase the software from Elite and train myself.

    To further the point that very few HVAC contractors are performing the necessary calculations, I was recently contacted by a woman in Northern VA building a 13,000 SF home. The contractor that sized and installed the system, Harvey Hottel, complete with radiant floor, fresh air ventilation, and all geothermal, was unwilling to do the Manual D. They simply over-sized all of their ductwork. This woman came to me desperate because she is wanting to get LEED-H certified and needed to document the system with Manual D, as is definitely still required. Clearly there's a reason that even a well seasoned HVAC contractor that performs residential and commercial work avoids this type of work, however unclear that reason is.

    This brings up an interesting point though. In my experience it seems that though the USGBC requires the Manual D calculations, unless one is going for the optional points under EQ 6.3 (3rd Party Performance Testing) there is not technically a requirement that the system installed matches the system designed. This may be a responsibility of the Green Rater, but my sense is that the raters are very lenient in this area because they understand the tremendous lack of experience, knowledge and training that exists in the HVAC installer industry. My sense is that the USGBC is at a point where they are simply trying to promote the use of Manual D, but they're not out there really busting chops about its application.

    As a last note, referring to the fact that the current 2006 IRC requires the use of "Manual D or other approved methods." In my experience as a builder it's VERY clear to me that the last 4 words of that sentence leaves the door wide open for an inspector. They can, and very often do, use a very wide latitude in their interpretation of the accepted building code. Furthermore, I have heard stated by inspectors that in that particular sentence it specifies "fabricated", not "installed." In other words, they interpret this section to mean that the ductwork and equipment should be manufactured under those standards, but not necessarily installed that way.

    There's a VERY wide gap that still exists between the theoretical and actual installation of HVAC equipment.

  16. Bob Ellenberg | | #16

    Calculations vs Rule of Thumb
    In 2004 and 2005 I built 2 houses that were well sealed, heavily insulated (2x6 walls covered in 2" of polyiso to eliminate thermal bridging, etc.) and with conditioned crawl spaces. I had them built by a modular manufacturer that specializes in custom homes and they were both 2 story houses 3,500-4000. I searched the internet and found an engineering firm somewhere in the midwest that would spec out the HVAC system. My concern at that point was not the duct layout but that the systems would end up being oversized. However, they were very specific right down to the model # for each supply grille (and they were not all the same). The local HVAC contractor I hired to complete and connect the system said it was grossly undersized and simply wouldn't work. They worked beautifully, running all the time on the hottest days and the cost were way down compared to comprable sized houses being built in that area (Northern New Mexico). What we found to be a fantastic benefit to how we built the structure was how even the temperatures felt throughout the house and particularly the naturally heated floors in the winter over the conditioned crawl space. In the larger house the upstairs was marginal on hot days. We added inline booster fans in the upstairs supply that had a main power switch that could be shut down in the winter and that helped a lot. I do not remember who the engineering company was (and those records are in storage). I sent them plans and specs. and they called and asked alot of questions about how well we were following details on sealing, etc. They charged by the SF and if I remember correctly it was about $300-$400, but I'm not sure.

    I haven't built anything new since then but have purchased and renovated 4 houses. In two of them I modified the HVAC systems and replaced them in the other two (hot humid climate). The HVAC contractor I am using has been doing it all of his life (74 and going strong) and does everything by the seat of his pants. He is unbelievably good, and everything he has done has worked perfectly. I am about to build something new again but they will be small single story houses and I think paying someone to run the duct calculations will be a waste of my money (when I did it in NM they were large 2 stories with complicated duct work). We intend to put the equipment in the center and do a radial system in the crawl with straight runs. My biggest challenge is getting my HVAC contractor to size the equipment small enough.

  17. MICHAEL CHANDLER | | #17

    I agree that on the new small single story homes the duct design may be non-essential, but you still will get value by having your Energy Star guy or the engineering form you mentioned do the manual J in order to get the air flow per room and the overall sizing for your HVAC guy to use.

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