Manual D is about comfort, not energy efficiency. Its requirement in LEED-H makes green certification too expensive to justify the benefit.
By Michael Chandler
I've been building solar and green for thirty years and 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 and it doesn't seem likely that I will unless I get a client who specifically requests the LEED-H program over the alternatives. The reason for this is that my homes use oversized ductwork with air flow controlled by butterfly dampers and the LEED-H program requires that airflow be controlled through implementation of 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 and 6" for medium sized rooms and 8" for larger rooms. In my market it’s 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). It 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 towards 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 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 issue 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 www.chandlerdesignbuild.com
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3 comments:
It's been a few years since you wrote this, do you still feel the same way about Manual D? If your HVAC guy is using software to generate Manual J then correctly set-up the Manual D calculations are done instantaneously, so not sure where the extra cost comes in...if he is using rule of thumb (which is what your 4, 6, and 8 inch rule sounds like - more than Manual D results) is that better? If he is caculating Manual D with a duct ruler/wheel, charts and spreadsheet then it will take more time. People do still dig trenches with a shovel, but most have moved to backhoes and excavators.
Beyond all that... is comfort not a part of green? If we have a house that is super energy efficent, durable, built of recycled products, etc. but uncomfortable to live in during the summer, will any one want to live in it?
I can make all houses energy efficient, if you ignore comfort, just turn the AC off on them all! Instant savings.
I think the point is that if you are going to install ducts, do it right. Lack of comfort results in complaints and unhappy owners. Do you want to have spent all the time and effort on a complete project and skip one part?
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