As a HERS Rater who tests duct systems all over the San
Francisco bay area and the Silicon Valley, I confront system failure all too often.
Every day I see brand new heating and air conditioning systems that are woefully
sub-optimized and fail to comply with California’s 2013 energy code. When those
systems finally do pass they are usually skating just under the legal limit of
duct leakage and air flow for air conditioning.
Yesterday I attended a PG&E sponsored energy efficiency class,
Optimizing Residential HVAC System Performance. The class reminded me just how
much that we, as an industry, know about HVAC systems, their optimization and
have the ability to build, if we care enough to do so. The presentation showed
a group of HVAC contractors who build their duct systems so air-tight that the
lowest setting on their test equipment shows “error”. These contractors take delight in the fact
that almost no other contractors can match their specs when it comes to air
tightness. At this performance level, there is almost no competition.
Other highlights of the class, the instructor showing how he
took an 18,000/40,000 btu per hour furnace and disconnected the high end. The reason he could do that was because he
actually did the load calculations and knew just how much heating and cooling
the house needed and provided just that. He actually designed the return system
so that it did not constrict airflow and complied with the 2013 California code.
He made the duct system as short and low resistance as he could and then built
air registers that were designed to throw the air at the far walls with
engineered grills. After completion, he commissioned the system and did a room
by room airflow test and balanced the system. Wow!
It was such a change from what I usually encounter that it
seemed like a different industry. In the field I see contractors struggle to
get 350 cfm per ton of air conditioning. In the class, the instructor noted
that California’s dry climate allows us to shoot for much higher airflows per
ton such as 500, 600 or more cfm per ton. To do this one must learn how to hot
rod the air handler. It is do-able, and it provides more energy efficiency and
comfort per energy dollar spent. The longer run times provide better filtration
and heat the house more evenly than an oversized system.
One class technique was really interesting in terms of
comfort provided. The instructor noted that he designed his heating system to
blow air around 82 degrees F rather than 120 F. This, and the longer run times
provides room air temperatures that are within a few degrees from floor to
ceiling. Hot air stratifies in a room, warm air, not so much. Again, this gives him a market edge, he can
guarantee that there is only a 2 degree difference from floor to ceiling;
market advantage. His competitors can’t match this.
In a few seconds I can usually get a good idea if an HVAC
system is going to be code level air-tight. If I see mastic on all the
connections, I can be nearly assured that the system is going to pass without
my helping to troubleshoot the leaks in the system. In my experience, people
just don’t realize how thin and slippery air is. I’ll ask, “Did you use mastic?.”
I’ll often get answers like “What’s that?” or “Oh that stuff… it’s really
messy.” Then they go back up into a hot attic and try to and find the leaks.
This is rarely a good experience. Doing it right the first time is a better
strategy for profitability.
We have a lot of tools at hand to help us build better HVAC
systems and would benefit us, as an industry, to use them. The first is the energy
code, anything below which, is illegal. The first thing one should do is to
read the California
2013 Residential Compliance Manual. Another tool is the ability to do load
calculations and duct design software such as Wrightsoft
. My field experience shows me that load calcs are all-too-often crude guesses.
This fixes that problem.
HVAC optimization is thus, measure a building’s actual need
for heating and cooling with load calculations. Specify efficient equipment
that matches load calcs. Actually design a compact, low restriction duct
system. Install the system so that it does not leak any measurable about of
air. Install register grills that puts the right amount of air where it is needs
to go at the right speed. Commission the system by measuring and balancing room
by room duct air flow, duct leakage and total airflow at the return.
We have the ability to build highly energy efficiency
comfort systems if we use the tool s at hand. Using tools such as utility
sponsored trainings, understanding code requirement and using available
software design are the way to get there. Doing so can make a contractor more
desirable to customers and hence profitable. It can also give one the
existential pleasure of doing something well. Building an HVAC installation industry
that does its job well can be a powerful tool toward building an energy
efficient economy and low carbon future.
George Matthews
Building Energy Compliance Testing
San Francisco/Silicon Valley
www.bect.us
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