Goal #5 Right-Sizing the HVAC

By Beverly Smirnis with special thanks to input by Allison A. Bailes III, PhD and Zero Energy Homes Designer, Consultant & Instructor Armando Cobo

Part 1: Airtightness and Keeping it Dry are Goals #1 & #2

Part 2: Making Houses Less Prone to Damage in a Natural Disaster is Goal #3

Part 3: Fewer Components, Less Waste is Goal #4

Part 4: Right-Sizing the HVAC is Goal #5

Only after dealing with the building enclosure and related passive comfort strategies is it time to consider the mechanical systems. And those decisions will then dictate the HVAC requirements.

Building net zero, carbon neutral homes does not necessarily mean going all-electric and getting rid of natural gas entirely.  The good news is that electricity, natural gas and solar energy have all made clean energy advancements in recent years. 

Most important of all is installing the right sized equipment that will stay on longer and blow air into the house at a lower rate.  When the heating or cooling system is oversized for the design load, your systems will come on for a short time and then go off.  Variable capacity heating and cooling equipment helps address the problem of short cycling.  When the load is smaller, the equipment runs at a lower rate.


HVAC 101: The Conditioned Air Delivered by your Equipment Must Be Delivered Properly

When designing a high-performing house, it is critical to know what type of HVAC system you are going to use so you can decide the location of the mechanical room or closets, have allocations for furr-downs or truss chases for all ducts, or what type of conditioned attic to keep all systems within the conditioned space.

Installing HVAC equipment in a ventilated attic in Texas is a poor choice. Our ventilated attics are hot, humid, poorly ventilated, and exposed to extreme temperatures, which can make HVAC systems work harder and less efficiently, thus, allowing for higher energy bills and shorter equipment lifespans. Unprotected ductwork can collect moisture and cause problems as well.

Proper ventilation is also principle to energy-efficient home design. A well-ventilated house can reduce the need for artificial heating or cooling by allowing fresh outdoor air to replace indoor air, controlling humidity and maintaining moderate indoor temperatures — thereby conserving energy.

Energy recovery ventilators (ERVs) can help maintain this balance by transferring heat and moisture from incoming and outgoing air streams. In colder months, ERVs recover heat from exhaust air and transfer it to incoming fresh air, reducing the need for artificial heating. Conversely, during warm seasons, ERVs extract heat from incoming air, essentially “pre-cooling” it to reduce load on air conditioning.

Ducts need to be properly sized and located.  And in Texas climate zones, they also need to be insulated.


“Home plans should include full HVAC designs complete with ventilation and dehumidification and/or humidification, or both. I truly believe that our chances for mistakes go way higher when we miss including those details,” says Cobo.

EDUCATION, EDUCATION and EDUCATION! is the bottom line when it comes to building high performing homes.

Did you Know? An Austin neighborhood paved the way for a culture of central air conditioning! This is an interesting read about some of the first air-conditioned homes in Texas.

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