Net-Zero by Design – HVAC Sizing

Net-Zero by Design – HVAC Sizing – Part 3

In Part 2 of the Net-Zero Energy Home series we discussed the importance of the home’s orientation as well as the uniqueness of the effort to get to Net-Zero. Energy Smart Home Plans is very proud of the fact that we’ve designed so many Net-Zero Energy Homes! Now let’s get into some detail about designing a Net-Zero Energy Home and how the customer’s wish list is a factor.

HVAC-sizing-insulation-compareHere are just a few questions I ask my clients. Will the new Net-Zero Energy Home have a swimming pool? How about a heated spa? Will the cost of the energy for both of these amenities be mitigated by solar power? If so, this will certainly impact the size and costs of the solar array. Think of it in terms of the number of solar panels sufficient to replace the pool and spa’s roughly $50 energy costs every month. Heating the swimming pool is another issue and often is satisfied with a relatively inexpensive thermal system, commonly seen on South Florida roofs.

HVAC-Sizing-pros-consFor optimal HVAC Sizing we go way beyond Code Minimum which typically conditions only 500 sq. ft. per Ton. Our Energy Smart or “Green” Homes conditions 1,395 sq. ft. per Ton.

Reducing Your HVAC Sizing

But the real meat and potatoes of Net-Zero Energy Home success in hot and humid climates is arresting the size (and cost) of the HVAC Air Handling or Mechanical System. How do we do this? We design the entire Mechanical System within the home’s conditioned space, preferably below a well ventilated attic, that’s properly insulated with R-38 above the ceiling. All of the ceiling drywall must be taped and tight, with no “communication” with the attic space above. We use a Cantilever Truss, this allows the insulation to extend across the exterior wall or top plate. The HVAC or air-conditioning systems in houses designed and built this way can be 60% smaller than what’s commonly used, precisely because the house is so much more efficient. It’s routine for our Energy Smart Home Plans to be built achieving a 45 HERS, before any solar PV is added! Click this link for a HERS explanation. And also understanding HERS Scores this interactive link shows just how great the annual savings are! We’ve had Net-Zero Energy Home success with 2×4 and 2×6 frame walls, block walls and ICF walls. The R-Value in the exterior walls in South Florida matters less, due to the low Delta T, or the low average outside & inside difference in temperature. So “exotic” expensive exterior wall solutions aren’t in my opinion worth the money in our climate zone.

HVAC-sizing-ducts-in-conditioned-spaceEnergy Smart Home Plans are designed with the AHU (Air Handler Unit)​ ​in an interior closet, along with all of the ductwork within conditioned space. A c​ommon practice everywhere, code minimum builders p​lace the AHU in the garage​, simply because the house was not designed with the HVAC system in mind. ​

This is ​not only ​extremely wasteful in terms of energy loss​, but the garage is the worst place for your HVAC, your home’s “lungs.” All of the air your HVAC puts into your living space comes from that hot, humid, garage, which also happens to be where gasoline, solvents​, paint, etc. are stored. Furthermore, with the HVAC in a hot garage and the ducts in an even hotter attic, every time your HVAC cycles off, that air inside heats up. So every time your HVAC cycles back on, it pushes that hot-humid air into your living space. There is simply no way to make good, on this worst practice. Placing the HVAC and the ductwork in the conditioned space is healthier, more durable, safer and far more energy efficient. All Energy Smart plans use this best practice and gets the best HVAC Sizing bar none.

Then, there is spray foam insulation. It means “easy energy-efficiency” to South Florida builders. Pointing out the inferior R-Value (R-19, half of what’s optimal) and the increased cost of material and higher energy bills for the life of the home, usually garners disgusts and scoffs from true believers. To them I say that we do recommend spray foam insulation and a non-vented attic when the home is on or near a large body of salt water. This is often the case here in SW Florida. Taking a hit on increased energy costs due to spray foam’s reduced R-Value and the increased volume of air we must now cool – becomes a secondary concern. It’s very important to eliminate the chance of salt laden air entering the home’s attic. We’ve also designed Net-Zero Energy Homes with a non-vented, spray foamed attic too, we just needed additional Solar PV to achieve an energy neutral home.

Lastly, a few words about solar hot water as part of the Net-Zero Energy Home strategy. Being a thermal system rather than photovoltaic, the solar hot water’s panels orientation is not as critical as the location of the solar PV array as discussed earlier. Not only that, but I’ve yet to hear a customer report that they’ve spent a dime on hot water, with a solar hot water system. Once installed, hot water is free for the life of the system. I installed a solar hot water system in my first home in 1984 and to my knowledge, with minor maintenance, it’s still providing “free” hot water. Solar hot water is a good idea!

In Part 4 of the Net-Zero Energy Home blog series, we’ll continue with more details about the envelope, windows, air tightness and more.

Dave Jenkins

Dave Jenkins is a leading designer of Hi-Performance homes for the residential market. His award-winning home designs boast the nation's most energy-efficient homes available. Dave has over 25 years of experience in high-end, residential design and illustration for luxury builders and developers.

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