Equipment Size, Ducts, and Building Envelope…Oh My!
What do these three have In common?
For HVAC contractors, everything. However, they are actually listed in reverse order. Many contractors and homeowners talk about HVAC systems. However, an HVAC unit is not a system; the house is the system. The HVAC unit is actually a subsystem that also includes ductwork and the building envelope. Getting equipment, ductwork and building envelope working together as a team is he key to maximizing home performance and indoor comfort.
Missing the Forest for the Trees
Many homeowners feel like they’ll get an HVAC unit with all these bells and whistles and they’ll save all this money and energy, but reality frequently falls short of their expectations. This misconception reminds me of an episode from the Tim Allen show Home Improvement when he wanted a larger motor for his lawnmower to get the job done faster.
However, 70% of the time, deficiencies are not equipment related. Instead, a faulty building envelope is often the culprit. Installing a new HVAC unit without dealing with the building envelope is like installing a NASCAR engine in a 1970s-era Pinto with flat tires. On the other hand, fixing infiltration and exhilaration faults even without replacing HVAC equipment will outperform a new HVAC system alone all day long.
Infiltration and Exfiltration
Homeowners come to me saying, “I have a cold room and a hot room and a damp basement. Can you fix it?” My response is, yes I can.
The first step is to determine where the problems originate. It’s standard operating procedure when performing a Manual J calculations to include the square footage of the conditioned space along with ceiling height, windows, doors, ceiling and floors. It is also customary to calculate R-values for insulation in walls and ceilings, as well as U-values for windows and doors. However, most Manual J software has a significant defect to account for Infiltration and exfiltration rate, the data is hidden and unless you know where to look, you will not know to enter it, as this rate is one of the most important values.
Infiltration and exfiltration rate are directly related to the building envelope. Knowing how well the building envelope is performing or not performing is key to equipment sizing. Yet many contractors ignore this basic fact. If we do not verify our insulation levels and ASSUME that the size of the existing equipment is correct, we are not serving our customers well.
At Minnick’s, we perform a complete home audit that includes inspqections to detect drainage issues and mold along with a blower door test to measure infiltration and exfiltration. Knowing the blower door number and the correct insulation value can change the equipment sizing needed for a home to a half to 2 tons larger than the existing equipment size. Once the unit is sized correctly, we confirm the ducts are sized correctly per Manual D. We find that returns are undersized 99% of the time.
For instance, if the infiltration values are not changed when doing a Manual J., the default is average, which means it is set up for 8 total air exchanges per day during a 24 hour period. But if your house has 32 exchanges in 24 hours, you would need a system that can be up to two times larger to compensate. However, a larger unit also requires larger ducts and more energy. And unless you correct the factors responsible for the excessive air exchanges, much of the conditioned air is directed toward the outdoors. Not only does this contribute to needlessly high utility bills, but it also contributes to global warming.
Oftentimes customers want to replace windows to eliminate drafts. But it often makes more sense to seal up leaks in the attic instead. For instance, many attic hatches are no more than a piece of plywood with no weather stripping. Plus, where do you want your home’s air exchanges to come from – a moldy attic or crawlspace or the windows?
Contractor Pushback and Customer Resistance
Many contractors don’t want to invest the time and effort associated with learning techniques associated with solving building envelope issues. They just focus on selling a bunch of equipment, and then they wonder why they’re losing so much money on callbacks.
Some customers are also resistant to investing the money associated with fixing ductwork or building envelope issues. To an extent, that’s understandable. It can cost an additional $7,000 to $12,000 on top of what they would spend on a new HVAC unit. However, addressing building envelope issues often allows customers to put off buying a new HVAC unit for another year or two because they’re no longer requiring the unit to run 24/7, which offsets the cost.
The Cherry on Top
Realizing a full return on the investment required to correct building envelope issues can require as much as 10 years, although consumers start to notice a reduction in their utility bills after a year or so. One customer reported back to us that she had saved $6,000 in a year and a half after we corrected her building envelope problems. In her case, the electric heat pump was cycling constantly, which really jacked her utility rates up.
While that kind of result isn’t typical, what is typical is that our customers are surprised how much better their home is performing after we’ve addressed infiltration and exfilitration problems. We’ve had customers who were able to stop using asthma inhalers because of improved indoor air quality. Once the house is healthier the comfort and energy efficiency will come, not the other way around. Efficiency is the cherry on top.
- Making Homes Healthier for Families - May 8, 2018
- Equipment Size, Ducts, and Building Envelope…Oh My! - November 22, 2017
- Leaky Ductwork Makes Systems Work Harder - August 23, 2017
Posted In: Building Performance, Residential Buildings
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