As our team finalizes details of the mechanical plan for the Queen of Zero house (see JLC’s Case Study 2024), we are faced with the unique challenges of designing an optimal heating, cooling, and ventilation system for a tight, high-performance home.
Mechanical Design Challenges
Homes aspiring to a Zero Energy Ready standard, such as the Queen of Zero, will achieve envelope air tightness of less than 2.0 ACH50— in other words, the home’s total volume of air will naturally ventilate less than twice per hour when subjected to 50 pascals of pressure through a blower door test. Passive house standards have traditionally required air infiltration performance of 0.6 ACH50 or less. Building mechanical systems to serve these types of high-performing homes present a range of design challenges and benefits:
- Air tightness combined with superior insulation and energy-efficient windows reduces the demand for mechanical heating and cooling throughout the space. Thus, standard HVAC rules of thumb (e.g. 1 ton of air conditioning equipment capacity per 500sf of floor space) do not apply.
- Tighter homes require more sophisticated ventilation systems to maintain optimal indoor air quality. Controlling humidity, removing contaminants from the air, keeping air balanced to avoid a negative pressure situation, and circulating filtered fresh air throughout the home must all be addressed by a complex system of equipment and duct work.
- While increasing the number of mechanical components to ensure comfort and wellness, a high performing net zero home also works to minimize energy consumption and fossil fuels. Thus, the design should prioritize all-electric energy-saving strategies such as heat pumps and shorter duct runs (or ductless minisplits).
Furthermore, the industry is currently lacking a robust workforce of HVAC professionals with the level of expertise, specificity, and accuracy needed to address the complexities of high-performance mechanical design. Take, for example, the ACCA Manual J tool which estimates the heating and cooling loads based on building size, glazing, orientation, climate zone, air tightness, R-values, occupant preferences, and other factors. Municipal permitting offices are increasingly requiring Manual J calculations to determine mechanical equipment sizing, but unfortunately many reports are fudged.
Building science expert, author and mechanical designer Allison Bailes explains, “A lot of HVAC contractors aren't used to doing real design. They work by rules of thumb, and if a building department says we need the Manual J, then they'll crank one out. But if they don't do this regularly and know what they're doing, a lot of times they’ll manipulate the numbers until they come up with their desired load for the system.”
Yet, it’s not just the steep learning curve that complicates this brave new world of mechanical design. Building science experts readily admit that there is a lack of consensus on best practices in their industry, which continues to evolve through experimentation and new technologies. Home performance expert and TV show host Corbett Lunsford, who produces educational videos on mechanical design, asserts that despite the evolution of building science field, “there’s no manual on this,” noting how each project should be tailored to a vast array of home-specific variables.
Mechanical Design Tips
In my work building high performance homes, I find myself on some sort of reverse learning curve regarding mechanical design. The more I learn, the less I know as I continue to gain a deeper appreciation for the complexity of these systems. Yet, over the years, I have gathered some effective practices for builders to help smooth the process of designing an optimal mechanical system.
Consider these tips:
1) Understand the basics of mechanical design. At a minimum, builders should be able to read and scrutinize the ACCA Manual J, D and S reports, know the difference between load calculations and system sizing, understand the fundamentals of heat pump technology (including methods for properly commissioning the equipment), and become familiar with the strategies for maintaining optimal indoor air quality. Many states and localities are now offering free training on high performance mechanicals to support energy-efficiency and electrification. Check with your state’s energy department.
2) Start the mechanical design process early. So often, the consideration of equipment placements and duct runs is put off way too late in the development of architectural drawings. As HVAC units have become more compact and more plentiful in homes, the layouts have also become more complex. Importantly, Bailes advises his clients, “Don't design the house with just a closet for an air handler because it's really difficult or practically impossible to get ducts in there installed and sealed properly.” Also, designate appropriate space in the landscaping for heat pumps—preferably somewhat concealed, but with room to breathe as per manufacturer requirements.
3) Recruit the best team. I prefer to engage a third-party mechanical designer with expertise in high performing homes to create the HVAC plan and then hand it off to my installer. Other builders prefer a design/build contract with the HVAC contractor. In most cases, a design/build HVAC contractor will be hiring a third-party mechanical engineer to draft the design documents. Regardless of the contractual relationships, the builder, designer and installer should all collaborate as a team throughout the process.
4) Tailor the model to occupant behavior and comfort preferences. Thermostat set points, fresh air circulation through open windows or doors, number of occupants, and use of various spaces throughout the day are all unique characteristics of families that should be incorporated into the mechanical design model. Corbett explains, “…every house is different mainly because every family is different. And so you got people who are environmental sensitive, they will have a very different experience of the same house because one person will get bothered and the other person won't. They'll cook differently. I always like to ask [homeowners] what kind of cooking do you do? Because if I have a family who is my client, they're cooking for eight hours a day simmering…” which impacts the kitchen exhaust and negative air pressure in the home.
5) Prioritize superior indoor air quality. A common mistake in high performance homes is to bring in unwanted humidity through a fresh air ventilation system, such as an energy recovery ventilator (ERV). It’s also a common misconception that ERVs are sufficient to control indoor humidity across all climate zones. Thus, in addition to homeowner preferences, it is also critical to take into consideration the geography and climate of the home. For example, how does the ventilation system prevent outdoor contaminants, such as smog from wildfire smoke, from entering the home? In Corbett’s TV Series Home Diagnosis, he identifies five interrelated air quality functions of the mechanical system: circulation, capture and filtration, humidity control, air dilution, and pressure relief.
6) Incorporate smart thermostats and monitoring support. High performance systems should include wifi capability to provide notifications for filter cleaning, filter replacements, and other maintenance-related issues. If feasible, the system should also include multiple room sensors to monitor temperature and humidity. This data can be used to make tweaks to the system over time, as needed.
Whenever we talk about mechanical design with the Queen of Zero homeowner, he reminds us that he’s a “fresh air guy” who likes to keep the windows open, which I’ll admit always gives me a tinge of heart burn. How will our very intentionally designed mechanical system, which includes a whole house dehumidifier, perform with this frequent onslaught of unfiltered, unconditioned air “au naturel” in an area where average humidity is over 60% nine months of the year? The answer is: it’s a brave new world, we don’t really know.