Recirculating Hoods and Indoor-Air Quality

By Kristof Irwin, originally published in The Fine Homebuilding Magazine’s “Ask The Experts” Segment 

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A few years ago, Fine Homebuilding published a very energy-efficient house that had a recirculating range hood. The reason for the recirculating hood was to avoid punching an additional hole in the air barrier and to avoid the need for makeup air, if my memory serves me. Now I’m faced with a similar decision. Seems recirculating hoods won’t remove moisture from the kitchen—do they at least do an adequate job of filtering the air?

—Karen Dorsel, Lincoln, Neb.

Building science expert Kristof Irwin replies: The short answer is no. Recirculating hoods neither remove moisture from the air, nor do an adequate job of filtering. The reason the answer is no is because recirculating hoods only endeavor to move the air  further away from the breathing zone near the range to reduce the smoke/odor in that area. The filters mainly aim to capture grease so less of it hits your forehead when you cook. Some filters also use activated charcoal as a way to capture some of the chemical compounds released when cooking. In neither case are particles or moisture removed from the air.

A slightly more detailed answer starts with a couple of facts. The first is that cooking is chemistry and cooking effluents are indoor-air- quality pollutants. The second is that the building science perspective that originally motivated the use of recirculating range hoods has evolved. Let’s explore each of these briefly.

Understanding cooking effluents and pollutants

The term pollutants means that what is released into the air from cooking negatively impacts the air quality in terms of our respiratory health. If there was no negative health impact, we would just call them substances. Based on the work of the researchers at LBL and others, including the HOMEChem research here at the University of Texas at Austin, we now know that cooking indoors can have significant negative effects on our health. Please refer to those links for more detail as there is a lot more to the story.

As an entry into the evolving nature of building-science recommendations, consider the tagline here at Positive Energy: “Design around people, a good building follows” (credit to Robert Bean). The message being that in addition to focusing on outcomes of energy efficiency and durability, it is also important to think about direct impacts to human health and well being. The motivation for restricting the number of holes in a building’s air-control layer were associated with energy efficiency and durability. When we take health into account the answer shifts.

Viewing hoods as part of a system

A core issue here is faulty thinking about what a hood is and does. A hood is only part of a system. The functional role of the hood part of the system is to capture pollutants from cooking and exhaust them out of the breathing zone inside the home. Clearly a recirculating hood fails at this. Even when a hood exhausts to the exterior, it will not work properly unless building science and system thinking is applied. When seen as a system it becomes clear that, as we build increasingly well-air-sealed enclosures, we need to provide incoming airflow to make up for any air that we send out of the home using a fan. This makeup-air issue foreshadows a reality that many of us see on the near horizon—any powered device that intentionally moves air from inside a home to outside needs integrated makeup air. Together the exhaust hood, fan, ducting, and makeup air form a functional system.

Getting back to your decision, the short answer to the question, “what do I do about my range hood,” is to vent it to the exterior and provide makeup air. The slightly more detailed answer has a few key points, including:

1. Reduce the amount of cooking effluent (AKA pollutants) you create by using a lid and limiting the amount of high-temperature oil cooking you do indoors.

2. Increase the capture efficiency of the hood by making sure it has a deep sump. Without an effective capture geometry, the pollutants just spill out around the hood. Think square tires to get a sense of how silly it is to design a hood that’s flat at the bottom.

3. Move the right amount of air and the right speed to maintain pollutant entrainment in the air stream.

4. Make sure to use metal ducting that has a path to the exterior that is as short as possible with a minimum number of bends.

5. Finally, use the hood and turn it on whenever you cook.

The performance of your range hood impacts your health. It’s worth doing it right.

The Fine Homebuilding Interview: Kristof Irwin

By Aaron Fagan, Kristof Irwin, originally published in The Fine Homebuilding Magazine, Issue 300 - July 2021

A professional engineer offers a consilient view of building science that prioritizes human thriving.

Synopsis: In the fourth installment of the Fine Homebuilding interview series, Aaron Fagan interviews Kristof Irwin, an engineer who wants us all to rethink what it means to live indoors. By focusing on human thriving, Irwin says, and viewing our indoor environment as greatly involving human health, we can cultivate a new relationship with our homes and all that they embody.

A principal of Positive Energy in Austin, Texas, professional engineer Kristof Irwin has an expanded view of building science. He asserts that any definition of the discipline is incomplete without accounting not only for the house as a system, but also for the fact that a home operates as a node in a larger societal and planetary system.

“The paradigm needs to change,” Irwin said during our interview. “Fundamentally, homes should be about human thriving.” And he believes this is an attainable goal—one that isn’t reliant on unrealized technologies of the future. The building industry needs to undergo a cultural shift, he says, and all of us need to cultivate a new relationship to our homes and all that they embody.

According to Irwin, we have the tools we need already to create long-lasting, healthy homes. He asks us to consider this question: Is it time to stop focusing on doing things better and start focusing on doing better things?

AF: How would you describe the focus of your building-science practice and work as an engineer from a cultural point of view?

KI: I really want people to rethink what it means to live indoors. We view an indoor environment as though it’s merely visual, spatial, and economic. When I say we, I mean those within the architecture, engineering, and construction professions. As a result, homeowners and developers are complicit in that view. However, from a very pragmatic standpoint, when we talk about actually being in your home, what we really mean is you are in the air contained by your home, and that means being in a highly immersive tactile situation where visual, spatial, and economic concerns must be secondary to human health.

For example, phthalates are a class of chemical plasticizers used in myriad building products. Even if you put indoor breathing aside, we can still get a substantial transdermal uptake of these chemicals, which are now being linked to a wide variety of health concerns.

That is one of many facts we are going to have to confront in the building industry regarding the health of our built environments. The paradigm needs to change. Fundamentally, homes should be about human thriving. We cannot put the very systems upon which we provide energy and resources for our homes, which are in natural ecosystems, out of that view. In thermodynamics, for example, you define a boundary, and what we tend to do is define the boundary around the home or the lot. That myopia is inappropriate and damaging.

I’ve been rethinking what it means to practice building science, which has been conventionally described as systems theory applied to buildings based on the physical sciences. That last piece is very important; typically, it’s the classic sciences like thermodynamics, hygrothermal dynamics, or, more broadly, physics, chemistry, biology, geology, and engineering disciplines. However, what’s critical to systems thinking is accurate, timely feedback. That principle is huge for the role Fine Homebuilding plays in the culture of home building. One of the most crucial developments for the steam engine was the centrifugal governor. It provided accurate timely feedback of the pressure buildup, which turned it from a grenade to an engine. Culture change is complicated because the very systems we rely on for accurate timely feedback to our society are working through implicit biases.

Where building science is concerned, the elephant in the room, the “emperor has no clothes” reality, is that there are essentially no compelling constraints to keep us from making fantastic buildings. And I mean multiple simultaneous dimensions of quality. These buildings could last 500 years, they could provide flawless air quality, and they could help improve sleep, life expectancy, cognition, and emotional regulation. We know how to design environments to promote human thriving, but we don’t do it.

 AF: Why don’t we do it?

KI: Well, it’s not because we are waiting for some invention of appropriate materials. or technologies. It’s because society is not asking us for those outcomes. Society is stuck in outmoded visual, spatial, and economic ideas. Something is wrong with the system when builders and developers see the houses they build as an economic asset for themselves. 

So, my point of view is that systems thinking is important, but that it’s incomplete without social science. It needs to include behavioral psychology. We are offering food for thought here. 

Fine Homebuilding offers nutrients to this ecosystem, but the ecosystem needs to recognize these as nutrients and consume them. The field of building science has been offering food and society is saying, “I’m not hungry.” I think building science should be using architecture, engineering, and systems thinking to design and build beautiful buildings that achieve practical outcomes. And when I say systems thinking, I mean an expanded view.

AF: There is no sense in talking about window flashing and vapor control if we don’t know why we are doing what we are doing.

KI: Exactly. If someone says “vapor permeance” to me one more time, I think I’ll explode. It’s as if we have a group of architects and builders at a job site and there is a huge pile of dirt we need to load into a truck, and we are lost in conversation about the shovels on the ground. Does it have a long or short handle? Is it flat or pointed? Hickory or ash? What we need to do is pick up the shovel closest to us and get to work. People need to recognize that the dominant pollutant-source exposure in our society is the air breathed in the home. We breathe 30 lb. of air per day, and that’s if we’re not exercising. Where does it go? It goes into our blood. It quickly crosses from outside of me into something I call me. Those pollutant particles go from the air around us into our blood and they have myriad health effects. Covid-19 has gone a long way toward making the invisible substantive. We are really at a point in our societal evolution where homes can be an essential part of the solution to the challenges we face. The climate solution in particular. But we need to prioritize human thriving in the homes we build.

AF: Health risks appear inextricably linked to other risks.

KI: One of the most hopeful things I’ve learned recently is that firms like BlackRock—with nearly $9 trillion in assets under management—are recognizing the enormity of these environmental issues. Larry Fink, BlackRock’s CEO,wrote in his January 2020 annual letter that “climate risk is investment risk.” So, what we have is an extremely powerful system of systems—I’m talking about the financial and banking sector—and they are urgently calling for transparency regarding climate risk. Banks are cleansing their balance sheets of investments that they view as exposed to climate risk, because if they don’t know where the risks are, how can they make decisions for their investors? This has already happened in Europe. It’s happening in Asia. The financial and banking sector says it’s urgent for companies to disclose these risks. Now apply that to the building industry.

AF: That’s the paradigm shift. That will have cascading implications.

KI: That’s exactly right. Think about the way we generate and deliver electricity to our homes. Utility companies are still using outdated science from the ’80s and ’90s. That’s a major source of distortion. A report with detailed modeling was released last December by a group called Vibrant Clean Energy in Boulder, Colo. It will require a major investment to bring it to scale, but if we invest in clean-energy renewables and distributed storage, this model shows that we can save close to half a trillion dollars in the next 30 years. But we need to think differently and go away from traditional practices.

However, the inertia of traditional practices is significant. We have generations of mostly men in the construction industry who would have to face a lot of pain. It means developing a huge amount of humility in order to really get new momentum behind this transition. They need to say to themselves, “My actions and decisions over the course of my life and my career—including the ones I’m making today—are actually part of the problem.” I say this to myself, and it’s hard. It’s not easy, but I know it’s the truth.

I can face that truth, and it’s not comfortable. One of the most important changes the building industry needs to make is that its systems exploit the environment, exploit labor, and promote a host of other unjust practices. We can’t shy away from the negative emotions that conjures. That’s feedback. As we stated before, systems require accurate timely feedback.

I think there is no other reality than the fact that the climate is weirding. Humans need to radically—and I mean unrealistically fast—change their behavior. That’s really hard, and it’s going to be challenging. But what’s happening right now is that we’re not even admitting that we need to do that wholeheartedly and with unified voice. That last piece, “with unified voice,” means there is no way around the fact that—grudgingly, over time—every-one will have to stop the party, go through the hangover, face reality, hit bottom, and say, “Yes, this is real. It’s happening, and we need to deal with it.”

There is a subtle form of optimism in there in the sense that there is no other forward. There’s no other future than gradually people will come to face this reality and accept it. When that happens, powerful change can happen rapidly.

AF: How would you reframe our definition of building science?

KI: I really see that building science needs to expand its purview and its understanding of systems. And I see it in three main areas: planetary systems, human systems, and digital systems. We’ve talked a lot about planetary systems. The good news there is that climate risk is investment risk. Planetary systems are about how we avail ourselves, our families, and our entire society and economy of energy and resources harvested from the planet.

An example of a human system would be that I am an engineer, and I work with architects, builders, contractors, consultants, code officials, inspectors, appraisers,underwriters, bankers, insurers, legislators,commissioners, lobbyists, industry associations, and media outlets including magazines, podcasters, bloggers, and influencers. Every human interaction has an impact.

What my local HVAC distributor chooses to carry, for example, impacts my ability to design, which impacts my installing contractor’s ability to move into the future. What I’m driving at is this expanded building-science systems perspective, and it really includes each individual as a node in a giant mind. And when it comes to human systems, how do we avail ourselves of adjacent expertise.

Medical science, social psychology, behavioral science, behavioral economics, and even marketing and consumer behavior are subjects of expertise. So, there’s all these adjacent expertise sets, and here we are in 2021—it’s not OK with me that they’re siloed next to me. I really feel that for me to do my job, I need to understand that I don’t understand. I have a small purview; I need to ask questions. I need to be able to face the fear and doubt that recognizes that what I’ve done in the course of my career, while traditional, was unskillful. That’s a tough thing to ask for.

We have to ask ourselves what’s on our dashboard when we move through the world, because those instruments shape what we see. You can put profits and consumer preferences on there, but I want to add health and wellness, thermal comfort, operational and embodied energy, and community and environmental health. So, we’ve had the wrong dashboard. Can you blame people for not charting a skillful course? Is it any wonder when we look at what has been prioritized? No.

And then the last one is digital systems. The psychologists who work for big tech fuel the attention economy, and that has bred an age of distraction with huge side effects. This goes back to the beginning of our conversation: If you want to understand something, think about the intent.

Big tech, along with advertisers, are getting the outcomes they were after.

AF: There’s a lot of stored power and intention in tradition.

KI: We shouldn’t bemoan the fact that traditional practices are powerful. That is deeply built into our mammalian selves. Tradition is like guardrails. But, we’re supposed to be thinking beings, not just traditional beings. Traditionally speaking, th e practice has been to create buildings out of basically cheap interchangeable parts so that we can apply low-skilled, exploitable labor, and get a very low dollar-per-square-foot building that looks like a nice home but isn’t. It has interior finishes and gadgets, but don’t peel back the walls! We didn’t optimize it to be a nice home, we only optimized it to look like a nice home delivered at a low first cost. And given that that has been our intent, we’re doing a good job.

I talk about the important difference between doing things better and doing better things. We are engaged right now in the building world with a constant search for doing things better—better wall flashings, better insulations, etc. I think it’s a really timely opportunity to break from that tradition and start thinking about doing better things. Are we prioritizing embodied carbon? Are we prioritizing health? Doing things better has an implicit basis in traditional behavior. Doing better things means thinking, what aren’t we thinking about?

One of the things I think drives our deep fear and concern in the United States building industry—and has us resisting acknowledging it—is the fact that traditional practices are failing us. We don’t want to admit that. Why? That issue makes us bristle so much because in our heart of hearts we don’t trust ourselves to take care of it. We’re not sure that we’re up to the task.

Our behavior communicates that we’re not sure human nature is basically good, creative, intelligent, and caring enough to deal with this problem we’ve created. I will for the rest of my life stand firmly on the idea that we can do this. We can do better things. We can make this happen, but I can also feel the doubt. I think that doubt in humanity’s ability to fix this issue is a source of a lot of resistance to the willingness to change. We need a lot of people producing better things instead of just doing things better.

I can nerd out about European versus U.S. filtration standards, but what really enlivens me and what I really feel connected to right now at this point in my career is this question: How do we get society to admit that it’s due to do things differently? It’s fascinating. As an engineer who wants society to thrive, I am starting to recognize that it’s not by doing engineering, it’s by getting society to ask me to do better and better things. It’s a weird kind of place to be as a quantitative, technical person. It’s very clear to me that the problem is not quantitative or technical; the problem is deeply emotional. We need cultural things that help galvanize societal will.

How do we get there? How do we get past seeing a home as a visual, spatial, economic situation and see it more fully as a deeply tactile situation that influences our very cognition and emotional state? It’s so much more interesting, but we don’t want to go there. Fear is always a story in the mind about what happens next. There are different types of dopamine receptors in the brain, and one of them is associated with the anticipation of the cessation of suffering. It sounds kind of Buddhist. That dopamine trigger is like finding the solution to a problem, and thinking therefore that the problematic situation is nearly gone. That’s not the reality. We don’t think of the fact that we are going to put a mammal into this box we built, and our felt sense of an indoor environment is vastly dominated by unconscious inputs we can no longer ignore.

State of the Art HVAC: Five keys to flawless space conditioning.

By Kristof Irwin, originally published in The Journal of Light Construction, July 10, 2019

In an ideal building science based world, the Perfect Wall concept would be matched and complemented by a Flawless HVAC concept.

Just as there are fundamental physics based principles supporting the Perfect Wall, the same applies to Flawless HVAC, but, alas, they are far less widely known, understood, or put into practice. Many know to “keep the outside out, and the inside in” but what exactly is the “inside” we keep “in”? The “inside” is a volume of air that we immerse ourselves and our loved ones in. We live most of our lives immersed in fishbowl of air of our own making. The qualities of this air are readily controllable and impact our health, comfort and well-being. Alas, being invisible, air gets less attention but is no less important to understand or do well.

The Perfect Wall has Rain, Air, Vapor Thermal control functions. Flawless HVAC has Capacity, Distribution, Ventilation, Filtration and Dehumidification (in humid climates) control functions. More specifically, the basics are: (1) variable capacity heat pump equipment (also known as Variable Refrigerant Flow, or VRF), (2) rigid metal ductwork, (3) continuous balanced tempered ventilation air, (4) effective particulate capture, and (5) dedicated dehumidification.

I’m the principal of Positive Energy, a full-service building science consulting firm based in Austin, Texas. We have an amazing team and we know that both our technical skills and our ability to evolve the systems and processes that deliver buildings to society are important. Among the services we provide to architects and builders are heating and cooling system design, building pressure testing, duct pressure testing, and duct flow balancing and verification. We have the good fortune to serve a high-end custom home market where clients are willing and able to spend the money to get an hvac system the quality of which matches the quality of the rest of the building. Working in the residential space we avoid much of the split incentive situation that plagues the developer world, where the person making the decisions is viewing primarily through an economic lens. Accordingly, we don’t cut corners with low quality equipment or ductwork. In an industry where the lowest common denominator often controls, we are focused on providing top quality solutions for our clients. In this story, I’ll talk about the principles and practices that guide our designs for state-of-the-art hvac systems.

Five Rules for a Healthy Building

As building scientists, we recognize that the house is a system. What do these buildings do? They take inputs of electricity, water, gas, and data, and they output human beings: healthy, functional members of society. Our philosophy is that we design to optimize that human output. Our motto is, “Design Around People, a Good Building Follows.”

There are five principles to creating a healthy indoor environment for the space where we spend 90% of our lives:

• start with a good enclosure

• minimize indoor emissions

• keep it dry

• ventilate

• filter.

The first two items on the list aren’t part of the hvac system (although technically, the enclosure provides the connection between the supply and the return air, and so in a functional sense could be considered part of the mechanical system). The good enclosure is the builder’s responsibility, and minimizing indoor emissions is in large part the responsibility of the homeowners and building occupants. But keeping the building dry, ventilating, and filtering the air are part of hvac system design.

You’ll notice I didn’t mention heating and cooling. Those are important for comfort, but they’re not related to the top priority: the health of the humans living in the space. (Yes, of course, there are climates and weather events where temperature control is a life-safety matter. My point is simply that much of the time, heating and cooling is not a health matter.) But while every hvac system does heating and cooling, it’s shocking how many systems don’t address the vital health priority of supplying dry, fresh, filtered air.

Heating and Cooling Equipment

These days, we prefer to specify VRF equipment, which represents the future of the hvac industry. VRF stands for Variable Refrigerant Flow, and modern VRF equipment offers advantages in at least three areas: efficiency, occupant comfort, and zoning capability.

In the old days, air conditioner or heat pump compressors had two modes of delivering power. Either full on, or off. More recently, dual-stage and unloading compressors have come into the market that add a second option, at either 50% or 65% of full capacity. VRF is a generation ahead of that dual-stage equipment. What VRF supplies is the ability to continuously vary the capacity of the machine. Quick reminder that capacity (power) is a rate, not an amount (energy). The goal is to vary the rate of heating or cooling to match the rate of heat leaking out or in through the enclosure.

I use a car analogy to explain the difference. Suppose you hop into your truck to go somewhere, and the rules are, you have to floor the accelerator all the time, and you control the speed of the truck by turning the ignition key on and off. That’s standard single-stage equipment. With VRF, you now have a gas pedal: You can smoothly vary the power output of the engine depending on how fast you need the vehicle to go. A four-ton VRF compressor like the Mitsubishi City Multi can smoothly vary its power all the way from 48,000 Btu/hr down to 15% of that, or anywhere in between.

This capability in the VRF equipment provides the ability to efficiently manage “part-load” conditions, when standard equipment suffers from the problem of over-sizing. ACCA Manual J is the industry standard manual for sizing hvac equipment. Manual J is often referred to as a load calculation. A word is actually missing there: It’s a peak load calculation. The Manual J load is representative of the peak heating and cooling loads you’re going to see in your climate zone for 1% of the hours throughout the year. Designers size their equipment to handle the peak load. But the vast majority of the time, your building will not see loads that high. It will see loads at what we call “part-load” conditions, when one-stage equipment runs in less efficient stop-and-start mode, that also causes more wear and tear on components. VRF equipment with its ability to give variable capacity is able to meet part-load conditions more efficiently.

Matching power to the load is not the only reason that a VRF compressor such as the Mitsubishi City Multi is more efficient than a standard compressor. The other reason is the design of the compressor motor. The electronically commutated motors in these units are driven by an inverter, and the inverter has the capability of adjusting not just the frequency of the current being delivered to that motor, but also the voltage. By playing with those two parameters in concert with one another, the motor achieves the highest power factor possible at any given speed and any given load that the motor is under. This improves the Energy Efficiency Ratio (EER) of the equipment (which expresses how many Btus of heat are moved for every watt of energy that you have to purchase). Simply put, you’re getting more heating or cooling per watt out of the VRF equipment at any speed. We’re getting more mechanical work than we were with the previous generation of equipment, for the same amount of power. So even at peak load, a 4-ton VRF system with inverter drive runs much more efficiently than a 4-ton single-stage or dual-stage system sitting next to it.

You can think of this in terms of the amps required to start and run the compressor motor. A standard single-stage four-ton unit will take about 100 amps of power to get started, and then will run at about 40 amps continuously once it gets going. A four-ton Mitsubishi City Multi will start out at about 2 amps, then it will ramp up slowly if necessary to meet the demand, up to about 24 or 26 amps. When the temperature in the space approaches the set point, the VRF unit will slowly reduce power and creep up to the set point, and, guided by its software, will then run just hard enough to maintain the temperature at exactly that set point. The traditional equipment will overshoot the set point, shut off, and then wait until the temperature rises above the set point again before it starts up again.

In practice, the lower amp draw combined with the the precise control of the VRF unit adds up to a savings of 20% to 40% in energy consumption. And because with a properly functioning controller the unit maintains a rock-steady set point, it also provides better comfort, without swings in temperature.

One last automotive metaphor that fits here. Remember carburators? They went away. They’re no longer used not because they did not work, but rather because fuel injection systems performed the same functional role more of distributing fuel to the engine efficiently and reliably. If you take nothing more from this article, please reset your view of VRF. VRF (which has been around since the 1980s) is not new, it’s not a fad that will die out. In fact, it could be that not to switch to VRF is the risky decision. Consider this, in 10-15 years when you need replacement parts, what will be occupying the shelf-space in distributors’ warehouses? Beyond the availability of parts, as someone who used to rebuild carburetors, both the parts and the installer expertise are needed to make things work. Will future generations of installers resist or appreciate the ability to connect a computer?

Air Handlers and Zoning

The outdoor compressor is linked to indoor units by refrigerant lines. Depending on the size and model of the compressor, a VRF compressor can handle anywhere from several indoor units up to dozens of units (in the case of big commercial equipment running on three-phase power). The homes we’re designing for typically have single-phase power, so we’re restricted to the equipment that can run on single-phase. We typically call for one or more Mitsubishi City Multi S-Series compressors, rated at 3, 4, or 5 tons, each of which can serve 8 independently controllable indoor units.

The indoor units could be anything from wall-mounted units or ceiling cassettes to variable-speed vertical or horizontal air handlers (commonly known as “multi-position”) much like the form-factor of air handlers for a traditional system. Our clientele has not embraced the visible wall-mounted units, so we typically specify one or more Mitsubishi multi-position air handlers and conventional ductwork. This form factor also leverages our ability to impact architectural decisions early in the design process. Again we benefit from non-split-incentive decision making: most homeowners understand that impairing access to their AHUs impairs the ability to provide quality installation and maintenance.

When it comes to zoning and duct design, there have to be conversations with the owners and the architect. Many in the industry, particularly residential, have grown accustomed to a process based only on an installation and not on any planning during the design stage. Architects don’t always consider the ductwork when they’re drawing house plans, but they should. I want architects to be thinking about the ductwork early enough in the process that the ductwork can be allowed for. Not leaving room for the “lungs of the home” or building is not really a full design. Perhaps calling it “ductwork” conceals that fact that we are talking about the distribution system that delivers thermal comfort and indoor air quailty. By “leaving room” I mean two things: Both room in the design process, and room within the architectural and framing designs. The simple concept of an integrated process, one that aligns architectural, structural and mechanical designs, is catching on strongly because it’s simple, effective and improves outcomes.

As for zoning, that requires a conversation with the customer on how they plan to live in the space as well as an analysis of the building. We zone the building by load profile and use profile. Load profile means, for example: “This room is facing east. That room is facing west. Those are different load profiles. This room’s on the first floor with very little exterior load and glazing. This room’s on the third floor. So those are different load profiles.”

You can also zone based on use profile: “This is the bedroom, it’s not occupied during the day. This is the central core. It’s rarely occupied at night. Those will be different zones. This is the man’s office. He wants to have it at a certain temperature. Or this is the woman’s sanctuary inside the house. She wants to keep it in her comfort zone. These two rooms are occupied by a teenage daughter and an 8-year-old son. They’re not going to want things the same, so give them each their own control.”

In the case of the east and west sides of the building, we may choose to give each zone its own outdoor compressor. That way, during a season with chilly nights and warm days, if the sun starts to overheat the east side in the morning while the west side is still cool enough to need heating, we can handle both needs at once.

But most zones aren’t going to have opposite needs, so multiple zones can usually be run off the same compressor using refrigerant lines and controls. In that case we give each zone a dedicated air handler and air distribution system that serves that area. Because we can have multiple air handlers served by the same outdoor system, VRF gives us the flexibility to do that and keep the initial cost down. This also minimizes the footprint necessary for all the equipment.

Sometimes, we get into a situation where the zones are too small even for the smallest air handler. In that case, we do “air-side zoning” — we zone the areas using dampers and controls in the duct system served by a single air handler. And occasionally, there’s a point load that is best handled by a wall-mounted unit, such as a laundry room or a garage.

Duct Systems

Duct board box plenums and flex-duct supply lines are typical in the industry in our market. We don’t do things that way: We specify metal duct for all our designs. In our view, flex-duct and duct board air distribution systems need to go away. Why? Well, think about it. People put a lot of effort into constructing a durable, functional enclosure. You have one chance to get it right, and then it’s inconvenient to fix it forever. The ductwork is the same way: It’s a durable, functional, passive assembly; you have one good chance to get it right, and then it’s inconvenient to fix it forever. And together with the enclosure, the duct system defines the breathing zone of the occupied space. The air distribution system is a permanent, durable part of the home that serves you well forever, or serves you poorly forever. Metal duct is appropriate for that situation.

Metal is a durable material. It will last the life of the home, if attached well and done well. And it’s a recyclable material, so at the end of its life cycle there is something we can do with it.

Metal has a natural galvanic action that retards the growth of indoor micro-biological organisms. That includes mold and bacteria, and even viruses and protozoa and all kinds of little living creatures. With air quality in mind, we always aim for fiber-free air distribution systems. The nooks and crannies of ductboard and turbulence created by flex duct spiral pressure liners do not help keep distribution systems clean.

Clean is another way of saying free of food, or substrates on which to grow unhealthy indoor microbiomes. If you think flex duct and ductboard is “fine” please keep in mind that your assessment is not an immutable physical law. It’s an assessment based on comparative metrics. Be clear on what your comparing to and what outcomes are priorities. You “eat” air all the time, is poor IAQ “fine”? Perhaps in the way that a greasy burger and fries is a “fine” diet compared to starving in sub-Saharan Africa. What really makes flex duct and ductboard the norm is the fact that it supports a beneficial economic outcome. Our industry is based on both interchangeable parts and exploitable and interchangeable labor. But that’s a topic for another day.

You get one good chance to get it right. This is perhaps the key consideration: an air distribution system moves tens of thousands of pounds of air every day. It will do so with either a lot of friction, very little friction, or somewhere in between. Using low-friction metal distribution systems based on the principles of fluid mechanics is analogous to having the right amount of air in your tires. Rolling resistance resists motion. So does friction in duct systems.

If you buy an efficient car, but then you drive on tires that are nearly flat, you’re going to lose a lot of the efficiency in that vehicle to rolling resistance. Of course you can always inflate your tires. You’re not going to roll around in your Prius with your tires half flat. But if you have ductwork with high friction resistance — like most duct board and flex duct the way it is typically installed today — you’re stuck with it forever. Just because you and your clients don’t see or value the ducts does not mean they don’t matter. Air distribution systems matter for the life of the home. It only makes sense to do it right when you’ve got the chance.

Filtration

Why filter the air in a home? It’s just dust, right? Oh, if only it were “just dust” — bits of leaves or soil, or even gross things like skin flakes. But dust is like a candy-coated M&M, and the candy coating is things like chemical pollutants and biotoxins. You breathe those things in with the dust, and if the particles are small enough, they can lodge in your lungs. The best way to keep from being exposed to those toxins is to filter the air, with at least a MERV-13 filter.

The MERV-8 filters that a lot of installers put in are touted as being 99% effective at catching dust. But all they catch is larger dust that your bronchial cilia are capable of catching and expelling from your system. MERV-8 filters are there only to keep the air conditioner coil from fouling. They’re not there to protect the health of the people in the building. Based on our expertise in IAQ and also per ASHRAE standards, we specify MERV-13 filtration at a minimum; these capture most of the smaller particles that your bronchial tubes won’t catch and clear. If the clients are sensitive, we may go up to MERV-16 or even to a whole-house HEPA filter.

The state of residential filtration provides a simple but powerful illustration of how far from human health principles our industry mainstream has drifted. We know that capturing particulate pollutants is important for health and should be happening whenever the home is occupied. Do we do that? Not so much. Our industry has somehow decided that the right time to filter the air is either when the temperature is too hot and we need cooling, or too cold and we need heating. The impacts of our societal and industry lens of home as a visual-spatial and an economic asset has a powerful distorting effect on our decisions and actions.

Fresh Air and Dehumidification

Humidity control is important for occupant comfort, and also for building health. If you maintain the air relative humidity (RH) in an acceptable range of 35-55% or 50-55% in hot humid climates, the occupant’s thermal comfort will be satisfied over an expanded range of sensible temperatures. That can make up for situations like an overheated sunroom: If I keep it dry, I am able to evaporate moisture off the occupants’ skin, which is part of cooling.

Controlling moisture helps maintain the stability of trim, or of musical instruments in the house.

But most importantly, dry air is critical for the health of the human occupants of the building, because humid air supports the growth of all the organisms in the “microbiome” of the home. Fungi, bacteria, and other organisms battle for supremacy in a humid environment, and they release biotoxins that cause human health problems. If we keep the air dry, we take away a major factor in that health threat.

And here’s the thing: As the energy code evolves, it’s increasing the need to independently manage humidity. Tighter enclosures, more insulation, and better windows are reducing the sensible load in the house. That means air conditioners — which are the only dehumidification equipment in most houses — are running less often. In essence, the code says, “Thou shalt run thy air conditioner less.” And if the air conditioner is not running, you’re not removing humidity. Meanwhile, required fresh air ventilation is bringing moisture into the home during much of the season.

So for our clients, we always specify a dedicated dehumidifier with its own controls. Typically that is an Ultra Aire unit, because we have a good relationship with Ultra Aire, have the ability to access their technical teams, and we have a solid track record with their product. We pull air from the conditioned space into the dehumidifier, and send it to the supply air distribution system. We also use a dedicated damper-controlled ventilation port on the dehumidifier unit to draw in fresh air and distribute it also using the heating and cooling air distribution system. Note that this system needs to be designed to account for the additional volume of dehumidified air.

The dehumidifier runs in response to relative humidity in the house. It doesn’t run only when the air conditioner or heat is running. But it doesn’t require the air handler to be running — the fan in the dehumidifier unit is sufficient to get the dry air where it needs to go.

Summary

The term air conditioning is so familiar that perhaps we don’t hear it. Conditioning does not mean cooling. Conditioning means that we are creating an indoor environment that is conditioned to be suitable for human occupancy.