Every car can do five of the 6 Functions of HVAC, but most homes can’t do any.  

These 6 Functions, done well, often lead clients to think, “I can’t believe this is the same house!”  

They are also much more likely to refer their friends to you, and much less likely to have expensive callbacks. Our lives should be happier as pros because of higher tickets, hopefully higher margins, along with lower marketing and callback costs. 

This all requires a different way of thinking than the typical changeout or service call. It needs a bit more engineering or holistic thinking than the mechanic’s mindset of finding a broken part and replacing it.  

It’s designing and building a winning racecar, not just getting a commuter car back on the road. 

It’s often much more fun, too. The work is interesting, and the client feedback is fulfilling. This is where you’re likely to hear that great line, “I can’t believe this is the same house!” 

What does this look like? Let’s dig in. 

The Aha Moment 

Years ago, my mind was wandering thinking about the simple HVAC system in my wife’s little Scion hatchback. It ran all the time and delivered excellent comfort once the car was warmed up or cooled down. The air got filtered. It brought in outdoor air to keep it healthier and less stinky. It dehumidified really well. In short, it was awesome.  

But what about houses? The systems are on or off, leading to mediocre comfort from either blasting heat or a/c, then nothing. The filters usually only filter out rocks and small children. No fresh air is brought in. As we move to more efficient air conditioners, they can’t dehumidify well either. In short, they suck.  

I thought about her little hatchback and realized there are 6 things I’d like every HVAC system to do: load matching, filtration, dehumidification, fresh air, mixing, and humidification. Here’s what it looks like on a car’s control panel: 

[insert automotive HVAC]

Load Matching   

Every moment, cars (and homes) need a slightly different amount of heating or cooling. Cars can adjust both fan speed and how much heat or cool is coming out. Those allow you to match the output of the HVAC system to exactly what the vehicle needs at that moment. 

This leads to the surfaces having more even temperatures. About 60% of human comfort comes from surface temperatures (radiant energy) around us. Being able to even out those temps by adjusting heating and cooling output is the single most important piece of delivering comfort. I didn’t believe it either until I did a few client homes with right sized variable speed heat pumps and felt it. But most homes don’t have those. Hatchback 1, Home 0. 

Filtration 

A good air filter in a vehicle knocks most of the road dust, exhaust fumes, and pollen out of the air before it gets into the cabin. Tesla is famous for their “Bioweapon Defense Mode” that takes this to an extreme — it’s comparable to a MERV 16 filter.  

Most homes have a crummy 1-inch “rock catcher” that lets the equipment get dirty, contributes to drain snot, makes the inside of duct work dirty from bio-growth, and leads to more dust inside the home. Hatchback 2, Home 0.  

Dehumidification 

Automotive HVAC systems tend to run icy cold coils that can suck the moisture out of the air like a cold glass of lemonade on a humid day. I remember ice shooting out of the vents of my dad’s 1989 Ford F-350. Because the a/c stays on, that cold coil keeps doing its work, even if you turn the air down. 

On the other hand, while old-school home a/cs were good at dehumidification, higher SEER equipment often pretends that humidity doesn’t exist, which makes homes wetter and eventually makes clients sicker. Hatchback 3, Home 0. 

Fresh Air 

Unless you tell it not to, your car’s HVAC system is bringing in outdoor air that gets filtered, then heated, cooled, or dehumidified as needed. This keeps CO₂ levels low inside the cabin so you and your passengers are less likely to get sleepy. If someone didn’t shower, it’ll help reduce body odor or any other, um, smells inside the cabin.  

Meanwhile, only a small fraction of homes have systems to bring in outdoor air. Basically, they are set to recirculate, which can make the air gross. If you use an indoor air quality monitor, you’ll find that in shoulder seasons the inside of homes get nasty. Hatchback 4, Home 0.  

Mixing 

Vehicle HVAC lets you point the air where it mixes the air in the car best — i.e., towards your feet in winter or your face in summer. The air in the car gets stirred like a vinaigrette to reduce hot and cold pockets. Most of the time, it does this with the fan turned down to 25-75% speed, so it isn’t noisy.  

On the other hand, single-stage furnaces and a/cs in homes are usually running the fan at either 100% or 0%. Rooms begin to see large temperature differences, and if you monitor CO₂, you’ll note that bedrooms get quite stuffy at night. Basic PSC or X13 motors are often poorly suited to run all the time. They often run too fast to mix the air effectively and quietly. Hatchback 5, Home 0.  

Humidification 

This is the only one of the 6 Functions of HVAC that cars and trucks can’t do.  

Sometimes we want to add moisture to the air inside a home to keep skin and lips from cracking, hardwood floors or string instruments happy, or to reduce the spread of viruses. Hey, the hatchback can’t win them all! Granted, most homes don’t have humidifiers either… 

Everyone Should Have This

As the pieces clicked into place, I became more and more convinced that every homeowner should be able to buy a system as good as the one in that little hatchback. 

While our first job is to have heating or cooling turn on whenever clients ask for it, arguably our more important job is to provide them with comfortable, healthy homes.  

This doesn’t take perfect air sealing, insulation, or duct work to do these (although it helps).  

Last year, we were staying at our Wizard House Airbnb to enjoy it and fix things up. It’s a small 1952 ranch home in a mountain climate with no wall insulation, only 2 inches of attic insulation, and the mediocre duct system it came with. (I did this on purpose to prove a point.)  

My wife commented that it was the most comfortable home she’s ever been in. That’s the “I can’t believe this is the same house!” comment, but in a different form. That house has a fairly simple HVAC setup that I’ve come to call Common Sense HVAC. 

Common Sense HVAC can tackle load matching, filtration, dehumidification, fresh air, mixing, and humidification with one system that is only slightly more difficult to sell and install than a standard single-stage system.  

Most consumers just aren’t aware of what a good HVAC system can provide. I’ve found the car-versus-home analogy to be very helpful in connecting the dots. Many homeowners become open to better systems and better installs if you use this comparison. Hopefully, it made you think about HVAC in a different way, too. 

Next time, we’ll break down what Common Sense HVAC looks like in a house. In the meantime, if you want to dive deeper into the 6 Functions of HVAC, read the free HVAC 101 chapter of my book at natethehousewhisperer.com.