Last week I attended the HVAC/R Symposium in central Florida. Three of us from Energy Vanguard went, so we rented an Airbnb. Of course, one of the first things we did upon arrival was scope out the house, especially the heating and cooling system. It was no surprise that we found it had a platform return plenum (defined below). Nor was it surprising to find it in the garage. Florida is the land of the garage platform return, you know. The bad news is that those two things can seriously harm a home’s indoor air quality.

The video above is a brief introduction to this type of ducting and some of the problems with that particular installation. Let’s look at this issue in greater detail.

What is a platform return?

A platform return plenum is one of the easiest HVAC concepts to understand. It’s exactly what the name says it is. Before I continue, let’s look at this annotated photo, which shows how a forced-air heating and cooling system moves air.

There’s a lot going on here, but if you follow the arrows, you can trace the airflow. Starting at the upper right, air from inside the home gets pulled back to the furnace and air conditioner. That’s called the return air. The return plenum combines all of the return air before it goes back for more heating or cooling. The system shown above is fully ducted with approved duct materials.

In contrast, a platform return uses site-built ducting. It’s just a big box with the air handler sitting on top. The lead photo of this article shows our Airbnb’s platform return plenum highlighted in red. In addition, the inside of the return plenum is often connected to building cavities, like the wall behind the HVAC system.

Leaks and indoor air pollutants

Here’s the thing about platform returns, though: Because they’re built on-site using nonstandard duct materials, they often don’t get sealed up well. And because they’re on the return side, those site-built, potentially leaky ducts are under negative pressure. Remember, the blower is pulling air back from the house through the return side of the duct system.

So what could go wrong? The best-case scenario is that the platform return is inside the house and sealed properly to prevent pulling in dirty air. That’s kind of what we had in our Airbnb last year (photo above). The garage was on the other side of that air-handler closet, though, so there may have been some connections to the bad air there.

But when you put the system in the garage on a platform return, you’re just asking for trouble. When the return ducts leak there, a lot of bad pollutants can get pulled into the system and then distributed throughout the house. That could include:

• Carbon monoxide
• Water vapor (not a pollutant per se, but often a problem in humid climates)
• Emissions from pesticides, paint, or gasoline
• Automobile exhaust

Worse, if the garage has a natural-draft gas water heater, the negative pressure created by return leakage could cause it to backdraft. That simply means the exhaust gases from the combustion don’t go up the flue and out of the house. Instead, they get pulled into the garage. From there, they can get pulled into the duct system from the return leaks and then be sent into the house. A clothes dryer in the garage also can cause the water heater to backdraft.

Better ways to move air

The good news is that the kind of platform return we saw in our Airbnb last week is not allowed in new homes. Here’s what the 2021 International Residential Code (IRC) specifies for ducts in a garage:

R302.5.2 Duct penetration. Ducts in the garage and ducts penetrating the walls or ceilings separating the dwelling from the garage shall be constructed of a minimum No. 26 gauge (0.48 mm) sheet steel or other approved material and shall not have openings into the garage.

The IRC, however, is simply a model code. The code in effect in your location is set by the state or a local authority, which often does something different. That seems to be the case in Florida, because a contractor I know there told me platform returns are still being installed and approved.

As I’ve written here many times, the best place for ducts is in conditioned space. Not the attic. Not a vented crawlspace. And definitely not the garage.

Even in conditioned space, though, you want to have your ducts sealed. This ensures that your air moves the way it’s supposed to move. It also helps keeps the pressures in the house balanced. Negative pressures anywhere can cause problems.

If you have a platform return plenum—especially one in the garage—you should line it with approved duct materials and make it as airtight as possible. And don’t forget the filter slot!

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Allison A. Bailes III, PhD, is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia.  He has a doctorate in physics and is the author of a bestselling book on building science. He also writes the Energy Vanguard Blog.  For more updates, you can follow Allison on LinkedIn and subscribe to Energy Vanguard’s weekly newsletter and YouTube channel.