The Case of the Duct That Wasn't There
I saw the photo below on the Structure Tech Facebook page recently. (They're a home inspection company in Minnesota that appreciates building science.) In this case, everyone can see one of the problems. Even if you know nothing about building science, you can see it, right?
That obvious problem, however, isn't the only one. It may not even be the worst one. The other problem here is that the register you see installed in the toekick of the kitchen cabinets is not connected to any duct work.
According to the guys at Structure Tech, this one actually does have a duct above the floor level. Many don't. Installers will run the duct in the crawl space or basement, connect it to the boot, and just let the air flow into the space beneath that cabinet.
The problem with that is that the conditioned air often finds other ways out of that space, so you've got a lot of duct leakage. The kitchen may be less comfortable. You could also have pressure imbalances in the home as a result of this, and that can lead to more infiltration.
If that's not bad enough, in summer you're pumping cold air into that space. The cabinet with the toekick supply vent is often the one with the kitchen sink. Sinks leak. Cold materials stay wet longer. Voilà! You've got a biology experiment in your kitchen cabinet.
Oh, yeah. It's also a violation of the building code not to connect all of the supply vents with ducts.
If you must have that vent in the toekick, here's how it should be done. Install a real duct across the subfloor, and then protect it with something like the OSB you see below. (Note: The OSB you see below is not the duct. Look closely and you'll see the sheet metal inside near Jeffrey's fingers.) If you don't protect it, that duct may not make it through the construction process.
Simple concept, really. Channel your air to make it flow where it's supposed to. If you don't, it will act like an undisciplined child and get itself into all kinds of trouble.