Are Smart Vents Safe?

So many smart home ideas purport to save you money on energy and make your house more comfortable, but sometimes they can cause more problems than they solve. At National Geographic, Wendy Koch’s CES coverage mentions "smart vents," vent covers for your heating and air conditioning system that let you set the temperature in your rooms by opening and closing the vents. She notes two companies that made a splash at CES:

The Keen Home offers “Intelligent heating and cooling.... Use your smartphone to take control of your air flow. Or sit back and let our Smart Vents sense when it's gotten too hot or too cold, then adjust automatically to keep your entire home comfortable." The Ecovent will “Eliminate hot and cold spots in your home. No smart thermostat can do that....Kids off to college? Guest Rooms? Stop heating & cooling that EMPTY SPACE!”

Not So Smart Vents

Except that it's a really bad idea to close off vents to a room. As energy expert Allison Bailes III notes on his website, most home heating systems are not designed for this. "... the system is designed for the blower to push against some maximum pressure difference. ... If the filter gets too dirty or the supply ducts are too restrictive, the blower pushes against a higher pressure.”
So when you close vents in rooms you're not using, the blower will just push harder. Since the air pressure is higher, the leakage is greater. Depending on what kind of fan you have, different things happen, both of them bad.

The more vents you close, the higher the pressure in the duct system goes. The ECM (electronically commutated motor) blower will use more and more energy as you do so. The PSC (permanent split capacitor) blower will work less but not move as much conditioned air. In both cases, the duct leakage will increase further. In air conditioning systems, it can cause the whole thing to turn into a block of ice. In heating systems, lower flow can cause the furnace to overheat and even crack. “When that happens, your duct system can become a poison distribution system as it could be sending carbon monoxide into your home.”

Bailes concludes about a standalone smart vent: (not one being discussed here) "This is an HVAC product developed by people who don't know some very important principles of heating and air conditioning. Let's hope they don't kill anyone." That's strong language, but there is science behind it.

Do Any Smart Vents Work?

But what about these specific smart vents?

The Keen Smart Vent is the first of a number of products the company claims is designed "to bring proactive intelligence to a home's core systems." The website gives little information, but if you go back to the company's Indiegogo campaign, there is this note: "To avoid damage to the HVAC fan, only one third of all vents in a home should be Keen Vents."

There is not enough information to determine if the Keen units go further than that, or if limiting the number of vents is the only protection. However shutting off a third of the vents could make a big difference in the amount of air being pumped.

Unlike Keen, the Ecovent people are not trying to build a connected house; they are just trying to solve this one problem, and they take a very different approach. For one thing, you have to do the whole house. “Since every vent in the system works together, you should replace all your vents.” Then when you go into the FAQ section, there is this zinger: “I’ve heard closing vents is bad for your HVAC system.” This links to a white paper the company has prepared looking at the issue. It starts with this:

We have found that home-owners who close vents in systems that lack dynamic monitoring of system load, flow conditions, and system efficiency may experience increased system pressure, increased noise, inefficient air leakage, decreased comfort, and the potential for system damage and shortening of equipment life. In short, things that are not the best for an HVAC system.

They conclude:
To truly address HVAC zoning at the vent register level, an equipment manufacturer must account for the real-time conditions affecting the home and its mechanical systems. A mechanized vent register that does not incorporate dynamic sensing and control will put the system at risk and will not address the concerns we reviewed previously.

So clearly, those devices that you plug into the wall to control the vent in each room are not only talking to your phone but to each other; if you shut one room down, the system is going to open another. Their plug-in units monitor temperature, humidity and proximity and feed it all back to a central hub that controls the whole thing together. This is definitely smarter.

Start With the HVAC

Both systems go back to the fundamental problem that was addressed in an earlier post on smart homes: that this technology isn't really needed in a home that was built right in the first place. A properly designed and balanced HVAC system in a decently built and well-insulated house wouldn't have all these hot and cold spots. The heat loss from an empty room would be minor. So far as I can tell, all systems like the Ecovent can do is add a little more air in one room and a little less in another and hope that the algorithm is smart enough to avoid damage to the HVAC unit. No doubt at some point soon it will actually talk to the blower and turn it into a complete variable air volume system and elmininate this risk, but it's not there yet.

The Ecovent is a clever and well thought-out system, but let's not oversell it; it's still just a Band-aid for badly designed systems in leaky houses. But it is a step in the right direction — and it is clear that they know their engineering.