Don't block return-air vents
26 March 2005
A. First, if at this moment you have covered any of your air-returns, put the paper down and go tear those covers off then come back and finish reading this.
Your heating and cooling system, and the vast majority of residential forced-air heating and cooling systems, is designed as a closed-loop system. It takes air that’s in the house and pulls it into the conditioning part of the system via the return ducts. That’s why they are called return and not exhaust. You can usually tell which grilles those are as they tend to be located high up a wall-- but not always—and are the ones that will collect a bit of dust on the leading edges of the grille vanes because air entering the grille from the house will deposit some household dust on it as it passes into the system.
The air then gets conditioned in the air-handler that we’ll find in the basement, crawl space, attic or in a utility closet, depending how smart or dumb the builder was, then is returned to the house heated or cooled. Forced-air systems are inherently leaky and I don’t like them in crawlspaces or attics where they can pull nasty air into the systems but as long as the model codes permit it there’s little I can do about it.
You have a finite amount of air in the house and if you close off a return vent for any reason unless you have opened another path of air-return of the same size somewhere else then you are strangling your system and it will not deliver heated or cooled air properly for you. It was designed to pull air from the house at a rate of about 400 cubic feet per minute (CFM) per ton of air-conditioning and to return it at that rate. You frustrate that design by doing what you propose.
Some folks will attempt to “tune” their heating and air-conditioning by closing off supply vents in unused rooms or lower rooms in the lower level during hot spells and vice versa during winter. The theory is the conditioned air will follow the path of least resistance.
What your air system is doing as it operates is trying to balance the air in the home relative to its temperature. Measure the air temperature as it enters the return grille and compare it to the temperature of the air exiting a supply register. The difference will be around 14 to 18 degrees. Normally, the thermostat is located somewhere near a return-air grille and the design is that the thermostat will call for heating or cooling until the air migrating back towards the return ducts equals the set temperature telling the thermostat that the temperature request has been satisfied and shuts the system down. What you are trying to do is to balance the air temperature from one level of the home to another and what you’re battling is the natural stack-effect of warm air tending to rise. If it really bothers you and you can stand the sound of the fan running a lot turn the setting on the thermostat from the AUTO setting to the ON setting and the fan will run continuously keeping the air mixed and the temperature differentials from one level to next should only be a degree or three. The amount of extra electricity you’ll be using is the equivalent of leaving a couple of lights on. The goal is comfort. We are slowly but surely getting better at building tight, well insulated houses with windows that aren’t big heat gainers or losers into which we can put heating and cooling systems that will work quietly and efficiently to the point we won’t even notice them. But that day is still way off in the future for most of us.