Will closing vents save energy?

By Arnie Katz
Will closing vents save energy?

One study found that in many cases closing registers would actually increase energy use.

Q A friend recently sent me information about a product that automatically closes heating/cooling registers based on the temperature in the room and asked me what I thought. The manufacturer claims substantial energy savings and comfort improvement. The units, which retail on the Web for $50–$60 each, including the wireless remote control unit, are said to pay for themselves in energy savings in less than a year.

A The theory goes as follows: many homes have systems that overheat some rooms in the winter and overcool some rooms in the summer. By closing the registers in the rooms that are too hot or too cold, you'll be more comfortable and you won't waste all that energy making those rooms too hot or too cold. At the same time, the hot or cold air is now "directed" to the rooms that need it. The end result is — according to the theory — balanced temperatures throughout the house, leading to higher comfort and less energy waste. This is very appealing and certainly sounds logical, but does it make sense?

The answer is probably not. A study by Lawrence Berkeley National Labs a few years ago looked specifically at energy savings from closing registers. They couldn't find any. In fact, they concluded that in many cases closing registers would actually increase energy use. Why would this be?

It's not really a mystery. Closing some registers increases the pressure in the ducts. Increasing the pressure in the ducts increases the amount of air leaking from any holes or gaps in the duct system. More duct leakage means the furnace or air conditioner has to heat or cool more air, which will make it run more, using more energy.

The study showed that whatever savings were gained by reducing the amount of space to be heated or cooled was more than counter-balanced by the increased duct leakage. Obviously, if the ducts are air tight — like they're supposed to be — this wouldn't be much of a problem.

Are ducts in North Carolina leaky? Some of the first studies in the nation on this question were done here in the Old North State and showed substantial leakage in most duct systems. Follow-up studies over the years have indicated we're getting better, but most of our systems are still very leaky. About 20 percent of every dollar we spend to heat and cool our homes is lost to the attic, crawl space or yard due to leaky ductwork. That's $1 out of every $5.

Even if you have very tight ducts, it may not be a good idea to close registers. If the ducts are tight, closing registers will reduce the air flow across the heat exchange coil, which can damage the unit. Even if it saves you a few dollars, if it leads to premature failure of the compressor you'll wind up paying dearly.

Typically, the rooms that overheat in the winter are upstairs, and the rooms that are too cold in the summer are downstairs. That means you would need either to buy twice as many of these automatic closing devices, or else move them around every spring and fall. Probably not a big deal if your registers are in the floor, but what if they're located in a cathedral ceiling 14 feet off of the floor?

Just for fun, enter "closing heating vents" into your favorite Web search engine. You'll find lots of advice, conflicting claims, sales pitches and a few actual studies. As far as I can tell, though, no one has published a study that reported on monitoring actual energy use in actual houses with actual people living in them. One of these decades, we'll be able to base our decision-making on actual data. In the meantime, I would invest in duct sealing rather than in gizmos that sound good but probably won't deliver.

About the Author

Arnie Katz is the former building science consultant for Advanced Energy in Raleigh. advancedenergy.org

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