When an air conditioning system is supplying just the right amount of cooled air to each room in a home so that an even temperature is maintained throughout the home, the system is called “balanced” by HVAC (Heating Ventilation and Air Conditioning) contractors. Balancing a system is not easy, and some floor plans are more difficult to balance than others, particularly long ranch-style and two story homes. A home with rooms that are always colder or hotter than others has an unbalanced system.

   Because the air conditioning indoor unit, called the air handler, is usually placed in the garage or utility room at one end of a long ranch-style home, it becomes a design challenge to get sufficient air flow to the rooms at the opposite end of the house—sometimes 50-feet away—without blowing too much air out of the nearby ducts. And a two-story home presents a doubly difficult task: getting air to go up to the second floor (or down to the first floor) in a way that provides equal air flow to each floor, while simultaneously allowing for the fact that cold air falls and hot air rises, so the first floor receives cold air continuously migrating down the stairs.

   A good air conditioning contractor uses progressively smaller duct sizes, along with carefully planned supply and return air registers (vents), to achieve a balanced system. But sometimes that’s not enough, and dampers are installed to further tweak the air flow. Dampers can be recognized by the little metal handles sticking out of the side of a duct used to adjust the amount of air flow.

   Also, an inadequate route for air to return back to the furnace or air handler from each room can cause temperature differences, especially in rooms with closed doors. A door undercut, return air register, or grille over the door (called a “transfer grille”) can improve the air flow going back out of the room.

   If you have an unbalanced air conditioning system, an a/c contractor can balance the system through adjusting the blower fan speed, installing dampers, adding insulation, or other techniques. The cost runs from only a few hundred dollars to much more. But your problem could be related to simple maintenance issues. Because you call for service, check the following:

1) A dirty filter reduces air flow. Check your filter. Many newer houses have filters at multiple return air registers around the home—at two separate hallways and the master bedroom, for example—so make sure you know where each filter is located and confirm they are all clean.
Double-filtration is not recommended, but occasionally we visit homes that have a filter at or next to the air handler, and also other filters at return air registers. So, again, be sure to verify that they are all clean. However, because having the return air pass through two filters can reduce air flow, you may want to eliminate one set of filters.

 2) Make sure all the registers are open. They don’t have to be open all the way, but completely closing a register is problematic for two reasons. First, it can unbalance your system. And, second, a closed register—with all the slots locked up—tends to develop condensate on the surface of the slats, and the back-pressure causes air leakage at the connection of the duct and the register (as shown in the infrared photo at right), with the subsequent mold growth over time. Also, look into each register for any debris behind the slats that might be blocking air flow.

3) This may sound obvious, but check to make sure windows are fully closed and not leaking air. Also, closing blinds or curtains can help stabilize the temperature of a room.

4) If your ducts run through the attic, poke your head up into the hatch opening and look around with a flashlight for any damaged ducts, specifically at connections Ducts can pull apart at connections over time and small animals that occasionally get into an attic can tear holes in them. If your attic is nearly as cool or warm as your house, investigate further. Also, the newer flexible ducts (like the ones shown in the photo above) sometimes get crimped by their hangers or an abrupt bend around a truss chord, and can get trampled down by careless workmen in the attic—any of which can cause reduced air flow to a room.

   Don’t expect perfection. There will always be one room that’s a little warmer or cooler. But if it’s more than you want to tolerate, and the above measures don’t work, call your favorite a/c service company for a solution.

    Also, see our blog posts Why is my air conditioner not cooling enough? and Will closing doors reduce my heating and cooling costs? and Frequently Asked Questions (FAQ) about Air Conditioning Troubleshooting.

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  To learn more about heating and air conditioning systems, see these other blog posts:

• How can I find out the SEER of my air conditioner? 

• My air conditioner won't turn on. What's wrong? 

• How can I find out the size of my air conditioner? 

• How can I tell whether the condenser (outdoor unit) is an air conditioner or heat pump? 

• Where is the air filter for my central air conditioner and furnace? I can’t find it? 

• Does an old air conditioner use more electricity as it ages? 

• How did homes stay cool in Florida before air conditioning? 

• What is wrong with an air conditioner when the air flow out of the vents is low?

• Why has the thermostat screen gone blank? 

• Why does it take so long to cool a house when an air conditioner has been off for a while? 

• What are the most common problems with wall/window air conditioners?  

• Will closing doors reduce my heating and cooling costs? 

   Visit our HEATING AND AIR CONDITIONING page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.