Balancing Forced Air Systems Q&A

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Dear NH,

I have a small problem when our heating or AC system comes on; the hot/cold air blows through all of the vents except one. How do I rectify this, as it does make one of the rooms either the coldest room or hottest room depending on the time of year?

RD from Grand Terrace, CA

There are two ways to solve this problem. The easiest and cost-free way is to simply partially close down all the other vents to force the air to flow more readily to the "starving" register. This is a primitive but effective way to, as they say in the biz, "balance the system". This is a common way to make seasonal changes in a two floor house with a single zone. In the winter months, you want more heat downstairs, but it tends to rise upstairs. By partially closing the upstairs registers, the downstairs registers blow more heated air… which eventually rises upstairs anyway. In the summer, the reverse is done. The upstairs registers are left open wide, while the downstairs ones are partially closed. Of course, you will have to fine tune this method through trial and error to get the best seasonal balance.

The other way, which would require more work and expense, would be to install a boosting fan for that register. These are designed to turn on when they sense air movement or a temperature change. They can be installed as part of the heating system, and are tied into the central unit. There is another type that I have seen available in some mail order catalogues and home stores. It is installed in place of the register, has a temperature-sensitive switch and receives power from a nearby outlet.

Dear NH,

This really isn't a question. Just a thought on the guy who said his office never gets cooled as much as the rest of the house by the central ac. Our son's room was like that and finally an ac repair person pointed out that his room is above the garage which, while it is well insulated, it is not air conditioned and the heat impacted on my son's room. We added an extra ac vent that helped a bit (at least when the ac is on), but there was only so much that could really be done. As I said just a thought.

Dear J,

It's tough to face reality sometimes. Though we would like our homes... inanimate objects that they are... to knuckle under to our wills, Mother Nature has a way to keeping us honest.

From your letter it is unclear if anyone checked to see if the ceiling above the garage was insulated. Sometimes these spaces have room for up to 10 inches of insulation and only have a paltry few inches. If your home was built before the 1970's, chances are having more insulation pumped into the space between the garage ceiling and your son's floor will work wonders!

Dear NH,

As a new homeowner and a handyman-wannabe myself, I must thank you for your outstanding page. With your help, I just finished re-installing our guest bathroom toilet from the wax ring up. And since it still doesn't seem to flush very well, it's off to the roof to attack the vent.

After this, I must try to figure out why my office is the only room in the house that doesn't get cooled efficiently by the central A/C system. It's most likely because I'm the one in the family who likes it the coldest! Unfortunately, the soft duct leading to the room goes from the attic up into a crawl space over a finished room and the builders didn't include any access panels to look for clogs or kinks or leaks. I'll have to find him and thank him sometime. Any suggestions for how to tackle this one before next Summer?

Again, thanks for your excellent page and great sense of humor - toilet humor, as it was :)

You may be right that the soft duct is slightly crushed. It is also possible, though, that the flexible duct is just too long. Those ducts cause lots of resistance to air movement... far more than rigid metal ducts...which might explain the low air volume received at a distant location.

You can try to compensate by "balancing the system". This is a fancy name for the creative closing... fully or partially... of all registers to the colder rooms (during AC season, that is... the reverse for the heating season). This forces the HVAC fan to overcome the resistance in the duct and force more air to the distant locations... in your case the office!

This simple method of balancing is "seasonal", meaning that you may have to change your adjustments with the seasons depending on whether you are heating or cooling!

Dear NH,

Dear J,

It's tough to face reality sometimes. Though we would like our homes... inanimate objects that they are... to knuckle under to our wills, Mother Nature has a way to keeping us honest.

Dear NH,

I live in a two-story house with the foyer open to the second floor balcony. Heat from downstairs rises and tells the thermostat located at the top of the stairs that it is 70 degrees. When we get up in the morning, the master bed and bath are freezing but the hallway is warm. Would relocating the thermostat further down the hall help read the actual temperature. By the way, if we turn down the heat downstairs, it's unbearable at breakfast!

LM from Conyers, Georgia

Dear LM,

You must sleep with the bedroom door closed at night! A good practice, really, since it offers you some protection from smoke in the hall in case of a fire... but you can still hear the smoke detectors wailing! You do have a smoke detector in each bedroom and the upstairs hall, don't you?

I would suggest that instead of moving the thermostat down the hall, you run the thermostat wires through the wall into one of the bedrooms. This would allow you better control over the temperature in them, and allow you to leave the downstairs thermostat set at a comfortable level. Just hand a picture over the old hole until you repair it or leave it if you decide to return it to its original location.