Balancing Ductwork

Old 01-10-02, 11:33 AM
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Balancing Ductwork

I just recently purchased my first house. The house is CBS construction and is 1025 sq.ft with a 500 sq.ft. addition (regualr stud walls with vinyl siding) on the back. I have come to the conclusion after the past couple of weeks that the Heating/Cooling system is not balanced well at all. I am relatively sure that when the addition was built, the A/C was not resized.
I can have a temp of 67 in the addition, while it is 71 at the thermostat and 74 in the master bedroom. Im looking to try to balance the ductwork myself, if possible. Can anyone point me to a good reference on how to calculate how the system should be balanced?

Help is appreciated...

Cocoa, FL
Old 01-10-02, 02:02 PM
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I'm no HVAC person, but I believe that there is no balancing calculation for residences due to duct work types and the difficulty in measuring cfm. Are there accessable dampers (auto or manual) on the ductwork? It is a matter of trial and error.
Old 01-10-02, 06:02 PM
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From your question, I take it that this is new construction and you have an outlet in the added room. Ordinarily, the added room usually is the room thats warmest. However, to start with you should try and check out your ductwork visually, taking note to duct sizing. In other words the largest duct work would be right at the blower outlet supply air and be reduced in size throughout the house depending on the number of outlets and their distance from the blower. Since the added room is at the rear of the house, I can assume its probably the furtherest from the blower. If so, then you have duct design problems, or something inside the duct system that is diverting and/or restricting air flow. In your area, its common to have flexible ductwork and if installed incorectly, there could be some ":squeezed down" duct runs or too sharp a 90o turn. Any a/c system is no better than its duct design and unfortunately, good residential duct design and installation techs are few and far between. It wouldn't be the first time to find a piece of leftover duct material left inside the ductwork. If you have a new house, then you should jump on the warranty. But to balance your system's air flow in the rooms without the luxury of a velometer or a sling psycrometer then I would tell you to set your, fan for continuance, get a step ladder and feel the air coming out of the registers with the back of your hand, which is more sensitive than the palm. Open all the registers in every room. Start at the closest to the blower and start dampering each down until you feel the furtherest has about the same velocity as the closest. The last in line may be wide open whereas the nearest one to the blower might be half-closed. Its time consuming but about all you can hope for is to get relatively close temperature in all rooms. Don't forget, your t'stat thermometer could also be out a degree or two. Check it out with a good thermometer. When you find it too cold in one room and warm in another, damper down the cold one , putting the air in the other rooms. If you have cheaper type registers that have horizontal dampning blades that turn down, you won't get very good results. Whereas if your have opposed blade horizontal dampers then balancing is a lot easier. I don't mean to confuse you with this answer, but you've got a duct problem. Good luck.
Old 01-10-02, 10:27 PM
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duct balance

Another thing that you haven't mentioned is the return air. The air from the ducts has to get back into the system somehow. If the addition doesn't have it's own return duct, you can really mess up the balance by simply closing a door. If this is the case, make sure that the doors have a 3/4 inch gap at the bottom of the door for the return air. Balanceing is often easier to do with return ducts rather that the supply ducts.

Just my 2 cents worth.

Old 01-11-02, 11:27 AM
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Florida systems tend to be alittle oversized due to the humidity/heat factor. At 1500 sq ft you should be operating with at least a 2.5 ton system. If you have less than that, no balancing in the world will cool or heat your home.

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