HVAC ducts

I have an upflow forced air furnace to which I added a 3-ton air conditioner. The uncased A-coil sits in the plenum above the furnace. I have a question regarding airflow. Approximately 5 ½ feet above the top of the furnace is a square right angle to carry the air horizontally across the bottom of the first floor joists. Approximately 4 feet from that right angle is a second square right angle directing the air into the main duct for distribution throughout the house. My question relates to these right angles. Would it be beneficial to the airflow to change the second square right angle to a curved one? Would a curve not improve the flow? My concern stems from the fact I have 3 X 10 registers throughout the house, since originally there was no air conditioning. I have been told I should have 4 X 10 registers for the A/C. I know of no way I can make the change, therefore I’m looking to find any way I can to improve the airflow. If there is a way for me to change the registers, I’m prepared to do that. The unfinished basement portion is no problem, it’s the finished area of the house that is a concern. I’d appreciate any suggestions you might offer. Thanks.
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Every 90 is the same as 10 feet of straight duct. Turning vanes MIGHT help some. Eventually the duct pressurizes and the air travels where the pressure is lowest.
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I'm not familiar with 'turning vanes'. It sounds like something that would be installed in the existing square right angle? If that is the case, then would the curved corner (ie curved right angle turn) I mentioned be as good or a better option? I'm thinking if the duct has to be opened up to install vanes of some sort, it will probably cost me more than to have a curved corner duct fabicated and installed. The big question is whether or not I'lll see improved airflow...no matter which option is chosen.
SQLit wrote:

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This is Turtle.
Taking out two square angle turn in a plenum box run to go to two curve turn type turns would improve about 5% to 10 % but if you did not increase the discharge duct sizes. Your effect would be really next to zero.
So Changing the discharge size duct sounds like the answer.
Now you do need a Manual D run on the duct work system to tell exactly what you need. then a Manual J for your heating and cooling needed. Then you would know for sure about everything.
TURTLE
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I'm not getting this at all. Have you run this system through a full air conditioning season? If so, what problem did you experience?
If the problem is that "someone" told you that you need bigger registers, perhaps you should try living with the system for a while to see how it works for you. You may have no problem at all.
The simple answer to "improving airflow" is to install a bigger, more powerful fan. But that may or may not solve a comfort problem if you turn out to have one.
--
Doug Boulter

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First get someone to MEASURE air flow with a flow hood or other device, NOT their hand. you should have around 400 CFM per ton. CFM=Cubic Feet per Minute. Installing bigger ducts would help, but the entire duct system needs increased, not just 2 fittings. A 90 degree elbow is equal to from 30 to 80 feet of straight duct if it is rectangular duct and 10 feet omly if it is round duct. note that to double air flow with a larger fan, you have to have 4 times the duct static pressure (square the increase) and 8 times the blower horsepower (cube the increase in air flow). These are called the FAN LAWS. The only way around that is a more EFFICIENT blower. but the best thing is to measure the air flow first. Note also that you may want to install a larger coil which would have a larger surface area and therefore a lower pressure drop. But that may cause a humidity problem, depending on where you live (climate).
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