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Old 03-25-2007, 02:54 PM  
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Default Ductwork Mysteries

I'm just getting started on a large, low-budget rehab project on a farmhouse that was built before they knew what modern heating was. For the past thirty or forty years it's been heated (??) by two propane wall furnaces and a bathroom space heater. Less than half the house is heated directly, and while tearing out ugly plaster in three rooms, I plan to install 3x12 ducts between the joists for the installation of a new heating/cooling system. So far I can't find much info on the volume of ductwork needed to insure adequate airflow. Anything would be better than what it has now, but I don't want to pay for energy that isn't being used efficiently. Any suggestions?

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Old 03-25-2007, 09:41 PM  
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Hello Cboley and Welcome to the Forum:
HVAC contractors use a duct calculator to size their jobs. It is like a little slide rule that gives instant answers. I worked for a National Distributor of HVAC-R for 3 years in the early 90's but, somehow, I have lost my calculator.
The best I remember a 3 X 12 duct will not carry over 100 cfm; same as a 6" round duct.
Unico, Inc. makes a system that is well suited for older homes.
They start with a high velocity/ high pressure unit and use an 8" square duct for a trunk line, a hose that looks like vacum cleaner hose, silencer adapters at the terimnis and use a register that is no larger than a porcelain bulb fixture. They are very convenient to snake through walls, ceilings and floors. I have talked to customers who had them and they were very well satisfied.
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Old 03-25-2007, 10:39 PM  
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you are getting into the black arts as far as sizing the furnace and such also . take some time, you will NOT want to do this again
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