Re: Installing duct work

Posted by David - OR on November 02, 2009 at 09:50:10 from (208.67.204.213):

In Reply to: Installing duct work posted by TimMiller on November 01, 2009 at 21:10:21:

The industry standard for ductwork design is published by the ACCA and is entitled "Residential Duct Systems Manual D".
The book can be bought over the internet or from a good technical book store.

After reading the book, I personally reduce the methodology to this:

Do a Manual J analysis of the heat and cooling load of the whole house, and for each separate room.

Size the equipment appropriate to the total load.

Divide the load of each room by the total load, then multiply this ratio by the total CFM of the equipment air handler. This gives you the CFM you need for each room. Choose the larger of the heat load or cooling load to size the duct.

Size your branch ducts to achieve less than 700 fpm flow velocity if metal, less than 500 fpm if flex. Consider running two smaller ducts rather than one large one if a room is particularly large. Choose a register to deliver about a 400 fpm face velocity to achieve adequate "throw", and place the register near windows or doors if possible (where the highest losses are). Be sure each room except bathrooms and kitches has a return duct back to the air handler. Alternatively, use a "pass through" to connect rooms which otherwise wouldn't have a return.

Size your main ducts to achieve around 600 fpm flow velocity. Size your main return ducts for around 500 fpm.

Double check that the airflow resistance of the overall system is within the fan curve of the air handler. (It should be, with these velocities, but if you have too many elbows, etc it is possible to mess this up).

Try to run ductwork within the conditioned space of the house, rather than attics or crawl spaces. If you must send your warm air "outside" (e.g. to an unheated attic),be sure to seal each joint very well with mastic or metal tape (ordinary duct tape is worthless), and insulate to code.

Nothing is all that complicated, but overall there is quite a lot to this. I designed the ductwork for my own home, and am quite pleased with how it turned out, but it took a great deal of reading and study -- maybe 100 hours or so. OK, so I'm a nerd and it was a hobby activity.

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