Duct Layout for 25' X 50" home 1

[mikeo56]

I am running new duct work in the attic of my Collier county, Florida home. The air handler is 2.5 tons and the home is 50' X 25'. There is a new plenum attached to the air handler with no perforations yet. Can I use flex throughout? I am concerned with if I need to vary the duct size to get an equal distribution of air to the 2 beds, 2 baths, closet, utility room living room and kitchen.
Thanks, Mike.

 

[imok2]

The size of takeoffs to each room is determined by the amount of CFM required for each room. The recommended length of flex duct is about 6FT from outlet fitting to hard duct and the flex duct must not have any sharp turns or kinks.It is imparative that you seal all fittings and there are no leaks in the distribution ducting

 

[CountOlaf]

Mike,

First a couple of comments on your terminology (just for fun--I see you're structural!). The "air handler" isn't 2-1/2 tons. The dx cooling coil within the air handler and corresponding condensing unit (or heat pump) have a capacity of 2-1/2 tons. The air handler includes a blower and filters (and heating coil or burner if you didn't live in Florida) in addition to the cooling coil. I assume by "perforations" you mean the branch duct take-offs. It's not as if you're going to punch a whole bunch of small holes in the plenum and let it blow the air freely out of these holes!

Anyway, you can use flex duct all over the place if you want. The problem is there's a pressure drop associated with ductwork and the smaller and more convoluted the ducting is (not to mention if it's ribbed like flex duct), the greater the pressure drop. This means that you will start losing desired airflow at the registers (air outlets). The blower (fan) is rated for a certain flow rate at a certain "external static pressure". If the system static is made too large (by excessive flex ducting, etc.), the airflow rate (CFM) will drop off and certain areas which are farther away from the blower will not get adequate air/cooling. That's why you see mostly rigid ducting with only a small amount of flex (if any at all in residential applications).

Anyway, assuming you don't have too far to go, the way you get an equal distribution of air is by making the longer runs with larger ductwork and the closer runs with smaller ductwork--to a degree. The other way (and possibly in addition to the former) is to have each branch equipped with a manual volume damper which gives you a way of balancing the airflow--dampering down (closing off) the runs closer to the blower and opening up the farther runs.

Good luck.

 

[lilliput1]

There is advantage to ducting off individual branches from the plenum with each branch serving only one air terminal (instead of a branch of varying sizes, serving several air terminals in series). Ajjusting the volume damper of a terminal would have minimal effect on the other terminals.

 

[imok2]

If you decide to use all flex duct on runs longer then six ft. you need to follow the manufacturers recommended
intervals, supported at no greater than a
distance of four feet. Maximum permissible sag is 1/2 inch per foot of space between supports.
A connection to rigid duct or equipment shall be considered a support joint. Long horizontal duct runs with sharp bends shall have additional supports before and after the bend
approximately one duct diameter from the centerline of the bend.

 

[mikeo56]

Thanks, For the help, Mike.

 

[cme]

There is 'cheap' flex duct like ATCO UPC-models or premium like stuff Thermaflex corp makes

 

[cme]

If you choose sheetmetal ductwork, I would suggest that you insulate it with Armacell rubber type. Since you are in an attic and in humid Florida, the Armacell has the greatest resistance to moisture penetration. You have to make sure that if you use fiberglass (with its scrim jacket), the installation is done properly and sealed. Just my 2 cents.

 

[lilliput1]

We do not allow use of more than 1/2" thick elastomeric type insulation indoors because the fire & smoke rating will exceed the 25/50 limit.