How big should the intake duct be?

joshoc · 06-15-2010, 09:02 PM

How big should the air intake duct be for a 3 ton HVAC unit heating and cooling 1800 square feet of first floor space in a house?

Thanks,
Josh O.
Nashville, TN

**beenthere** · 06-15-2010, 09:08 PM

Depends on how long it is. How many ells in it.

joshoc · 06-15-2010, 09:27 PM

From the intake opening on the wall to the HVAC unit is about 30 feet. The intake opening on the wall faces the direction of the HVAC unit, so I'm guessing the duct does a 180 degree turn to get going the right direction. That could add a few feet to the length of the duct in addition to the 30 feet.
Don't have any idea what you mean by ells.

**beenthere** · 06-15-2010, 10:06 PM

ells. Look at a plumbing pipe. That makes a 45 or 90° turn. The fitting used to make that turn is an ell.

A 180° turn is equivalent to about 45 foot of duct. How is it attached to the furnace.

joshoc · 06-16-2010, 06:57 AM

The duct just goes straight into the HVAC unit. It is an outdoor unit on a concrete pad next to the house. The only turn in the duct is that one 180 degree turn at the intake opening. The rest of the duct is straight.

**beenthere** · 06-16-2010, 07:04 AM

Min of 28X8, or 22X10.

Because you will also need to make a transition to the units opening. Which also causes resistance.

joshoc · 06-16-2010, 07:52 AM

What do those measurements refer to? The opening of the duct at the point of intake is round. How many inches in diameter should the opening be? The duct is round.

**beenthere** · 06-16-2010, 07:56 AM

16 inch.

joshoc · 06-19-2010, 10:53 AM

Ok. How do you calculate that?

**beenthere** · 06-19-2010, 10:57 AM

CFM needed,air velocity desired for performance and noise reduction, friction rate/pressure loss through length of duct, and 180° turn.

jack of none · 06-20-2010, 01:50 PM

Hey been there,speakin of intake duct,my old airhandler/furnace was oil,and the return was the door unit,so theres no return ,and i have limited space,the unit bein in a closet,how short of line is ok,i have about 6 ft, horiz. and vert. to install,thanks.

**beenthere** · 06-20-2010, 02:04 PM

Shortness only hurts as far as noise.

2 90's helps to reduce noise. But there is no limitation to shortness.

A single 90 could be equal to 15 foot of length. Unless its an ell attached directly to the furnace. Then its equal to closer to 135 foot, if it isn't a radius ell, or have turning vanes.

jack of none · 06-20-2010, 05:25 PM

hey Beenthere,the line set goin under the house is not gonna happen,i think im gonna run it up the outside wall,thru the soffet,and back down to the air handler,have you ever used slim duct? if not,are there any other ways to hide the line set? If i only go up 10 feet,will i need an oil trap?

**beenthere** · 06-20-2010, 05:34 PM

Slim duct is nice stuff. Nice price tag on it too. You can also use down spouting. So it looks like its part of your rain gutter system.

Technically. An oil trap would be put in the high point of the line, before it goes down to the condenser. Since you are creating an inverted trap in the line. This can be just a slight bend in the line. Doesn't need to be a fitting.