Size ductwork to furnace or a/c ?

[orangemeatball]

Should ductwork be sized to furnace or a/c. New 80k 90% gas furnace has 2, 2 1/2, 3 ton blower. Two ton a/c needs 800 cfm, furnace tag indicates blower is 1200cfm? should duct be sized for 1200 or 800?

[amickracing]

Well technically it should be sized to the house (heating/cooling load). That said I generally go with the 400 CFM per ton, so I go cooling.

[trane]

Amick is right , the ducts should be sized for the required cfm of the house . Also you may have trouble with the temp. rise if you size this for 800 CFM.

[coolguysfl]

Ditto.

Add to that:

1) Size to .1"WC (by the time it's installed you'll be well over that)
2) Do not exceed TTL @ .5" WC
3) Upgrade your IAQ now
4) Use two ductaltors - 1 for hardboard 1 for flex

[framehvac]

amickracing
Professional Member

Registered: May 2004
Posts: 372
Well technically it should be sized to the house (heating/cooling load). That said I generally go with the 400 CFM per ton, so I go cooling.

You get the furnace & a/c size first then you make the duct to to fit to the sizes of furnace or a/c.

I think this is what you meant.

You wouldn't won't the duct to big or small for the furnace.

[orangemeatball]

Just to clarify, cooling load @22,000 heating load @68,000. If only installing air handler with 2 ton coil would size for 800 cfm. Since furnace indicates blower is 1200 cfm, not sure if ductwork should be sized for this instead? Don't want to exceed temp. rise. I think rise was 45-70. Would it be say 70 at 2 ton speed @800 cfm and 45 at 3 ton speed @1200 cfm? Or would you size duct system for 3 ton and slow speed down for a/c operation to acheive 800 cfm. Space is a concern with duct sizing. (not as much of a concern as the correct installation) At 1200 cfm duct would be @ 10" x 20" @ 900 fpm? At 800 @650 fpm with same size duct. Thanks for the responses!

[dash]

You can't just size the duct to .1" of WC,or 900 fpm.

You need a Manual D calculation to size it correctly.

The furnace being capable of 1200 cfm ,is if you needed 3 tons of cooling,what cfm is required for heating???

[wendel]

A crazy thought--could you have a dampered, 10" bypass duct around the 2 ton coil(furnace output to supply plenum)? Run the AC on low fan speed(800 cfm) with the bypass closed. Run the heat on high speed (1200 cfm) with the bypass open. The house ducting could be a compromise designed for 1000 cfm. Register velocity would be a little high when heating and a little low when cooling.

[bornriding]

In my world ( L.A. ) ( Lower Alabama ). we size the ductwork for tha max cfm that the system is required to carry, which is the cooling cycle for us. The velocity drops for the heating cycle but thats probably ok since you need less velocity for heat than cooling

[beenthere]

On average, a 80,000 furnace should move 960 cfm, for heat.

In this area, the cooling cfm is alsmost always greater then the heating.

When we do run into the heating being greater then the cooling, we design our duct for cooling, on med, or low speed fan, and set the blower to run on the higher speed for heat to keep temp rise in the middle of the data plate.