# Thread: Is this return big enough?

1. Regular Guest
Join Date
Sep 2012
Posts
37
Post Likes

## Is this return big enough?

I have a 2.5 ton AC on an 80,000 BTU gas furnace. The existing (single) return consists of a 20 x 20 filter grille and approximately 20 feet of 14" round pipe with two 90 degree elbows. This connects to a return plenum under the furnace.

I know I should increase the filter size to either 20 x 24 or 20 x 25 (the 20" width can't be increased, but I can accommodate some extra height). Is the 14" pipe large enough for this installation?

Thanks!

D.

2. Professional Member*
Join Date
Dec 2002
Location
SouthEast NC ICW & Piedmont Foothills
Posts
7,702
Post Likes
nope

3. The pressure drop of the return duct (assuming it's galvanized sheet metal) is only 0.036, and that's on the high side. Not knowing the plenum configuration (= system affect), type of filter, grill etc. it's hard to answer your question. Off the cuff, the 14" dia. duct is ok as pressure loss in duct is minimal.

4. Professional Member*
Join Date
Mar 2011
Location
Galveston Texas
Posts
530
Post Likes
If it was me neither would be big enough. A 20x20 grill is only large enough for a 2 ton unit, and the 14 in duct if it is galvanized would only handle ~700 CFM so that's less then 2 ton at 400cfm/ton. The 20X25 R/A would be just big enough for a 2.5 ton unit, and a minimum of a 16" duct is required (and that's still a bit light but only by ~20cfm) if galvanized and an 18 if flex (again it's still a bit light by 25cfm). These are for a bare minimum 1000cfm of airflow.

5. 20' of galvanized duct at 1000 cfm: .09 p.d./ 100 ft. =.09*20/100=0.018 loss. That is nothing. It's not "how much air a duct can handle", the only thing the fan cares about is how much static pressure it needs to overcome. I agree the filter is small, based on velocity, but all that does is reduce filter effectiveness of the filter and add to total system static pressure losses, well, it also increases NC (noise criteria).

6. 16 inch

7. Regular Guest
Join Date
Sep 2012
Posts
37
Post Likes
This broad range of answers is an interesting illustration of both the value and challenge of getting multiple professional opinions on most anything. It would appear that there is no absolute answer, though I suspect most of the responders would disagree.

D.

8. When I am selecting a fan motor for a 20K cfm air handler I need to know what the total system pressure is. I need to find which branch has the highest static pressure loss, both supply and return. Knowing what my loss is the only way I can ensure I select the proper horsepower of motor to overcome the total system static pressure. This is why manufactures have fan charts. Heck, try asking a vendor to give you a selection on a 2000 cfm roof mounted exhaust fan. The first question they will have is "what's your static pressure".

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•

## Related Forums

The place where Electrical professionals meet.