You are here:

Heating, Air Conditioning, Fridge, HVAC/Return air flow for 3 ton residential a/c

Advertisement


Question
We've lived in this 3400 sq ft 1-1/2 story house in OK for over 25 years, built in 1969. All ducting is the same since we have lived in the house, and I assume it was built that way.  About 20 years ago, I had a 3 ton Trane installed to replace upstairs 1200 sq ft a/c. Less than 6 mos ago replaced with Amer Std. silver 3 ton (compressor not 2 stage). Return air is by a 16" flex duct through the attic to the blower/heater located over garage at first floor level. Duct is attached to a 12x24 opening standard grill cover in ceiling of the hall in the center of 2nd story. There are also four 8x24 grill covered openings in wall at floor level in different rooms for air(?) to return up between 2x4 studs. I had assumed all these years that was plenty of return.  However, I was told with most recent a/c installation that all the 8x24 spaces are blocked in the attic - i.e., air in the space from these floor vents goes no where - not connected to the 16" vent.  That's 768 sq in. (5.3 sq ft) less of return area than I thought all these years.  Now my wife wants an air filter put in the 12x24 grill in the ceiling to reduce dust, but now I am worried about return air flow volume.  I have read that from 144-200 sq in/ton return area is needed for a/c in warm climates, which would = 432-600 sq in total.  The ceiling hole is 12x24, but there is insulation lining the box so the actual opening is 10x22 = 220 sq in (about 1.4 sq ft). The area of a 16" duct is also about 1.4 sq ft. That area should be 1/2 to 1/3rd of what is needed, but I know that it's been that way over 25 years.  I have a relatively inexpensive digital anemometer (China) and the measured air flow at the opening ceiling level is 500 ft/min but, holding the fan probe up in the duct itself, the flow is 1500 ft/min. Should be the same sq ft area, so that difference in velocity doesn't make sense.  Nevertheless, 500 ft/min x 1.4 sq ft = 700 cu ft/min and 1500 ft/min x 1.4 = 2100 cfm. If a/c needs 400-450 cfm/ton, then 3 tons = 1200-1350 cfm needed return.  Taking the air flow measured in the 16" duct, there is more than enough, and I should be able to put in a dust filter. But, something about this still seems strange, and I read how one of the most common a/c issues is insufficient return air flow, so I still wonder if I might have a problem.  Residential a/c people don't want to talk much about it, I think because they see labor intensive ventilation work in an old attic which is hot work and less money than just replacing units or installations in new construction.  There are also a lot of people who sound good, but don't know what they're doing, and I'm not confident in my ability to tell the difference.  Can you give me any insights from this information?  Thanks very much.

Answer
Air flow can be an issue in homes. Some of that comes from the size of the unit.  The home may have been set up for a slightly smaller unit.  It seems people like to make the unit bigger, because they think bigger is better, although the unit that was installed by the builder might be on the small size.  Over the years, there have been changes made to the standards, but for the most part the units basic design has not changed.  
Typically there is 400 CFM/ton.  Your unit should move about 1200 CFM.  If all 1200 CFM are moving though the 16" duct, you should be moving at a rate of 859 FPM.  Current standard max is 700.  The device you are using is not the best thing to use, as it can be affect by restriction from the device itself and your hand.  A hood is going to be the most accurate way to measure the flow.  You can use this link: http://efficientcomfort.net/asp/ResDuct_Web/ResDuct_Web.asp  

At this point I would not put a filter in the return, but use the filter at the unit.  A filter in the return is only going to restrict more air flow. The location of the filter is not going to reduce the dust.  You can install a better filter, but that will should be near the unit.  The return grille surface likely collects dust, and filter there will not stop that.  

Heating, Air Conditioning, Fridge, HVAC

All Answers


Answers by Expert:


Ask Experts

Volunteer


Craig HVAC Expert

Expertise

I have been in the HVAC field for the past 18 years. I can help with most HVAC questions. I work on commerical buildings for the most part, and have yet to find anything I could not troubleshoot and repair, when repairable. I work on small 1 ton units to a 2500 ton chiller. Troubleshoot air flow, elect, and control problems. I attend regular classes to keep up with the latest and greatest.

Experience

I work on commerical sites, hospitals, gov't buildings. I can troubleshoot just about anything in the HVAC business.

Education/Credentials
5 years trade school, VFD training classes, Liebert factory training, some York and Trane factory training.

©2016 About.com. All rights reserved.