AboutFred Weldin Expertise I can't help you if your whatsis won't work, people (Especially if it's a refrigerator); I have no experience with appliances, and I haven't been involved with H&AC service and repair since March 08, 1996 (Thank the Lord); I reject appliance questions and H&AC "Service and Repair" questions (about 18 a week), so if you want to know why your whatsis isn't working like it used to, ask someone else. I delete answers that are unread after one week (about three a month).
If you have questions about how big a unit you need, if one room's warmer than the others, if you want an opinion on bids received, etc. just ask. Do visit askweldin.com, there's a lot of good information on sizing, ductwork, efficiency, as well as some techniques accomplished DIY-ers can use to troubleshoot and improve their H&AC.
Finally, I want to thank all of the kind, serious people whom I meet here; I enjoy working with you.
Experience 50+ years in the business. See askweldin.com
Education/Credentials B of ME U of D 1965, numerous classes and seminars since.
Question Fred,
I recently had a 2 1/2 ton Trane split system installed and the a/c tech stated the return air grill located in the hall was undersized. Is there a rule of thumb for sizing return air grills based on the tonnage of the system. I have heard 144 & 100 square inches of return air per ton.Do you reduce the net area of the grill for metal grills ? ( e.g. take 70% or 50% of the total square inch area of the grill face?)
Answer Tim:
We limit grille velocities to 400 FPM to reduce resistance to flow and eliminate air noise.
If you're in a climate with normal moisture levels, you'll design for 400 CFM/Ton, or 1,000 CFM; you'll need 2.5 SF FREE AREA (1,000 CFM/400 FPM), or 360 sq. in (OHMYGOSH, that's 144 sq. in. per ton!!! - The 100 number yields 576 FPM, and that's way too high).
The grille selection will depend on fin spacing and angle: A H&C #672 has 1/2" spacing at 40 degrees, and you'd need a 24x20 or equivalent (That's 75% Free Area); a H&C #650 has 1/3" spacing at 20 degrees, and you'd need a 20x20 (That's 90% Free Area). The 2006 IMC suggests 75% as the free area for metal louvers when the spec is unknown, and you can't go wrong with that; 50% is used for wood louvers.
In any event, it's best to check the published data from the manufacturer you buy; look for an AK (That's A sub K, or area factor) of at least 2.5 SF.
