How many return air vents should a 1700 sq foot house have ?

Size of the AC unit needed can't be determined with the information given, but that size, single floor provides maximum exterior surface area, a 2.5 ton could easily be too small.

Location of return vents will affect uniformity of cooling, but number of vents is less relevant than total size. One strategically placed 20 x 20 vent may work adequately if relief from individual rooms is provided by gaps at the bottom of the doors or other means.

As I understand it the size of your return is within IRC minimum requirements, but the installation instructions of the specific unit override the code minimum.

IRC M1602.2 Required area.

"...The minimum unobstructed total area of the return air ducts or openings to a central air–conditioning unit and/or heat pump shall be in accordance with the manufacturer’s installation instructions, but shall not be less than 6 square inches (3870 mm2) for each 1,000 Btu/h (293 W) nominal cooling output rating."

So if you were to get a 4 ton unit there are and there is 12,000 btu's per ton, 4 tons x 12,000 btu, x 6 sq in = 288, the code minimum size of square duct would be (rounding up to even numbers) 18" square or 20" round.

A 2.5 ton central air should have about 10 supplies. A 3 ton unit needs 10-14 supplies.

Count all the heat supplies and use the rule of thumb of 4 supplies per ton of air. DO NO oversize the unit or it will not remove the humidity from the home and will short cycle (run and turn off quickly) and not cool the house.

Have a HVAC contractor perform a 'Heating & Cooling Home Analysis for the home. That will determine the precise size of a new furnace and central air conditioner. That is the BEST way to determine sizing. If your contractor does not know how to perform the analysis find another contractor.

Return Air Vents

Based on the fact that you have average insulation and windows in the home at 1700 sqft. You should have a 3 ton unit indoor and out.(your indoor coil no larger than a 1/2 ton than the outdoor). You should also have an average of 12 vents(+/- 1 vent). No matter how you spread the intake air duct it should be at minimal 15"x15" square. 1000 cfm. Remember state laws will change so you will want a minimum of 14seer but if you want rebates most likely your state will require a minimal of 15seer. Good luck!

Air Flow Is King! All systems are designed to blow out and return 400 cubic feet per minute ( cfm) of air per ton. 4 Ton = 1600 cfm, 5 ton = 2000 cfm. Typical home requires 1 cfm per sq.ft.

The amount of return air is based on 2 things: (1) duct size, and (2) grille opening size. A duct chart will show that 8" duct = 160 cfm, 10" = 300 cfm, 12" = 480 cfm, 14" = 700 cfm, 16" = 1000 cfm, etc.

The "grille opening must be larger than the duct and is = to appx. 2 cfm per sq.", so a 14" 700 cfm duct would lead to a 20" x 20" 800 cfm grille.

So, for 4 ton system, you would use 1 - 14" (700cfm) duct with a 20x20 ceiling grille and 1 - 16" (1000cfm) duct with a 20x25 (1000cfm) ceiling grille.

Important: If using "wall" vents, cfm per sq. " is limited to wall space, typically 14x4 = 64 sq" x 2 = only 128 cfm.

I believe there is another reason for your system to keep running. With so many reasons, a tech will have to be there to check the pressures and temperatures of the line set, the supply and return air temps, the motor voltage and amps, check the ducts and plenums, look at the indoor and outdoor coils. You can shade the windows, make sure all the supply vents are open, keep all the doors open, and change the filter before he shows up. The return can only suck in a certain amount of air but you can try it.

I definitely would not go larger than a 3.5 …..Due to short cycling….bigger is not better when it comes to cooling.

With that said ….any specifics on your home must be determined from the home condition >> outside air leakage/infiltration, duct location, insulation levels, sq inches of glass, solar orientation…. etc.

If your home were just horribly inefficient you possibly could go to a 4 ton (400 sq ft per ton), but in my opinion one of the major problems you have is your return air ….or lack thereof. Ideally you should have a return air in every major room in the house …placement and size should be determined by the HVAC contractor …the unit you get… and ease of installation.

Return Air Grille Sizing Chart

they are generally recommended for every room except bathrooms and laundry rooms

homes that have a single central return don't generally have proper airflow unless you leave the doors open all the time, people do this to save money on ductwork and its installation but are usually dissatisfied with performance.

More returns are not actually needed if there are gaps under doorways for air to flow. Returns in every room may make for more even heating/cooling distribution, but I have been in homes with one large return that work very well.