AC Unit Freezing

Background:
Location: Colorado Springs, CO
3,000 sqft. Ranch (1,500 ft. 1st floor, 1,500ft basement finished with bedrooms)
3.5 ton Rheem AC Unit
Home never had AC

Issue: AC unit freezes

I went with a newer veteran owned local company for my AC/Furnace installation when in hindsight I should have used a well established HVAC company. The company is now potentially out of business (number is disconnected). My AC unit keeps freezing up. It will run okay for about 20-25min., but then freezes. For the past 2 years the company that installed it has tried/recommended various things such as using a cheaper filter to allow for more air flow. They resorted to putting a freeze stat on the unit which stops the unit from freezing.

I only have two return air vents in the house, one panned return upstairs in the central hallway (29.5 inch x 15.5 in) and one in the basement (15.5in x 19in.). Both of these vents I made bigger, however my concern is even though I made the return grilles bigger, the unit is still not getting enough air because the duct work/panned joists restrict the amount of air flow. The return air plenum in the basement is 20in x 8in x 86in.


I had another company come out, and they believe the unit is too big for the house and that is why it is freezing. They checked the refrigerant (so did the installation company) and said it was good. They lowered the blower fan speed seemed to help delay how long the unit ran before it froze. They recommended I replace the AC unit with a 3.0 ton.

Would adding other returns in the house help with the unit freezing? Would I need a bigger return plenum if I add more returns? I know it may to run perfectly, but I am hoping to avoid replace the unit since it is only 3 years old and has been barely used. Any other ideas?

Thank you for any insight/ideas you all may have.

Brian

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