20 Degree Rule

You need to have a heat gain/ loss calculation preformed on your house to see what size unit you would require. Bigger is not better. Actually, using a new higher eff. unit you would need a smaller unit to replace an older R-22 system and also would need to increase return duct size. Dependng on where you live in Texas the outdoor design temperature for cooling is between 89 and 101 deg. F. Laredo AFB design is 101 deg., Vernon and Wichita Falls AP is 100 deg. F. All other locations design is 99 and below. Based on sq. footage you are probably oversized by approx. .25 to .5 tons with the 4 ton unit. Also normal Indoor design cooling is 75 deg. F @ 55 % RH.

Thank you for the input, Stan. I just wanted to add that requiring 70 deg. F on the hottest summer day on record (103 sounds close to that) is going to result in a system that runs inefficiently most of the rest of the year. I would guess that part of the issue with such low temp requirement has to do with dehumidification. Perhaps the poster should look into adding a portable dehumidifier for the room(s) that are actually occupied during the day and leave the thermostat setting on 76 - lower humidity will result in a more comfortable environment despite slightly higher ambient temperature. It's hard to tell if this approach will result in lower energy usage on the hottest day of the year, but it's a guarantee that on the days when extra dehumidification is not required (i.e. likely the rest of the year) it will be more energy efficient than upsizing the AC.

Thanks for the added input homeowner, always glad to have it. Also poster should look at making the home for energy efficient before considering the HVAC system. Have a home energy specialist audit it in.Look for air leaks in the return duct system and air infiltration into the supply ducts. Have the unit cleaned and serviced, run a capacity test on the unit to be sure it is properly charged and performing to the orig. manufacturer's spec's.The 4 ton unit should pull the home down to 70 deg. at some point. I am reasonably postive the problem lies outside the size of the A/C.