@Julie Walker,
I’ve been a commercial/industrial HVAC service technician for 25 years. It sounds like it’s undersized. The company’s that said they did a manual J calculation most likely did not, ask them to show you the Manual J sheet they filled out, I’m willing to bet they don’t have it. The guys that stopped out are service technicians, not engineers. They just did an estimate.
A quick way to see if the AC is working up to its capacity is to measure the supply air verses the temp of the house. It should be 20 degrees cooler. Example; if the house is 78 the supply air blowing out of your registers should be 58. If it is, it’s doing everything it can do.
Make sure the thermostat it is set to maintain temp 24 hours a day. If it is, and its blowing a 20 degree temperature difference when running, and it’s still 78, it is undersized, no need to have another service call.
From an efficiency standpoint, the more times an HVAC unit cycles the less efficient it is. Both furnaces and AC units waste energy starting and stoping. It’s more efficient to run a smaller unit continuous for an hour to maintain a temp, than to have an oversized unit cycle on/off 5 or 6 times in an hour to maintain that same temp.
