Hi April, In your past posts you stated the house has 2400 sq. ft. It is difficult for anyone on this forum to state what is the correct size for your air conditioner. The load calculation is the only way to do this properly. I would have much more faith in the contractor who took detailed measurements. It is likely the cooling load somewhere between 3 and 4 tons. One contractor may be rounding up, and the other down. The 2-stage Infinity AC would be a good choice. On the low stage it would be operating around 2.8 tons. You would be comfortable and do a great job of removing humidity. How many days a year does the temperature go above 90 degrees in the summer? I personally think a 21 SEER system is overkill for your part of the country. It is unlikely you will ever see a pay back of the additional cost over the 16 SEER unit.

I found this note on page 11 of the installation manual under upflow units: NOTE: Where the maximum airflow is 1800 CFM or more, both sides or the bottom must be used for return air. This is saying the bottom return by itself will be enough. There was note under the upflow unit section, but I assume it also applies to a horizontal installation. You want the filter and grill cover to be at least the size of the opening to the furnace. Making the grille bigger is not going to help if return itself is smaller. I don't know how this is going to work with the filter size you quoted. I would think you would need two 14X24 filters. It would be nice if you get get a 4 inch filter instead of the 1 inch. Otherwise you will be changing the filter every month. The 6x6 stud obstruction should be removed and you want a tightly sealed return duct. The return duct should not have a right angle. It should be a curved elbow to make the 90 degree transition. Sharp angles create more air resistance. How big was the original house and AC, and much area are you adding? Most houses don't have the properly sized duct work to handle a 5 ton AC. Do you have confidence in this HVAC contractor?

mike_home, Thanks for the info. I will consider a heat pump. This year the state of new jersey now requires all ac installers to receive training and state licenses in ac installation. In my research it has been stated quite a few times, the ac will work only as well as it was installed. Lennox has premier installers and only a very few of the other brand installers are NATE trained. But the majority are still not ac trained licensed installers. The majority are plumbers and electricians. My concern is the duct work and the refrigerate not being done to specifications. Many hvac contractors offer only one year labor. Unfortunately a number of homeowners found themselves shelving out more money 2,3,4, years later, just for labor. The installer is as important as the ac with 10 year limited warranties. I can accept an annual maintenance contract, but want the ac to work as smoothly as the Luxaire had for 35 years.

"He had the same conclusion as yours says it is short cycling due to leaking of freon." I did not say the short cycling could be due to a freon leak. I did say it could be due the condenser being increased in size. I don't understand how a loss of freon could cause a condenser to short cycle. Perhaps you misunderstood what the contractor said. But if he made that statement then my confidence in his ability continues to go down. What exactly was replaced? So far I know it was the condenser. I hope he replaced the coil in the attic. Was the furnace also replaced? What about the line set (the copper tubing running from the attic to the condenser outside}? Normally on an attic installation that is not replaced since it is running inside walls. The old line set can be reused but it must be flushed out properly. You can't mix the old and new refrigerants. But, if the condenser was increased in size, the line set may be too small (diameter) to work properly. This is another reason not to be increasing the size of the condenser. Has the contractor returned yet? If there were air flow problems while running the AC, then there will be air flow problems with the furnace. Those have to be fixed. Make sure the air flow works well in the spare bedroom. Just because you rarely use it does not mean it should not heat and cool properly. All bedroom vents should have approximately equal air flow. This should not be too difficult with the furnace installed in the attic.