Regards to an air conditioner being able to cool the indoor area no more than 20°F lower than the outdoor temp, that depends entirely on the capacity of the unit. Air conditioning doesn't so much MAKE cold, as it MOVES heat from one place (inside the house) to another (outside the house). The capacity rating of a unit (such as 12,000 BTU) refers to how much HEAT a unit can MOVE over a stated period of time (one hour). Each 12,000 BTU of capacity is also referred to as a "ton." This comes from 12,000 BTU being roughly the amount of heat required to raise (by adding) or lower (by removing) the temperature of one ton of ice (water at 32°F) by 1°F. Now, keeping in mind the concept of MOVING heat ... if the unit is too small to overcome the amount of heat that's coming INTO the house (or apartment) by air leakage around doors and windows, radiation through walls and ceilings, etc., along with what's being generated inside by people's bodies, lights and appliances operating and so forth ... then obviously it won't be able to drop the indoor temperature or even maintain a temperature point (the indoor temp will rise as more heat filters in than the unit can remove). HVAC contractors size units to a "design temperature," which is a specific indoor temperature that the unit can be expected to achieve at a specific outdoor temp, taking into consideration the construction and design of the building (insulation, doors/windows, sun exposure, etc.) ... such as 78°F indoors at 98°F outdoors, which there's your 20°F difference. There's also the dehumidification factor. Dehumidification is a large part of air conditioning. Dry air feels cooler to the skin (sweat evaporates more readily) than damp air. A unit that's too large won't run enough to dehumidify properly, resulting in a reasonable indoor temperature but a clammy feel. A unit sized just barely large enough to handle the load will run longer (and even continuously when the outdoor "design temp" is reached/passed) and dehumidify better. Units are sized accordingly such that they work fine the majority of the time, but may fall short when hotter-than-usual weather occurs. Larger-capacity units cost more to buy and more to run, and in these days of conservation concerns and people no longer running their air conditioning at meat-locker temperatures ("design temperatures" are higher than they used to be), smaller-size units are the norm ... except perhaps in the case of public buildings that tend to be overcooled, and homeowners who purposely want their systems larger to better handle weather extremes. Folks living in apartments unfortunately are unlikely to have any say in what the building owners install. Anyway, the point I'm making is that a blanket statement that "no air conditioner can cool the indoors more than 20°F below whatever is the outdoor temperature" is absolutely wrong.

Tar Heel You have relatively cheap nat gas compared to electric. Even at 80% efficiency furnace, nat gas is cheaper. Below s a fuel comparison that is to be used as a guide only. 2.75 COP for electric, 80% for nat gas. Cost per 100,000 btu of useable heat Electric baseboard: $3.59 Heat pump: $1.45 Natural gas: $1.24 Again this is to be used as a guide only. While you live in a location that has relatively moderate winter weather, you normally do not place high eff condensing furnaces 95%+ in unconditioned attics because of freeze ups even however remote. If your attic does not have certain insulating properties, it simply is not advisable. With the fuel comparison calculator, I probably would not recommend a heat pump. I would go with a properly sized 2 stage 80% var speed furnace paired with at least a 15 SEER AC condenser. Just for info, if you went straight HP with air handler for upstairs, normally you place the condenser, air handler, and heat strip on separate circuits. Only a dealer with qualification in this area could determine if you have the capacity. If not, an upgrade would be required which probably would not be cost effective. If you went dual fuel with heat pump and 80% furnace, then your electric service would not be an issue. Hope this helps even tho I don't care for the UNC Tarheels. Just kidding. IMO

"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.

There are a large number of things that can influence how a forced air system performs dependent on its position - maybe such a large number of for it to be an accurate science. Cool air will constantly sink to the lower part of a room, and except if you're enhancing the climate control system with a fan to course it around the room, introducing it higher will help cool the room all the more equally. In case you're just introducing a solitary climate control system unit, consider where it will be generally valuable. The open air units of split framework forced air systems are by and large introduced against the external mass of the room being cooled, however a reasonably qualified and authorized installer will have the option to bring up the best area for every circumstance. Outside units ought to be mounted in stable spots where they won't vibrate, where they have a lot of free space on one or the other side to permit wind current, where they're effectively available for support, and where they aren't probably going to be presented to destructive substances. On the off chance that you have open-plan living zones, fans ought to be introduced over the focal point of parlor, cooking or feasting territories. Again, the situating of fans is never off-base given that they're coursing air around you and others in the room. There is also an option for a standing ac which takes less space and works perfectly fine.