Hi Jim, There are a tremendous number of variables between homes, as to insulation, air-infiltration solar heat barriers, etc. Pickup trucks & cars have much more AC tonnage per cubic foot of air space than homes. If a manual J were done on your home it would probably show a very low rate of heat-gain as compared to those that could not satisfy the room t-stat. Many central units are not performing to their nominal tonnage ratings! - udarrell

It depends on how much slop is present in the Manual J, and that often depends on the age of the house. A house built from abut 1970 on in many areas had at least some code required insulation in the walls and ceiling. Houses older than that can be very hit or miss. How was the R-value of the exterior walls determined? A small error here can often have a large impact since there are so many square feet of wall involved. Attics can be easily checked in most cases. Infiltration is often a large portion of the gain/loss, but without a blower door test can only be estimated roughly. There are a number of methods using previous year heating and cooling bills, together with degree days to further refine the calculation. Even if you have never had cooling, the losses can be determined from heating use and then used to help determine cooling.

As a long time retired HVAC contractor, my advice is to have a thorough audit of your home to determine everything that can be done to reduce the heatload. Heating & cooling costs will probably continue to rise! There are many new effective ways to greatly reduce that heat-gain, heat-loss calc. By reducing the heatload of your home, they need to get the tonnage of the equipment down as low as possible! Only 2700-sf, there are ways to minimize the effect of the 18 foot ceiling areas & reduce the tonnage requirements. The proper sizing installation & setup of the new systems is everything! Duct layout & airflow is critically important. In the relative dry Dallas climate they could go to a wet coil of 450-CFM airflow per/ton of cooling or more. The low speed setting of that air handler might work with a 2-ton condenser & an a matched larger tonnage evaporator coil. - udarrell Here is a link that might be useful: A/C System Sizing for Optimal EER & SEER Efficiency

So wouldn't zone cooling create a pressure back up too? Yes, it would if not designed and done properly. There are a myriad of ways to do this. You can oversize the ducts. Which is fraught with misery during 10% of the season when you (home owner) decides I must cool these other rooms. Because the ducts are over sized you unintentionally have reduced velocity and throw of the air to cool properly. There are dampers that let out some air. Not that this is wrong, right or otherwise... but my opinion of a zone system is to cut waste and allowing air thru to areas you don't want to cool is counter productive. But with that said some *Locations* this really doesn't matter that much. There are systems that use what is called barometric bypass to send air flow back thru the system. If pressure builds up enough it forces open this bypass damper. I really don't care for this method either because this is a position sensitive damper. They really just don't perform well enough in my opinion. All these things relate to design of the system and what ever method one chooses to relieve (air) pressure of the system it must be designed properly not only to handle low load temps like 80 degrees OAT on up to heat wave temps of 105 degrees OAT. This conundrum is what befuddles most who attempt at zoning especially in my neighborhood. For a neighborhood like Ohio it would be quite a bit easier, but only because I know what it takes to do it in Katy, Texas. Questions about this chart: I know you said this is based your area and 95 degrees/75 degrees and ambient temperature. Does humidity play a factor in this (obviously not on this chart). I'm not sure if you're area is humid, but in Ohio we get our share of humidity. If the OAT entering the condenser is 95 degrees and 90% humidity, doesn't the humidity actually help with heat exchange? Humidity doesn't help with anything. It tends to make problems more than it helps. If you reduce humidity heat is less of an issue (to a degree if like pun) Too much humidity can be just as bad as too little. Humans tend to be most comfortable in the 50% RH range. Lower than 40% and things start drying out that shouldn't really be dried out. Yes, my area can have 80% plus humidity sometimes all year long as experienced the last 2 years throughout the winter here. If the OAT entering the condenser is 95 degrees and 90% humidity, doesn't the humidity actually help with heat exchange? No. That humidity will find it's way into the structure. If your AC is a single speed air conditioner it will have little effect to 'effectively' reduce high humidity like that. Any help that humidity provides with heat exchange would be a negative on the inside of the structure that it is trying to cool and or relieve the humidity that is infiltrating it. I think the point he was making was that the AC in cars driven in high humidity areas generally have colder air than cars in Phoenix. Cars are a completely different application, but humidity is not a factor. The factors are Ambient Temperature and design of the system. If you lower the ambient temperature (outdoor temp) any system will cool much better. The ambient temperature is a load. When you mist water over a condenser it lowers the ambient temperature of the air. You can do this your self when you are hot... pour some water over your head. It feels cooler all of a sudden. Now sit in front of a fan. Hello? free air conditioning. The way I put this makes you feel silly I know, but I am not trying to ridicule you just so that you fully understand the answer to the question you are asking. Look back to that chart... look at the lower outdoor ambient temps. What happens to capacity? It goes up when ambient temps are lower.