My new Carrier has been doing that this summer. With 100+ temps and unbearable humidity, there has to be a breakeven point. If the systems could keep up with that weather, I would think they'd be way oversized for the rest of the year.
Oddly enough, it's 101 today. I've had the a/c off all day and doors/windows open. The humidity is so low it's pleasant.

philip 11, you don't mention where you're from. It just might be that yo are now above the system design temperature. Thats the uppermost temp. the designers expect the unit to run in. Depending on where you live in the country the design temp can be higher or lower. If once the outside temps drop to the 80's and 90's the problem is still present you may have a problem requiring getting the contractor back

Same story I keep hearing/reading this summer. Call it what you want: "system design temperaure"... I call it being too hot inside. I as have posted before, even more reason to be politically incorrect and "oversize" (I don't view it as being oversized). In am in SW Florida, and have a 1,600-1,700 sq. ft. single story with 2.5 ton 7year old RUUD that has never kept up (78-81 without rain). I know my heat gain is a contributing factor (lacking reflecivity/insulation), but please!

I am in the process of eductating myself and finding a contractor who will not give me the oversize/not remove humidity/all you need is variable speed line of bull...

If I can make it through this summer, I will find a contractor who will do exactly what I want (nothing more or less). At least 4 tons, ducts sized appropriately, returns in each bedroom plus in family room, simple configuration (no variable speed anything) 14 SEER Copeland Scroll... It is difficult enough finding a half honest contractor, but requiring that plus one that will do exactly what you want is nearly impossible.

Again, my simple cure for being "oversized", cold, clammy, or humid is a Tstat setting of 70. Sorry to be so stuck in the dark ages, but I would rather be on the chilly side (@70) on 100 degree days than sweating (@80).

I have read so many posts over the last few months of folks spending $5,000-$10,000 on a shiny new system that runs 22 hours a day to get their house to an unacceptable 79 degrees in the afternoon. I have determined not to be one of them.

Well when you rewrite the book an standard air conditioning practice maybe you can give us all lessons.

I am not educated enough on any of this the give anyone lessons. I appreciate your sensitivity to my sarcasm and critical nature. I do not wish to offend. I appreciate expert opinions, and learn a lot from them. I have also learned a lot from so many posts who have spent thousands of dollars on new systems and cannot get below 80 degress in the late afternoon. That will not be me. I am simply educating myself so I will be able to cool my house to 70 degrees (not 79) in hot Florida August. I am simply tired of hearing from many out there it is not possible. I get the whole oversize/high humidity concern. I get it! I have simply realized that it is up to me to calculate the tonnage to hold 70 degrees, then work backwards from there by designing the ducts/returns/registers... No offense mr.hvac. I have learned from some of your posts the most...

Thanx for the compliment, I try, I probably PO as many people as I help but hey, "whaddya want fer nuttin/" :-) If you've been following the posts here eths (how do you pronounce that?) then surely you know that "ITS NOT THE BRAND THATS IMPORTANT, BUT THE QUALITY OF THE INSTALLATION" Soooo, with that said even if you oversized excessivly, if the people designing and installing the air distribution network don't have a clue as to what they're doing or the ductwork is inadequate to begin with in the case of a system replacement then warm and cold spots and stratification makes for a very unhappy camper. On top of that if the technician doing the start up is a bimbo the extra tonnage may not be utilized anyway.
FYI, and somebody correct me if this is wrong, but i believe the design temperature for florida is 105 where as in my state its only 90. Believe me 15 degrees is a lot in the world of A/C

I think the money would be better spent upgrading insulation, tinting windows if they are not already and sealing air leaks in the home. My guess is that your problem lies there.

I'm in Houston, homes in our area ~3000 sqft give or take. I keep my home about 80' that's my preference. Many here keep their homes 70'-75'.

Here is a link that might be useful: Saving energy in newer Houston homes.

I have an 1800 sq-ft split foyer in Baltimore. I realize that a 2-story home is much better to cool than a 3-story. The t-stat is on the same floor as the bedrooms so there is no 5-7 degree difference in the bedrooms as there would be in a 3-story with 1 a/c unit.

This year I replaced an old heat pump with a new 3-ton unit, same size. I have decent insulation and all-metal ducts. I chose the 3-speed air handler (they can keep that variable speed stuff). I can freeze meat if I want to, even on a 100+ degree day.

Many systems today don't put out their full-rated capacity. A 3 ton unit (36,000 btu-hr) may only put out 32,000 btu-hr. This info is buried in the manufacturer's performance data and most homeowners do no have the info available to them. Couple the under-achieving capacity with an undersized unit with a v-s fan and consecutive 100-degree days and you have a lot of unhappy home-owners who paid big-bucks for the latest and greatest system.

If eths has a typical older Florida home it's no wonder he
can cool it with the SQ Ft. sized system. They had virtually
no insulation CBS (CBS, cement block stucco)walls, no vapor
barrier, little or no attic insulation, Jalousie and or single pain Awning
windows and Whole House attic fans without weather stripping.

Well, I sympathize with ALL of you posterÂs that bought new higher SEER units with those apparent results. It doesnÂt have to be that way.

Reductions in the four main sources of heat gain would also make a tremendous difference. They are: Conduction; radiation; infiltration; & convection.

I hope all the equipment had Scroll compressors & TXV refrigerant metering devices.

The way the high SEER equipment is engineered - links it to Humidity control.

Irrespective of unit sizing, the engineering design of the condenser/compressor & evaporator & refrigerant metering device combinations do make a difference in latent (humidity) removal.

There are four major factors in humidity removal:
They are: coil size selection ratio to compressor Btu/hr; airflow; refrigerant control device; and the superheat control of the refrigerant gas.

The evaporator coil size in relationship to the compressor Btu/hr is a large determinant of the latent heat removal ratio.

The larger the coil size to the compressor Btu/hr capacity, the lower the humidity removal rate.

Irrespective of equipment BTU/HR size, in some situations, a smaller coil can increase humidity removal by up to 30% or more, resulting in greater dehumidification.

An oversize compressor, as compared to an undersized compressor in relationship to the evaporator coil and the condenser coil makes a huge difference in the pressure temperature relationship of the evaporator coil.

Therefore, the seer rating of the match system is a major factor in the dehumidification ratios. Some say a variable speed blower motor and a slower, 350-CFM airflow can compensate for the oversize coil and the higher temperature evaporators.

If any of the above factors are not optimal for dehumidification you can expect humidity problems.

There is one more factor and that is the thermostat. What is required is a thermostat with a wide temperature differential setting, so that you could set the room TH for 77°F ON , and 73°F OFF.

This would allow much longer run cycles irrespective of somewhat oversized system.

Use low wattage fans to circulate air only over the occupant occupied areas, especially while the unit is on the off cycle.

Now regarding equipment sizing, if you use all of above factors in an optimal way, you can achieve adequate dehumidification with a somewhat oversized BTU per hour system & get COOL. GO for IT!

(Due to bad fingers, I used my voice typing software.) - udarrell _ 2007

Here is a link that might be useful: udarrell.com

The computer program that was used to do the Manual J calculation for my house had 95 as the maximum outdoor temperature. I asked the contractor to assume 98 when calculating mine. Probably made no difference in the unit size. We had a couple of days at 101 this Summer, several at 100. Have had as high as 105, but very few such days. My systems performed very well.

Some good information... In my case, I plan on adding insulation, already have new windows, but will still insist being more oversized than undersized (but not too much). Trying to make it through the summer and have work done in Fall or Spring.