I’m getting tired of sweating inside my house for 3-4 months out of the year, and I’m not confident that my usual HVAC company will do anything other than to suggest I buy a new unit.

The particulars: I’m in South Florida where it is currently 92 degrees with 64% humidity. My house is single story CBS/stucco, around 2700 sq ft. The attic has batt insulation that looks to be in good condition, and 3 large skylights in different rooms.

I have 2 separate AC units, one cools the main living/kitchen area and the other cools the bedrooms. It is divided by a pocket door. I replaced the living room AC in 2008, to the best of my recollection it is a Rheem 2.5 ton 13 0r 14 seer, I replaced the bedroom unit in 2019, due to excessive repairs, with a Rheem 2 ton 17 seer 2 stage variable speed unit.

On humid days my house will not get below 79-80 degrees, set at 74 and running all day. It will finally cool off in the evening, but climb again the next day. I assumed it was due to the old unit, but the new one has the same issue on the other side of the house.

I’m willing to replace the older unit if needed, but I’m hoping to identify why both are having the same issue.


UPDATE: Thx for all the advice and suggestions. I guessed at my tonnage size for the original post and undersized by half a ton. The living room unit is actually a 3 ton and the bedroom 2 tons. I replaced both my thermostats with Nest's, it seems to help with the humidity levels while using the cool to dry feature.

My service tech checked the living room compressor and said that I have a leak. He suggested that it would be more economical to replace it with a new unit than to track down and repair the leak, South FL is very hard on outdoor equipment. I decided to try and get another 6 months or so out of the unit before replacing. He added 3 lbs of refrigerant and suggested an acid wash of the condenser coil. I noticed that one side of the coil was completely covered in a heavy matt of dryer lint. Of course the builder located the dryer exhaust right next to the unit!

I will have a manual J done before I replace the unit. For now, the service call seems to have fixed the issue. He did mention that my attic insulation is good, but the local trend has been to overfill lately.

Also in south Florida, this is always an issue. The last few weeks have been nuts, we have our system set to 75 during the day, and it runs all day. We have to turn it up to 78 to stop running.

We did install two mini splits and new windows and doors and it helped, however this crazy heat and hot sun we have had has been taxing our hvac as well. We are actually getting quotes for a new system, prices are nuts due to lack of supply.

Sounds like lack of insulation and/or return airs or oversized units.

Assuming you have lived there since 2008, does your home have a history of being that warm during the summer months? I would have someone come check the refrigerant of the older unit. Check your condensate lines and make sure they aren’t backed up. Have them check the capacitors.

I don't remember if I had the issues we\hen I bought the house, but they have been like this for at least 4 summers. The fact that it affects the new unit makes it more confusing.

The condensation lines used to be an issue, but I rerouted them due to excess elbows in the lines, and flush them every few months

4.5 tons for 2700 sq ft?  That sounds just a bit small to me.

I have 7.5 tons for 3000 sq ft here (about 1 ton oversized).

Texas is different than S. Florida.... but what are your ambient temps during the heat of the day?  AC units when sized properly are designed to run 100% of the time to be able to hold a target of 75 degrees on the hottest days of your typical summer.  If yours cannot achieve/hold 75 degrees, then your units are undersized, or are not performing as they should.  Get a laser thermometer, and look at the output temps at the register that is closest for each unit.  Then measure the air temp at the return.  What is the differential on these days?

Get someone to do a load calculation (Manual J) so your not guessing.  If you have to replace the older on go with a staged unit.

My mom's house in Tampa had the same issue.  I replaced the single pane windows and old sliding glass doors with high efficiency units and added insulation to the attic (there was only 4 inches).  After that I went around with a rented thermal camera and sealed up several places where heat was coming in.  

These few things allowed her existing HVAC to keep the house comfortable even in August and reduced her electric bill by nearly 40%.  I suspect that the bulk of the efficiency gains were from the attic insulation and sealing up the air leaks, that is also MUCH cheaper than new windows and doors.

Do the rooms all have send and return ducts, or just sending ducts?  My upstairs is more humid than it is downstairs, and i wouldn't be able to cool it down if the air didn't have a way to circulate.

Not FL, but SC high humidity and heat issues.  Also have two systems in my house.  Bedroom unit never would keep upstairs and master bedroom on main level cool.  Upgraded from 2 ton to a 3 ton unit along with all new windows in the house and on the basement level where the garage is, we put up insulated garage doors.  All these made a huge difference.  
Sometimes it’s more than just the AC unit.

The 'shoulder' seasons are always a problem for older style systems.

The unit operates in bang-bang (on off) mode only.

During spring and fall it is way oversized and does not run long enough
effective humidity control.

You end up cool and damp.

Newer systems with speed control can dial back there capacity and
will run longer to achieve humidity control.

Commercial systems cool and dehumidify years round,
then heat the air and humidify it.

They are complicated and expensive to operate.
Using things like variable metering of refrigerant from a
storage tank into the evaporator coil does wonders.

The $$ are pretty big though.

Best bet is to run manual J heat load calculation.

You can short cut and sacrifice some accuracy by using highseer dot com and selecting region, sq/ft, ceiling height, windows, etc.

It will give you a rough btu count. Ignore the equipment suggestion, just note the btu.

Divide by 12000 and that's roughly how many tons you need to effectively cool using your regions average temperatures.

If your systems are close to the tonnage, take a look at the evap coil on each unit to see if its dirty and clogged. Check the blower wheels for the same condition. Check the condenser fins outside for dirt and debris, hose off the fins if they are dirty.

Cross reference your return air filter sizes with online charts for cfm of air flow. 400cfm per ton is the typical amount needed to run the hvac efficiently. Insufficient return are is pretty common.

Now you have a good idea of the situation and can go about solving any issues.

Check out attic foil radiant heat barrier film for your attic to block a lot of the heat coming in.

Attic Foil

I’ve watched several independent YouTube videos on it and read a bunch of reviews and I’m convinced.

Might help you with a multi-faceted solution like the other guy did for his moms place.

No financial interest here.

I’m adding a dedicated dehumidifier to tackle that.

The scientific reason is that your AC is short cycling and not running long enough to dehumidify. This is either because the outside temps aren’t much different from your target indoor temps,  or because your unit isn’t sized correctly (usually oversized).

I have several months where the weather is great for me temp wise, but humidity sucks.

OP has a 2 stage variable speed main unit.  It won’t short cycle as the small stage manages that.

Ummm...  re-read the OP.

Outside temp 92, set point 74. Unit runs 24 hrs per day and actual temp sometimes reaches 80 inside. His unit is NOT short cycling.

Sounds like charge pressures need to be checked.

Missed the 92 degree part somehow.

In Florida, a dedicated dehumidifier still doesn’t sound like a bad idea.

I would check the system pressure and check the coil for frost.

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