Right now and all Summer long, regardless the scorching heat and humidity, I run my small 1,400 SF home's central AC 3-ton 14-SEER to keep cool. I have a guage in my kitchen that reads; 41% relative humidity. Extremely comfortable.

Now here's the question.....

One of my Brothers has a small home in Omaha, NE similar SQ. FT. of 1,400 SF. also has a 3-ton AC unit, but is suffering a 96% ambient humidity in the home. He stated that it's cool, but could start raining at any moment. Ha! The unit a Carrier - complete furnace and outside AC unit 1-year old.

Getting the company back out for a re-check has been a challenge. Last year, my Brother related that within a week of it's install, it was similarly very humid in home. Company came back, adjusted something, and it was good for last Summer.

This Summer, guy came out Monday, and said that he thinks the unit is too big for the home, thereby cooling the home too quickly, without removing the humidity from the air.

My Godfather, many years ago put a 6-ton AC unit on his 1,600 SF home in the '70s, because he was big into comfort, but it always felt humid in his home too. Reason? Too much tonnage for the small square feet of home.

What could be going on with my Brother's home? Could the humidifier that operates on the gas portion of the furnace assembly in the Winter, which is also houses the AC be kicking in? How else could an ambient temperature in the 90 percentile range be achieved, even by too much tonnage AC power?

BTW, he has a portable type de-humidfier cranking in the homes basement, that is collecting 1.5 gallons of water every 12 or so hours!

Our humidity fluctuates with the outside humidity.


Then again, we have no AC.


The stores we visit to get away from the heat do, and they seem to have a low humidity. Almost make me think that if I worked there, I would get a nose-bleed .

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2 Samuel 12:13: "David said to Nathan, “I have sinned against the Lord.” And Nathan said to David, “The Lord also has put away your sin; you shall not die."

Joe, I see one of each condition in our home. We have two compressors. One cools the ground floor and the second compressor cools the upstairs. The air handler for the ground floor sits in the basement and that system can drop the RH to about 35%...very comfortable. The air handler for the upstairs unit sits in the attic () and the RH is around 50% or higher upstairs. I've always attributed this to the high ambient temp environment where the air handler is located and the temp drop its trying to achieve.

Both compressors sit outside, next to each other.

No AC at my place either. I'm currently at 75 degrees and 30% humidity with a fan pulling in cool outside air and another fan in a different room pushing out hot inside air. Later this morning when I go to bed I should be down to 25% humidity. I close the house up during the hot part of the day, and can get up to 45% before it cools down outside and I get the fans going again.

As for the OP's question, it depends on how much outside air the AC is pulling in and the input and output temperatures of the unit. I would have to get the book from my college HVAC elective out to give a more definite answer. It would have something to do with the AC output temperature (lower pulls out more water) and the in house process (load IOW). I can imagine low load in relation to the unit's capacity resulting in poor humidity mitigation.

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When I purchased my duplex the inspector told me one unit had an A/C that was too big and that was exactly what he said the problem with big units were. They dont remove the humidity as well because they run too quick. The other unit on the otherhand never shuts off.

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if he has a basement he needs to plumb a hose off the tank of the de-humidifier to just run the water right down the drain,saves having to empty it every day.most of em these days have a fitting to put a garden hose on em.

It also allows the dehumidifier to run when it would otherwise be off due to a full tank.

I hope that he is keeping an eye out for mold.

Relative Humidity: The sweat that rolls off your sister's back as you two are getting busy.


96% humidity? What is he running? A sauna? Something is wrong there. Even when I was in Asia, the humidity wasn't that high and we were living on a small island.

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His unit may be too big and acting just like you describe. (Rule of thumb I always heard is about a ton per 600 sq.ft.) If his house is well insulated and doesn't have a lot of windows or sun exposure it will exacerbate the situation. If there is a humidifier for winter use simply shut off the water supply to it and see if it improves.
One other possibility is the unit is undercharged. This can cause icing of the evaporater coil and as it melts the humidity is blown right back into the dwelling.
Just a couple of guesses. The pros should be able to nail it right away.

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"Good" is the worst enemy of "Great".

Thought he has a 3 ton unit for 1400 Sq Feet? Shouldn't be that much since the rating is 1800 Sq Ft. Now if he had a 5 ton unit, it would hit that rule of thumb you mentioned. Is that extra bit that critical? IDK.

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Ya, not sure that's a hard and fast number. Also the windows, solar exposure, insulation, etc. can shift that. If he's well-insulated, with few windows and in the shade his tonnage required would be much lower.
For a/c you are better off slightly too small.

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1983 M-B 240D-Gone too.
1976 M-B 300D-Departed.

"Good" is the worst enemy of "Great".

The fundamental problem with most a/c (typically window units) is the fact that they can reduce the temperature of the space very quickly. I have one running now from from a start at 86F and within two hours it's reduced the room to 76F.

All well and good, and during this process the humidity will drop as the compressor is running full time.

But, the moment the compressor stops and the unit is effectively idling, the humidity starts back up and will continue to go up until the compressor restarts.

In a whole house setup, you'd prefer to have the compressor running at least 75% of the time. Unfortunately, this leaves little margin on the top if the ambient temperature climbs to above a 20 degree differential (the max that the unit should be designed for).

Additionally, homes with very humid environments would benefit by materials (some paints can even do it) that will resist the transfer of moisture through the walls.

I watch the humidity in the bedroom here. When the compressor runs full time, it can drop from 70% down to 50%, but, the moment it stops, it will climb back above 60% relatively quickly.

It's not easy to keep the humidity at tolerable levels (60% or less) when ambient is above 90%.

The unit could be too big for the house or the house could have a lot of outside air coming in and raising the humidity or he could have a damp crawl space or the like flooding the house with humidity.

At my house we have nearly 5000 SF including the finished basement running on a 3.5 ton unit. when it wears out I will put in a larger unit because when its really hot it does not do the job if we have in a bunch of people. My unit is now 22 years old and still the original equipment, though the blower motor has been changed. Its not a particularly high efficiency unit....maybe a seer 11 or so.

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The system is probably sized wrong and the house is probably built wrong and it is probably the wrong size too.

First is to keep moisture out of the house to start with. Water should drain AWAY from the house on all FOUR SIDES. Many houses are built with poor or marginal drainage and after some plants get planted drainage stops. Exhaust fans in bathrooms, kitchens, indoor pools, etc. Roofs can leak and nobody knows. DON'T open windows when it is cool and humid. You don't save money because then you have to dehumidify the house again. Don't change the thermostat when you are gone, let it do its job.

Read about Latent Heat of Evaporation. You have to dehumidify before you cool. Have to.

It may be possible for someone to change the fan speed in the unit to make it dehumidify better. Evaporator coil should be near freezing for maximum dehumidification.

You may be able to run the heater and the air conditioner at the same time for dehumidification.