Relative Humidity Signficant Differences Upstairs vs. Downstairs

Old 06-05-17, 04:50 PM
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Relative Humidity Signficant Differences Upstairs vs. Downstairs

Hi guys, looking for some advice. I live in South Florida, never paid attention to relative humidity, but always noticed that even though sometimes I keep the temperatures the same downstairs and up, that upstairs always felt warmer.

Right now it's summer time and it has been raining daily with the temperature in the 80's during the day.

I have a 1.5 ton system upstairs (18 yrs old), and a 3 or 4 ton system downstairs (4 years old).

When the temperature is at 75 downstairs, relative humidity is 53% and feels fine. Upstairs at 74 degrees, relative humidity is 72% and it makes it feel a bit muggy.

What's going on? I'm replacing the system upstairs very soon, but I'm afraid there's more to it than that. Thoughts or suggestions?
Old 06-05-17, 04:54 PM
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The biggest problem with A/C on the second floor is the heat and air infiltration coming down from the attic.

Is your attic well sealed from the second floor and insulated ? Could it use more insulation ?
Old 06-05-17, 05:44 PM
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The intricacies of RH and its control are often arcane. Your two A/C systems are most likely programmed to operate from a thermostat to maintain a set temperature. When that temperature is achieved the compressor shuts down and all dehumidification stops within seconds. It could be something as simple as carpeting and bed linens adsorbing (I think this is the correct usage of the word) moisture from the air and the A/C system simply does not run long enough cycles to remove the excess moisture, at least not without dropping the temperature significantly.

Also, do you leave the fan control in "auto" or do you place it in "fan on" mode when using the A/C? Leaving the fan in continuous operation causes the condensed moisture on the evaporator (cooling coil) to immediately be re-vaporized and sent through the ducts raising the RH.
Old 06-05-17, 06:36 PM
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It sounds to me like the AC is dehumidifying downstairs because the cold air is pooling there. Are the upstairs doors open and the cold air spilling down a staircase?
Old 06-06-17, 04:29 AM
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Like Furd said make sure you have your fan in the auto mode. Make sure you have a heat load done on your home when you replace. Might be working fine might just need to slow the fan down or go to a 2 stage equipment when you replace. Remember the humidity will rise so make sure you use bath fans at least 20 min after bathing, fans when cooking etc etc. Respiration could be an issue when taking the readings.

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