Will too big of an ac unit cause the humidity to stay to high??

Question

Will too big of an ac unit cause the humidity to stay to high??

Answer 1

I've heard that, but I don't find it to be true. I usually get the largest AC that will fit, and for which I can supply power. This means the room gets cooled quickly when I turn it on, and I have excess capacity for open doors, etc.

It works fine, the humidity goes down OK.

Lets try to analyize it. Consider an AC that can just handle the load. It will be running continuously to just maintain the set temperature. That means the cooling coils will be not too far below the room temperature, which means there will be less water condensed, and thus the humidity will be higher.  To state it another way, the dew point will be the coil temperature, close to room temperature, so the humidity will be high.

Now an AC that is way overpowered, will run on a short duty cycle, so when it runs it has to do a lot of cooling. Which means the coils are very cold, close to the freezing point. Which means lots of moisture is condensed out, and therefore you get low humidity.

So my rough analysis says the opposite. Too big an AC causes a lower humidity.

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Answer 2

Short answer - Yes, and it can even induce a raising of the room humidity. This effect is well known in the HVACR industry.



Long answer-You were doing fine, Billruss until your final paragraph.  The humidity will be high if the unit satisfies the sensible cooling demand before the air passing through the coil begins to condense. Sensible cooling will occur when the air passing through the coils reaches about 70% RH and then latent cooling and condensation will begin along with additional sensible cooling. The sensible cooling will first follow a horizontal line and then will follow the 70% RH state line. (It does not go straight to saturation and down, but to about 70% and curves down with it.)  This  condensation will begin to occur when the air is part-way through coil, after the first row(s) of fins. This can be seen represented on the Psychrometric charts that used to be published by Trane.

If the unit is over-sized, that is, it the air flow is too high, the  velocity will be too high and the air will not have enough contact time to reach the 70% RH before it has exited the coil. Adequate condensation will not have occurred before the thermostat (sensible heat) shuts the unit down. The room humidity will creep-up, depending on the amount of moisture in the room. If there is a substantial latent load, the humidity will increase faster until it reaches a state where that 70% will occur at a higher sensible temperature. Having a chart would make it easier to demonstrate.

( With deeper coils and higher fin count the condensing curve beginning at 70% will then proportionately approach 90% exiting, but will never reach saturation.)

Example - Dedicated computer room AC units are designed for a higher than usual face velocity (intentionally oversized for volume) so that there will be no dehumidification through the coil. Since the computer room is nearly all sensible load, the room RH will stabalize. The reason that condensation is avoided is to eliminate any need to re-humidify which involves the injection of water or steam which you just do not want in there. (You do not want the computer technicians putting their desks in there either-  they will just want to be comfortable and computer room AC is not intended for comfort, but for reliability and consistency.)