Two months ago, I posted about our upstairs a/c not cooling all of a sudden. I ended up replacing the capacitor for the blower fan and the contactor, which did the trick. AC cools!!! However since then, there has been increased humidity upstairs (>60%), and I just can't figure out why. We keep the temperature between 75-78, and live in NC. It seems to be related to this unit, as the downstairs air at the same temperature, has lower humidity (<60%).

As I understand, humidity comes out of the air as it is cooled across the coils, in the form of condensation. It is captured in the pan under the evap coils. There is a drain to evacuate water to an overflow pan from the pan beneath the coils when it gets to a certain level.

Please tell me what else I am missing from my understanding of how air is dehumidified in a central ac unit (without a separate dehumidifier). I don't know if there is a fan that "evaporates" off the condensation like there is with a refrigerator or freezer.

This is what I've inspected/done:

  • Inspected drain pan, which was bone dry.
  • Accessed the evap coil, and found 1/2 inch of water which I drained. No blockage to the overflow pan. There is some rust on coils and at the bottom of the pan, but I didn't see any pitting or obvious leaks.
  • Fins over the eval coils were pretty clean - no dust.
  • Cleaned the fins on the coils in the outside unit which were dusty

There must be something fundamental I don't understand, because if unit is cooling to the correct temperature, it seems that we don't have a cooling or a handling issue. But why would humidity be an issue now, if it wasn't before, and particularly, only on this one unit.

enter image description here rusted coils; but fins look clean

4 Answers 4


Most residential AC system thermostats only turn on based on temperature. And most residential AC systems only dehumidify when then AC is cooling. Therefore, it's very possible that the AC is cooling the house faster than it can dehumidify. Try turning the thermostat down a couple of degrees.


As stated by Longneck, An AC unit has to be balanced for the load to run long enough to remove humidity. Yours is now functioning so well that it cools down too quickly. I would just add an stand alone dehumidifier, or turn it to a colder temp. The fact that it isn't production very much water is a clue. Did you verify the humidity with a second meter? I have seen measurement variations up to 10%. 60 is not bad. I shoot for around 50 in my basement.

  • thanks for your thoughts. I have verified with a stand alone meter recently purchased. would your answer/thoughts change if I told you the unit was really old (>10 yrs)? Is there anything else that could keep it from extracting humidity from the air?
    – eliza1
    Sep 8, 2015 at 20:58
  • I posted too quickly...sorry...What I was going to ask is whether the rust in the pics looked excessive, and whether that could be a contributor in any way. Also, the blower has 3 speeds (H, M, L), and as I understand it, the a/c is wired to the "High" speed. I could wire it to run from the med speed so it would take longer to cool. Would that make sense?
    – eliza1
    Sep 8, 2015 at 21:02
  • Does the air seem cold enough? The colder the coil, the more moisture is going to condense on it. Whats the air temp? If you lower the fan speed, that would keep the coil cooler, I'm not sure if that's a good idea or not. If the unit is not operating efficiently enough, it might need a refrigerant check. All that being said, a couple of degrees lower on the thermostat might provide enough humidity reduction to get through the summer and until the weather breaks for fall. One more thing. Is there a source of humidity in the house like a shower without a bathroom vent?
    – BrianK
    Sep 9, 2015 at 1:55
  • Brian- air temp per a stand alone temp/humidity meter equals the temp on the thermostat at roughly 75-78 degrees. But before we had this issue, it felt colder in the upstairs area than now. And you are right, we have been turning the temp down lower and it results in lower humidity, but we haven't had to do that before now. No source of humidity that is new or producing more moisture than before. And on the refrigerant check - this may be dumb, but if the temp is getting to where it is supposed to be, is it still possible that we are low on refrigerant? thank you - I appreciate the education.
    – eliza1
    Sep 11, 2015 at 5:02
  • From all clues, sounds like it could be low on refrigerant
    – BrianK
    Sep 11, 2015 at 11:44

Indoor relative humidity over 50% can cause problems. High humidity from A/C operation typically arises from one of two causes:

[1] The unit capacity is substantially oversize for the space being cooled.

In this condition, moisture is removed much more slowly than the temperature set point is achieved, such that the unit runs for short periods, cycling off before removing much moisture. To check this, compare the square footage being cooled to the unit cooling capacity. Capacity usually should be 12000 BTU for every 400-600 square feet (400 for old poorly insulated homes, 600 for higher efficiency homes).

[2] The unit has an evaporator temperature that is a little too warm.

To get good moisture removal, the evaporator temperature must be less than 55F. Upper 40s to 50 works well. Using a thermometer, check the discharge temperature at a register near the unit when the A/C has be running continuously for 5-10 minutes (turn the thermostat way down). Warmer than 55F probably means your evaporator temp is too warm to dehumidify effectively. Evaporator temp too warm usually indicates either too much blower for the compressor capacity (select a lower speed) or the system is a little undercharged (to correct this on an older R22 system you need a licensed tech). There are other more serious underlying causes like the compressor wearing out. If the system is generally OK, you can safely select a slower blower speed to lower the discharge air temp. If you do this make certain the discharge air leaving the coils is not colder than 25F below the inlet and that no frost forms at the refrigerant liquid inlet side of the evaporator coil. (This is an approximation. The accurate professional way to do this is with a set of gauges.)

For the curious:

Why no colder than a 25F temperature drop at the evaporator? If the house is 72F, then the discharge air temp at the evaporator would be 47F (25F colder). Compressor mfgs require 15F superheat in the vapor returning to the compressor. Roughly speaking, 47F air temp => 50F vapor temp of refrigerant returning to compressor (the evaporator is a cross-current heat exchanger so yes, the vapor temp to the compressor can be warmer than the air leaving the coil) => 50F vapor less 15F superheat means there is a 35F saturated suction temp. The saturated suction temp must be a few degrees above 32F or the unit will collect frost and turn into an ice ball instead of taking liquid moisture out of the air. By the way, if you lower the blower speed and the underlying cause of the problem is that the unit is low on refrigerant, then the unit will be less efficient than with the correct refrigerant level (it will use more electricity to get the same job done). On older units, the condenser temperature split should be 25F-30F. When the outdoor temp is at least 80F, measure the air temp going into the condenser and measure the air leaving the condenser. The air leaving should be at least 25F warmer. If not, then your system is probably low on charge or the compressor is worn.

  • Chris - thanks for your insight. On #1, the unit is not oversized, so we'll put that one to rest. On #2 - great insight - I can't wait to check this out tomorrow/Saturday. I will report back.
    – eliza1
    Sep 11, 2015 at 4:48
  • What continues to bug me is that we didn't have this humidity problem before I "fixed" a different issue a couple of months ago. The fix involved a new capacitor on the blower, a new contactor, and a new thermostat. Ultimately, I don't think the old thermostat was bad, but I changed it out before I had done diagnostics on the other parts. So let me ask you this: could the new thermostat be turning the system on more frequently than the old one, and therefore the system is now running for less time than before? Like there is less tolerance in temperature variation from the programmed temp?
    – eliza1
    Sep 11, 2015 at 4:52
  • It's possible that cycling due to the thermostat is an issue but that's not high on my list of suspects (unless the tstat is directly under a supply vent). If I were looking at this I would check the discharge temp, condenser split, airflow, refrigerant charge and compressor performance. Matching the airflow to a low charge or a worn compressor may compensate somewhat. The 25F evaporator temp split is a hard limit. 20F would be safer. New units sometimes run about 15F (and they remove less moisture). When the ambient air is dry the evap temp split will increase compared to being humid.
    – user39367
    Sep 11, 2015 at 16:02
  • Having too much air for the compressor does cause higher humidity than when the two are correctly matched. If the old blower capacitor was weak, then when you installed a new one you increased the airflow and now the compressor cannot keep up.
    – user39367
    Sep 11, 2015 at 16:14

How much dehumidification an air conditioner provides depends on a few factors, but only one of them is easily tunable - air flow. The lower the air flow through the unit, the colder the coils will become, and the more moisture you will remove.

The standard rule of thumb for airflow is 400 CFM per ton of air conditioning. With a lower fan speed, you get more dehumidification, and with a higher fan speed you get less dehumidification. It is generally considered safe and reasonable to set the air flow down around 325-350 CFM to improve dehumidification. It may be possible for your unit to go lower, but the risk of freeze ups will increase.

  • I like your answer but I am wondering, how would a homeowner measure airflow without an airflow meter (since an airflow meter is generally specialty equipment)? Do you have something in mind?
    – user39367
    Sep 11, 2015 at 0:28
  • Cardboard box with most of the ends cut out and a weighted swinging flap hanging across the entire box. Calibrate it by holding out a car window while you drive at various speeds. Bingo, poor man's airflow meter. Sep 11, 2015 at 2:36
  • @chris - By reading the specs for your furnace/blower, basically. You won't get an exact answer unless you measure the pressure across the blower, but it will still give you a fairly close estimate.
    – Zhentar
    Sep 14, 2015 at 15:01

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