Heating, Air Conditioning, Fridge, HVAC/residential AC RH


Hi Jim,

I've read that, generally, the AC air speed should be on high and the heat air speed somewhat lower.  However, I've also read that it is common practice to reduce air speed to extract more moisture because the "newer, more energy efficient AC units" generally extract less moisture to begin with.

Question 1. I've found that for my AC to extract the most moisture, and for the evap coil to get to the low end of its cold range, I have to put the blower on the lowest of 4 speeds.  Is this "normal"?....Is it true that newer AC units inherently extract less moisture by their more energy efficient design and that the only way to get them to extract more moisture is to lower the air speed?

Question 2.  Assuming the AC is functioning as well as it ever will, what can be done to extract more moisture; e.g. smaller/bigger AC,  differently designed AC....the reason I ask is because, even though my AC functions within normal parameters with the air speed on the lowest setting, the RH in the house in the summer still stays between 60-65 with the AC set on 70-74.  I assume this is because the house is on a slab and the air vents are under the slab.

Question 3.  What difference between outside and inside RH can be expected of any current residential central AC in summer when outside temps/RH is around 80-90 deg F and outside RH is 80-90. (I know the temp difference is around 20 deg, but not sure about RH.)

Thanks for your help!

Hi Skip,

Question 1. No, it's not normal to run the fan on low in AC mode. I have install lots of high efficient systems and have never had a problem with humidity. Factory settings are set to a higher fan speed. I haven't heard or read anything that high efficiency systems won't remove humdity properly.

Question 2. I suspect that your system is oversized. It is not running long enough to remove the humidity. Your system is designed to cool at a certain CFM. By lowering the CFM you are losing some efficiency. Have you seen any frosting on the outside unit or at the coil inside?

Question 3. The humidity difference depends mostly on how "tight" your house is.

If you can send me the make and model# of your system I think I can give you some more info.  

Heating, Air Conditioning, Fridge, HVAC

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Jim S.


I can answer questions on gas or electric furnaces, ac units and heat pumps. I don't work on boilers,chillers,or home appliances.


I have been in the HVAC trade for over 35 years. For the past 15 years I have owned my own business. I am semi-retired so I can answer questions in a timely manner.

I attended trade school for 5 years on weekends and evenings. This was very hard to do, but well worth it!

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