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



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!


AC speed is determined by the tonnage, climate and (of course) duct design: 350 CFM/Ton in moist climates, 400 for normal climates and 450 for dry climates. Heat speed is determined by the "ATR" (air temperature rise) on the furnace nameplate: Design CFM is calculated at the midpoint of the ATR. I assume you have a furnace because you mentioned different cooling and heating speeds. It's not uncommon to see a job where required heating CFM is higher than cooling CFM.

Question 1. I haven't heard anything about "newer, more energy efficient AC units generally extract less moisture to begin with". As stated above, reducing the CFM improves humidity control. Operating the furnace blower its lowest speed for cooling is highly unusual. You might want to have a reputable contractor check the refrigerant charge.

Question 2. A smaller unit will run longer and improve humidity control. Click on "More Is Worse" on the website and read about timing the compressor. The only way "...on a slab...under the slab" would come into play insofar as humidity control goes is if the underground ducts contain groundwater.

Question 3. Industry Standard, regardless of climate, is RH maximum of 55% at 75F indoors. The associated outdoor condition would be the "1% Value", which is 90F and roughly 54% where I live. When it gets hotter, indoor temperature might "float" a degree or two, but humidity control should still be good. Virginia outdoor humidity design is 55% (Hampton) or less, by the way.


---------- FOLLOW-UP ----------


Thank you so much for your help!  For question 3, are you saying that with outside temp at 80-90 deg F and outside RH at 80-90 percent, inside RH should still be a max of 55 percent with inside temp at 75 deg F?

FYI, on the more energy efficient design I mentioned reading about, "newer" is actually about 20 years old now; when energy efficient "everything" got to be trendy and the Government passed new laws.  The general theme of the article was that the cooling part of AC got cheaper at the expense of less efficient moisture extraction.


Psychrometric Chart
Psychrometric Chart  

Virginia Outdoor Design Conditions
Virginia Outdoor Desig  
I suppose indoor humidity could float, as does temperature when outdoor conditions exceed design. However, I can't recall many (I'd like to say "any", but that's such a restrictive word) times the outdoor humidity reached 80% or 90% when outdoor temperature exceeded design. I checked our weather records for last July and August, learned the maximum outdoor humidity did not exceed 60% on the hottest days: July max. 96FDB, 77FWB, 59%; August max. 89FDB, 77FWB, 45%. This is a hypothetical question, right?

I can tell you I've never seen my indoor humidity over 55% when I use my AC. You'll see some observations in "More Is Worse".

I've been selecting AC equipment since the 60's, and that "moisture extraction" was reduced in the early 90's must have escaped me.

Here's a Psychrometric Chart. DB temps are vertical lines, WB temps slope down from L to R and humidities slope up from L to R. I've also enclosed VA's outdoor designs. Enjoy!

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Fred Weldin


I can't help you if your whatsis won't work, people (Especially if it's a refrigerator); I have no experience with appliances, and I haven't been involved with H&AC service and repair since March 08, 1996 (Thank the Lord); I always send a "standard" reply to appliance questions and H&AC "Service and Repair" questions (about 20 a month), so if you want to know why your whatsis isn't working like it used to, ask someone else. A lot of my answers arenít read by the questioners; in that event, I always send reminders to read the answers for a month or more (the word "idiots" comes to mind - actually there's another more appropriate word, but discretion prevents its use here). If you have questions about how big a unit you need, if one room's warmer than the others, if you want an opinion oas to scope of work on bids received, etc. just ask. Do visit www.askweldin.com, there's a lot of good information on sizing, ductwork, efficiency, as well as some techniques accomplished DIY-ers can use to troubleshoot and improve their H&AC. Finally, I want to thank all of the kind, serious people whom I meet here: I enjoy working with you. My wife says I'm "snarky" from time to time; as for you others, please use your imagination as to the responses swirling on the snarky side of my brain when I read your questions.


53 years in the business. See www.askweldin.com


B of ME U of D 1965, numerous classes, seminars and a lot of "Hands On" learning since.

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