It took me awhile to learn about dewpoint and some basic concepts of psychrometrics. Thanks to the people on this board who steered me in the right direction. The following is something which has been there all the time, but I just figured out what I was seeing.
My ACCA Manual J book gives design conditions for Houston AP as dry bulb 94F, wet bulb 77F. That equates to 71F dew point or 115 grains/pound absolute humidity. It is very common to have daily high approximately the design temperature, but what bugs me is how very common it is to have dewpoint 75F or higher. That means the difference in indoor vs. outdoor humidity is 20 grains greater than Manual J assumes.
Is this possibly some kind of a flaw in the theory behind Manual J? Or is it just not important? I understand the concept that the design DB is expected to be exceeded 2.5% of the time. However it seems the DP measure is exceeded rather more than that, while the DB is still up around 94F.
This is not uncommon in Dallas either. Manual J design conditions are for 65F DP while it sat at 72 DP all through a weekend visit there. Using Weather Underground it appears to be 72 DP as I write this, although the forecast DB high is a few degrees below the Dallas design temp of 100F.
Is this a thing?
Thanks -- Pstu