Duct sizes contributing to high humidity?
My relatively new Amana APH1536 first floor package unit is not keeping my RH below 70%, particularly at night. My second floor fancy 20 SEER Am Std split system just barely gets it below 60% upstairs. Here are my house/HVAC specs:
First floor 1850 sq ft
Package unit: Amana 3 ton HP
CFM: 1100 (dipswitch setting B)
Return: 16" round flex, about 25' long, sealed with mastic
Grille: 20"x25", centrally located in home with CLEAN MERV 6 filter
Supply: 12 equal runs, nicely tapered toward far end from HP, sealed with mastic
Drain pan and condensation pipe in great shape w/ P-trap
T-stat is digital touch screen but no RH sensor
T-stat set at 75 degrees constantly
Fan is in Auto mode
Encapsulated crawlspace (CleanSpace)
Crawlspace Santa Fe Advance Dehu set at 50%
Energy star windows
Humidity range: 75% at 6AM 64% at 6PM
Temp range: 73 at 6am 75 at 6PM
Based on recommendations from my tech, I am considering adding a second 12" return duct and filter grille from my foyer area and then joining it to the existing return box. Then he would install a new 18" or 20" return duct from the existing return box to the package unit. Will this help my RH? Other suggestions???
Answers to questions so far
Awesome posts, everyone! Answers to several questions on the replies are:
1) Location of home - North Alabama. RH issue is by far worse at night.
2) Upstairs is 1400 sq ft
3) Upstairs unit is a 3 ton split system Am Std, 20 SEER Communicating. It almost never needs to run at "full blast"
4) Upstairs has a bonus room over a garage with heavy heating/cooling load. A single 1-ton commercial-style supply and register are dedicated to that room.
5) Upstairs has no return duct, just a box - unit is in a closet right beside the grille.
6) A load calc was done on the 1800 sq ft downstairs at the time of unit installation. Result was 3.5 tons. I talked the tech down to a 3.0 ton based my known RH problems and what I learned from this forum :-)
7) Cycle time downstairs is not too long -- ~20 to 30 minutes each cooling run.
8) Insulation in home: average to above-average; crawlspace insulation is like a finished basement (foam on the inside walls).
9) Note: My walls have no OSB sheathing; they have 1" foil-faced foamboard on the outside of the walls covered by brick veneer
10) Yes, am interested in fresh air supply during calm weather but don't know how to solve that.
11) Leakage at the unit is always a possibility. But both my supply and return ducts are tight with mastic and good takeoffs.