jay wrote:
I live in Northern NJ. I'm beginning to monitor humidity levels in my
basement to determine whether or not I need a dehumidifier. Today, I put a
hygrometer and the relative humidity of my basement was around 66%. Today
was not warm enough to use AC, but I decided to turn the blower motor of my
furnace/AC system on, and since there are return ducts in the basement, the
added circulation caused the basement humidity to drop to around 59% after
leaving the fan on for an hour or more.
Not a big change. Did you warm the basement?
Removing moisture from materials can take a long time.
...The humidity at the rest of the house was still at the low 40% range.
If I can keep the basement humidity below 60% by simply leaving the fan on,
do I really need a dehumidifier?
Maybe not.
Does leaving the blower fan on all the time use up too much electricity or
cause blower motor to wear out very quickly?
I'd say so... maybe 400 watts, $400/year.
Then again, you might store dryth in the basement on dry days.
Kurt Kielsgard Hanson's 142 page catalog of sorption isotherms
as LBM technical report 162/86 under
http://www.byg.dtu.dk/publications/reports.htm
says concrete stores about 1% moisture by weight as the RH of the
surrounding air increases from 40 to 60%, and it weighs about 150 lb/ft^3,
so a 4"x1000ft^2 50K pound floorslab might store 500 pints of water as
the basement RH increases from 40 to 60%. Mold forms in about 2 weeks,
above 60% RH.
The catalog also says the equilibrium moisture content of some woods
is about 30% by weight of the RH of the surrounding air, so a 25 lb
cubic foot of dry hem-fir might weigh 28 pounds in 40% air and 30 at 60%,
after storing 2 pints of water. Paper and clothing also store water.
The calc below says outdoor air is dry enough to keep a house below 60%
at 70 F for all but 360 hours in NREL's TMY2 Typical Meteorological Year
in Phila, with at most 75 "wet hours" in a row near the end of July.
20 PH=.6*EXP(17.863-9621/(70+460))'house vapor pressure ("Hg)
30 WH=.62198/(29.921/PH-1)'house humidity ratio
40 DAYSTART=150'display start time (days)
50 DS=DAYSTART*24'display start time (hours)
60 RANGE=3000'dISPLAY RANGE (HOURS)
70 LINE (0,0)-(639,349),,B:XDF=640/RANGE:YDF=3.88
80 FOR TR=60 TO 80 STEP 10'temp ref lines
90 LINE (0,349-YDF*(TR-10))-(639,349-YDF*(TR-10)):NEXT
100 CFM=2470'whole house window fan cfm (Lasko 2155A)
110 OPEN "ecayear" FOR INPUT AS #1:LINE INPUT#1,H$
120 FOR H=1 TO 8760'hours of typical (TMY2) year
130 INPUT#1,MONTH,DAY,HOUR,TDB,WIND,TDP,IGLOH,SS,WS,NS ,ES
140 PA=.6*EXP(17.863-9621/(TDP+460))'ambient vapor pressure ("Hg)
150 WA=.62198/(29.921/PA-1)'ambient humidity ratio
160 PSET(XDF*(H-DS),349-YDF*(TDB-10))
170 'PSET(XDF*(H-DS),349-YDF*(TDP-10))
180 IF WAWH THEN WETSTRING=0:GOTO 230'dry hour
190 WETHOURS=WETHOURS+1'accumulate wet hours
200 WETSTRING=WETSTRING+1'accumulate wet string length
210 IF WETSTRINGWETMAX THEN WETMAX=WETSTRING'measure max wet string length
220 LINE (XDF*(H-DS),290)-(XDF*(H-DS),300)'mark wet hours
230 IF DAY=1 AND HOUR=.5 THEN LINE (XDF*(H-DS),349)-(XDF*(H-DS),345)
240 NEXT H
250 PRINT WETHOURS,WETMAX
wet hours per year: 360
longest wet string: 75
Nick