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. If I shut the fan off for an hour the humidity would rise into the mid 60s again. 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?
Does leaving the blower fan on all the time use up too much electricity or cause blower motor to wear out very quickly?
By the way, my basement is finished, and I don't want mold problems. If you think I should get a dehumidifier to further reduce humidity, then I think the only convenient logical place to drain the condensation would be to install a T fitting into the existing narrow PVC A/C condensation drain pipe. (and presumably some dehumidifiers have the ability to pump the condensation upward through a tube which I would connect to the T fitting. If I want to avoid having to empty out tanks, then is the setup I just described a good setup?
(If so, then hopefully basement humidity would not be a problem during the heating season, because the only practical location is within the furnace room, and since the furnace room gets hot/dry when the gas furnace is running, the dehumidifier would not function correctly during the winter months. Presumably I would only need to use the dehumidifier in the warmer months)
I have a dehumidifier running in my Pennsylvania home basement in the summer. If you have a clothes washer down there - you can run the drain tube into the washer drain tube. That way you will not to empty the tank. You will have to mount the dehumidifier high enough to drain by gravity.
I run a dehumidifier in the summer as it seem to take the musty smell out of the basement and mine is finished, and there is nothing wrong with running the fan on the furnace constantly, mine runs 24-7 365.
Not a big change. Did you warm the basement? Removing moisture from materials can take a long time.
Maybe not.
I'd say so... maybe 400 watts, $400/year.
Then again, you might store dryth in the basement on dry days. Kurt Kielsgard Hanson's 142 page catalog of sorption isotherms as LBM technical report 162/86 under
formatting link
concrete stores about 1% moisture by weight as the RH of the surrounding air increases from 40 to 60%, and it weighs about 150 lb/ft^3, so a 4"x1000ft^2 50K pound floorslab might store 500 pints of water as the basement RH increases from 40 to 60%. Mold forms in about 2 weeks, above 60% RH.
The catalog also says the equilibrium moisture content of some woods is about 30% by weight of the RH of the surrounding air, so a 25 lb cubic foot of dry hem-fir might weigh 28 pounds in 40% air and 30 at 60%, after storing 2 pints of water. Paper and clothing also store water.
The calc below says outdoor air is dry enough to keep a house below 60% at 70 F for all but 360 hours in NREL's TMY2 Typical Meteorological Year in Phila, with at most 75 "wet hours" in a row near the end of July.
20 PH=.6*EXP(17.863-9621/(70+460))'house vapor pressure ("Hg)
30 WH=.62198/(29.921/PH-1)'house humidity ratio
40 DAYSTART=150'display start time (days)
50 DS=DAYSTART*24'display start time (hours)
60 RANGE=3000'dISPLAY RANGE (HOURS)
70 LINE (0,0)-(639,349),,B:XDF=640/RANGE:YDF=3.88
80 FOR TR=60 TO 80 STEP 10'temp ref lines
90 LINE (0,349-YDF*(TR-10))-(639,349-YDF*(TR-10)):NEXT
100 CFM=2470'whole house window fan cfm (Lasko 2155A)
110 OPEN "ecayear" FOR INPUT AS #1:LINE INPUT#1,H$
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 WA
Yes, there are windows, but opening them is not very convenient, and leaving them open could worsen the problem if it is a humid day.
By the way, simply by keeping the blower motor of the heating/ac system running continuously, the circulation of the air throughout the house, so far, seems to be keeping the basement's relative humidity within the mid to upper 50s (as opposed to sometimes creeping up into the mid 60s if the blower motor is turned off). The humidity in the rest of the house is still well below 50%.
Is a basement relative humidity level in the upper 50s too high if the goal is to prevent potential mold/mildew in my finished basement?
Normally running the furnace fan constantly to reduce humidity during the summer is a bad idea. I did it in my house as a test, using data loggers to measure and record relative humidity. When I ran the blower constantly one summer, it raised the relative humidity 10% to 15% over letting it run just when the compressor run. TThe extra relative humidity is from water re-evaporating from the cooling coil and drain pan.
But I live in Myrtle Beach, SC and humidity is a big problem here. Since you are measuring the humidity in the rest of the house and it is staying at an acceptable level. I would say that it is OK to continue to do so. Just continue to monitor the humidity in the basement and house. Do whatever works in your climate and your house. The furnace blower should use less electricity than a dehumidifier.
I am running my dehuminifer at 45% humidity level in my basement, and I am also in Northern New Jersey.
I have two things that I need to mention about the use of a dehuminifer:
Seem like its air moving capability is not that great even at high fan speed. I have a feeling that it is only dehuminifing the adjacent area. This "may" be a problem if the basement is partitioned in multiple rooms or closets. Honestly, I am not sure about this. After using it for two years, I don't have any mildew problem in my basement (I had mildew problem in my basement before I started using the dehuminifer). I guess it is doing something good in my basement despite my concern about its coverage and despite the fact that my basement is partitioned into two large rooms (with opening between rooms) and with two closets.
Running dehuminifer can generate enough heat that we can feel uncomfortably warm in the summer even with reduced humidity level. This is especially bad if we have an exercise room in the basement. You "may" need to add air-conditioning in addition to using the dehuminifer. Still, using the dehuminifer should be the first choice over the use of air conditoning to reduce humidity level because I am under the impression that the air conditioner will stop as soon as the room temperatur has dropped down to the pre-set level regardless if the humidity level is reduced enough or not (and basement tends to be cooler than the rest of the house to begin with; hence the air conditioner will only run for a short while and probably will not dehuminify enough).
A dehumidifier would add significant heat to the house in summertime...
Here's a calc for the minimum basement slab temp needed to keep a basement less than 60% RH for a typical year in Baltimore, using long-term monthly weather data, assuming a fairly airtight house. It looks like the basement has no problem staying below 60% in wintertime (months 1-4 and 11-12 below), when outdoor air is dry and the 55.3 F ground supplies enough heat to make the natural slab temp more than the min slab temp required for 60% RH, given the moisture in outdoor air.
Psat = e^(17.8653-9621/(460+53.2)) = 0.413 "Hg at 53.2 F and 100% RH in January, and the vapor pressure of outdoor air with a w = 0.0025 humidity ratio is Pa = 29.921/(0.62198/w+1) = 0.120 "Hg, so RH = 100Pa/Psat = 29%, approximately, for a slab that doesn't store moisture.
20 ASLAB=1000'slab area (ft^2)
30 RGRND=10'R-value of deep ground (h-F-ft^2/Btu)
40 TGRND=55.3'deep ground temp (F)
50 CLEAK=10'air leakage rate (cfm)
60 RHMAX=60'max basement relative humidity (%)
70 TAC=80'AC temp (F)
80 RAC=50'AC RH (%)
90 DATA 31.8,.0025,34.8,.0027,44.1,.0037,53.4,.0052,63.4,.0083,72.5,.0115
100 DATA 77.0,.0134,75.6,.0131,68.5,.0106,56.6,.0070,46.8,.0048,36.7,.0032
110 DIM T(12),W(12)
120 FOR M=1 TO 12'month
130 READ T(M),W(M)'average outdoor temp and humidity ratio
140 PA=29.921/(.62198/W(M)+1)'vapor pressure of outdoor air ("Hg)
150 TSMIN=9621/(17.863-LOG(100*PA/RHMAX))-460'min winter slab temp (F)
155 TSNAT=TGRND+(T(M)-TGRND)/(1/CLEAK+RGRND/ASLAB)*RGRND/ASLAB'nat slab temp
160 PRINT M,T(M),TSMIN,TSNAT
170 NEXT
month outdoor min slab natural slab air (F) temp (F) temp (F)
The average daily max in May (month 5 below) is 74.2 F. A 66.2 F slab might lose 24h(66.2-55.3)1000ft^2/R10 = 26K Btu/day to the ground, which might come from a minimal 230 kWh/mo of indoor electrical use or a 90 W 2470 cfm intake window fan running 26K/(2470(74.2-70)) = 2.5 hours per day, or less, on a dry that's warmer and drier than an average May day.
5 63.4 66.21149 56.03636
It looks like AC will help for the next 3 months, with basement-house air circulation, which would reduce the AC load. At 80 F and 50% RH, a 400 cfm basement return would make the slab about 55.3+(80-55.3)(1/400+0.01)0.01 = 75.1. With Pa = 0.5e^(17.863-9621/(460+80)) = 0.524 "Hg indoors and Psat = e^(17.863-9621/(460+75.1)) = 0.890 "Hg near the slab, RH = 100Pa/Psat = 59% in the basement, approximately. The slab might give (75.1-55.3)1000ft^2/R10 = 1980 Btu/h of sensible cooling.
The average daily max in September (month 9) is 78.5. This might be another ventilation month, or an air conditioning month, if it's warmer than average, or a dehumidification month, if it's cooler.
9 68.5 73.23895 56.5
The average daily max in October is 67.3, so it's a dehumidification month, with possible help from ventilation.
10 56.6 61.41242 55.41818
And we might do nothing again in months 11 and 12.
A TMY2 hourly simulation would show less energy use, since some days are drier and/or warmer than average in winter and drier and/or cooler in summer. And concrete can store lots of water, about RH/2K % by weight, in RH% air, and it takes little additional house heat to reduce the basement RH and dry out the concrete and desirably increase the house RH in wintertime. A 4"x1000ft^2
50K pound slab can slowly store and evaporate (0.03-0.01)50K = 1000 pints of water as the RH of the basement air rises from 30 to 60% and falls back again.
Hour-by-hour smart ventilation controls that manage heat and moisture storage in a basement might reduce the AC and heating load and eliminate the need for a dehumidifier.
We might automatically change a $69 window AC from AC to dehumidification and back by adding a box around the outside with a $50 2 W (Honeywell 6161B1000) motorized damper with a hinge h at the top that opens for AC and closes for dehumidification, with horizontal partitions p and one-way passive plastic film dampers Di and Do on each side of the AC in the window mount flanges. It might look like this in the AC damper position, viewed in a fixed font: -------- ceiling | | | window | | ---|---------------------h---damper--- (AC) | |. | . | |.Do f c| . | |. a o| => . | | n n| . | | d| . | |--------p------|--p--. | .| condenser | . | Di.| inlet air | d | | n n| a | | d| m | |--------p------|--p--p | .| condenser | e | Di. \\ inlet air |
What nick is saying nobody will ever know as he advocates flooding your basement floor in winter for humidity and shrink wrapping your house to stop air exchanges. I use a basement dehumidifier it only ads a few degrees of heat which I welcome as my basement is always cool. My dehumidifier costs 3-4$ a month to run, a furnce fan running 24x7 would cost me 35 $ a month.
Not sure I understand this, but am I right in interpreting that you are basically saying that a dehumidifier is going to be relatively helpless against a cold slab with warm, humid outside air? (Plus we have the added comlication of dampness (not puddles) seeping in during rainstorms?
So if a large cold slab stores so much moisture, what is the best thing to do? - Heat the slab? - Circulate *in* hot air from outside (even if it is humid) to attempt to heat up the slab? - ????
It would work, if the air leakage is not too large. For instance, if C cfm of outdoor air with wo = 0.0134 pounds of water per pound of dry air leaks into a 55F 60% RH wb = 0.0051 Baltimore basement on an average July day, a 40 pint per day dehum could keep up as long as 24hx60C0.075(wo-wb) < 40, ie C < 45 cfm, or more, if the dehumidifier and the incoming air warm the basement, BUT that uses lots of electricity and makes lots of heat, about
1.6x40x1000 = 64K Btu/day. You might run a 5K Btu/h window AC 13 hours per day to remove all that heat :-)
It can be a lot more energy-efficient to use a fan to circulate air between the basement and the house when the basement is humid, warming the basement and cooling the house.
Maybe we can adjust the gutters and downspouts and slope the soil away from the house to improve that.
The long term moisture content of concrete might be 5% of the RH of the air surrounding it. Concrete weighs about 150 lb/ft^3.
Maybe a little, in wintertime, by moving air from the basement floor up into the house. A Baltimore basement with 10 cfm of outdoor air leakage might be 53 F in January, with Pa = 29.921/(0.62198/0.0025+1) = 0.120 "Hg vs Ps = e^(17.863-9621/(460+53) = 0.440 "Hg at 100%, and 100Pa/Ps = 29%. Circulating some air between the house and the basement could lower that and add desirable humidity to the house, with a little more heating fuel. (Unless the home is 100% solar-heated :-) Outdoor air warmed to 70 F would have 100Pa/0.748 = 16% RH. A 4"x1000ft^2 50K pound slab can slowly store (0.03-0.01)50K = 1000 pints of water as the RH of the basement air rises from 20 to 60%.
I don't think so, if it's more humid outside, in the absolute sense.
Why not subfeed the Heat/AC blower with a circuit, " Humidistate>>Relay>>Fan Switch>>Motor". Set Humidistate to close circuit at about 55%>>fan runs>>52% circuit opens>>fan stops....
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.