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Posted by RobertPatrick on June 24, 2005, 6:57 pm
nicksanspam@ece.villanova.edu wrote in
>
>>So far I can keep the Basement's humdiity level below 60% as long as I
>>leave the blower motor on the Furnace/AC system turned on all the
>>time. This causes some of the basement air to circulate throughout
>>the rest of the house (with the rest of the house still having
>>humidity below 50%). Would I be better off with a dehumidifier rather
>>than leaving the blower motor on all summer?
>
> 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)
>
> 1 31.8 34.03717 53.16364
> 2 34.8 35.98914 53.43637
> 3 44.1 44.13639 54.28182
> 4 53.4 53.22431 55.12728
>
> 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.
>
> 6 72.5 75.61597 56.86364
> 7 77 80.12378 57.27273
> 8 75.6 79.4524 57.14546
>
> 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.
>
> 11 46.8 51.05957 54.52727
> 12 36.7 40.35092 53.60909
>
> 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.
>
> Nick
>
>
Dry heat feels better than humidity + heat. It's very humid here and it
makes the heat feel even worse.
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