A 2400 ft^2 x 8' tall house with 0.5 ACH leaks 160 cfm. At 30% RH and 70 F,
Pa = 0.3e^(17.863-9621/(70+460)) = 0.225 "Hg and wa = 0.62198/(29.921/Pa-1)
= 0.00470 pounds of water per pound of dry air. Outdoor air has wo = 0.0025
in Phila in January. Air weighs about 0.075 lb/ft^3, so keeping a house at
30% RH requires evaporating 60x160x0.075(wa-wo) = 1.584 pounds of water per
hour, which requires about 1584 Btu/h, ie 38016 Btu/day, about 11.1 kWh or
$1.11 per day at 10 cents/kWh, with electric heat.
As an alternative, we might use the earth's heat to evaporate water from
a basement floorslab and move humid air up into the house as needed. The
deep ground temp in Phila is 54.3 F. If the air near the slab is 54.3 F
with 60% RH (not higher, to avoid mold) and wb = 0.00541, we might move
1.584/(60(wb-wa)) = 37 cfm of basement air up at an energy cost of about
37(70-55) = 558 Btu/h, ie 35% of the heat energy needed above.
Or maybe less, since humid air diffuses and rises, so some cool water vapor
will move upstairs without moving any cool air with a fan and a duct near
the floor. We might dampen the floorslab with a soaker hose and solenoid
valve whenever the house is less than 40% RH and the slab is less than 60%,
and only turn on the fan when the house RH drops to 30%.
Solenoid valves can be scrounged from discarded dishwashers and washing
machines. Herbach and Rademan (800) 848-8001 http://www.herbach.com sells
nice $4.95 Navy surplus humidistats, item number TM89HVC5203, with a 20-80%
range, a 3-6% differential, and a 7.5A 125V switch that can be wired to
open or close on humidity rise.