blueman wrote:

writes:

jay wrote:

...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...

...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?

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.

(Plus we have the added comlication of dampness (not puddles) seeping
in during rainstorms?

Maybe we can adjust the gutters and downspouts and
slope the soil away from the house to improve that.

So if a large cold slab stores so much moisture...

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.

...what is the best thing to do?
- Heat the slab?

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%.

- Circulate *in* hot air from outside (even if it is humid) to
attempt to heat up the slab?

I don't think so, if it's more humid outside, in the absolute sense.

Nick