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re: circulating basement-cooled air - should i?
20 may 2004
ms  wondered...

>...if it's advisable to circulate the cool basement air throughout
>the house, using the blower and ducting from the forced air heater?

probably. altho those furnace blowers are inefficient (300 w for 400 cfm?)
you might prefer grainger's $73 4tm66 5850 cfm 90 w window fan mounted in
a basement stairwell. 

>i would need to install an auxiliary air intake into the return duct...
>in its basement run...

or open a plenum and stairway door?

>i would also need to figure out how to control the blower.

you might want it on when the house is warm (eg 75 f) and the basement is
cool (eg 70 f) and off if the basement temp tb is ever less than the house
air dew point temp td, to avoid condensation on basement walls. air at temp
t has td = (t+460)/(1-(t+460)ln(r)/9621)-460, where r = rh/100. for example,
if the basement walls are 70 f and the house air temp is 75 f with 80% rh,
td = 535/(1-535ln(0.8)/9621)-460 = 68 f, so it's ok to turn on the blower. 
(you can also measure the dewpoint by stirring a glass of warm iced tea
with a thermometer and reading it when you see dew on the glass.)

>my biggest concern is the potential for some musty smells traveling
>from the basement (the usual basement smell) throughout the house.

you might keep the basement clean and dry, eg less than 60% rh. wrote:

>your basement will not act like a heat sink, in enough of a manner to make a
>difference for more than about an hour, then, the basement temps will be the
>same as your home....warm.

exactly one hour? :-) basements have lots of thermal mass, and the soil
surrounding them is cooler than summer air, and basements can be cooled
with outdoor air at night. replies:

>>geothermal systems, (basically what you are wanting to do) that use nothing
>>but buiried pipe to remove heat from the air, can have several hundred feet
>>of pipe buried deeper than your basement, and still require an evap and
>>compressor to remove, or control humidity.

>so, in effect the underground pipe can only replace the condenser?

basements can cool without compressors...

>i didn't realize the cooling capacity of the basement "operates" on
>such a limited efficiency (one hour or so?).

stephen@screweduponpurpose is sometimes wrong about hvac. for instance,
he's posted that there are no rules of thumb to calculate ac loads and
that evaporative cooling is mostly worthless.

>...a dehumidifier is in place and collects about 45 pints in 24 hrs.

wow. where does all that water come from? maybe you need some poly film
on the floor and walls. dehumidifiers generate sensible and latent heat.
better to use an ac in summertime. wrote:

>my underground 25x25 basement never gets above 72 even with a
>dehumidifier and 90+ outside  , i cut in returns as you are thinking
>about.  i have central air and it helps balance and circulate air a bit.
>a sawsall, jigsaw a closeable vent and 15 minutes are all it takes. 
>if you need a new dehumidifier a 70 pt sears is quiet and operates to
>45f. i just bought one. the cooling benefit depends on your basement ,
>location, ground temp, size of  windows , type,   amount underground
>and air infiltration. 

sounds reasonable. wrote:

>why not just apply the $150 to a window-rattler ac for the upstairs? 
>that way you would be directly cooling your living space with a machine 
>designed to do just that?

even more reasonable, with a low indoor airflow (a dirty filter?)
so it mainly acts as a dehumidifier, dumping the heat outdoors...

if your 32'x32'x8' tall house has r32 (8" sip) walls and 96 ft^2 of r4
windows and 0.5 ach of air infiltration and a 300 kwh/mo electric bill,
its conductance from indoor to outdoor air is 96ft^2/r4 = 24 btu/h-f
for the windows plus (1024-96)/r32 = 29 for the walls plus 1024/32 = 32
for the ceiling plus about 0.5x32x32x8/60 = 68 for 68 cfm of air leakage,
a total of 153 btu/h-f. with 2048 of masonry surface, the basement might
have 10k btu/f of thermal capacitance in series with a 3k btu/h-f air film
conductance. the house might have 5k btu/f of capacitance in series with
another 3k btu/f of conductance. we might add these to make 15k btu/f
in series with a 6k btu/h-f conductor, approximately. 

nrel says an average july day in phila has an 86 f daily max and a 67 f
min, with humidity ratio w = 0.0133 pounds of water per pound of dry air,
which corresponds to an ambient vapor pressure pa = 29.921/(1+0.62198/w)
= 0.626 "hg. the 1992 ashrae-55 comfort zone has a 68 f wet bulb temp
upper limit, and 68 f air at 100% rh has pw = e^(17.863-9621/(460+68))
= 0.699 "hg, approximately. at the dry bulb temp t, bowen (1926) said
100(pw-pa)/(tw-t) = (69.9-62.6)/(68-t) = -1, so t = 75.3 should be comfy.

if it were 86 f for 12 hours per day and 67 for 12 hours at night in
a simplified weather world, we'd have an equivalent circuit like this 
during the day (viewed in a fixed font):

86 f ---www-------- 75.3 f
         ---    |
     |--|-->|---|                 ie is the internal heat gain from 
         ---    |                    electrical use... 300 kwh per month
         ie     w                    is 10 kwh or 34120 btu/day, or
                w  1/(6k btu/f)      2483 btu/h, if all use happens 
                w                    over the 12-hour day.
               --- 15k btu/f

here's a (thevenin) equivalent daytime circuit:

tt   ---www-------- 75.3 f     tt = 86+2483/153 = 104.6 f.
        --->    |
         i      |                    
                w                   if i = (104.6-75.3)153 = 4480 btu/h
                w  1/(6k btu/f)     at the end of the day,
                w                   tb = 75.3-4480/6k = 74.6, max.
                |- tb
               --- 15k btu/f

during the day, rc = (1/153+1/6k)15k = 101 hours.

if 74.6 = 104.6+(tbmin-104.6)e^(-12/101), tbmin = 70.6 f. 

at night, we have (approximately):

67 f ---www-----
                |                 70.6 = 67+(70.6-67)e^(-12/rcn)
                w  1/(6k btu/f)   makes rcn = 16.6 hours 
                w                           = 15kr, so r = 0.00111
                |- tb             and r-1/6k= 0.000907, so cfm = 1102.
               --- 15k btu/f

we could make this more precise with a simple tmy2 simulation...


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