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re: cooling the home
19 jul 2002
cm  wrote:

>ceiling fans are designed to move a large mass of air at a low
>velocity and very low pressure differential. they would not work well
>as a whole house fan...

they might, with no shutter, and lots of basement windows with one-way
plastic film dampers. a simple 4'x4' hole in the ceiling with 16 ft^2
of windows 8' below might move 16.6(16ft^2)sqrt(8x10f) = 2375 cfm with
a 10 f dt and no fan at all. 

>also, having a large hole in the ceiling with no shutter could be an
>open invitation for attic critters to get in when the fan is off
>(i've had wasps in the attic).

maybe it needs an insect screen or a motorized shutter above,
eg a 4'x4' piece of 3" foamboard hinged on one edge, with
a motor and pulley to raise it. 
 
>my whole house fan moves air from open windows around the house when
>it's cool, and by cooling the house at night and early morning it
>dramatically reduces the ac load during the day - but i have to make
>sure it's off and the windows closed when it warms up outside.

i'd like to see that happen automatically... 

you might use an ac blower without the compressor to pressurize a
basement and make a new duct to supply the house registers with air
from near the basement floor, using the thermal mass (20k btu/f?) and
earth contact of the basement to cool the house in summertime when
the basement is cooler than the house and the outdoor air is warmer.

and modify the controls to inhibit the ac and turn on the whole house fan
and draw in outdoor air through basement windows when the outdoor air
is sufficiently dry, and cooler than the house air, ie cool or heat
the basement and house with outdoor air to save air-conditioning or
heating energy. the basement windows could have passive plastic film
dampers that only allow air to flow into the house.

the controls might monitor the indoor and outdoor air temperatures 
ti and to and the basement temp tb and relative humidity rb, and operate
the whole house fan f, the furnace blower b (alone, to circulate air
between the house and the basement), the ac compressor a and the gas
heater h. 

a crude ac-only control might work according to this table,
(view in a fixed font):

     ti     to    tb    rb  | f     b     a      notes
   -------------------------|-------------------------------------
1.  <68   | -   | -   | -   | off | off  | off   do nothing
2.  >74   | -   | -   |>60% | off | off  | on    ac: too humid
3.  >74   |<63  | -   |<55% | on  | off  | off   economize
4.  >74   |>69  |<68  |<55% | off | on   | off   circ basement air
5.  >74   |>69  |>74  | -   | off | on   | on    ac: too hot

line 1 does nothing, if the house is comfortably cool. line 2 turns on
the ac if the house is too warm but the basement is too humid. line 3
turns on the whole house fan if the house is too warm and the basement
is not too humid and the outdoor temp is sufficiently lower than the
house temp. line 4 turns on the furnace blower if the house is too warm
and the outdoor air is warm but the basement is cooler and dry. line 5
turns on the ac if the house is too warm and the outdoors and basement
won't help. 

we could implement this with 3 of grainger's $12.93 2e158 22 amp spdt line
voltage thermostats (with a 2/4 f heat/cool diff.) and an h&r (800) 848-8001
$4.95 navy surplus hair element humidistat (tm89hvc5203) with a 20-80% range
and 3-6% differential and an spdt 7.5 a 125 vac switch: 

                           >60                     >74
                      ----------------------------------- ac 
               >74   |   |             >69   |   |
          -----------|rb |        -----------|tb |
         |   |       |   | <55   |   |       |   | <68    furnace
120 v ---|ti |        -----------|to |        ----------- blower
         |   |                   |   | <63
          -------                 ------- whole house fan 

this might be worth trying, but it can be improved. make the lower and
upper comfort limits tl = 68 and tu = 74 changeable by the occupants,
for one thing. eliminate the undesired dead bands, try to save heating
as well as cooling energy, and use differential vs fixed limits. for
instance, in line 3 of the table, it would be more efficient to use
the whole house fan vs the ac if the house were 80 f and the outdoor
air were 74 vs 62 f. why wait till the outdoor temp drops to 62?

we could also save more energy by trying to use ventilation to make
the thermal mass of the basement (which might include some additional
4" sealed water pipes under the rafters) close to some target temp tt
which is closer to the upper comfort limit tu in wintertime and the lower
comfort limit tl in summertime, so the mass can store more useful heat
in a larger temp swing. the more complex table below reflects this,
using a 3-day-averaged outdoor temp toa to determine the target temp tt. 

     ti     to    tb    rb  | f     b     a     h     until ti
   -------------------------|-------------------------------------
1.  tl+2
          |     |     |     | warm house with furnace
          |     |     |     | basement too cold, last resort 
          |     |     |     |  
2.  60% | -   | -   | -   | on    >tl+2
          |     |     |     | warm house with furnace
          |     |     |     | basement too humid, last resort 
          |     |     |     |  
3.  tl+4|<60% | -   | on  | -   | -     >tl+2 
          |     |     |     | warm house with basement 
          |     |     |     |  
4.  ti+4| -   |<60% | on  | -   | -   | -     >tt
          |     |     |     | warm house with fan  
          |     |     |     |  
5.  >tt+2 |tu   |>ti-4|tu   | -   | -   |>60% | -   | -   | -   | on    tu   |>ti-4|>ti-4| -   | -   | -   | on  | -     
      


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