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Nick Pine
June 30th 03, 04:05 PM
>You might make an apartment less massy and desirably increase its thermal
>resistance to outdoors by gluing lots of $11 4'x8'x1" sheets of Atlas
>Energy-Shield double-foil polyisocyanurate "R7.2" inside the drywall...

OTOH, as Steve Baer says, thermal mass is good for natural cooling, as in
this average July day in Phila, with 67.2, 76.7, and 86.2 F min, 24h, and
max average temps (view in a fixed font):

1/Gapt 1/Gdry
----------------www-------------------www----------- T drywall temp (F)
| | 1/200 | 1/6000 |
| | | |
| | / 1/Gfan | |
| ---. .-----www---------- I |
| fan on 1/3000 (cfm) |
|+ at night |
--- Cdry --- 2000
| ~ | Ta = 76.7+9.5sin(wt) --- Btu/F
--- outdoor temp (F) |
| |
- -

10 PI=4*ATN(1)'BASIC fun
20 W=2*PI/24'angular frequency
30 GAPT=200'apartment conductance to outdoors (Btu/h-F)
40 GFAN=3000'approx 3K cfm fan conductance (Btu/h-F)
50 CDRY=2000'drywall capacitance (Btu/F)
60 GDRY=6000'drywall air film conductance (Btu/h-F)
70 DT=0.1'time step (h)
80 T=76.7'initial drywall temp (F)
90 FOR D=1 TO 2'simulate for 2 days
100 FOR H=0 TO 23 STEP DT'in 6 minute time steps
110 TA=76.7+9.5*SIN(W*H)'ambient temp (F)
120 IF TA>T THEN GS = GAPT ELSE GS = GAPT+GFAN'conductance to outdoors
130 I=(TA-T)/(1/GS+1/GDRY)'heatflow into drywall (Btu/h)
140 T=T+DT*I/CDRY'new drywall temp (F)
150 IF D<2 GOTO 210
160 IF H>11.2 AND H<11.6 GOTO 190'pick interesting results
170 IF H>18.7 AND H<19.1 GOTO 190
180 GOTO 210
190 IF GS>200 THEN FAN = 1 ELSE FAN = 0
200 PRINT H;FAN;TA,T
210 NEXT H
220 NEXT D

hour fan outdoors indoors

11.20001 0 78.67514 78.24278
11.30001 0 78.43121 78.2446 max indoor temp
11.40001 1 78.18611 78.2385 fan turns on
11.50001 1 77.93998 78.20735...

18.70004 1 67.3591 67.46273
18.80004 1 67.40762 67.45697 min indoor temp
18.90004 0 67.46251 67.45703 fan turns off
19.00004 0 67.52373 67.45768...

The fan would run for about 7 hours per night. We could estimate the
expected yearly cooling and heating energy of a conventional AC system
and furnace with and without smart night and day ventilation schemes with
a TMY2 simulation using NREL data for every 8192 hours of a typical year
in Phila, turning the fan off and the AC on if the house becomes too hot
or humid (say more than 60% RH) or too cold, or the drywall temp becomes
less than the dew point, threatening condensation on the indoor surface.

This could work better with bigger fans and more thermal mass, eg
some water-filled poly film ducts between floor joists or rafters.

Nick

Nick Pine
June 30th 03, 04:05 PM
>You might make an apartment less massy and desirably increase its thermal
>resistance to outdoors by gluing lots of $11 4'x8'x1" sheets of Atlas
>Energy-Shield double-foil polyisocyanurate "R7.2" inside the drywall...

OTOH, as Steve Baer says, thermal mass is good for natural cooling, as in
this average July day in Phila, with 67.2, 76.7, and 86.2 F min, 24h, and
max average temps (view in a fixed font):

1/Gapt 1/Gdry
----------------www-------------------www----------- T drywall temp (F)
| | 1/200 | 1/6000 |
| | | |
| | / 1/Gfan | |
| ---. .-----www---------- I |
| fan on 1/3000 (cfm) |
|+ at night |
--- Cdry --- 2000
| ~ | Ta = 76.7+9.5sin(wt) --- Btu/F
--- outdoor temp (F) |
| |
- -

10 PI=4*ATN(1)'BASIC fun
20 W=2*PI/24'angular frequency
30 GAPT=200'apartment conductance to outdoors (Btu/h-F)
40 GFAN=3000'approx 3K cfm fan conductance (Btu/h-F)
50 CDRY=2000'drywall capacitance (Btu/F)
60 GDRY=6000'drywall air film conductance (Btu/h-F)
70 DT=0.1'time step (h)
80 T=76.7'initial drywall temp (F)
90 FOR D=1 TO 2'simulate for 2 days
100 FOR H=0 TO 23 STEP DT'in 6 minute time steps
110 TA=76.7+9.5*SIN(W*H)'ambient temp (F)
120 IF TA>T THEN GS = GAPT ELSE GS = GAPT+GFAN'conductance to outdoors
130 I=(TA-T)/(1/GS+1/GDRY)'heatflow into drywall (Btu/h)
140 T=T+DT*I/CDRY'new drywall temp (F)
150 IF D<2 GOTO 210
160 IF H>11.2 AND H<11.6 GOTO 190'pick interesting results
170 IF H>18.7 AND H<19.1 GOTO 190
180 GOTO 210
190 IF GS>200 THEN FAN = 1 ELSE FAN = 0
200 PRINT H;FAN;TA,T
210 NEXT H
220 NEXT D

hour fan outdoors indoors

11.20001 0 78.67514 78.24278
11.30001 0 78.43121 78.2446 max indoor temp
11.40001 1 78.18611 78.2385 fan turns on
11.50001 1 77.93998 78.20735...

18.70004 1 67.3591 67.46273
18.80004 1 67.40762 67.45697 min indoor temp
18.90004 0 67.46251 67.45703 fan turns off
19.00004 0 67.52373 67.45768...

The fan would run for about 7 hours per night. We could estimate the
expected yearly cooling and heating energy of a conventional AC system
and furnace with and without smart night and day ventilation schemes with
a TMY2 simulation using NREL data for every 8192 hours of a typical year
in Phila, turning the fan off and the AC on if the house becomes too hot
or humid (say more than 60% RH) or too cold, or the drywall temp becomes
less than the dew point, threatening condensation on the indoor surface.

This could work better with bigger fans and more thermal mass, eg
some water-filled poly film ducts between floor joists or rafters.

Nick

July 9th 03, 06:56 PM
Steve > wrote:
> I just moved into my first apartment.

> It is cooled with a large window unit air conditioner that will auto
> cool to a particular temperature.

> Is it cheaper for me to leave it turned off while I go to work or
> leave it running all day?

Cheapest is to not use the AC at all. ;-)

If you live in a humid area, the most comfortable thing to do is leave it on
all day to keep the air inside the house dry. A small apartment with a big
air conditioner will allow you to quickly cool the place, but it won't take
the humidity out of the air. You need to keep the compressor running as you
pump the air through the AC to take the humidity out of it. I prefer
using "undersized" efficient airconditioners and leaving them on all
day at a slightly warmer than comfortable temperature. I then turn the
thermostat down when I get home.

July 9th 03, 06:56 PM
Steve > wrote:
> I just moved into my first apartment.

> It is cooled with a large window unit air conditioner that will auto
> cool to a particular temperature.

> Is it cheaper for me to leave it turned off while I go to work or
> leave it running all day?

Cheapest is to not use the AC at all. ;-)

If you live in a humid area, the most comfortable thing to do is leave it on
all day to keep the air inside the house dry. A small apartment with a big
air conditioner will allow you to quickly cool the place, but it won't take
the humidity out of the air. You need to keep the compressor running as you
pump the air through the AC to take the humidity out of it. I prefer
using "undersized" efficient airconditioners and leaving them on all
day at a slightly warmer than comfortable temperature. I then turn the
thermostat down when I get home.

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