Night ventilation for cooling

Discussion in 'Home Power and Microgeneration' started by [email protected], Feb 22, 2005.

1. Guest

Phila has an average daily min and max of 67.2 and 86.1 F in July. Two A ft^2
vents with one-way plastic film dampers and a 16 ft height difference and a
DT (F) temp diff would allow 16.6Asqrt(16DT) cfm to flow at night, cooling
a house by cfmDT Btu/h. What would the average July temp be in a house with
10K Btu/F of thermal capacitance and 200 Btu/h-F of conductance and no internal
heat gain and 2 4'x4' vents with a 16' height difference, and no fan?

10 PI=4*ATN(1)
20 TH=76'initial house temp (F)
30 FOR D=1 TO 100'simulate 100 average July days
40 FOR H=0 TO 23 STEP .1
50 TA=(86.1+67.2)/2+(86.1-67.2)/2*SIN(2*PI*H/24)'outdoor temp (F)
60 IC=(TH-TA)*200'conductive loss (+) or gain (-)
70 IF TH>TA THEN IV=16.6*16*SQR(16)*(TH-TA)^1.5 ELSE IV=0'vent cooling (Btu)
80 TH=TH-(IC+IV)/10000*.1'new house temp (F)
90 NEXT H
100 NEXT D
110 PRINT TH

75.96505 (close to the average daily temp) with no vent

69.50536 (2.3 F above the min) with 2 4'x4' vents

With a higher house conductance, the vents make less difference, eg 73.3
vs 69.5 F at 1000 Btu/h-F. The vents above would move a 24-hour average
16.6x16sqrt(16(76.7-69.5)) = 2851 cfm, comparable to a whole-house fan.
With humidity sensing and motorized dampers, they could help heat a house.

Nick

2. Dr. HardcrabGuest

They said there wasn't gonna be any math on this test..........

;-]

3. Edwin PawlowskiGuest

I don't dispute any of the calculations. I have, however, lived in Philly
in the summer. While the average max may be 86.1 degrees, that is the
temperature taken at the weather station. In the city, it can easily be 90+
for days on end and lows in the high 80's as the heat sink of a brick row
house and concrete sidewalks holds the energy.

Northing beat an air conditioner for comfort.

4. Robert MoreinGuest

I, too, have lived in Philly. Come July you won't feel like doing those
calculations

In the days of the British Empire, Philly was considered a "hardship post",
due to the extreme summer heat. I have had friends from New Dehli, one of
the hottest places on the planet, say that West Philly, where they resided,

5. BughunterGuest

I don't live in Philadelphia. What should I do?

7. p j [email protected] _my _sig _for_address.comGuest

Paul ( pjm @ pobox . com ) - remove spaces to email me
'Some days, it's just not worth chewing through the restraints.'

HVAC/R program for Palm PDA's
Free demo now available online http://pmilligan.net/palm/
Free Temperature / Pressure charts for 38 Ref's http://pmilligan.net/pmtherm/

8. p j [email protected] _my _sig _for_address.comGuest

Who do they ship with, Airborne ?

Paul ( pjm @ pobox . com ) - remove spaces to email me
'Some days, it's just not worth chewing through the restraints.'

HVAC/R program for Palm PDA's
Free demo now available online http://pmilligan.net/palm/
Free Temperature / Pressure charts for 38 Ref's http://pmilligan.net/pmtherm/

9. Oscar_LivesGuest

Don't use a humidifier. Those things are too expensive to run.

10. Robert MoreinGuest

snip calcs

The solution for the Philadelphia climate is big-ass Trane or Carrier.

11. Guest

I think we can ignore humidity if ventilation is only used for cooling,
vs heating in shoulder seasons. Can the outdoor dewpoint Tod be greater
than the indoor wall temp Tw, if the outdoor dry bulb temp To is less than
the indoor dry bulb Ti and Ti is close to Tw? Tdo = To/(1-TolnRo/9621),
using a Clausius-Clapeyron approximation, with Tdo, To, and Ti in Rankine
degrees and the outdoor RH Ro expressed as a fraction.

To/(1-TolnRo/9621) > Ti makes

To > Ti - TiTolnRo/9621, or equivalently,

9621(To-Ti) > - TiTolnRo.

This can't happen, since the left side is negative, since To < Ti,
and the right side is positive, since 0 < Ro < 1.

QED.

Nick

12. Guest

Chinese peeled garlic costs \$2 per pound.
And igloos would melt.

Nick

13. ~^Johnny^~Guest

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

Very good! But it's in the molding, you know.

-----BEGIN PGP SIGNATURE-----
Version: PGP 7.1

iQA/AwUBQhzO0AIk7T39FC4ZEQLSPgCeO73I8m6nZaNyB2y86dGum7DDGusAoOms
iQB1DAGOEGJUlF8sVM1+lC1d
=ZKTO
-----END PGP SIGNATURE-----