Insane House Cool-Down Scheme

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Doug Miller writes:

Begone, inimical troll.
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That scheme is unlikely to be efficient, with lots of thermal resistance (insulation) between the living space and the roof, and it's the wrong time of day to collect coolth, but Harry Thomason did something like this in the 60's for hundreds of houses, including his own in Washington, DC (a bad climate for evaporative cooling) pumping water over the roof at night and storing and distributing coolth during the day. He also ran an AC at night, when ACing is more efficient.
In "Solar Space Heating and Air Conditioning in the Thomason Home" (Solar Energy Journal, Vol 4, No. 4, Oct. 1960, pp 11-19), Dr. Thomason wrote "...the water on the north sloping roof flows slowly in an almost perfect thin sheet from top to bottom. The granules of sand in the asphalt shingles help to spread the water evenly and to retard the rate of flow... On the night of 14 July the rate of cooling was 25,700 Btu/h at the peak when the sky was overcast, but there was a breeze of about 15 mph. Outside air was at 68 F, water to the roof was at 73 F, with a return temp of 65.5 F and a flow rate of 5 gallons in 41 seconds. Humidity was about 65%."
Thomason used a 1600 gallon steel tank surrounded by rocks for heat and coolth storage and distribution. A homeowner with a basement might cool a house by pumping water over a roof at night and collecting it in a gutter and letting it flow back into some 4" thinwall PVC pipes or poly film water ducts tucked up between basement ceiling joists and circulate air between the house and the basement during the day.
NREL says an average 75.6 day July day in Sterling, VA (close to DC) with an average daily 64.1 and 87.0 min and max has a humidity ratio w = 0.0136 pounds of water per pound of dry air and an average V = 6.2 mph windspeed.
With average night temp Ndbt = (75.6+64.1)/2 = 69.9 F and vapor pressure Pa = 29.921/(1+.62198/w) = 0.640 "Hg and dew point Tdp = 9621/(17.863-ln(Pa))-460 = 65.5 F, the approximate wet bulb temp Twb = (Ndbt+Tdp)/2 = 67.7 F. With lots of T = 70 F water flow, Phil Niles says A = 0.002056*Tdp+0.7378 = 0.8725 for a radiation loss Qr = 1.63E-09*((T+460)^4-A*(Ndbt+460)^4) = 16.5 Btu/h-ft^2 plus convection loss Qc = (0.74+0.3*V)*(T-Ndbt) = 0.3 Btu/h-ft^2, with B = 3.01*(0.74+0.3*V)*((T+Twb)/65-1) = 8.753 and evaporation loss Qe = B*(T-Twb)-Qc = 19.8 Btu/h-ft^2, for a total Q = Qr + Qc + Qe = 36.6 Btu/h-ft^2, or 12Q = 440 Btu/ft^2 per night.
A small 1000 ft^2x8' house with 200 Btu/h-F of thermal conductance and 0.5 air changes per hour with 600 kWh/mo of indoor electrical use that's 80 F with w = 0.012 indoors would need about 70K Btu/day of cooling, which might come from 70K/440 = 160 ft^2 of roof.
It would need about 7100 Btu/h of peak cooling. If that comes from 1000 ft^2 (about $150) of ducts in the basement ceiling, the min duct water temp would be about 80-7100/(1000x2x1.5) = 77.6 F. If the house needs 350K Btu for a warm humid week, with no significant cooling at night, 350K = (77.6-70)C makes C = 45,851 Btu/F, eg 23 tons of water in 8.8" deep ducts (a good ballast foundation :-) More roof or basement ceiling surface would reduce the water depth.
As an alternative, we could cool static basement ducts without plumbing or pumping with a whole house fan on cool nights. Dr. Thomason also used a small dehumidifier.
Nick
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snipped-for-privacy@ece.villanova.edu wrote:

...
I believe that, rather than collecting coolth, he is trying to reduce the infiltration of heat during the day. He is not trying to collect the runoff water, which has been heated, not cooled. It doesn't cool the house, but just reduces the solar heating. It would be more effective if you have minimal ceiling insulation and minimal venting between the insulation and the roof (as is often the case in many "flat" roof houses).
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wrote:

You are correct, Kind Sir.

You are again correct, Kind Sir.

Not the case. Standard-looking gable roof, R-30 insulation. Little exhaust fan doesn't cool crawl-space much. Whole-house fan is not used except late at nite and very early in the morn.
I'm still inclined to think that drastic reduction of solar heat on roof surface would be very helpful.
Lots of folks water their lawns/gardens. I just wanna water my po' scorchingly-hot roof. :-)
Anybody got ideas on how to mount, say, 2 sprinklers and a hose on a gable roof without piercing any shingles?
Cheers, Puddin'
****************************************************** *** Puddin' Man PuddingDotMan at GmailDotCom *** ******************************************************;
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wrote:

resistance
this
house,
insulation
In Spain & Portugal, stallholders use water running down the canvas stall canopies, collect the water in a tank and pump it up again. It really does cool it underneath. That is a fabric tent like roof. It must make a difference on any roof though. How much, well I don't know.
Heavy insulation to keep the heat out must be a good thing.
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sounds a bit like a >>desert cooler<<. I saw one in India, cools about delta = 8C
Paul
News wrote:

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Leave the guessing to those who have taken the plunge.
I lived in a 14 X 70 house trailer w/ heat pump air/heat.
I discovered the first summer that just a simple $10.00 oscillating water sprinkler on the roof durring the hottest part of the day worked wonders ( gave the kids something to play under also).
(my .02 worth)
Rick Eisner N4NKR Rick
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You might collect the water in a pool with a $40 400'x1/2" pipe spiral to preheat water for showers...
Nick
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Here in Florida your roof would be covered with an ugly thick coat of gunky, slippery green slime (mildew? algae?) in about a month.
JustDave
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On 10 Jun 2005 08:06:48 -0700, snipped-for-privacy@slr.orl.lmco.com wrote:

Yeah, there's an exhaust fan up there ...
I dunno anybody understands. On a clear 90+ degree day, the shingles get more-or-less too hot to touch. I can direct the garden hose on 'em and the steam just rolls off ...
I think it's the extreme heat that fries the shingles (dries 'em out, curls 'em, etc). Will assume that they'd last longer 'till I have evidence to the contrary.
Still haven't figgered how to rig a cheap system. Can't be too obvious or expensive: water dept. may well shut me down.
Puddin'

****************************************************** *** Puddin' Man PuddingDotMan at GmailDotCom *** ******************************************************;
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wrote:

This is Turtle.
Good now we have heard from the alt.hvac Troll and give out his fool words for today.
TURTLE
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It would be easier for you to live in your basement during the summer.
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On 13 Jun 2005 12:16:38 -0700, scott snipped-for-privacy@my-deja.com wrote:

Tell me about it.
I spent most of the aftnoons down there on hot days last summer.
But I can't move everything down there ...
Puddin'
****************************************************** *** Puddin' Man PuddingDotMan at GmailDotCom *** ******************************************************;
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Couple thoughts: Why bother cooling off the whole house when the only thing that really needs cooling is _you_? With that in mind, leave the cold water running in the shower and stay in there during the hottest part of the day.
Or - a looping configuration of copper pipes encircling the La-Z-Boy with constant cool water circulation?
Just brainstorming here.
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Why not combine these two ideas? Strap the wookie (http://www.passedoutwookies.com /) down in his la z boy, and connect one copper tube to his neck, another one at his foot. Route the one coming out his foot to a floor drain, and the one going in his neck to the water supply; turn water on.
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thing
water
day.
Actually, just coiling the copper pipe around Puddin's neck - much like the rings worn by certain African tribeswomen- would probably keep him cool. Fitting the copper to a lengthy flexible hose would also allow him the mobility to leave his La-Z-Boy to make sandwiches and such.
Drainage may be problematic, though.
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snipped-for-privacy@Gmail.com (Puddin' Man) wrote:

I've seen horse stables here (Idaho) cooled with a similar system. Works OK, although our humidity is a lot lower than the Chicago area.
My *guess* would be that you might cause the roof to fail prematurely -- asphalt paths that are watered (sprinkler) on a daily basis wear out a lot faster than paths that are kept dry most of the time. Of course, keeping the roof cooler may more than make up for keeping it wet all the time.
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Puddin' Man wrote:

[snip]
Nothing, but don't expect much.
It's a strong function of the temperature of your water coming out of the hose, which is probably a bit under 70F if your lucky. But it will take ALOT of water, and flowing pretty quickly. It will also tend to cool the peak of the roof the most. It's also hard as heck on the shingles. Do you have any roof ventilation? That's gonna help the most.
Planting grass up there would probably do more, but the roof probably can't take the weight.
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Plant trees around the house. In 10 years from now when you go insane from the heat the house will have lots of shade.
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Puddin' Man wrote:

http://mirror.lerfjhax.com/www.eng.uwaterloo.ca/~gmilburn/ac /
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