SOMEONE TELL ME WHAT I’M MISSING
I’ve got almost all of our windows covered with heat rejecting film and am looking for the next energy saving project. The payback time for the film is about 1 to 1.5 summers, and I’m looking to beat that. I’m inclined to try misting the roof.
According to this site, the evaporation of one gallon of water is about the same as one ton of A/C capacity. A large home A/C unit might be four tons. So evaporating even a single gallon of water could remove a large portion of the heat load to the home.
There are all sorts of expensive commercial systems for spraying water on the roof. I’m not going that route. What I’m considering is using a simple electronic timer that attaches to the spigot. The outlet would be connected to a regular garden hose that I’d run up the downspout. Finally, the hose would connect to one of those flat misting hoses that you can pick up at Walmart for a few bucks. I figure the whole set-up should cost less than $50.
And here’s where I’d install it. The spigot is about 2 feet to the right of the downspout.
Seems easy-peasey to me. So what am I forgetting?
UPDATE: I just sprayed some water on the roof with the garden hose and let it run off into the downspout. The excess water was definitely warm.
7 Responses to “SOMEONE TELL ME WHAT I’M MISSING”
 Comment by Nance Confer July 27th, 2008 at 4:10 pm |
How much flat misting hose are you planning to use? Are you just looking to spray the corner in the picture or the whole house? We have a misting hose under the trellis and it is remarkably cooling. Lovely on a muggy day. 🙂 Nance |
 Comment by Daryl Cobranchi July 27th, 2008 at 4:27 pm |
For this test, I’m just going to run it up the valley in the pic. If it seems like it’s accomplishing something, I’ll come back with a more elaborate system next spring. |
 Comment by Rob July 28th, 2008 at 5:45 pm |
Our air conditioning system consists of the following: It’s probably hit 90 a time or two, but never above 75 in the house. I’ll also occasionally just point the nozzle skyward and wet down the roof in the evening. It does seem to help the house cool off a bit quicker once the sun goes down. |
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Comment by Daryl Cobranchi July 28th, 2008 at 6:06 pm |
On the same page linked above, they note that spraying water across the heat exchanger in your A/C accomplishes much the same thing. I tried it today. Our home A/C usually struggles to keep the family room below 80 ‘F. Right now with water trickling down one side of the A/C unit (maybe 1 quart per minute) the room is a very comfortable 76 ‘F. I wouldn’t want to run that much water all the time, but I think it does demonstrate the feasibility. |
 Comment by speedwell July 29th, 2008 at 2:09 am |
What you may be forgetting is that you’re probably using purified municipal drinking water to run the evaporative system. Even water from your garden hose is safe to drink. But there’s a cost in energy and environmental impact, not to mention taxes, in using that water to do nothing but sit around and dry off. I’m not saying I have a solution… There may be easy ways to mitigate the impact, though. How warm is the runoff, for example? How clean is it? Could you use it instead of water warmed by your water heater? Could you use the heat from the water some other way? |
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Comment by speedwell July 29th, 2008 at 2:16 am |
OK, my mind is clicking a bit on this. Placing an underground storage tank and filter to clean and hold the runoff water and recycle it to feed the mister, for example, might allow the relatively constant underground temperature to cool the water somewhat before it’s sprayed up there again. Potential savings in water and energy there. Add one of those nifty water-mill style wheels to catch the falling runoff water, and you might get it to run itself. Perpetuum mobile! (I kid, and I deserve to have my mouth washed out with soap by a mechanical engineering professor for it, too. LOL…) |
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Comment by Daryl Cobranchi July 29th, 2008 at 3:50 am |
Well, we’re on a well. 🙂 No chlorine but relatively high iron. So staining is a possibility. Also I’d have to calculate the energy required to pump a gallon from 25′ below grade. But whatever runs off goes back into the very porous soil here (sand, really) and ends up back in the water table eventually. |