Sweaty Houses Cool Faster: Evaporative Cooling From Stored Rainwater

"Six gallons (22.7 L) of water evaporating has the same cooling effect as a typical (3.5 ton-hour) home central air-conditioner." - Alliance for Water Efficiency

My home air conditioning system is cutting edge technology... or was in 1965 anyhow. It is very inefficient, loud, and it doesn't keep up on really hot days. Since I'm faced with a much needed HVAC replacement, I have decided to try a novel cooling approach to help the old unit limp along until I can get someone out here to replace it.

The initial results are good; I can say for sure that evaporative cooling works great when it's not extremely humid, and it still helps the system keep up when it is extremely humid. On moderate days, full sun and 75-80 degrees, the air conditioner used to kick on regularly, but now it stays off until the temp gets well into the 80s.  For a more quantitave result, see below.

It requires 21 gallons of water per day, covering about 600 sq ft of roof with 2.5 gallons each of the eight times it runs. Since tap water is costly in the summer, I am using water from my rainwater storage system which holds enough water (150gal) to run this system for about 7 days before running dry.

Building It

The system itself is very simple. I have a 1hp submersible dirty-water pump (with float) that sits inside my black rain barrel and feeds a standard garden hose through a hole drilled in the top which runs to the roof peak. From there the garden hose connects to a 25' “soaker hose*” ($10) which is made up of a semi-porous rubber and is designed for drip-watering plants.

Since I only need to give the roof a coating of water, and don't want any wasteful runoff, it runs for exactly one minute about once per hour, using a programmable timer ($10). That's enough to evenly distribute water across my asphalt shingles. It takes the water anywhere from 15 to 45 minutes to fully evaporate off of the tiles depending on weather conditions.

Of course mine runs even when it's raining, but who cares, the rain is refilling my barrels at the same time at a much faster rate. It wouldn't be hard to add a relay in between that prevents it from running when the sun isn't out, or the temp is too low, you only need a few electronic components to create a circuit like that.

Measuring Progress

If you're like me and you'll build first and adjust later, you may want to know how much water you're going to need, or how much time you need to run it. This is solved easily with some grade school math.

My first concern was with how much water each cycle was using. Since my barrels permit me to see the water level, I placed a mark at the point the water was at before the system started running for the day, and then measured from that mark to the new water level after it had run its 8 cycles during the day. My barrels also have markers to at 5 gallon increments, so it was a matter of measuring how many inches of water level amounted to one gallon of water ( distance between gallon marks divided by the number of gallons represented by the distance). Once I knew that, I could instantly figure out how precisely how much water was used that day (inches between first and second marks times the previous gallons-per-inch figure) and then divide that by 8 cycles so I know what each cycle uses.

As for measuring the effectiveness of the evaporative cooling itself, that is something that someone with a MS in physics can probably help you calculate, but not me!

I have to rely on degree day based calculations.  In comparing this June with last June, the degree days increased by 17%.  At the same time, my electric bill increased by 4% over last June.  So it appears that it may be an effective supplement to my overall cooling system, though this is not a rigourously scientific analysis.  In any case that's the best I'll be able to do, I just had a new 16 seer AC unit installed which renders fair comparisons with prior years impossible from here on out.  If you try it, tell me how yours does.

Downsides

Some negative consequences of using this sort of system could include extra shingle wear from the constant wet-dry cycling (I don't know for sure, so far so good), strange looks from neighbors while they get adjusted to your new “green economy,” and a modestly expensive set-up.

The pump is by far the largest cost, mine was $60 from Harbor Freight (part 93819). It is rated to lift water to 26 feet, so at my 16' roof peak there's still plenty of pressure. When selecting a pump it is important to consider how much force it needs, but if you try this with city water you don't need a pump and pressure will probably not be an issue at all. Of course, the water bill is going to be pretty high in that case, I'll easily recover the cost of the pump and everything else in one summer by using only rainwater.

* Note: I do not recommend using a soaker hose in the long run as it will decompose quickly, rather a uv-protected pvc tube with small diamond shaped spray holes would be a better choice for a permanent installation.