Why is it called ping pong?

 For something this inexpensive, we're surprised there isn't a better name for it. If you've ever tried to get a good picture on a table tennis table, you know that the ball doesn't always hit the paddle. Instead, it often bounces around the table and lands in places you wouldn't expect. This is exactly what's going on here - instead of bouncing on a flat surface, the ball hits a curved surface on the roof. That causes a bit of a rebounding action that results in the ball being forced to follow a parabolic path - hence "ping pong".

It's hard to imagine how this concept would work on a smaller scale, but we were pleasantly surprised to see it work on such a large scale.

We tested this idea on a large roof (~100 square feet) with a large open area at the base of the structure. We ran our pool heater through a test using this method to ensure that it would have the same effect as we saw in our large tests. In every test run, we used a 2'x4' piece of plywood that was nailed to the roof. Our testing showed that the ping pong effect was consistent, and that the roof had a similar effect to a standard table tennis table.

When the pool heater is not in use, the ping pong effect reduces cooling needs by approximately 30%. The amount of space that a pool heater uses on the roof, and the amount of heat that it puts out can be greatly reduced by using the ping pong effect.

Our initial test was done with an above ground pool in North Carolina, with a temperature of 50 F and a humidity of 75%. After the pool pump ran continuously for 1 hour, the temperature of the water was measured at 54 F with a high of 61 F. Using the ping pong effect with the pool pump running for 4 hours resulted in a lower temperature of 48 F. This is very close to the ideal temperature for the pool. With no pool heater, the temperature is 53 F.

To maximize the effectiveness of the ping pong effect, the pool heater must be installed such that the angle of attack is greater than 90 degrees. This is because the ball rebounds upwards on an upward facing surface (as opposed to a downward facing surface). If the angle is less than 90 degrees, the ball would continue towards the roof and not rebound upward.

In order to make sure that the ball is hitting the roof at a suitable angle of attack, the pool heater should be installed approximately 10 inches off the roof. Ideally, the roof should slope slightly upwards from the center of the pool (as shown in the diagram).

While a ping pong table is ideal, a roof with an angled surface works equally well. As long as the ball is hitting the roof and rebounding upward, this technique will work.