I was wondering if I could heat the water in my swimming pool using the heat in the house's attic and I started messing about with a large <s>heatsink</s> thermal battery. The water for my hotwater heater has to pass through it and picks up free calories which saves on the power bill quite a bit of the year. Even on winter days it adds some heat to the water and every calorie free is one I am not paying for...

http://senseless.livejournal.com/152881.html

Step 1: Make a Big Box.

I was worried about condensation on the pipes and also concerned about freezing even though I live in Florida so I decided to try a large box of sand. I used about two cubic yards and mixed it with two sacks of mortor mix to prevent it from turning into a giant hourglass. In the event the house is knocked down in a hurricane, it will just crumble and that will be one less thing I have to clean up.

Step 2: Stick in Some Pipe

I tried CPVC first because it was easy.

Step 3: CPVC Has a Surprisingly High R-Value

I began to experiment and after modifying a large tank I began to circulate hot water to try and warm the box of sand. After burning everything I could find, I began to realize that for the water to run in hot and back out hot and the sand to remain cool I was not transferring any heat.

Step 4: Boyle's Law

I forgot to keep an eye on it and add water. When the water got low enough for it to start circulating steam instead of liquid, the pressure spiked and it made a very loud noise.

The sand was still cool...

Step 5: Never Fear!

Just make it bigger.

I added 150 feet of 3/4 copper tubing primered and painted to protect agaisnt corosion.

This doubled the original size of the experiment but it began transferring heat.

Step 6: Let It Warm Up

There is enough mass so that the tempurature will only swing up or down a few degrees a day, so in the summer it averages about 90 to 100 degrees. In spring ,fall and most of the winter it is still above the tempurature of the well water, so it is stil benificial.

Step 7: Go Around It in the Cold

I can open an irrigation valve and bypass the water around the box if it ever gets colder than the well water here. I'll use a sensor in the box to open the valve, otherwise it is shut by default.