The Problem

We live in the northeast where the summers only last for 3 months. We open our pool on Memorial Day and then need to close it by the end of August. My lovely wife likes the pool water like a hot tub, so, she never goes in. I prefer the water cooler it's more refreshing when I get in, yet below 70 is a bit brisk for my liking. How can we add heat to the pool to increase the temperature and extend the summer swimming, free?

The Experiment

Using some materials around the house, and some PVC connections from a local hardware store, build a passive solar heater.  

Parameters & Constants

  • Pool Diameter = 21 feet, and it holds about 10,000 gallons of water.
  • It takes 1 btu to raise 1 pound of water 1 degree Fahrenheit
  • 125 ft of black 0.5 inch diameter hose = 295 in3
  • 1.28 gallons of water in hose, or 10.64 lbs

System Design

The design is very simple, take a black garden hose and loop it around itself till it reaches the end. This is the size plywood in diameter that you need plus a couple of inches. I painted it black then cut four 2x4's to screw down from the bottom on the outside edges. Drill a hole in the 2x4 for the input of the hose, and a hole on the other side for the output. Put the output of the hose in the pool, and run the input from the filter into the black framed box. On top, I had a piece of 1/8 inch plastic that I screwed to the top. In a second box I used an old window as the top, then I put the two boxes in series.

Passive solar heaterCredit: My pool, my picture

It's critical to include an adjustable flow rate on the output of the filter. Notice the 2 valve system provides a mechanism to turn off the flow of water through the black hose when its night-time, and a second valve that is slightly turned that increases or decreases the flow. By partially closing the larger valve, more water will flow through the black hose.

Valve to change flow rate for Solar HeaterCredit: My pool, my picture


Heat transferred is what we want to maximize here and heat, Q(dot), is equal to:

Q(dot) = m(dot) * Cp * ΔT, where - m(dot) is mass flow rate, Cp is the specific heat of water (39 btu/lb degF) and ΔT is the temperature change.

Some basic testing showed 0.85 gpm volumetric flow with a temperature change (ΔT) of 7 deg F and with zero flow the temperature change is 13 deg F (the water just sits there and heats up to 90 degrees). Knowing that the curve is a first order differential equation we can do a regression analysis and fit the data to a curve, in this case Temperature change as a function of volumetric flow. The mass flow rate is linear (8.34 lb / gallon), therefore the energy recovered is maximized at the highest flow rate. In other words, picking up a little heat over many gallons is better than a lot of heat in a few gallons.

This experiment was successful, we added significant heat to the pool over the summer by utilizing the free solar heat.


Design suggestion is to maximize the hose in the sun, more hose more volume. Clamp the hose on the side of the pool to make sure it stays in the pool, since its connected to the output of the filter, if it came out of the pool it would drain the pool down to the filter intake. Maximize sun exposure to the solar boxes. Turn off the valve at night, or put the entire system on a timer. Cover the pool at night to keep the heat in the pool.