Step 9: Mount it and turn it on
Thats it! Plug it in and watch any joints for leaks. It will take a minute or three for the water to travel the 150' of line before coming out the bottom. For best results, put a thermostat with a sensor in the panel to turn on only above 100F, or a timer to only turn on during the daytime (or both, to make sure your panel doesnt act as a radiator and cool the tub off in the evening after running the tub at temp. A future project will be a pic/audrino type micro-controller with temp sensors in both the tub and panel, to turn on the pump via relay when the panel is warmer than the tub. Mine is currently on an X-10 outlet with a timer programmed to turn it on/off with the sunrise/sunset (and delays to account for shadows from neighboring trees/houses), which also allows me to remote control it if the weather turns cold/cloudy.
One note on this design: with the PVC top sheet, you will want to run water through it any time its sunny outside to prevent it from going soft and possibly melting.... I left mine off one day and came back to a sagging lid with impression marks in the valleys from touching the tubing. Previous experiments using PVC pipe as line splices (1/2" pvc fits irrigation line inside it quite well, actually) showed similar results: the splices held up until run at temperature, then they started to bend and crack from the heat. Lesson learned: PVC has a low tolerance for heat.
Other note, and Warning: The pump I used in this is a fish pond pump. While it is a grounded pump and works fine without electrocuting your fish, the pump itself is NOT rated for pool or spa use (specifically states so in the manual). The temp of a spa at usable temp is also out of bounds of operation stated in the manual. While I am fine with this for my own personal use, you should evaluate this risk on your own. I take further precautions, like running this on a switched outlet that is ALWAYS OFF with the pump unpulgged when I get in the tub, I also remove the pump from the water (leave the hose ends under the surface to keep from draining the panel). Also, ALWAYS USE GFCI! As the mythbusters tested, GFCI can save your life. Any precautions you take could be moot if the power line insulation gets nicked or some internal conductor becomes exposed to the water. If the GFCI is working, it will trip... if you find it tripped, CHECK YOUR EQUIPMENT and figure out why before using it again! You are basically dropping a 110V power cord into a big tub of water you will jump into, if that doesnt scare you.........