How to Make a Simple Solar Spa Heater on Your Roof

Even in the summer months, it's nice to have hot water for the shower, doubly so if you have a spa or hot tub. To help you keep things steamy while keeping your electricity bills down, here's a surprisingly simple DIY project. For about $60 worth of materials, you can make your own water heater, installed on your roof.

This project deals with heating a spa, but with some tweaks near the end of the process, you could use it to heat a pool, sink or bathtub. Here's how.

This project is the work of Instructables user Greg Horejsi, aka petastream. Special thanks to him for his permission to post it here.

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Why a Solar Spa Heater?

Well it's that time of the year again here in Southern California time to fill up the spa and break out the BBQs. This has become a yearly ritual as well as a scathing reminder that electricity is not free. Anyhow, my wife has been wanting to move in a greener direction and has been rather matter a fact about it as well. We now compost our organic waste and recycle that into our garden. Her new kick was "How can we heat the spa without using the electronic heater?" She got up on the roof with 50 feet of black hose and ran it back and forth a few times, hooked it to a pump and came up with the proof of concept. Yay! (The original idea was sourced from my coworker Gary who has a similar setup for his pool.) Today we decided to move from conceptual proof to full on production. This Instructable will walk you through the process that we went through and try to help you avoid some of the gotchas that we found along the way. Hope you enjoy reading this as much as we enjoyed building/documenting this project.

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What You'll Need

For around $60 you can build this neat little system using the following parts purchased at your local mega hardware store:

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Setting Up the Frame

This step is simple. You take the 20ft of PVC pipe and cut it down to four 5ft long sections. Glue them each in to the 4-way fitting using blue PVC cement or some similar form of adhesive. Let it dry a bit, and voila! Your framework is complete. Wish it was that easy creating frameworks at my day job! (I'm a software developer if you're wondering.)

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Your Work Area

Make sure you have room to move around the work area, as you will be doing a lot of walking (mostly in a circle). We set up a ladder with a pipe crossbeam which held the drip hose. This setup didn't work quite as well as we had hoped, but did help keep the coil from getting horribly tangled. The frame was set up on a trash can that we filled part way with water to stabilize it from falling over. The spool unwinding was managed by my wife for the most part and my son helped guide the hose while I walked in circles like a carnival mule.

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Spiral Construction, at a Snail's Pace

The hose was spun on to the frame starting at the middle and working outwards. We took a lead of around 6ft and attached to one leg of the frame, then we guided it in to the middle to begin the spiral. There were issues with kinks and it worked out that at around 5in from the lip of the fitting, the curve was easy enough that we could form the shape without kinking the hose. I marked out this 5in using a marker and fastened the hose to the frame to begin the spiral. Working in chunks of around four to five rotations seems to work well, until the later stages of construction. After each set of rotations you loosely fasten the hose using zip ties. Working from the last tightened point, you guide the coil so that the hose sits evenly next to the previous loop. Too tight and it will overlap, too loose and it will cause grief on the next loop.

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Completing the Spiral

When we made it to the outer reaches of the spiral, the weight of the hose was causing the frame to bow and was making it difficult to properly set the hose in relation to the preceding loop. To remedy this we moved to the ground for the remaining loops. To do this we removed the remaining hose from our make shift spool mount and my wife walked the hose out while I followed behind her fastening the hose to the frame.

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Mounting the Spiral and Hooking It Up

Once the work was completed, we carefully migrated the beast to the roof. We set up on the south facing side of the house. We fastened a rope to the center of the frame and tied it off on the north side of the house to prevent the coil from sliding down the roof. Once it was secured, we connected the garden hose adapters to each of the leads coming from the coil and hooked up the cold water feed and hot water return hoses to the coil. (Note: Both our connectors were female, so we ran the return hose backward with the female end in the spa.) After the hoses were connected, we hooked the cold water feed hose up to a regular garden hose faucet and charged the system, so to speak, using the household water pressure. Once the water was completely through the system we connected the cold water feed to a pump in the spa and ran it for a test drive. Voila! It worked: The pump was pumping water through the coil and it was returning to the spa.

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Integrating the System

This part of the process is completely custom, based on your particular spa and how everything is set up. In our case the spa is on our back porch area next to the house so plumbing some PVC and running it up the wall was incredibly easy. We started by mapping out the current way the water cycles through the internal spa heating system. After some trial and error we ended up adding a 3/4" barbed T fitting inline with the internal heater pump. The return from the roof is fed back in to the spa system and enters the water through a small vent at the bottom of the spa. The pump that is currently installed is a small magnetic pump that is installed to simply help cycle the water through the siphon system. The overall flow is around 1GPM through the solar loop. A check valve was installed to help prevent water that has been heated being recycled through the coil, and valves were installed on the outside of the spa to allow us to isolate the coil plumbing from the spa for maintenance. The rest of the plumbing was pretty straight forward, we went a little over the top by using pipe insulation but hey if your going to do it, you do it right!

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The Results

Here's how it worked out: Day #1 Time - Spa - Water from Solar coil 09:32am - 82.2 - 93.9 11:11am - 85.8 - 93.9 01:03pm - 91.0 - 101.1 01:57pm - 93.9 - 104.3 * Peak reading 03:37pm - 96.8 - 106.8 04:18pm - 98.6 - 103.8 Total increase over the time we monitored: 16.4 degrees fahrenheit. Special thanks to Greg for his permission to re-post his instructions for this project. Be sure to follow his Instructables page for great DIY projects!