Introduction: Solar Spa Heater

Picture of 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.

Any how 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 in to our garden. Her new kick was "How can we heat the spa without using the electronic heater?" She got up on the roof with 50ft 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 which 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.

Step 1: Parts List

Picture of Parts List

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

20ft of 1/2in diameter PVC pipe
1 x 1/2in 4-way Cross PVC Fitting
PVC cement for gluing PVC sections to 4 way fitting
500ft 1/2in diameter black drip irrigation hose
2 x drip irrigation to standard garden hose coupling
Around 200 outdoor 8in zip ties

2 x 25ft or so normal garden hoses to use as water feed and return (Not included in $60 est as we had 2 on hand)
1 water pump to push water up to the roof and through the solar coil for heating. (Also not included in $60 est as we had one on hand)

Step 2: Frame Setup

Picture of Frame Setup

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 your wondering.

Step 3: Work Area Setup

Picture of Work Area Setup

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 setup 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 setup 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.

Step 4: Spiral Construction at a Snails Pace.

Picture of Spiral Construction at a Snails Pace.

Photos don't show the first stages of construction as the idea to create an instructable from this project came slightly later in the project.

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 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 5 in using a marker and fastened the hose to the frame to begin the spiral. Working in chunks of around 4 to 5 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.

Step 5: Completion of the Spiral

Picture of Completion of 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 proceeding 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.

Step 6: Spiral Deployment and Finishing Touches

Picture of Spiral Deployment and Finishing Touches

Once the work was completed we carefully migrated the beast to the roof. We setup 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 connector where 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 a regular garden hose faucet and charged the system so to speak using the house hold 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.

I will update this instructable with thermal findings once we get a full day to test the system.


Step 7: Full Integration

Picture of Full Integration

This part of the process is completely custom based on your particular spa and how everything is setup. 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 feed 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 had 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! :)

Step 8: Results

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 


tgvoss (author)2017-10-28

It seems by the photos that they used a sump-pump (or a utility pump) similar to this one:

Anybody make this work with a pump similar to this or another?

MikeC133 (author)2017-06-19

what pump do you recommend for this?

tgvoss (author)MikeC1332017-10-28

I tried this setup with this ( pump and it did not work even with the coil on the ground tilted up 45*.

It seems by the photos that they used a sump-pump (also called a utility pump) similar to this one:

Anybody make this work with a pump similar to this or another?

BF4 made it! (author)2015-07-06

thanks for sharing your projet, I did the same in Quebec, Canada. Today it's my first test, I hope this will works great! Thanks again!

By the way, I used electrical PVC for the structure, it also works great.

Elio23 (author)2015-06-30

I want to say thank you to you and other great insctructable who make me
very happy. See my photo. Thank you so much, Elio from Italy

npambudi (author)2013-07-15


Nice reading your project. I have question regarding on this solar water heater. I live in Indonesia and have a project to build swimming pool with area 10X8 meter and the deep is 1,5 meter. The volume is 120 meter cubic. The water temperture average in Indonesia is 25 C. I want to increase the temperature around 33 C. My question is how long the tube ? do you have any suggestion to run this project ? Thank you

sparkydave (author)2012-05-02

I have made one of these but used a small 12 volt water pump and small 12 volt solar PV panel to run it. that way it doesnt cost anything at all to run and the pump only runs when there is sun! (both purchased on eBay for about $70 combined)

bblaske (author)sparkydave2013-06-09

Could you send some links to the things you have purchased? Thanks!

bblaske (author)2013-06-09

Hi! Really cool idea! Could you please give some hints about the pump that would be required to push the water through the system? Thanks!

7808 (author)2013-05-27

very cool, wish my pool and or spa were a little closer to the south side of my house. i would really like to try this with my 16ft round above ground pool and maybe just lay the spiral on some corregated sheet metal roof piece or somthing because my pool is just too far from my house . what was the ambient temps on the days you took those readings? im wondering if the sun up here in MN will not work so well, also we just plain dont have as many sunny days so im not sure its worth building

srobbins813 (author)2013-04-03

I have built this solar spa heater, and it works even better then I thought it would. I have also add a timer to turn the pump on and off daily, The thing that is not working well is each morning before the pump comes on I have to go out and prime the heater. Both the intake and discharge lines are under water so that air can not enter the system, but each day when running the garden hose into the system theres air. the pump I have can push up to 9' and the solar heater is only at 8' or less.
what size pump do you have?
why or how does the air get in the lines?

Jaygo (author)2013-03-31

I live in East Texas- Beacoup sunshine but still need a heater for early spring and fall's shorter sun hours. This (very nice instructable, thanks!) gives me an idea but I would like to trot it through here. I have one of those old humungous satellite antennas of years past, right behind the pool as it happens.
What if I took that dish, painted it black then wound your hose round and round in that? Our pool area gets full sun most of the day (in high summer it's more like a hot tub!) so I wouldn't have to mount it far from the pool pump at all.
What do y'all think of that?

EcoMotive (author)2012-08-18

According to your data at peak output which is a spa temp of 93.9F (can be used as an input temp), an output temp of 104.3F and a flow rate of 1GPM, you were raising the temp of 3.79 kilograms of water by 5.78C every minute. That would be a total energy increase (using q=mc(delta t)) of 91 509 joules to your spa every minute. Divide that by 60 seconds you get a power output of 1525 watts. Very impressive.

kellyawilkins (author)2012-05-01

Did anyone consider the "unbreakable" PEX for hydronic systems? the best of plastic and metal And cost yay! I saw it at the hardware store and in the farmtek catalog.

PEX is a great type of plumbing. The only problem with it is its susceptibility to breakdown due to UV. Anytime PEX is run, it should be run where sunlight can't affect it.

One suggestion would be to wrap the PEX in a pipe insulation of some kind to protect it from UV. PEX is great in areas where freezing temperatures are an issue and is extremely flexible.

prphntm (author)danielmitchem2012-07-21

I've had mild success running a PEX line inside of a slightly larger schedule 10 PVC pipe (used for low pressure drains) when it has to be in a UV exposed area

guitarpicker7 (author)2012-06-04

comment on ganaanaq2 post:

you missed the mention of OUTDOOR zip ties in the parts listing - as they are less UV sensitive, they make a LOT of difference in the life expectancy of the fasteners.

petastream (author)guitarpicker72012-06-07

Yes, we did use Outdoor zip ties. We will be updating the list as well as pictures for all the work we have done to it since the original posting.

petastream (author)2012-05-05

Figures after we build this solar project the sun goes in to hiding for a week and a half. Anyhow today looks like it will be clear yet busy for us. I'll take a new sample set tomorrow if the sun stays out!


petastream (author)petastream2012-05-13

We had a good week of sun this week, we were able to raise the Spa temp from ground temp of around 72 degrees fahrenheit to a peak of 106 degrees fahrenheit over a 3 day period. Not bad :)

Note: Still working on the SpaDuino project and instructable to match...

cowboynwh (author)2012-04-30

Do you turn the heaters on at night and use this as a supplement heat during the day?

sconner1 (author)cowboynwh2012-05-02

If you want hotter water than the sun can provide at any given time, just boost with the electric heater.
Any amount you heat the water without electricity is money saved.
It would be a good Idea to bypass this system at night or on cloudy days because it will radiate heat into the air/space when the sun isn't shining.

petastream (author)cowboynwh2012-04-30

We plan on only using the heater as a last resort, our target temp is around 102 degrees fahrenhieht in the spa.

gana_anaq2 (author)2012-05-02

i like the project ideea , simple and practical. I see only a hidden mistake, from the fixing the plastic tube. You used plastic self locking clips and from my experience this after a few years of beeing kept on the sun , the plastic become very hard and fragile ... so i sugest to put a metalic wire between the pipe and the metalic cross. Otherwise it is possible to se one time a big black flower in the roof :))

waylonis (author)2012-04-30

Very nice!

Perhaps you could hook up a power measuring device (e.g., Kill-A-Watt) to your pump to indicate the energy use? Maybe compare with the energy consumption of the spa without solar warming?

Also, maybe put some reflective insulation panels underneath the coil on the roof to further heat them as well as isolate the heat from the roof?

veloboy (author)2012-04-30

NIce job!
I made something very similar for my wood fired hot tub.  I was using it on the ground so that the warm water could passively thermosiphon into the tub. I found that I needed to insulate it from the ground as it would loose too much heat from conduction.  In terms of pay off however the gains were pretty small. What I got from a days worth of solar heating could be achieved with an armload of wood in an hour.

Victor Moser (author)2012-04-29

"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")?
Scales in celsius?

Total increase over the time we monitored: 16.4 degrees fahrenheit

But what was the final temperature?

petastream (author)Victor Moser2012-04-29

In our initial set of readings all values are in fahrenheit and the final reading of the day was 98.6 in the spa with water coming from the coil at 103.8. I am currently working on the Netduino based project that will log this information to an SD card for analysis. If anyone is interested there is some initial hack code up at GitHub. I will be posting a follow up instructable regarding this project in the near future.


Victor Moser (author)petastream2012-04-30

Wonder. I await the final results!

petastream (author)Victor Moser2012-04-30

We are waiting for a good sunny day here. Now that the project is ready the clouds moved in. Inevitable I suppose.

That si pretty cool

Oops I mean "That is preety cool"

gcai_fwb (author)2012-04-29

good instructable!

I built a pool heater using basically the same idea a long time ago - I put 5 expanded 100' coils of 1/2" PVC (500 feet of it in total) into five 2'x8' wooden boxes connected in series, with glass 1 side mounted on backyard shed roof. The pool pump (3/4 hp 300 GPM) drove water through a controllable 'Y" connection (restrictor) - flow required regulation so that water would heat up sufficiently else it acted as radiator - the exact opposite of what was desired. This could have been a function of ambient temperature at my location (43 degrees N)

It worked reasonably well however its demise as Duplo above mentioned was UV from the sun - even with the piping behind glass it started to break down (became brittle and developed pinhole leaks) after 5 or 6 years and requiring many patches, too many :-), and I scrapped it.

I'd strongly suggest using UV stabilzed PVC pipe - much more expensive though

petastream (author)gcai_fwb2012-04-29

The tubing we are using for the coil is this I used it once before for a drip system at my parents home about 3 years ago and that system seems to be doing fine. We are also pushing very low volume through this system, using the new magnetic pump the flow is around 1GPM which allows us to cycle 3/4 of entire spa through the system during the sunny hours of the day.

flyingpuppy (author)gcai_fwb2012-04-29

Would spraying the pipe/hose with UV protectant spray keep it from breaking down? Would it cut heat absorption?
How hot did the water get & other stats?

gcai_fwb (author)flyingpuppy2012-04-29

no idea if UV protectant would help - build one, treat it and wait 5-6 years :-)

it would be easier just use the UV stabilzed PVC if you are going build one - either that or make the PVC pipe easily replaceable - my design did not allow for this - live and learn - Petastream's design would be better suited to periodic replacement IMO

as to heat added - I'd guess delta between inflow and outflow was about 7 degrees C - not huge but noticeable considering my pool is heated only by the sun.

the roof temperature (under the shingles) hit ~80 C on a nice sunny day

danzo321 (author)2012-04-29

Seems like you could use the water pressure in the system (plus gravity) to fill hottub with hot water as you push in cold water for the next time you want some hot. OR are you re-heating what has already been heated? I just know if I turn on my garden hose, first flow would kill my plants it's so hot.

petastream (author)danzo3212012-04-29

We run the water through the system when we first fill the spa up, after that we use the system to recirculate the water during the day. If we leave the water stagnate on the roof for a few hours then turn the system on the water coming from the return line is around 150 fahrenheit.

ferralll (author)2012-04-29

Have you had any problems with vapor lock in your heating coil?

Also, how strong in the pump you used? (GPM)

kyle.marsh (author)ferralll2012-04-29

Looks like his pump is inside the hot tub pushing rather than above it pulling; I think that means that there shouldn't be any vapor lock problems at all. If you wanted to put the pump up on the roof so you didn't have it kicking around inside the tub you might run into some trouble.

petastream (author)kyle.marsh2012-04-29

The initial run we used the submersible pump to help the water cycle through the system. That pump was pumping at around 1.25 GPM which yielded the results seen on the last step of the instructable. We have since moved to a lighter duty pump and have tied the solar system in to the internal spa plumbing. This part of the project was completed today and I will be appending that process to this instructable.

chaitanyak (author)2012-04-29

i dont have a pool but will try it anyway.. for one of the bathrooms

MsLynne707 (author)2012-04-29

As good as this is about 20 yrs ago, I built a system quite similar with the pvc and glass and it worked great for about 3-4 yrs and then we went to a change and used a old hot water heater stripped down without the jacket and we put tha into a box with mirrors inside to reflect the heat onto the tank and we had this for about 7yrs after that we incorporated the whole system together and we now have way more HOT WATER than we ever can use so just keep up the great work and continue on the joirney towards an ALL GREEN ENVIROMENT.......

mrfoltz (author)2012-04-29

love it when I see posts like this! Good for you! And your Wife and child!!! ;0]

triumphman (author)2012-04-29

I would love to pre -heat my hot water for showers & sinks! Most hot water is used for these two uses. My oil furnace is a full time /always on system! It wastes a lot oil oil! Do you think i could rig up a coil for a pre-heater? It woul really save a lot of oil! The cost of home heating oil is never going to go down! My roof slope looks just like yours! I just personally put new shingles up there! That goodness for compressor powered nail guns!

buju357 (author)2012-04-29

polycarbonate will last 1-2 years in the sun then disintegrate , rather use 4mm perspex , that has +- 10 years life span in the sun , or best to use 10 mm glass , life span will be 20 -30 years.
I have a very similar system running for 2 years now , I use 1/4 inch garden irrigation hose to heat water for the washing machine , basically push cold water from the tap into the pipes and return it to the inlet of the washing machine , best temperature Ive got is 55 deg cel.
After much r&d , the problem is pressure , combined with heat it easily bursts the pipes or connection so a pressure relief valve is mandatory.

Well done , and lets all keep up the green movement.

poco424 (author)2012-04-29

By going to yard sales one can pick up two 52" patio tables one can disassemble for the glass. Using this glass one can make one's own trapped air space for this assembly cheaply and easily. The pvc pipe and cable ties are not necessary in that the black piping can be laid directly on the glass and glued to it and the return hose can be taken directly out of the umbrella hole.
A note of caution: the trick to working with tempered glass is that while one can almost beat on the face of the glass with a hammer the edges are especially tender. Protecting the edges until the assembly is placed on the roof requires attention.
Also, there are table tops with holes in the center and those without. One without a hole would probably be best for the lower glass.
Just an idea. Used to wholesale shaped glass most of which was tempered glass table tops for patio tables.

Duplo for Daddies (author)2012-04-28

Nice work. Comment about the polycarbonate. Glass has an unbeatable weather resistance, poly does not. My experience is that the more expensive weather resistance poly will empty your wallet. Poly is definitely more convenient, but if your lucky you might come by a big round glass table, which most likely is made by tempered glass. Safe and durable!

About This Instructable




Bio: I'm a software developer and systems analyst for a small company in Southern California. I have a loving wife and wonderful son that are ... More »
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