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.
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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)
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npambudi1 year ago

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
sparkydave2 years ago
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)
Could you send some links to the things you have purchased? Thanks!
bblaske1 year ago
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!
78081 year ago
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
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?
Jaygo2 years ago
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?
EcoMotive2 years ago
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.
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.
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
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)  guitarpicker72 years ago
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) 2 years ago
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)  petastream2 years ago
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...
cowboynwh2 years ago
Do you turn the heaters on at night and use this as a supplement heat during the day?
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)  cowboynwh2 years ago
We plan on only using the heater as a last resort, our target temp is around 102 degrees fahrenhieht in the spa.
gana_anaq22 years ago
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 :))
waylonis2 years ago
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?
veloboy2 years ago
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.
"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 Moser2 years ago
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.

Wonder. I await the final results!
petastream (author)  Victor Moser2 years ago
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_fwb2 years ago
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_fwb2 years ago
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.
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?
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
danzo3212 years ago
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)  danzo3212 years ago
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.
ferralll2 years ago
Have you had any problems with vapor lock in your heating coil?

Also, how strong in the pump you used? (GPM)
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.marsh2 years ago
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.
chaitanyak2 years ago
i dont have a pool but will try it anyway.. for one of the bathrooms
MsLynne7072 years ago
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.......
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