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Cheap solar hot tub/spa/pool heaterby tmack0 on July 25, 2010

Table of Contents

Cheap solar hot tub/spa/pool heater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Intro:   Cheap solar hot tub/spa/pool heater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Step 1:   Parts and Tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Step 2:   Frame . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Step 3:   Attach the Tin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Step 4:   Insulate the Back . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Step 5:   Paint the thing black . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Step 6:   Making the Coils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Step 7:   Laying Pipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Step 8:   Put a lid on it . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Step 9:   Mount it and turn it on . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

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Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

http://www.instructables.com/id/Cheap-solar-hot-tubspapool-heater/

Intro:  Cheap solar hot tub/spa/pool heater

This instructable covers the solar heater I made from parts available at the local hardware store (or salvage) for cheap. I have yet to do true empirical measurements onits output/efficiency, but it will raise the temp of my hot tub (~460gal) from 70 to 80 in two sunny days, and keeps it in the 90s during the summer without using the tub'sheating element. This allows me to keep it warm and only use the electrical element to boost the temp when I want to jump in (saves quite a bit of $$ on electrical bills),after which this will keep the temp up in the 100s for a day or two on its own. This is the result of a few experimental panels, and the finished product turned out to beabout the easiest of them all to do. A more refined version with fewer connections could be made if properly planned out.

It can be built in an afternoon, possibly just a couple hours if you have the parts ready to go. The longest wait time is for paint and sealant to dry/cure.

*Trying to find more of the pics I took while building this, taken over the course of about a year (hence the new look of the wood at the start, and OLD/weathered look atthe end)

** Evidently the photo tagging thing likes to move my tags up from where I put them.... working on getting it fixed, for now just imagine them a good bit lower than wherethey are

Image Notes1. not quite done yet in this pic...

Image Notes1. the panel, laying on the foam insulation board2. tub3. gfci outlet w/x10 appliance module

Image Notes1. tripawd2. hoses going to/from panel and into the tub3. GFCI outlet with X-10 appliance module to allow remote control and timercontrol of the pump4. hottub

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Image Notes1. hot tub2. finished panel mounted here3. plums!4. feed lines are secured to a support post and fed through a hole in thepaneling to go to the roof5. testing a coil

Step 1: Parts and ToolsThe bulk of the cost was the pump, which could be obviated if you plumb this in directly to your pool/spa pump, though that makes it harder to power independent of thespa. Make sure your pump is rated with enough head to pump the water up to the panel with enough left over to overcome the head loss of all the piping. At first I lookedat solar powered fountain pumps, but couldn't find any in my price range with enough head to work. I ended up with a 320gph pond pump* with ~10' head. I am mountingmine on the roof over the hot tub, about a 7' rise, so the 10' rating is needed (and gives it a good flow rate). Properly sealed, once the water is up to the panel the siphoneffect should reduce the head back to just the loss from the flow through the pipes themselves, but the water still has to get up there first. You want the pump to be ableto do this on its own without priming or other assistance because the panel will eventually drain if there are leaks, a hose comes out of the water, or you let the water sitand it boils in the pipes (has happened to me!). Manually priming it to get it going again sucks, and letting it sit in full sun without water to cool it off can soften or evenmelt some parts (PVC specifically).

*WARNING! (see my more detailed warning at the end of the i'ble. POND PUMPS ARE NOT RATED FOR THIS USE! YOU COULD ELECTROCUTE YOURSELFUSING ONE! PLEASE USE COMMON SENSE AND GET A PUMP RATED FOR POOL/SPA USE TO BE SAFE!!!!! Continue strictly at your own risk (think: submergingthe end of a LIVE 110V (220v?) power cord in a tub of water and jumping in).

Parts: 6' galvanized roofing tin panel, 2@ 8' 2x3 or 2x4, 12 wood screws (long enough to hold the boards together, so ~2.5" or be prepared to drill countersinks for shorterones), 1" Galvanized roofing nails or corrosion resistant screws and washers (I used hardy-backer screws left over from a tiling project, with galvanized washers) , High-Heat Flat Black Rustoleum (aka Grill or engineblock paint), 150' 1/2" black irrigation line, Pump,2 @ 8' 1x2's and some "Great Stuff" type spray foam insulation (or similar2part expanding foam) and 2-3" thick 2'x4' (or larger and trim to fit) Foam insulation board, 6' Clear Corrugated panel (poly carb or pvc, poly carb is preferred but costs abit more. Glass is best if you can adapt this to it, be careful!), 4x corrugated to flat insulator foam strips (2 to fit the tin, two for the clear panel), wire/string, silicone caulk.

Tools: Drill, 1" bit, smaller pilot bits, hammer, screwdriver (or bit), caulk gun, saw, snips, measuring tape.

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Image Notes1. Head. Note that this is a small/weak pump, I used a much more powerful onefor the final project. This one was fine for testing at ground level, but would barelyget flow even then.

Image Notes1. 6' roofing tin2. 1/2" black irrigation tubing3. 2" styrafoam insulation board4. 2x4s, 8' long each

Step 2: FrameThe panel itself is light weight, so the frame doesn't have to be super strong, just solid enough to keep its shape. The frame also acts as the walls of this panel, trappingthe hot air inside to improve convective heating from the surfaces not directly heating from solar radiation or conduction from the metal backing through the piping, andprevents convective losses from air colder than the water in the pipes (we hope) blowing through. The frame is a simple rectangle. Measure and cut the 2x3 or 2x4boards at 6' each, this yields 2x6' and 2x2' boards. Screw them together to form a simple rectangle the size and shape of the roofing tin. The tin should fit nicely withminimal overlap. Make sure you do this on a flat level surface or the frame might come out twisted or irregular. To make it more air-tight and rigid, you can useconstruction adhesive in the joints along with the screws. Drill pilot holes or countersink them as you see fit.

(its easier to do these next steps now before the tin is in place,Drill 2 1" holes near one corner of the frame, side by side close to the bottom edge (where the tin will be attached), to be for the inlet and outlet lines. 1" is large enough topass the lines and a hose adapter or line splice, so if you dont plan to use one of those, or have them outside the frame completely, use a 3/4" or other bit to mode snuglyfit the lines.

Place several nails, or more preferably screws about 1/2" from the bottom (tin side) along the inside of each board, leaving 1/4-1/2" sticking out. Space them about every1-2'. These will be used later to tie down the coils. Its much easier to place these now than after the tin is in place.

Image Notes1. Ignore this, from a preliminary experiment with smaller tubes...2. Pilot holes and counter-sinks drilled

Image Notes1. hardibacker screws. coated to prevent rusting

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Image Notes1. Tin fits neatly to the frame (dont secure it in place yet!)

Image Notes1. 1" spade bit

Image Notes1. inlet and outlet holes2. counter-sunk screws3. plums

Image Notes1. only 3-4 are really needed, this pic is from one of the experimental setupsusing lots of smaller tubing

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Image Notes1. Some along the side are needed too. Use screws, the nails pull out (fasterthan the milkman hearing a car in the driveway)

Step 3: Attach the TinLay the tin over the frame, lining it up for best centered fit. Optionally put a bead of silicone caulk along the top of wood where it will interface with the tin to better seal thepanel from drafts. Fit the corrugated foam thingy between the tin and the wood on both corrugated ends, you will probably need to stretch it to fit properly. Using theRoofing nails or some short screws, secure one corner, double check alignment and the foam (it likes to move if you don't hold the panel down), then secure the diagonalopposite corner and end of its foam strip. Nail the other corners to secure the tin and foam, and then nail down every valley in the corrugated tin starting in the middle andsecuring the middle most valley each time. Going along the flat/long sides nail the tin every 6" or so also starting in the middle and dividing the distances until you get toabout 6". This is to keep any slop from being propagated along to one corner where it becomes a large gap. Again, make sure the frame is on a flat surface for this,nailing while its not flat will give you a twisted frame (mine came out with a minor twist).

Image Notes1. 1" Galvanized roofing nails

Image Notes1. pre-formed foam thingy goes between the wood and the tin here and at otherend.2. Secure corners first, then divide the length in 2 for each subsequent nail (only inthe valleys for the ends).3. slop, cause I did it wrong (nailed across from the other corner, leaving a big bitof slack in the tin)

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Step 4: Insulate the BackNow we will make a frame to hold the foam and insulation board on the back of the panel. Cut your frame material into the same dimensions as the main frame, using the2' sections made from the 1x2's (gives an extra 1" for the corrugated tin to fit) and longer sections with the 1x3's. Build the frame the same as you did for the main onetoo. Secure the frame in place with wood strips cut from the leftovers of the 1x2's and 1x3's, nailed/screwed to the side of the main wooden frame, and thennailed/screwed into the sides of the new frame. To seal the ends, use two or the foam insulation strips (the ones that fit the tin's corrugation).

Now the fun: use your spray foam (the stuff for "Big Gaps" is probably best here) and go back and forth along the troughs in the corrugation on this side. One can will notcome close to filling the box, so use it mainly to fill the troughs and act as glue to hold a slab of foam board insulation.

Step 5: Paint the thing blackUsing the high-heat paint, give the interior of the panel a nice flat-black coating. Use several light coats of paint to get a nice even finish. Since the bare tin is bright andshiny its fairly easy to find the missed spots. Spray the wood as well, though pay more attention (and use more of the paint) on the tin itself. Once done, let it dry beforemounting anything in it. The next step can be done while waiting.

Image Notes1. First coat Rustoleum High Heat flat black

Image Notes1. second coat

Step 6: Making the CoilsThe next step is to setup the pipes that go into the box. This is the part I did the most experimenting with. I tried a few variations of pipe setups, including an array of thetiny 1/4" lines running from a manifold, but found that the more complex the setup, the only things gained were leaks. Power from the sun is a blanket rate over an area,so the more surface area of panel, the more power you get into your system. The size of the elements doesnt matter as much as the surface coverage. The 1/4" test alsoconfirmed that. Since its such smaller diameter, it took a lot more to cover a lot less of the area of the panel, and choked off the flow rate, and ended up not as efficient asmy final design.

This design is simple, easy, and very immune to leaks. Since I experimented, I ended up making 3 purchases of 50' spools of the irrigation line, rather than a single 150'(or 200' as they tend to sell it) spool, so I had to splice them back together. The splice you want to use if you have to do this is the simple screw-down compression type,NOT the black tube with a colored end you stick the hose into (these will leak like crazy, its only holding the pipe in by a sharp lip whereas the other one clamps the pipe).If you use a single piece of line, you don't need to worry with that at all, though putting in a splice at the input/exit of the panel will allow an easy way to disconnect thepanel from the feed lines so the panel or its feed can be taken out for maintenance.

All that said, the design itself is simple: 3 flat coils of the line packed into the panel. To get the line to actually stay flat and coiled, you will need to tie them into it usingstring/wire. I used old/salvaged solid core Ethernet wire strands to tie them up. You will want to measure all this before hand to make sure your coils will fit properly withthe ends just coming out the holes of the frame (or with enough of a tail to reach the source/destination).

To start: lay out the length of tail needed to reach the panel exit (or source/destination feed). Start coiling the first coil, from the outside-in. The tubing comes pre-coiled,so its fairly easy to do this, you just need to guide it and keep it flat. Its easier if you let the tubing warm up by leaving it out in the sun a while before you start, as it willsoften a little and be more flexible and less prone to kink. Get the center of the coil as tight as it will go without kinking, with the total width of the coil just under the insidewidth of the panel (about 12 loops). Put a board or something else flat and heavy across the coil to hold it in place when you have it set right.

With the long end of the tubing (the end/rest of the tubing going on the the next coil) , set its direction out of the center of the first coil so that it will be easy to start thenext coil (ie: tangent to it). Weight it down in that position to hold this all in place while you secure it with the wire. Starting at the outer edge of the coil where this tail

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crosses over the top, wrap the wire around the tail and twist it to itself to secure it. Then run the wire under and around the outermost coil of tubing and back over tail,making the wire cross between the tubes (sorta like a figure 8). Continue this for each coil trying to keep them snugly next to each other as well. Once at the center,secure the end of the wire similar to how it was started.

Move to the section where the short tail goes off the outside towards the exit and start another piece of wire there. To hitch this section together we will use half-hitchesaround each coil. To do this, bring the wire around the tubing, then send it under and around the wire itself. Each hitch ends with the wire pointing to the next coil. Tokeep things tight (keeps the coil, coiled, which is what we are going for here), pull the wire snug after wrapping under the tubing, then start the wrap around the wire abouthalf way between coils, using your fingers to take up any slack it tries to put into the system. Pull gently to snug the hitch (wire will snap if you pull it too tight here,especially if its soft/copper like I used). Continue for each coil. Do this at least once more to get three sections tied off. Repeat the coiling and tying for coils 2 and 3, eachtime going from center of one coil to outside of the next. The final coil should end with the tail going up the sides of the other two coils and out next to the other tail so thatboth inlet and outlet lines are next to each other. Secure the tail to the outermost loop of the other coils, and to the other tail with a couple wraps of wire at crossings.

Image Notes1. the coil, using a cast-iron grill to hold the loops flat while I make it

Image Notes1. coil tied together2. Anchored to the wall and return line3. return line running up the side4. pvc lid bent back while I work on the inside5. out from the center, on to the edge of the next coil (the coil above this one wasdone first, and thus, different), not tied to the rest of the coil though...6. foam insulation thingy7. tail tied to the coil with the figure-8 like wraps8. both tails exit here together (hidden by the lid)

Step 7: Laying PipeOnce the paint is dry, you can mount the tied up coils in the panel. Three coils of the proper width should fit snugly in the box. Position the coils in the panel with thecrossings and wire hitches on top, and the inlet/outlet lines fed through the exit holes in the frame.

If you want to put in a splice or fittings for a quick disconnect, do this before securing the coils. Make sure they protrude through the frame long enough to enable you tohold them steady while connecting the other half (remember, once done you will not be able to touch anything inside the panel without taking things apart.

With the coils placed how you like, use more wire with the nails in the frame to run strands over the coils to hold them down and in place. You can also run segmentsaround the outer most loop of each coil to tie it in place.

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Image Notes1. coil tied together2. Anchored to the wall and return line3. return line running up the side4. pvc lid bent back while I work on the inside5. out from the center, on to the edge of the next coil (the coil above this one was done first, and thus, different), not tied to the rest of the coil though...6. foam insulation thingy7. tail tied to the coil with the figure-8 like wraps8. both tails exit here together (hidden by the lid)

Step 8: Put a lid on itOnce all the coils are reasonably in place (tilt the panel, making sure they stay as coils and relatively in place, they will probably slide a little), its time to put the lid on. Ifyou have a sheet of glass to use, good luck, I wont go into detail on that as its not what I did and can get complicated (simple version: silicon caulk, nail down a frame tohold the glass in place), and is much more dangerous. For the corrugated pvc/poly carb, its much the same as installing the tin on the bottom. Line it up so the edges areflat on the sides and centered best you can, apply a bead of caulk between the sheet and the wood, make sure the foam strips are in place, secure the corners, then nail*or screw the panel in every corrugation valley and every few inches along the longer (flat) sides (again working from the middle out). If you use screws, also use a washerto spread the load on a wider area to avoid cracking the plastic, and only tighten it enough to hold it firm to the wood for the same reason (I used 2 washers, a spring/splitwasher above a flat washer, and only tightened until the split washer started to flatten). If using nails, be VERY careful to only hit the nails. Hitting the panel can shatter it(if you do this, clear packaging tape is an ok temporary repair, but will eventually dry up and flake off, requiring you to put another piece over the break or replace thewhole lid). Allow the caulk to cure and your panel is ready to mount and plug in.

*Note that using screws can make it much much easier to open up again should you need to. For mine, I used screws on one-half (split length wise) of the panel so I canremove the screws on that side and bend the lid back with the corrugations.

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Image Notes1. foam insulation strip thingy2. sorta hard to see: several screws holding down this side of the lid3. oops.. missed a nail and shattered the edge. The white is moisture under the packaging tape I used to fix it

Step 9: Mount it and turn it onAlmost done! Find your mounting spot (preferably above the surface of the water to prevent it from siphon-draining your tub/pool if things go wrong) and set the panel inplace. Angle towards the path of the sun if needed, keeping the inlet/outlet at the bottom and note that the head needed by the pump is measured to the highest point ofthe panel, and connect the source/output lines. One line goes to the pump (doesnt really matter which), the other should come out under the surface (prevents drainingthe panel). To attach to a pond pump, the 1/2" adapter will probably need to be increased in size for a tight fit. I use a wrap or two of foil tape to do this (electrical tape istoo soft especially at hot-tub temps, the adhesive will turn to goo and it will fall off), making sure none of the tape protrudes over the outlet nozzle.

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 athermostat 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 radiatorand 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, toturn 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 leftmine 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 NOTrated 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 finewith 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 withthe 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 someinternal 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 itagain! You are basically dropping a 110V power cord into a big tub of water you will jump into, if that doesnt scare you.........

Image Notes1. ducky says its warmer over here2. submerged pump, puts out a decent flow. *NOTE: these pond pumps are specifically NOT rated for pool/spa use for temperature and grounding reasons. Pleasedo NOT do anything in the water with the pump in and plugged in, it might electrocute you. Always use it on a GFCI circuit!3. ziptie to keep the inlet/outlet together. This helps hold the return line in place with the pump anchoring them both.

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Comments

13 comments Add Comment

 cobinrox says:  Aug 1, 2013. 2:19 PM  REPLYDo you think this would work with a garden hose rather than the irrigation pipes? I have about 100 ft of black garden hose and thought this would be a gooduse for it. Thanks.

 tmack0 says:  Aug 5, 2013. 3:49 PM  REPLYIt probably would, though it might kink easier (especially when it heats up) and not transfer the heat quite as well. If its just laying around, couldn't hurt togive it a try, and it will likely help some. Think how water in the hose comes out steaming hot at first if its been laying around in the sun, same principle isat play here. Just make sure it doesnt get too hot and melt holes in it!

 jad51 says:  Oct 8, 2011. 12:50 PM  REPLYIs there anyway of doing this but convection rather than using a pump?

 cdb1977 says:  Jul 23, 2012. 2:50 AM  REPLYAnyone?

 tinkerer4 says:  Jul 28, 2012. 8:10 PM  REPLYIf you had your water storage (hot tub in this case) above the solar collector, you could work through convection. You would also want to simplify theflow path of the collection piping. Instead of spirals, have vertical piping that are connected at the top and bottom. As the author stated, though, thiswill give far more opportunity for leaks.

 cdb1977 says:  Jul 30, 2012. 10:51 PM  REPLYOK just wondering. Thought maybe too much friction loss anyway.

 kolraw says:  Feb 6, 2012. 8:15 PM  REPLYWould using copper pipe help with the heat transfer thereby heating the water more? I know it would increase the cost but it sounds like it might allow thepanel to be in the sun without running water in it also.

 tmack0 says:  Feb 6, 2012. 11:14 PM  REPLYOh, and about the part of not letting it sit in the sunlight without running water: Its mostly a problem with the transparent PVC lid. The poly pipe itself getssofter, but isnt under pressure enough to cause problems (it might flatten out a bit from vacuum, but will return to normal when the water turns on again).The lid, on the other hand, sags from the heat built up in the box, which without any water running in the system to remove, gets quite hot, and wouldregardless of materials the piping is made of. This sagging is the problem I try to avoid, as it actually deforms the lid and starts collecting rain water in thedepression. The Polycarbonate option I mentioned might avoid this, as glass certainly would.

 tmack0 says:  Feb 6, 2012. 11:08 PM  REPLYCopper would dramatically increase the cost (and weight) of the project.The design is similar to commercial ones (wikipedia: http://en.wikipedia.org/wiki/Solar_thermal_collector ) but with the collection tubes in front of the"absorber plate" since I used a corrugated panel instead of flat, and plastic tubing instead of metal, all to reduce costs. Once you get into copper, theprice approaches/exceeds that of commercially available panels (see http://www.amazon.com/SW-37-Solar-Water-Heater-Panels/dp/B0041VM58E ). Ifyou have it laying around though, sure, use it instead, paint it black (just be sure to use a pipe bender to avoid kinking it.

There would be gains in conductivity (coefficients of 400 vs 0.5), and since the copper is thinner, there would be gain in that as well (Q=kAΔt/x , heatflux= conductivity * area * temperature difference / thickness). http://www.engineeringtoolbox.com/overall-heat-transfer-coefficient-d_434.html has a gooddescription of how this works out. It is a bit more complicated because instead of a (relatively) simple heat exchanger, its a heat exchanger with aradiative input power on the air side boundary layer. This adds in another Q on the right hand side of the equations.

Disclaimer: I was an ME in school, but fluids and thermodynamics were not my favorite courses. I could work all this out, but it would give me a headacheand the numerical answers are already mostly done if you search the internets.

http://www.instructables.com/id/Cheap-solar-hot-tubspapool-heater/

 cmac91000 says:  Aug 21, 2011. 3:55 AM  REPLYSounds like a great idea but in the UK, irrigation piping has holes in it. Is this correct or do you mean what we would call hose piping which doesn't?

 tmack0 says:  Aug 21, 2011. 10:46 AM  REPLYDefinitely do not want leaks in the panel, as any water that gets out of the piping will not make it back to the tub, and will also create condensation on thetransparent lid (which is an easy way to tell if you have a leak), and will eventually fill the box with water and will start leaking everywhere. This is the nonperforated tubing, sometimes its also called "Poly Pipe" as its made of polyethylene, or "Funny pipe" since its like pipe but flexible. Its sold in rolls andcan be bent easily (more so if its warm).

 nanosec12 says:  Aug 17, 2011. 6:48 AM  REPLYI love this....I have been contemplating something similar, but your method is much easier to accomplish that what I had been planning. I think I will followyour instructible this winter and live up to the old motto of 'keep it simple stupid'

Thanks, 5 of 5

 Jaycub says:  Aug 13, 2011. 5:50 PM  REPLYThis a good idea. I saw something similar on a greenpowerscience video on youtube.