DIY: SOLAR-HEATED OUTDOOR BATH

Phase One: The Bath

I purchased a second-hand fibre-glass bath tub from TradeMe for NZ$57 and began from there. To this I added another NZ$350-worth of timber, stain, glue, urethane foam, bolts and plumbing (plumbing stuff is ridiculously expensive for what you get) to come up with a light-weight cabinet to support the bath tub. I wanted the whole shebang to be liftable with just two people, so the framing and panelling were as light as I thought would suffice. The framing is 25mm-square, treated pine and the panelling is 6mm marine ply.

The cabinet was assembled and then coated in two coats of epoxy (waterproofing) and three coats of exterior polyurethane (UV protection). The bath was sealed in with a generic calking compound and then the cabinet cavity was filled with spray-in urethane foam (provides insulation, rigidity and support).

Phase Two: Install And Test

Once all the urethane foam had set, I sealed the base with some builder's plastic and duct tape (to keep water and creepy-crawlies out). The base is recessed such that the panelling forms a 75mm-deep skirt around the base. Five lengths of 100x50mm treated timber were cut to fit this skirt snugly and so provide a set of 'sacrificial' feet that raise the bath and cabinet a nett 25mm off the ground. The feet are not attached to the cabinet and can be replaced when/if they rot (they can also be moved separately from the bath and cabinet, should I ever need to relocate the unit).

I then moved the unit into position beside our back door and installed some PVC piping to carry away the waste water to a drain around the corner. I also installed two pairs of solar-powered garden lights purchased for NZ$8.50 a pair and hacked up to separate the LED from the solar panel + battery unit. The idea was to hang a set of solar-powered 'fire flies' or 'stars' over the bath. At this stage a nearby indoor hot tap plus an outdoor hose were used to do a test run of the progress to date. Bliss! Next up, the fun bit...

Phase Three: Solar Hot Water

I wanted to use the principle of KISS on this phase of the project. I was inspired by an article in ReNew Magazine which detailed the construction of a 1000mm-square, 30mm-deep (i.e. a capacity of 30-litres) metal box, painted black, erected on an insulated base and plumbed in with garden hose. This was pretty much exactly what I wanted, with some minor adjustments. I scaled up the design to have a 50-litre capacity (1100x1100x42mm - see 'Solar hot water tank plans' image, below) and commissioned a local sheet-metal place to build it out of 1mm-thick aluminium (rather than the galvanised steel used in the article). I chose aluminium because it was lighter, cheaper, better at conducting heat and more weather resistant (we live beside the sea - anything other than marine-grade stainless steel would corrode before too long). Aluminium is not a stiff as steel, however, and I needed to provide some internal support within the tank. The sheet-metal guy I spoke to recommended making the box in two distinct halves (rather than one piece with folds, as per my plan) and so I went with his recommendation. The plan was for them to do the cutting and folding, and for me to do the drilling, cementing/sealing and riveting. I ordered two tanks, which cost around NZ$300 all up (NZ$100 per sheet of aluminium, plus NZ$100 labour). In hindsight I wish I'd stuck to the plan with the tanks. Two distinct halves, rather than one piece with folds, left me with more cementing/sealing and riveting than necessary, for no very good reason.

First up I drilled holes in three of the four corners of one half of each tank wall pair. These holes will become: the water inlet; the water outlet; the overflow. The holes need to be large enough to accept the plumbing fittings used to join the tanks to the plumbing. After cleaning (detergent, water and steel wool) around two of the three holes (the overflow hole is left as is, with no further fittings attached), I secured the fittings with a pipe wrench (including sealant). I then put each pair of tank walls together and drilled a hole every 50mm [2"] to take the rivets. I separated each pair and scrubbed the drilled sections inside and out with detergent, water and steel wool to get a oil-/dirt-free, scuffed surface to cement/seal. I also fitted (with 'blind' aluminium rivets) a Z-shaped internal support, running from top to bottom, inside each tank (with enough of a gap at the top and bottom to let the water circulate freely). I re-paired the walls and then lathered between the lapped aluminium with cement/seal. I chose to use Bostik Stick'n'Seal as my cement/seal because it had the highest temperature rating I could find, whilst claiming to cement/seal aluminium. Unfortunately it neither sticks, nor seals - it is utterly useless. Unfortunately this didn't become apparent until after I'd riveted everything together (with 'blind' aluminium rivets) and let it all dry/set. The overspill of the cement/seal pealed all too easily off the scrubbed aluminium. This worried me a little, but I held on to the hope that the 'seal' part of the product claim was better than its 'stick' claim. After all was dry/set I scrubbed the top side of each tank (the side without the plumbing fittings on it) and painted them with metal primer and three coats of matte black paint.

Phase Four: Putting It All Together

I made a frame from wood to hold the two tanks side-by-side and at an angle of 45°. I fitted the tanks such that: the water inlet fittings were at the top, centre; the water outlet fittings were at the bottom, centre; the overflow holes were at the top, outside corners. I then hooked up piping such that the pairs of inlet and outlet fittings were serviced by a single pipe each, with a T-splitter.

Testing the tanks was simply a matter of filling them with cold water and leaving them in the sun. I also made provision to capture any leaked water so that I could measure the rate of leakage (if any). And, boy, did they leak! I repeat - Bostik Stick'n'Seal is utterly useless! I was losing 5 litres [1 gallon] an hour - enough to empty the tanks in 10 hours. If the rate of loss had been less than 1 litre per hour, I might have been OK. As everything was riveted together, there was no easy way to undo and start again. I was pretty unimpressed. On the positive side, the water in the tanks quickly became hot (and the tanks themselves way too hot to touch), so the system has promise, assuming you can build water-tight tanks. I may try to use some 'Radiator Stop Leak'-type substance on them one day, but at this time this part of the project is dead in the water.

So I've been left with a fabulous outdoor bath that has to be filled from the hot tap. Not ideal, but still very, very pleasant on a moonlit night! One possible use for the tanks is to use them to heat air instead of water, but I've no real use for lots of very hot air whilst the sun is shining, and I don't think they would be of any use as part of a winter heating system. If I ever build a glasshouse/greenhouse, however...

Success? Failure?

★★☆☆☆

Pros: The bath is great - well insulated and well weather-proofed.

Cons: The tanks are next-to-useless, but the simple design seems to be sound in principal, if not in practice.

Enhancement Suggestions: A hot water system that works! Plus: perhaps some way of keeping the heat in after the sun goes off the panels (and before the water is used), e.g. an insulated cover.