Hm. Maybe not.


The heat exchanger box for the (still proposed) water heater will fit on the exposed rack left over on the solar panel ground mount, as I expected. I took that picture at about quarter to ten and there’s still some shadow on the solar panels, which isn’t great. But the rack will work as expected. Of course the rack ends need to be supported and I’ll need to do some modification to that drainage berm but I knew that.

In terms of how much hose the box will hold: I hoped to be able to conduct a very simple experiment this morning but that didn’t go so well…


In 2011 we ran a bunch of black flexible pipe over the ground during our attempt to start a little fruit tree plantation at Ian’s place. Longtime readers know how that ended up, and that pipe – which I think is just Pex – has been roasting in the sun ever since. I’m surprised by how well it has withstood all the UV radiation but there’s no thought of actually re-using it because it’s all rat-chewed. But by dragging this length over to my place I hoped to be able to determine how much I could coil into the box.

Good news/bad news/really bad news…


The box would hold a lot of it, that much is clear. Unfortunately this particular pipe is in such poor condition that getting an actual coil turned into an infuriating impossibility, but clearly the box would hold a lot. How much all that Pex would cost, for an experiment that might just be a colossal waste of time and money, I’m reluctant to even consider. Maybe black garden hose, if there is such a thing, would be a better choice? Don’t know.

ETA: That tiny drib of hose in that box measures approx 130 feet, which surprised me. The box would hold a lot of Pex.

About Joel

You shouldn't ask these questions of a paranoid recluse, you know.
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16 Responses to Hm. Maybe not.

  1. Ben says:

    Move the box to the strong end of your rack.

  2. Ben says:

    Is this to be a simple one-pas heat exchanger? Or do you intend to have a hot water tank and recirculating?

  3. Joel says:

    I’m going to make this end as strong as the other, since obviously it won’t work if I don’t. Though I am thinking it might be best to put the box on the cabin side of the rack just to get it closer to the cabin. And at least the first generation will just be a one-pass without frills because if I can conjure 6-10 gallons that’s more than I’d ever use at one time anyway.

  4. Phssthpok says:

    “Unfortunately this particular pipe is in such poor condition that getting an actual coil turned into an infuriating impossibility…”

    Your photo proves that to be demonstrably not true. You already have it coiled, just not in progressively smaller concentric rings. Instead of coiling it like a ships rope*, try keeping the rings the same size and overlapping them (imagine ‘smearing’ a slinky**…looks like you’re halfway there already). You might lose some solar exposure on the sides where it overlaps, but the box looks deep enough to handle the stacked thickness, and the glazing will help prevent loss of heat.

    I know one guy who simply used black garden hosed looped/draped over the roof of his Airstream as a water heater for a couple of years; no box, no glazing, no tight coiled patterns…just flopped up there.

    You are working with ‘free’ materials…don’t let ‘perfect’ be the enemy of ‘it works’.

    * https://thumbs.dreamstime.com/z/coiled-rope-1012688.jpg
    ** https://img0.etsystatic.com/000/0/6546473/il_570xN.303063308.jpg

  5. bmq215 says:

    If you’re only going for 6-10 gallons perhaps it’s best to use that as the determining factor. Ultimately to soak up the most heat you want to go with the smallest diameter hose you can, although if you go too small flow will probably be an issue. So calculate the length needed to get that volume for any given diameter and see where it gets affordable. Color shouldn’t matter too much as a can of matte black spray paint can take care of that.

    If I remember my maths correctly the volume should be:
    (3.14 * ((inner diameter / 2)^2) * length)
    Divide that by 231 to get gallons if you’re using inches, or 3785.5 if you’re using cm.

    Remember, empty space isn’t totally wasted. It’ll still heat the inside of the box and increase the amount transferred to whatever piping is in there.

    Quickly running the numbers suggests that 3/4″ PEX will give you 2.3gal for every 100ft. So maybe 200-300ft in total for a test? You might be able to get away with that for $75-150 which could be low enough to make it work?

  6. Wayne Dygert says:

    Stick with the PEX. The garden hose will break down after two or three years. (Ask me how I know)

  7. Dug says:

    Make sure you can drain it before the first hard freeze – I suppose an air compressor would blow most of the water out if you had a big enough air tank. Or just take the PEX out and drain it by hand.

    Back when I was interested in it, (In the Reagan era) they used copper pipe, and an aluminum extrusion to clamp onto the pipe, all painted black, to collect the solar and transfer it to the pipe. Probably expensive and not made anymore?

    Regarding the 6-10 gallons, how hot do you want it, and you might want to look at the steady state on a sunny day – how many feet does it need to run through to get warm enough. 200 feet may be overkill. I think that a solar oven can get hot enough to boil, so plan for a pressure relief valve, too.

    Did I talk you out of it yet? If not, have fun.

  8. Jim Price says:

    PEX is great stuff, and has revolutionized the plumbing industry. But it would not be suitable for a solar panel. It is very vulnerable to UV damage.

    https://www.doityourself.com/stry/how-long-can-pex-piping-be-exposed-to-sunlight

    “PEX piping is highly susceptible to damage by sunlight. If exposed to sunlight, the molecular structure disintegrates. This causes the piping to become brittle and rupture. Indoor PEX installations require a covered environment. This helps to prevent exposure to direct sunlight.”

  9. A. Jannelle Moseman says:

    For many years we heated our pool using looped black garden hose in a plex box mounted to the side of the shed housing the pool pump.

  10. Michael Gilson says:

    PEX tubing is also used for radiant heat floors, where the pex is attached to aluminum radiator panels. The purpose made panels might be more money than you want to pay, at least while you are still at the experimental stage, but maybe you could score some corrugated sheet metal as salvage? Have the sheet metal in the frame with the corrugations running horizontally, run your PEX back and forth in the grooves on the BACK side of the sheet so that the PEX is protected from the sun and paint the front side flat black. An idea for securing the PEX in the corrugated sheet metal, if you can get enough to fill your frame twice bolt two sheets back to back. If you can’t get enough to fill it twice but it is more than enough to fill it once, cut the excess into vertical strips that can be bolted onto the back of the main sheet to secure the PEX in place.

  11. TK421a says:

    Glad to see the project coming along Joel. A good alternative to PEX is a black flexible PVC pipe under a sheet of clear plexiglass. It works really well.

  12. Kentucky says:

    Gonna take a step back.

    If all you’re interested in is 6 – 10 gallons of hot water per use, why not reconsider a ten-gallon barrel painted black and mounted right close to the lair?

    Cheap and simple, right? Details would have to be resolved, but that’s also true with that “tubing heater box” thing under discussion.

    Just a thought.

  13. mbumgua says:

    Try black drip tubing much cheaper and will hold up well as it is made to be exposed to sun.

  14. Norman says:

    Random thoughts:
    PEX won’t hold up in direct exposure to UV. Regular window glass allows passage of UV-A but blocks UV-B and UV-C; how damaging UV-A is to PEX I don’t know. PVC doesn’t like UV either. (No idea what solar intensity is at your latitude and altitude, but I can testify at sea level in Florida it’s lethal to plastics and wood unless the wood surface is well treated, and re-treated frequently, to resist it. ).
    A highly efficient glazed and well sealed solar water heater at certain latitudes can produce water temperatures approaching the sea level boiling point, especially if water velocity through the tubing isn’t high enough. This is an argument against using one long endless loop of tubing and for individual tubes with distribution manifolds at each end. This would require multiple trips through the solar panel(s) which also means some sort of reservoir (aka “tank style water heater”) would be needed.
    In the photography world there are IR Pass Filters that block UV but allow IR; I’d guess smart engineers would use that kind of glass on hot water solar panels but at what cost I don’t know.
    I’d also guess the smartER engineers would use copper piping for hot water solar panels. Mine did.
    One can heat water with tubing laid in the sun but tubing inside a glazed box (aka “small greenhouse”) works much better.
    Freezing isn’t a problem with PEX because PEX will expand enough to tolerate water expanding into ice without damage. How many such cycles it will withstand I don’t know (I can personally vouch for “at least two.”). The freezing weak points will be the fittings, use brass not plastic (I can vouch for that as well).
    When you strengthen the rack don’t forget the weight of the water; using standard PEX dimensions, each 100 ft of (nominal) 3/4″ tubing = 15.75 lbs/100 ft.
    Copper is naturally anti-bacterial, plastics are not. Direct use (showering with the water that passes through the plastic tubing) creates an opportunity for inhalation of aerosolized bacterial growth and transfer (it likes warm, wet environments and shower spray and toilet flushing aerosolizes it) which is why solar hot water systems -usually- use a heat exchanger to transfer heat to the usage reservoir (aka “tank-style water heater”) rather than direct use. Swimming pools don’t usually have that problem because of the amount of chlorine used.

  15. Malatrope says:

    I surprised no-one has mentioned black poly piping. Farmers use it all the time, as it’s extremely cheap, resistant to sun, and nearly freeze-proof (it expands).

    https://www.homedepot.com/p/Advanced-Drainage-Systems-3-4-in-x-300-ft-IPS-100-PSI-UTY-Poly-Pipe-75100300/203294392

    I’ve seen a lot of people use this to heat water. You can just lay it out over the ground, although putting it in a box to keep the rats off would be a good idea.

  16. B says:

    I was gonna reccomend the black ply, but Maltatrope beat me to it.

    If you paint the back of the box black, and glue the poly to it, you will get a fair bit of efficiency in heat transfer/absorption. I would go with larger 1-1/2 diameter if all you want is a few gallons, or if you are gonna use a small pump and a resivour then use multiple turns of smaller (like half inch).

    Do you have enough photons to power a small pump?

To the stake with the heretic!