Solar Water Heating Panel For a Hot Tub/Spa

[voyager]

I've been thinking and planning on this for a long time now.
Recently, I've been looking at materials to make my ordering decisions.
I would really prefer to use a copper piping rather than a black plastic thin wall irrigation tubing in it for heat transfer efficiency.

My main concern is that somewhere along the line, I've been told that the chemicals in the tub's water will corrode the copper piping giving it a short lifetime.
Which also makes me wonder about potential toxicity from copper being dissolved into the water.

So, my question:
Is copper a suitable material for heating panel piping to pump hot tub/spa water through?

[khanh dam]

don't drink the water and you will be fine, lol.

Copper and Chlorine | Trouble Free Pool

[bob-n]

This article blames chloramines and sulfites for copper corrosion in common house plumbing:

https://sagewater.com/copper-corrosion-pits/

Using chlorine products (ie: dichlor, trichlor, bleach) to sanitize a hot tub results in chloramines in the water. Some people have switched to bromine instead of chlorine for their tubs. It depends what you use in your tub.

[J.P.M.]

So, my question:
Is copper a suitable material for heating panel piping to pump hot tub/spa water through?

Short answer: Yes, but it may be overkill due to cost. The greater thermal conductivity of copper over plastic is immaterial for solar heating applications involving low temp. water.

As for corrosion, for most locations, the copper in a hot tub application will last longer than you will. FWIW, to me, links such as those of Bob-n's reference are biased because they sell plastic repipe jobs. They have a dog in the fight. Follow the money.

Still, plastic collectors and piping made for the purpose are the usual choice, but mostly due to cost considerations.

Provided you stay below the material's temperature limits, including considerations for stagnation temperatures if the plastic is in a glazed collector, plastic will most likely be fit for purpose.

However, if you're thinking about simply placing some long, black, round stuff with a hole in it (tubing) and expect 104 F. spa water temp because you confuse heat with temperature, suit yourself. But if I was doing it (and I have) I'd not use what amounts to black irrigation hose alone, or any such method for that matter. Once you learn a few basics of solar thermal collector design, you'll find that black hose by itself makes a very inefficient pool or spa heater. While cheap $$wise, and while it does get hot in the sun, the amount of heat collected that'll go into the tub water will be far less than you may think. That means more area will be required with a lot more detail for flow pattern arrangement. Unless you're into redneck engineering, a little education and then simply getting a couple of pool heating collectors made for the purpose will be better/faster and, in the view of some, less of an eyesore.

Snoop around and you'll probably be able to find a couple of Fafco type used pool collectors for cheap and see how they work.

In any case, I'd not throw out your conventional method of spa water heating just yet. You'll probably always need some spa heat when the sun isn't shining for a day/two.

Take what you want of the above. Scrap the rest.

[voyager]

May not be moving fast, but I am moving towards getting a spa solar water heater made.
I was aesthetically stuck on a serpentine configuration pipe in the panel, but after looking around a bit, I'm now convinced a harp type of panel will probably be a more efficient configuration.
You've convinced me that copper pipe is a worthwhile and preferred pipe material.

I had been thinking of using 1/2" to 3/4" pipe.
But in looking around, I'm led to believe that a faster flow rate will give a more efficient heat transfer.
I may go to 1" maybe even 1-1/4" pipe to get a better flow rate.
I may need to add a better pump to it than I was planning.

I'm still planning to try keeping it as simple as possible.
I don't want to use added on electronics to control temperature, etc.
I'm thinking of tapping the piping to and from the panel into the spa's filter/heater piping, so that I can use the filter and temperature controls already in the spa.
With the panel on solar 12v DC and the spa's pump on 110v AC, some problems will need to be worked out yet.
Still need to think and work on that part.

The spa is less than 6'X6', under a 36 sqft surface, with almost 250 gallons in it.
Plus, there is a cover for it when not in use.
My greatest fear right now is having the panel too large and putting out too much hot water.
I still need to work out the panel's size.

I do feel that I'm slowly moving towards getting this done.

[LucMan]

Do not use copper! Hot tub water needs to be disinfected- no matter what disinfection system you use copper will not work.
Plastic or stainless steel for chlorine. Titanium and plastic for salt or ozone systems.

[voyager]

The consensus is that if the spa's pH is kept between 6.5 and 7.2 [about 7.0] corrosion of copper piping by the chlorine or bromine additives will be negligible.
But, the pH will require constant motioning and adjusting to keep it there.
The other possibility would be a closed system using a heat exchanger.
Haven't looked into that.
It seems likely to be more complex.


I had originally wanted to use aluminum piping.
It would be much stabler.
It is much harder to get out here.
And it's much harder to work with, wants TIG equipment and skills.

Which brings up the question:
How about using square instead of round cross section aluminum tubing?
Will the difference affect the heat exchange rate of the tubing?

[J.P.M.]

The consensus is that if the spa's pH is kept between 6.5 and 7.2 [about 7.0] corrosion of copper piping by the chlorine or bromine additives will be negligible.
But, the pH will require constant motioning and adjusting to keep it there.
The other possibility would be a closed system using a heat exchanger.
Haven't looked into that.
It seems likely to be more complex.


I had originally wanted to use aluminum piping.
It would be much stabler.
It is much harder to get out here.
And it's much harder to work with, wants TIG equipment and skills.

Which brings up the question:
How about using square instead of round cross section aluminum tubing?
Will the difference affect the heat exchange rate of the tubing?

Spa water needs monitoring same as a pool.

As far as the thermal efficiency design portion of collector design is concerned, the cross section of a fluid conduit makes little difference in thermal performance.
Copper has better thermal conductivity than aluminum, but that makes little to no difference in the design of thermal solar collectors.
Square cross section tubing will induce more pressure drop but that's probably not a big issue.
Aluminum is more reactive than copper. Usually, a tough oxide layer forms on the aluminum surface and "passivates" it and so the oxide layer provides a lot of protection.
Chlorine or chlorides will deteriorate that oxide layer more than other materials/compounds making aluminum a less suitable choice than copper from a metallurgical standpoint.

NOMB or concern, but if you want to design solar water heaters, stay out of trouble and get more education with respect to fluid dynamics, heat transfer and the solar resource before you waste time and money. If you do, you'll come to understand why using plastic pool heaters is a better, cheaper, faster and generally a more fit for purpose choice for this application.

Experimentation and learning is great. Study more before you build and build small when you start. It'll all come to a better end.

[voyager]

@JPM
Most of what you posted I am aware of and agree with, some is irrelevant, some is wrong, much is little more than a digression.
Not going to waste time and energy going into it.
Got other things to address.
Despite the foregoing, I do appreciate your input, all thoughts given help towards working this out.
------------------------------------------------

I did look into the possibility of the use of a heat exchanger.
Previously, I had only dealt with large building heating system HX's, made me dislike the idea for the spa.
Looking into Amazon and eBay, I now see that smaller, much more reasonably sized and priced HX's that could do the job, are available.

I'm beginning to think it's a very reasonable possibility, might only add a relatively small amount [~$200 (?) or so] in cost, would need a second pump and some extra insulated piping external to the panel.
The HX and all subsequent piping could be within the tub's base.

Because of the quantity of water being heated, the size of the spa and the heat requirements for it, it's looking as if a heat exchanger might be a good alternative.
Copper piping would only be needed inside the panel adding to it's effifiency reducing the size it would need to be while making a glyclo solution in it reasonable.

All external piping could be insulated plastic.
There are some design problems to be worked out in getting the piping from the panel to the tub, keeping it neat and clean.
Might have to cut a chanel to bury a couple of feet or so of piping in the slab to reach the tub, so as there won't be any piping hanging down openly from the roof or laid across the top of slab to reach it, might need to rent a concrete cutter to do that job.

The external piping could be kept down to 1/2 or 3/4" dia, keeping the internal piping larger, maybe 1".

[J.P.M.]

@JPM
Most of what you posted I am aware of and agree with, some is irrelevant, some is wrong, much is little more than a digression.
Not going to waste time and energy going into it.
Got other things to address.
Despite the foregoing, I do appreciate your input, all thoughts given help towards working this out.
------------------------------------------------

I did look into the possibility of the use of a heat exchanger.
Previously, I had only dealt with large building heating system HX's, made me dislike the idea for the spa.
Looking into Amazon and eBay, I now see that smaller, much more reasonably sized and priced HX's that could do the job, are available.

I'm beginning to think it's a very reasonable possibility, might only add a relatively small amount [~$200 (?) or so] in cost, would need a second pump and some extra insulated piping external to the panel.
The HX and all subsequent piping could be within the tub's base.

Because of the quantity of water being heated, the size of the spa and the heat requirements for it, it's looking as if a heat exchanger might be a good alternative.
Copper piping would only be needed inside the panel adding to it's effifiency reducing the size it would need to be while making a glyclo solution in it reasonable.

All external piping could be insulated plastic.
There are some design problems to be worked out in getting the piping from the panel to the tub, keeping it neat and clean.
Might have to cut a chanel to bury a couple of feet or so of piping in the slab to reach the tub, so as there won't be any piping hanging down openly from the roof or laid across the top of slab to reach it, might need to rent a concrete cutter to do that job.

The external piping could be kept down to 1/2 or 3/4" dia, keeping the internal piping larger, maybe 1".

Would you mind taking the time to tell me what you think is wrong about my post content and how you came to think it wrong. I sure like to get my information correct.