hot tub water heating

[dennisg]

I'm a noob. please forgive my dumb questions. I am going to use evacuated tubes to heat water in a hot tub (electric backup). I'm planning a closed loop with glycol (we live in a freezing climate). Is there any reason NOT to think of the hot tub as a hot water storage tank? Can I just install a copper coil for a heat exchanger in the hot tub, under the seats?

All the info I can find suggests using a closed loop with a heat exchanger and another loop for the tub itself. It seems to me to be unnecessarily complex and expensive (exchanger, pump, plumbing) when I could just run the coil in the tub.

Thanks, Dennisg

[Naptown]

No reason not to think of the tub as a storage tank.
A few caveats though
Base your collector size on the coldest conditions you will experience.
When it is cold turn off the current heater and track how fast it cools off. By calculating the cool down rate you will determine how many collectors or tubes you will need for maintaining the heat in the tub. Since the tub will most likely be used in the evenings when there is no solar gain as you most likely have a job and have to work turn the jets and air injection and run for an hour and see how much the tub cools down. Best to do this on a cold day.( make sure the electric heater is disabled) This will most likely produce the greatest cooling.
So the choices are to maintain temperature during non use hours ( minimal) or try to heat during use which will only occur during sunny periods.
Answer the above questions and the answer will be determined.

[mountain]

It should work well
...but

High chlorine levels typically exist in hot-tub water. Direct contact with copper tubing will results in a chemical reaction resulting in copper-chloride (green pipes). The corrosion rate of the copper pipe will be proportional to the level of chlorine and temperature. This is why hot-tubs use plastic pipe.

An external stainless steel heat exchanger doesn't cost all that much & your system will last much longer. If you use a double-wall heat-exchanger, then you don't need worry about the glycol in your primary loop leaking into the pool water.

[russ]

Mountain made an excellent point about the chemicals in a hot tub - you don't want that in the panel system.

You do need to go with the heat exchanger.

Russ

[dennisg]

Chlorine copper interaction

I plan to use an ozone generator and a mineral ionizer and then use bromine if needed. Does that change opinions about the chlorine/copper interaction? Thanks.

[LucMan]

I plan to use an ozone generator and a mineral ionizer and then use bromine if needed. Does that change opinions about the chlorine/copper interaction? Thanks.

yes you will have to go with a titanium & plastic HX instead of stainless.

[dennisg]

yes you will have to go with a titanium & plastic HX instead of stainless.

OK, can I ask why?

[LucMan]

OK, can I ask why?

Stainless steel will only last maybe a year with out springing a leak with the purification system that you are proposing. Don't know the exact reason, but know from experience. I know nothing about pool & spa water chemistry or purification I am a HVAC contractor.
I was servicing a pool heating system that was using salts and ozone, after replacing 2 leaking stainless HX in a year (under warranty), the heat exchanger manufacturer (Triangle Tube) recommended that I use their titanium model. Since switching over no problems for the last 3 years.

http://www.triangletube.com/TriangleTubeProduct.aspx?CatID=2&PID=12

If you do a search on ozone & salt purification you will see that titanium is specified.

If you do find out the details why stainless is vulnerable to this disinfection system please let me know for future reference.