HOT TUBS: Calcium (and Bromine)

[chem geek]

Thanks Evan. The information you provided is great. It all makes sense though it would be interesting to see if chlorine depletes faster than bromine for indoor spas not exposed to sunlight at all. Perhaps the depletion is a "good" thing in that it is oxidizing more organics or killing more pathogens since chlorine is a more powerful oxidant than bromine.

Are you sure about the calcium protecting the copper in heaters? Please refer to this post I wrote on another thread. I know that the conventional wisdom in heat exchangers is to use water balance to slightly scale calcium carbonate to form a protective layer, but that is typically for cooling towers where the water is already quite saturated and where parts of the system are acidic (pH < 7). It is not clear to me that in basic waters (pH > 7) that the protection from calcium is needed or works. If a thin film of scale is required, then the water balance would need to have lots more calcium than 100 ppm (with typical TA of 80-120). If the mere presence of calcium ion somehow inhibits corrosion, then that's new to me.

I think this issue of whether or not non-plaster/grout pools and spas need calcium is critically important to figure out correctly. There has been a lot of advice on this forum that vinyl and fiberglass pools do not need any calcium. If it is needed for spa heaters as well as pool heaters (which typically use copper pipes for heat transfer since copper is very efficient at transferring heat), then we'd better get this sorted out.

Richard

[chem geek]

In fact, I don't believe that hot water in the copper pipes in a home corrode very quickly. Even a hot water heater that has a sacrficial anode to prevent rust does so to prevent the steel tank from rusting -- it's not so much to prevent the copper pipes. In fact, steel has a greater tendency to rust when it is near copper or brass.

So it is possible that if steel is used in spa or pool heaters then rusting could occur, but I think that in these heaters only copper is exposed to the water, not steel.

I certainly want to know if I'm wrong about this or if I'm missing something.

Richard

[waterbear]

In fact, I don't believe that hot water in the copper pipes in a home corrode very quickly. Even a hot water heater that has a sacrficial anode to prevent rust does so to prevent the steel tank from rusting -- it's not so much to prevent the copper pipes. In fact, steel has a greater tendency to rust when it is near copper or brass.

So it is possible that if steel is used in spa or pool heaters then rusting could occur, but I think that in these heaters only copper is exposed to the water, not steel.

I certainly want to know if I'm wrong about this or if I'm missing something.

Richard

My understanding is that heaters for hot tubs do not have a holding tank. They use a series of tubes as a heat exhanger that the water flows through from the tub and back....much like 'tankless water heaters' used in homes.

[waterbear]

I personally think that pH is the most important parameter to protect the copper heat exchangers but until there is definitive answers one way or the other I see no harm in maintianing a calcium level in the water. Also most portable spas are acrylic and fiberglass and there seems to be some conflicting info on the need for calcium to protect the gelcoat finsish and help prevent staining. Bottom line is that many manufacturers recommend a certain level for their warrenty and THAT should be followed!
Calcium certanly IS needed for plaster or tiled spas and for now I guess we should follow the recommeded guidelines of 200-400 ppm.
For reasons I don't fully understand bromine santized spas have a tendendy to become acidic. Don't know if this is from the bromine itself or the widespread use of MPS in spas. Richard, perhaps you can shed some light on this.

My only real experinece with indoor spas on chlorine was the one my brother had in his old house in Miami. He needed to check his chlorine levels daily since they did not hold. He kept the spa at 104 degrees and used both liquid chorine and dichlor. His other water chemistry parameters tended to stay fairly stable except pH when he used dichlor. It would drop a bit and he would add sodium bicarbonate to raise it. His TA would raise somewhat but the spa had a waterfall that was always on when the pump was running so (I am guessing) the constant aeration would cause his TA to drop over time. I do know that the spa was in a very large bathroom of his house (there was also a sauna in this bathroom!) and there was a constant chlorine smell in the air in this room. Smelled like an indoor pool!

[chem geek]

For reasons I don't fully understand bromine santized spas have a tendendy to become acidic. Don't know if this is from the bromine itself or the widespread use of MPS in spas. Richard, perhaps you can shed some light on this.

My only real experinece with indoor spas on chlorine was the one my brother had in his old house in Miami. He needed to check his chlorine levels daily since they did not hold. ... I do know that the spa was in a very large bathroom of his house (there was also a sauna in this bathroom!) and there was a constant chlorine smell in the air in this room. Smelled like an indoor pool!

Certainly MPS is quite acidic so that would explain the situation when MPS is used. If MPS is not used, then the acidity is probably from outside sources, probably bather sweat (that's a guess).

Well, the chlorine smell (I assume you mean the "clean" smell of chlorine gas and not the "smelly" smell of chloramines) would indicate an outgassing of chlorine. If your brother don't use CYA, and with his indoor spa I would assume that he didn't, the outgassing is much faster (due to the higher HOCl concentration) and the typically higher TDS found in spas also contributes to the problem (higher Cl- chloride ion concentration). At a 77F temperature, I calculate the half-life of a 1000 ppm TDS to be 34 hours and if I boldly assume that the outgassing rate doubles with each 10C (18F) of temperature, then the half-life would be around 12 hours so each day you would lose 1-0.5*0.5 = 75% of your chlorine. This half-life rate is limited by the reaction rate of conversion of hypochlorous acid to chlorine gas and would normally be limited by aeration, but in a spa aeration is high so the net rate is somewhat closer to the reaction rate. I doubt that your brother actually lost 75% of his chlorine every day, but at least the calculations show that this theoretical maximum (with perfect aeration) explains his situation.

Richard

[waterbear]

Should have clairifed better....by indoor pool smell I meant choramines! He sold that house a few years back and now has an outdoor portable spa on bromine and a pool with a SWG.

[chem geek]

Boy, you sure know how to blow a theory right out of the water! Well, if it's chloramines then that means not achieving breakpoint and that's more like the problem Ben says that indoor pools have in general. Without sunlight and possibly good aeration, breakpoint doesn't happen as readily. Either that or there is something else going on we haven't yet figured out. If indoor pools and spas truly have a serious problem with chloramines, then the use of a maintenance dose of MPS would make sense since this oxidizes organics and ammonia really well and does so before chlorine gets a chance to form chloramines (and even some, but not all, chloramines get broken down from MPS according to my discussions with DuPont).

I just find it surprising that an indoor pool or spa without CYA has trouble achieving breakpoint as long as chlorine levels are sufficiently high. This is where the "real-world" butts heads with theory.

Richard