How bad is low hardness?

[Bill]

First off, I'm a long time lurker and have enjoyed/benefited from the many informative posts. Thanks to all.

I have a Marquis Epic and use Lake Michigan water right from the tap.  All parameters indicate in acceptable ranges and remain so with only minor tweeking now and then.  That is except for the hardness; it always read extremely low.  My dealer tells me that its not a problem and not to worry but I'm not convinced.  Can anyone shed some light on this and recommend treatments?  I use in-line Spa Frog bromine and mineral cartridges.
Thanks,
Bill

[Spatech_tuo]

First off, I'm a long time lurker and have enjoyed/benefited from the many informative posts. Thanks to all.

I have a Marquis Epic and use Lake Michigan water right from the tap.  All parameters indicate in acceptable ranges and remain so with only minor tweeking now and then.  That is except for the hardness; it always read extremely low.  My dealer tells me that its not a problem and not to worry but I'm not convinced.  Can anyone shed some light on this and recommend treatments?  I use in-line Spa Frog bromine and mineral cartridges.
Thanks,
Bill

The water can be aggressive when the hardness is low and especially so in your case where you say it’s “extremely low". When the hardness is very low you'd worry about the type & grade of metal used on the heater, Stainless Steel jet covers etc., as the water will affect certain metals. You may see your Ph start to bounce due to this. There are simple cheap additives to adjust the hardness so its not like you have little option and you don't need to necessarily get the reading all the way to the mid-point range but you don't want it to remain extremely low IMO.

[chem geek]

Low pH is damaging to metal, but low hardness or low TA are not.  The reason for saturating the water with calcium carbonate by adding calcium (i.e. calcium hardness, CH) and bicarbonate (i.e. total alkalinity, TA) is to protect plaster surfaces.  Being undersaturated dissolves such plaster (calcium carbonate) surfaces while being oversaturated can cause scaling.  Low CH will not lead to pH bounce -- it is low TA that can cause though it has to get very low unless you are adding strong chemicals that affect pH in the spa (such as Trichlor, but you generally don't use that in spas).

The idea of needing calcium carbonate saturation to protect metal surfaces is controversial.  You can read a discussion about it from corrosion experts in this link.  Even my tap water has a CH of only 55 ppm, TA of 80 ppm, pH of 7.7, TDS of 136 ppm, and temperature of 67ºF for a saturation index of -0.7, yet my pipes are not corroding.  The water district does add around 300-500 ppb of phosphates (that's 0.3-0.5 ppm) as a corrosion inhibitor, but they make no attempt to coat the piping with calcium carbonate.  It would be difficult to do in a spa anyway since the conditions in a spa change so dramatically with temperature changes of the water and the much higher temperature of the metal in the heat exchanger that is usually 30ºF or so hotter than the bulk water temperature but only when heating.

So your dealer is correct, assuming that you do not have a plaster spa or exposed grout in tile (you most likely have an acrylic spa or possibly fiberglass).

[Spatech_tuo]

Low pH is damaging to metal, but low hardness or low TA are not.  The reason for saturating the water with calcium carbonate by adding calcium (i.e. calcium hardness, CH) and bicarbonate (i.e. total alkalinity, TA) is to protect plaster surfaces.  Being undersaturated dissolves such plaster (calcium carbonate) surfaces while being oversaturated can cause scaling.  Low CH will not lead to pH bounce -- it is low TA that can cause though it has to get very low unless you are adding strong chemicals that affect pH in the spa (such as Trichlor, but you generally don't use that in spas).

The idea of needing calcium carbonate saturation to protect metal surfaces is controversial.  You can read a discussion about it from corrosion experts in this link.  Even my tap water has a CH of only 55 ppm, TA of 80 ppm, pH of 7.7, TDS of 136 ppm, and temperature of 67ºF for a saturation index of -0.7, yet my pipes are not corroding.  The water district does add around 300-500 ppb of phosphates (that's 0.3-0.5 ppm) as a corrosion inhibitor, but they make no attempt to coat the piping with calcium carbonate.  It would be difficult to do in a spa anyway since the conditions in a spa change so dramatically with temperature changes of the water and the much higher temperature of the metal in the heat exchanger that is usually 30ºF or so hotter than the bulk water temperature but only when heating.

So your dealer is correct, assuming that you do not have a plaster spa or exposed grout in tile (you most likely have an acrylic spa or possibly fiberglass).

I'll always defer to you on these things but is there really no need for concern even if the reading is "extremely low" (assuming his adverb is accurate).

[Dr. Spa™ Ret.]

"acceptable ranges" and "extremely low" are not scientific readings that would allow anyone to really give any professional advise or make any kind of recommendations.

[chem geek]

His CH could be zero and it wouldn't lead to metal corrosion.  Now if the TA were zero or close to zero, then that would be a problem because the pH could get very low too easily, but even a TA of 40 or 50 ppm is more than enough to prevent this if one is using the usual spa chemicals.  Just note that over time, the TA will drop from net acidic chemicals (such as Dichlor and bromine tabs) so one periodically needs to add baking soda to maintain the TA level.

[Chas]

From the practical chemist -

I have warned folks not to fill tubs with softened water for years, due to it's "aggressive" nature. Of course, it didn't take long to find out that most tubs had removed soft copper, iron, and other lower-grade components which could be eroded or corroded by this. (Soft copper thermowells allowed sensors to gain accurate information about water temperature, but tended to turn the tub turquoise and get eaten away by any aggressive water, and especially low pH).

I was encouraged to stick with the mantra, mainly due to the residual salt that most home softening systems left in the water. But again, it didn't take most makers long to find problem areas thanks to the chlorine or other halogens we commonly put in the tub - and make adjustments in their construction to reduce these problems. I'm referring here to 'chrome' trim and the like.

Now, I offer a salt system, the "ACE" system, and it USES salt water. And, it also USES soft water. In fact, we sell a water softener called "Vanishing Act" which you can use to soften the water in your tub if you don't have access to a softening system.

So - I agree with your dealer. In modern, non-plaster tubs, soft water is no longer an issue.

 

[Water Boy]

Low pH is damaging to metal, but low hardness or low TA are not.  The reason for saturating the water with calcium carbonate by adding calcium (i.e. calcium hardness, CH) and bicarbonate (i.e. total alkalinity, TA) is to protect plaster surfaces.  Being undersaturated dissolves such plaster (calcium carbonate) surfaces while being oversaturated can cause scaling.  Low CH will not lead to pH bounce -- it is low TA that can cause though it has to get very low unless you are adding strong chemicals that affect pH in the spa (such as Trichlor, but you generally don't use that in spas).

The idea of needing calcium carbonate saturation to protect metal surfaces is controversial.  You can read a discussion about it from corrosion experts in this link.  Even my tap water has a CH of only 55 ppm, TA of 80 ppm, pH of 7.7, TDS of 136 ppm, and temperature of 67ºF for a saturation index of -0.7, yet my pipes are not corroding.  The water district does add around 300-500 ppb of phosphates (that's 0.3-0.5 ppm) as a corrosion inhibitor, but they make no attempt to coat the piping with calcium carbonate.  It would be difficult to do in a spa anyway since the conditions in a spa change so dramatically with temperature changes of the water and the much higher temperature of the metal in the heat exchanger that is usually 30ºF or so hotter than the bulk water temperature but only when heating.

So your dealer is correct, assuming that you do not have a plaster spa or exposed grout in tile (you most likely have an acrylic spa or possibly fiberglass).

Chem Geek,

That's interesting on the city adding phosphates to the water. Phosphates can be a big nuisance on a swimming pools as phosphates are a food source for algae. Can be hard to get rid of sometimes.

[chem geek]

I had over 3000 ppb phosphates in my pool at one point and was able to prevent algae growth through the proper FC level for my CYA level.  There are others on some pool forums with over 5000 ppb phosphates as well.  Algae nutrients (phosphates and nitrates) are irrelevant if you maintain the proper chlorine level to kill algae faster than they can grow.  Regardless of how much "food" you give to the algae, they can only reproduce at a limited rate from temperature and sunlight, doubling every around 3-8 hours.  If the active chlorine level is high enough to kill them faster than this rate, then you won't get algae growth.

Now I will say that if these nutrient levels are high that IF you let the chlorine level too low for too long then the algae do seemingly grow quickly, though it still takes a day or so of "unusually high chlorine demand" before it typically becomes visible.  So phosphate removers should be seen in the same vein as other algaecides -- an insurance policy, nice to have but not necessary.

As for corrosion, chlorine is a corrosive oxidizer and higher salt levels are more conductive so increase corrosion rates.  In pools, the 3000 ppm salt levels for saltwater chlorine generators can be a problem for some copper heat exchangers.  Hot water corrodes faster so even lower salt levels (1500-2000 ppm) could also be problematic in spas (i.e. the Ace system) but as Chas pointed out manufacturers have often moved to stronger materials such as titanium or cupro-zinc alloyed heat exchangers.  The point is that negative saturation indices due to low pH are corrosive because low pH is corrosive, but a low CH by itself is not.