Why calcium carbonate in the hot tub?

November 28, 2008 5:09 PM

Why add calcium carbonate to the hot tub?

I just had my hot tub water tested, and in addition to the usual pH adjustments, buffering capacity increases, etc., I was instructed to add calcium carbonate to the hot tub. When I asked why, the answer was to increase the amount of calcium in the water. (*sigh*)

Google searches only tell me how bad it is to have extra calcium carbonate build up on the pump, water lines, heater and whatnot, so my question remains: why am I adding calcium carbonate to the hot tub?

posted by skybolt to home & garden (3 comments total)

If the water is too soft it will cause corrosion on finishes and metal fittings, including the heater itself.
posted by bradly at 5:53 PM on November 28, 2008

If your tub is tile this page suggests that insufficient calcium will cause the water to dissolve the grout as well as cement and similar compounds. That makes sense but if you do not have a concrete/gunite, tile, or similarly constructed pool I am not sure why. Perhaps the technician was told to add calcium for that reason in the past but nver really knew why and so just gives everyone that advice.
posted by TedW at 5:57 PM on November 28, 2008

Calcium carbonate is added to hot tubs to increase the mineral content (hardness) of the water. You want between 150-300 PPM of calcium in the water to prevent corrosion of metal parts. The posts in this forum will explain everything you want to know about hot tub chemistry: http://www.poolspaforum.com/forum/index.php?showforum=13

Personally, I'd recommend getting either a Taylor K-2006 or K-2106 (depending on if you are using Chlorine or Bromine for sanitation) test kit. They will test Ph, Total Alkalinity, hardness and sanitizer via drop counting, rather than color matching. I find them a lot easier than the strips. Also, note that some hot tub chemicals are available at the supermarket. Ph Up is often just Baking Soda or Soda Ash, both of which are available at the market. For Ph Down, Muriatic Acid is a lot easier to use than Dry Acid, which must be mixed with water before being put in the tub, and increases your dissolved solids.

If you have other questions, post on that forum I listed above. One of the regulars (chemgeek) is very knowledgeable about this stuff.
posted by fengshui at 3:38 PM on November 29, 2008