Calcium scaling is an all-too-common problem.
It typically appears when calcium attaches to carbonate in the
water and grows less soluble at higher temperatures. The calcium
eventually comes out of solution in the form of surface
Because scale worsens as water temperatures rise, some of the
most stubborn cases are found in hot tubs.
Once entrenched, scaling can be quite difficult to remove
— and efforts to do so may even damage the finish on a
To prevent scale, you must first understand the sources of
calcium in the water, and employ preventive measures through pH
control and specialty chemicals. Finally, continue to monitor any
calcium buildup, and be prepared to drain the spa when
High temperatures combined with increased aeration result in very
quick evaporation rates for spas. This evaporation, in turn, leads
to higher levels of calcium hardness.
The majority of calcium comes from the spa’s fill water.
Depending on the source, calcium levels may vary greatly from
region to region. The Southwest, for instance, typically generates
very hard water, and calcium levels straight from the tap may reach
But if the water source is especially soft, the spa often is
treated with calcium chloride, either through a powder solution or
straight liquid form. This chemical, unlike calcium carbonate, is
engineered to be more soluble.
Calcium chloride, however, usually is only added to the spa
during a refill.
A final source of higher hardness levels comes from
calcium-based sanitizers such as calcium hydroxide, commonly known
as cal hypo.
But more often than not, evaporation is the primary cause of
calcium carbonate buildup, particularly in climates with low
humidity. Because many inground spas are left uncovered (mainly for
aesthetics), a spike in calcium levels can occur in short
Calcium scale is tricky. It may appear at the waterline or, more
generally, in areas with poor water circulation. And it creates an
uncomfortable soak, potentially causing skin abrasions and
If left untreated, it could even damage the spa’s
The key, therefore, is prevention. The method: water
Scaling typically grows more stubborn as the water’s pH
level rises. Still, the spa should be maintained within the ideal
range of 7.4 to 7.6.
Though 7.0 is pH neutral and decreases the risk of calcium
coming out of solution, it still can be irritating to the eyes and
Use a mild form of acid, such as sodium bisulfate, to regularly
control pH in areas with naturally high calcium.
As an additional safeguard, add a metal and scale inhibitor each
time you refill the spa. These chemicals should be added before you
introduce calcium chloride to raise the spa’s hardness
Approximately 70 percent to 75 percent of the spa water’s
total hardness comes from calcium. The rest comes from mineral
salts such as iron and manganese, which may potentially stain the
But watch out because many products used to inhibit metal
staining and scale formation also contribute complex forms of
phosphate to the spa water. These eventually break down to a form
of phosphate that can harm overall water quality.
Spa water should be tested monthly for phosphates, especially if
they are tied into a salt system to generate either chlorine or
bromine. The water should be maintained at a phosphate level no
higher than 200 ppb.
Oil, bather waste and scum buildup above the waterline also can
attract calcium scaling. This may be prevented, however, by adding
an enzyme treatment or a natural-based clarifier each week to trap
and filter out contaminants, thereby reducing any such
Testing and draining
Testing for calcium hardness should be done regularly, much as you
would for alkalinity, pH and TDS.
According to the National Swimming Pool Foundation, the ideal
range for calcium hardness in a spa is 150- to 250 ppm. When
hardness exceeds 500 ppm, a service technician must closely
maintain and monitor pH and total alkalinity levels to avoid
Soon, however, it will be time to replace the spa water. In
fact, that’s the only effective way to lower calcium
Typically, spas are drained and refilled every six to 12 weeks.
Alternately, a formula based on the number of users per day can
help determine when the water should be drained and replaced:
Number of gallons in the spa ÷ 3 ÷ users per day =
number of days between drainings
So unless you’re looking to burn a few calories with
regular scrub-downs, be sure to test regularly for calcium, and
treat the spa with preventive chemicals as needed.