Cyanuric acid, or CYA, is great at protecting chlorine from the sun’s UV rays.
But if it isn’t regularly diluted with fresh
water, the chemical, a byproduct of trichlor and dichlor tablets,
can quickly escalate to unwanted levels, well beyond the 30- to 50
ppm industry standard.
“A lot of times, residential pools have much more than 100
ppm of cyanuric acid,” says Robert Lowry, owner of Jasper,
Ga.-based Lowry Consulting Group and author of the Independent Pool
& Spa Service Association’s Basic Training Manual.
In fact, pools using stabilized chlorine may
easily exceed CYA levels of 300- to 400 ppm.
That’s where the real trouble starts.
Too much CYA can trigger a number of chemistry
problems, including skewed alkalinity readings, which can
compromise your ability to maintain water balance; low ORP
readings, which can cause unnecessary use of chemicals; and stalled
chlorine, which can allow algae to sprout and spread.
What’s more, recent studies indicate that
large quantities of CYA can even damage plaster, making it vitally
important that service technicians understand what they’re up
against, consistently monitor cyanuric acid levels, and keep those
levels in check whenever possible.
Stabilization with Cyanuric Acid
Cyanuric acid works to preserve chlorine sanitation levels in
outdoor pools. But as the chart demonstrates, 25 ppm of CYA should
sufficiently guard against UV rays. Notice there’s almost no
difference in chlorine deactivation between 50 ppm and 100 ppm of
The problem: High CYA levels can skew total
alkalinity (TA) readings when the Saturation Index is used to
determine water balance. Incorrect TA readings can lead service
techs to inadvertently balance the water at a more corrosive level,
with the possibility of fluctuating pH levels.
How it happens: Bicarbonate ions
are key to stabilizing pH. These ions are measured through a
titration test for total alkalinity. Though cyanurates are not part
of this carbonate buffering system that affects pH, they will show
up in a test for total alkalinity.
Thus, any reading for total alkalinity will include the effects of
CYA, which does nothing to hold pH in check.
“[Testing total alkalinity], 25- to 35
percent is going to come from cyanuric acid,” Lowry says.
“We have to convert that entire reading into calcium
The calcium carbonate equivalence is what actually
stabilizes the pH, and it’s used in the Saturation Index to
To determine the adjusted TA, Lowry recommends
subtracting one-third of the CYA level from the total alkalinity
reading. In other words, a reading of 90 ppm cyanuric acid would
account for 30 ppm of total alkalinity. Thus, for a pool with 90
ppm CYA and 100 ppm total alkalinity, the TA reading should be
adjusted to the 70 ppm that actually affects the water’s
It’s important to note that at pH readings
of 7.5 and below, CYA has a diminished effect on total alkalinity.
For example, at pH levels of 7.0, CYA accounts for less than
one-quarter of total alkalinity. But with cyanuric acid levels that
peak above 200 ppm at this pH, the effect is still significant
because 50 ppm of a TA reading will do nothing to balance the
For pools with high amounts of CYA, service techs
must constantly compensate for skewed alkalinity readings. Without
these adjustments, the result will be unexpectedly corrosive water
and unstable pH levels.
Effect of Cyanuric Acid on ORP
Higher levels of cyanuric acid generally will lead to much lower
ORP readings, and adding more chlorine will not necessarily bring
ORP back up. As a result, some manufacturers recommend a maximum of
20 ppm CYA to lessen the effect on ORP.
The problem: ORP is a common test for
oxidation/sanitation in chemical controllers. But when paired with
cyanuric acid, the results can be imprecise. Low ORP readings can
lead to an overdose of chlorine, which is a waste of money and a
potential breach of the federal Environmental Protection
Agency’s chlorine limit of 5 ppm.
How it happens: ORP probes give a
very accurate reading of active oxidation potential. But the test
is instantaneous, which means it cannot sense the chlorine ions
attached to CYA molecules. These ions, while temporarily unable to
oxidize, still can be considered free chlorine because they will
“An ORP probe takes a still picture —
not a movie — of how much chlorine is free,” Lowry
explains. “So there may be hundreds of connects and
disconnects during that [one second] that the probe isn’t
The bond between chlorine and CYA becomes stronger
in the presence of UV rays, so this difference in readings is
intensified during daylight hours, notes Ron Akin, vice president
of sales at Chemtrol in Santa Barbara, Calif.
“If you look at data logs of controllers,
you generally see the ORP go up at night because the bond is
weakened,” he says.
One solution is to make a chart of free chlorine
readings (obtained through a DPD test) and the corresponding ORP
measurements. As long as the pH, total alkalinity and cyanuric acid
are stable, your corresponding ORP reading should be enough
sanitize the pool, regardless of the actual number.
CYA’s Effect on Bacteria Kill Time
Extremely high levels of CYA remain unstudied, but it’s clear
that even at a level of 100 ppm, the bacteria kill time can be
twice as long as in a pool with 50 ppm of cyanuric acid.
Because of CYA’s effect on ORP probes, Akin
advises service techs to avoid stabilized disinfectants altogether
on pools with
controllers. As more cyanuric acid is released into the water, ORP
readings will continue to drop, causing further confusion.
He also recommends using no more than 20 ppm of cyanuric acid in
pools with controllers.
“The idea is to create a ratio where
you’re not locking up too much [chlorine], but not allowing
it to burn up before it’s had a chance to do anything,”
The problem: Unlike ammonia, CYA doesn’t
actually lock up chlorine due to the weak bond. But too much CYA
can slow the effectiveness of chlorine, resulting in the spread of
algae and shorter filtration cycles.
How it happens: In theory, the
more time chlorine ions spend on CYA molecules, the less time they
have to oxidize bacteria and other contaminants.
“The more cyanuric rings you have in the
water, the more time, statistically, [they’ll spend
there],” says Ellen Meyer, technology manager at Arch
Chemicals’ Water Products in Charleston, Tenn.
And lower concentrations of chlorine appear to be
more at risk, according to studies (see chart above). For instance,
the effect of CYA on a pool with 0.5 ppm of chlorine is
exponentially greater than for a pool with 1 ppm of chlorine.
It’s still unclear to what extent CYA affects the performance
of chlorine. But many state health departments have set a maximum
level for cyanuric acid at 100 ppm for commercial pools to prevent
interference with chlorine.
Problems in plaster
The problem: Studies by Arch Chemicals and the
National Pool Industry Research Center on the effects of cyanuric
acid on plaster found extensive degradation, as well as large
amounts of algae, in the high-CYA (beyond 100 ppm) test pools.
How it happens: With high levels
of CYA, the acid actually leaves the water and attaches itself to
In 2004, Arch Chemicals studied this phenomenon by
testing pools with cyanuric levels of 0, 25, 50, 100, 250 and 500
ppm. Under electron microscopy, researchers first observed the
effect on plaster in water with 100 ppm of the chemical, Meyer
says. The tests were performed in lab tanks and pools, with the
same results: The higher the levels of CYA, the greater the
deterioration of the plaster.
“The cyanuric acid was no longer in
solution,” Meyer says. “It was on the plaster surface,
having some kind of effect.”Meanwhile, the impact on water
with 250- and 500 ppm CYA was more clearly visible to the naked
Beyond observations in a lab, plasterers have
noticed this effect, too.
“I’ve seen hundreds of pools in the
last year [with plaster problems],” says Kevin Kostka, who
performs pool start-ups and quality control for Alan Smith Pool
Plastering in Orange, Calif. “This cyanuric [issue] comes up
every single time. It’s obvious when you see enough of
The solution: dilution
Dilution is widely considered the most logical approach to lowering
Phil Sharp, owner of River City Pool Service in
San Antonio, says he’d rather drain and refill a pool
entirely than waste even more money on chemicals to rectify a water
Sharp tests his pools every other week for CYA to
ensure the levels remain under control.
“You’ve got to test for cyanuric acid,
no matter what you’re using,” he says. “If your
ranges are out of control, it’ll create huge
Service techs could always switch to using
unstabilized disinfectants, but for many, this is rarely an ideal
“When you use dichlor or trichlor,
you’re constantly [putting in] cyanuric acid. … But a
lot of people will use the dry chemicals because they have a longer
storage life,” Akin says.
Techs may prefer trichlor for its ease of use.
Also, without using stabilized sanitizers, cyanuric acid must be
added eventually because rain and splash-out would lower a
pool’s CYA levels over time.
But if you do use stabilized chlorine, testing for
CYA is critical.