Many pool and spa professionals are incorrectly sizing salt chlorine generators, according to a growing school of thought.

“They’re sizing based on the volume of water in the swimming pool, when they should be sizing based on the pool’s chlorine demand,” said Christian Vernhes, owner of Crystal Salt Pools in Jacksonville, Fla.

In Europe and Australia, where many commercial pools are equipped with salt chlorinators, the devices are selected by the amount of chlorine demand they can meet. This is the general practice for many pools in the United States as well, as long as they’re commercial.

“I’d say most of the manufacturers have said for years that commercial pools should be sized by demand rather than gallonage,” said Bob Harper, general manager at Pristiva, Inc., a salt system manufacturer in Overland Park, Kan.

However, many salt chlorinators are sold to residential customers based not on chlorine demand, but on the gallonage of the pool on which they’re to be installed. A particular model will be listed as having, for example, a 40,000-gallon capacity. The measurement is based on the amount of water a chlorinator can theoretically keep sanitized under an average bather load, when running for about eight hours a day in a cool climate.

“The U.S. is the only country in the world where salt chlorinators are marketed based on pool capacity, not chlorine demand,” said John Naquin, product manager at PoolCorp in Covington, La.

Manufacturers and dealers who use this gallonage-based system, Harper explained, often are just trying to make things simpler for residential customers unfamiliar with the concept of a bather load. “A dealer can tell a consumer, ‘This particular unit’s good for 20,000 gallons, and you have an 18,000-gallon pool, so this generator supports a gallonage above yours,’” he said. “I’ve seen that a lot, unfortunately, at the dealer level.”

But this system of sizing is actually multiplying long-term costs for consumers, claim an increasing number of industry experts. Trying to keep up with a pool’s sanitation needs can quickly wear down an undersized chlorinator, especially in warm climates where chlorine tends to break down more quickly, and in pools with heavy bather loads.

“You can have two identical 16,000 gallon pools — one with a family of six that has two dogs, and the other with a retired couple — and each of those pools requires a different salt chlorine generator system,” Vernhes said. “The size of the pool is not as important as chlorine demand.”

The only solution to this problem is education, Naquin said. He travels the country offering training seminars on salt chlorinator sizing, and has encountered stories of algae-filled pools whose chlorine generators were sized according to their listed gallonage capacity. After trying the chlorine demand-based sizing methods Naquin recommends, a growing number of pool care veterans are converting to this school of thought, he said.

“Any pool in New England is going to require a different amount of chlorine than the same size pool in Alabama,” Naquin said. “With salt chlorinators, though, I see guys all the time who base their decisions about which salt chlorinator to use on the [gallonage] figures on the box. But what’s printed on the box doesn’t always work.”