I don't think there's much of a mystery here. Let me describe what the pool service does in my local area where out of two stores (with many trucks) they service over 2000 pools. They use Trichlor tabs either in floating feeders or in inline chlorinators in order to maintain the chlorine level during the week. They then come weekly to brush the pool, clean the skimmer and pump baskets, clean the filter (if needed) and they shock the pool with chlorinating liquid.
As the season progresses, the CYA level builds up. At 2 ppm FC per day chlorine usage, this is around 36 ppm CYA per month. When the CYA hits 100 ppm, they usually do a partial drain of the pool, BUT we get NO summer rains in our area and most pools have cartridge filters due to water restrictions so there is NO backwashing. They target the FC level to be 4.5 ppm. So generally the pools are fine for preventing algae growth until the CYA gets somewhere above 60 ppm but the weekly shocking keeps things in check until 80-100 ppm CYA. When a pool has problems, even after shocking, then they use a phosphate remover. Some pools are open from Memorial Day to Labor Day which is only 3 months while some are from early May to late September which is 5 months. So 3 months is easy and it's only the 5 month pools that get a little dicey towards the end of the season.
In Florida, you get summer rains and that usually overflows the pool and dilutes the water. Also, many pools have sand filters where the weekly backwashing also dilutes the water. So for smaller pools, this dilution could extend the time before a problem occurs to 6 months or so. They likely use either a phosphate remover or a weekly algaecide to prevent algae growth. And we get tons of people reporting in on this forum around August in Florida when the CYA has built up from their use of Trichlor tabs/pucks, but they usually aren't using chlorinating liquid so their CYA is building up more quickly.
Why don't you test your pool's CYA level since that's really the key. Also let us know the FC level right after they've serviced the pool and again right before they service the pool. That will help us sort out what is being done. As others have noted, one can also shock the pool to a high chlorine level and if the CYA level is high it might last through the week, but the swing in FC will be rather dramatic. At 100 ppm CYA where the daily chlorine loss rate may be 15%, one can go to 14 ppm FC and the next week it may be at 4 ppm FC and that's without using any Trichlor to supplement. Obviously, some combination of techniques can be used to balance between the FC swing and the rate of CYA buildup.
There may be 10 million residential pools in the U.S., but there are already nearly 45,000 registered members on this forum and in the peak months of May, June and July this site gets over 300,000 unique visitors per month and these numbers are growing. In nearly every case with algae problems, it is due to the FC level being too low relative to the CYA level and this often occurs due to use of stabilized chlorine with the CYA level building up over time.
As the season progresses, the CYA level builds up. At 2 ppm FC per day chlorine usage, this is around 36 ppm CYA per month. When the CYA hits 100 ppm, they usually do a partial drain of the pool, BUT we get NO summer rains in our area and most pools have cartridge filters due to water restrictions so there is NO backwashing. They target the FC level to be 4.5 ppm. So generally the pools are fine for preventing algae growth until the CYA gets somewhere above 60 ppm but the weekly shocking keeps things in check until 80-100 ppm CYA. When a pool has problems, even after shocking, then they use a phosphate remover. Some pools are open from Memorial Day to Labor Day which is only 3 months while some are from early May to late September which is 5 months. So 3 months is easy and it's only the 5 month pools that get a little dicey towards the end of the season.
In Florida, you get summer rains and that usually overflows the pool and dilutes the water. Also, many pools have sand filters where the weekly backwashing also dilutes the water. So for smaller pools, this dilution could extend the time before a problem occurs to 6 months or so. They likely use either a phosphate remover or a weekly algaecide to prevent algae growth. And we get tons of people reporting in on this forum around August in Florida when the CYA has built up from their use of Trichlor tabs/pucks, but they usually aren't using chlorinating liquid so their CYA is building up more quickly.
Why don't you test your pool's CYA level since that's really the key. Also let us know the FC level right after they've serviced the pool and again right before they service the pool. That will help us sort out what is being done. As others have noted, one can also shock the pool to a high chlorine level and if the CYA level is high it might last through the week, but the swing in FC will be rather dramatic. At 100 ppm CYA where the daily chlorine loss rate may be 15%, one can go to 14 ppm FC and the next week it may be at 4 ppm FC and that's without using any Trichlor to supplement. Obviously, some combination of techniques can be used to balance between the FC swing and the rate of CYA buildup.
There may be 10 million residential pools in the U.S., but there are already nearly 45,000 registered members on this forum and in the peak months of May, June and July this site gets over 300,000 unique visitors per month and these numbers are growing. In nearly every case with algae problems, it is due to the FC level being too low relative to the CYA level and this often occurs due to use of stabilized chlorine with the CYA level building up over time.
