when to backwash

You should have a pressure gauge on the filter, when it goes up about 10 PSI from its base pressure from when it was freshly backwashed it is time to backwash again.

Ike

p.s. the typical pool pressure gauges are cheaply built if yours does not move either before / after backwashing it may be sticking
 
From Pool School: While most people know that a filter is a key component of a swimming pool's filtration system, quite a few do not realize that the filter needs to be cleaned periodically to insure proper equipment performance and cleanliness of the pool water. This article will discuss why maintenance cleaning is a good idea, then tell how to clean each of the 3 types of filters; cartridge (or cart), sand and finally both types of the Diatomaceous Earth (DE) filters.

When your filter was new the water could easily pass through it - the pounds of pressure per square inch (psi) shown on the filter's pressure gauge probably read ~8 - 20, depending on your pool and plumbing set up. As the filter does its job and filters, the debris in the water gets removed, there is less room for more new debris and the pressure rises. When the pressure rises too much (usually 8 - 10 psi) it's time to clean out the stuff that has already been captured in the filter so you have room to catch any new debris that enters the pool. Failure to keep the pressure in the normal range can result in cloudy water, poor circulation and excessive wear on the pool equipment.

Sand filters make use of a multiple position valve (multiport or multi) for cleaning the debris trapped by the sand. Whenever changing the position on a multi, you MUST have the pump off or you will break something. First make sure that any valve on the discharge line is open and any discharge hose is rolled out to where you want the dirty water to go. After turning off the pump, move the lever to the "BACKWASH" position and restart the pump. Most multis have a view glass so that you can see the debris coming out of the filter - when the water in the glass is ~ clear, turn off the pump and switch the multi to "RINSE" and run the pump for ~ 15 seconds. Repeat the "backwash/ rinse" cycles until the water comes out clear when you do the final backwash. Then reposition the valve back to the normal "FILTER" position, and you're all set until the unit needs to be cleaned again. Please note that this process removes water from the pool, so be sure to keep an eye on the water level and don't start the cleaning process if the water is already low! Some multis have the "BACKWASH" function, but not the "RINSE', this is also true of multis with a pull up - push down handle - in either of these cases, a 10 second "FILTER" cycle can be used instead of the "RINSE".
 
I have a variable speed pump. My new 60 sf filter (and new gauge) only measures about 1 psi when it is running. If priming (higher GPM) or if I adjust my run-time to a much shorter period (forcing a higher GPM), the pressure will increase with the higher water flow. I like to run my pump for a longer period of the day to keep sweeping leaves off so I have the very low PSI while running the pump.
My long-winded background gets me to my question: If I get less than 1 psi when clean and new, do I still use the +10 psi as my backwash/cleaning guide?
 
I believe the 10 psi rule dates back to single-speed pump days. I'd say when you gain 10 psi on high speed, it's time to backwash.

You can reduce a lot of that by stretching a knee-high nylon over your skimmer basket. I catch piles of stuff, looks like the dryer screen sometimes, in a nylon. I just carry it over to the trash can, flip it upside down and sort of brush it all off with my fingers. About every third time, the nylon shreds, so I replace it. There are also commercial skimmer socks for sale at the pool store.
 
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