The pipes connecting your pool skimmer to the circulation pump and filter and terminating at the jets form a closed system. When the pressure at the jets is lower than it should be, one reason could be that the system is leaking and allowing air and water to escape. Before suspecting this issue -- which may require professional help -- look for a number of other, more simple causes, including low water in the pool, failure of the pump to prime, a dirty filter and the wrong setting on the multiport valve.
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Start by Eliminating Simple, Easy-to-Fix Problems
Air can get into the circulation system through the skimmer when the water level in the pool is too low. For the circulation system to be truly closed, the intake ports have to be completely submerged, which means the pool level should be at least halfway up the skimmer opening. If it's lower than this, turn off the circulation pump and fill the pool before turning the pump back on again. While you're checking the water level, remove the skimmer basket and clean it. Leaves and other debris may be preventing water from passing through it.
Sand and DE filters have multiport valves with several settings that reduce or eliminate output from the jets. These include backwash, rinse, waste, winter and -- of course -- closed. To ensure proper circulation, the valve should be set on filter or recirculate (which bypasses the filter). Check the setting, and adjust it if necessary. You should also backwash or clean the filter if you haven't done that in a while. A dirty or clogged filter reduces water flow.
Air in the System
The pressure in the recirculation system should be between 10 and 15 psi. If the gauge reading is below 5 psi, something is wrong, and if you've checked all the easy-to-fix problems first, you should suspect a leak. One way to confirm this is to look for bubbles in the pump strainer -- if the system is airtight, there shouldn't be any. You might also see bubbles coming out of the return jets. Air can enter the suction side of the recirculation system through O-rings on the pump, cracked pipes or fittings or through the drain plug on the pump, and you may have to do a step-by-step inspection of the plumbing to find the leak.
If your pool water isn't as clean as it should be and you don't have much output at the jets, you may find that the pressure gauge reads more than 15 psi. This is the result of a clogged filter, but just cleaning it might not solve the problem. The debris in the pool making the water murky may quickly clog it again. If this occurs, you may need to flock the pool to settle out the sediment, then balance the chemicals -- particularly pH and free chlorine -- to prevent the growth of algae and other microorganisms that are making the water cloudy. You may also need to introduce an algaecide into the water.
Clogs and Pump Problems
When all else fails, it's time to check for a problem with the circulation system itself. Clogs can develop in the pipes if leaves and other large debris get sucked through the skimmer and get lodged in the pipes, usually on the suction side between the skimmer and the pump. Instead of using a plumbing snake to clear these -- which probably won't work -- use a bladder-type drain cleaner or an electrician's fish tape.
Once you've eliminated all other possibilities, you have to suspect problems with the pump itself. An impeller could be broken, or something may have lodged in the pump housing in such a way as to interfere with the motion of the impellers. If so, you -- or someone you hire -- may need to disassemble the pump to make repairs.
Reprime the Pump -- Pumps can lose their prime, especially when the water level in the pool falls too low. Before disassembling the pump, try to reprime it. To do this, turn it off, and then open the lid and fill the strainer with water. If it takes more water to fill the pump than the strainer actually holds, the water is going into the inlet and outlet pipes, and this is a good indication the pump wasn't properly primed. When you turn it back on, the pressure at the jets may return to normal.