Pentair Sm 20 2 Manual

Working with the Pentair SM-20-2 Salt Chlorinator

The Pentair SM-20-2 is a medium-output salt chlorinating cell designed for pools between 20,000 and 40,000 gallons. It connects inline to your return line and runs off standard 120V power through a separate controller. I've serviced probably two dozen of these over the years, and they mostly behave if you keep the water chemistry balanced and don't ignore the flow switch. You can find the full manual on Pentair's website under their residential pool equipment documentation section. Search for "SM-20-2" and the PDF will come up. It covers wiring, cell removal, and calibration. But honestly, the manual is pretty bare-bones on troubleshooting. It tells you what the error codes mean in a sentence or two. I rely on my own notes more than the official doc for anything beyond basic setup. Here's the practical part that the manual barely touches on. The SM-20-2 uses a flow switch to prevent the cell from running dry. When water flow drops below a threshold, the controller shuts down the cell output and displays a low-flow warning. This is by design and not a malfunction, but it trips people up constantly. I had a job last summer where a homeowner called saying their system was "broken" because the cell kept cycling off. The real problem was a partially closed return valve and a clogged skimmer basket. Once I cleared both and verified flow at the cell was above 30 GPM, the error stopped. The cell never produced that much chlorine in the first place because the pump was starved for intake.

The cell itself has five electrodes inside a PVC housing. Salt passes over them and electrolysis converts the sodium chloride into hypochlorous acid, which sanitizes the water. You'll want to check the electrode condition every few months. There's a quick way to do this visually. Remove the cell from the plumbing, take off the end caps, and look at the titanium plates. They should be relatively clean with a slight white or gray buildup. If you see heavy calcium scaling or pitting, the cell is degrading and output will drop significantly even if the controller thinks everything is fine. A healthy cell at proper salt levels and water balance will produce around 1.2 to 1.5 pounds of chlorine per day at 100% output on a 20,000-gallon pool. One thing the manual doesn't emphasize enough is salt level tolerance. The SM-20-2 operates optimally between 2,700 and 3,400 ppm. I've seen cells run fine at 2,500 ppm, but output drops roughly 20 percent at that level, and the controller may start throwing low-salt warnings. Above 3,600 ppm, you risk accelerated electrode corrosion and voided warranty. Use a proper test strip or liquid test kit. The built-in salt sensor on the controller is okay for rough guidance but drifts over time. I calibrate mine against a LaMotte salt test every six months, and it usually needs a slight adjustment. Wiring is straightforward but worth getting right the first time. The cell has two power leads that connect to the controller, and the controller plugs into a GFCI-protected outlet or hardwires through a transformer. Make sure you run the flow switch wiring separately from the main power. Interference between those two lines caused intermittent shutdowns on a system I installed in a garage-mounted equipment pad where the conduit ran parallel to the pump power cable for about eight feet. Separating them by at least six inches eliminated the issue entirely.

Cell orientation matters too. Install the cell so the arrows on the housing point in the direction of water flow, and make sure the flow switch is mounted on a straight section of pipe, not right after an elbow or valve. I learned this the hard way on a retrofit where the previous installer put the cell within 12 inches of a 90-degree bend. The laminar flow was disrupted enough to cause false low-flow readings even though actual flow was adequate. Moving the cell three feet downstream resolved it without any other changes. Maintenance is minimal but not optional. Clean the cell every three to six months depending on your water hardness and cyanuric acid levels. A 50/50 mix of water and muriatic acid in a bucket, soak the cell for 10 to 15 minutes, and rinse thoroughly before reinstalling. Don't use vinegar or any commercial cell cleaner that contains phosphates. Phosphates feed algae and defeat the purpose of having a chlorinator in the first place. I've seen multiple cases where well-intentioned homeowners used the wrong cleaning solution and then complained about green water weeks later. There are genuine limitations with this unit that you should know before buying or installing one. The SM-20-2 has no digital clock or scheduling capability built into the cell itself. It's either on or off based on the controller settings, which means you need a separate timer or a control system like the Intellichlor IC series or a Pentair E-Series panel if you want automated scheduling. The cell also doesn't compensate for temperature changes in output. In winter when the water is below 60°F, chlorine production slows considerably regardless of what percent output you set. I've seen pool owners blame the cell for poor sanitizer levels in early spring, when the real issue is just cold water and low UV exposure.

Another shortfall is the lack of a built-in ORP or free chlorine sensor. The SM-20-2 runs on a simple salt-based calculation, which means it assumes your cyanuric acid level is stable and your bather load is predictable. If either one changes rapidly, the system won't adjust on its own. You need to manually check free chlorine at least twice a week during heavy use and adjust the output percentage accordingly. A jump from 1,000 to 10,000 gallons of swimmers in a single weekend can wipe out your chlorine residual faster than the cell can replenish it, and the SM-20-2 won't warn you about that. If you need something with active chlorine monitoring and automatic feedback correction, you're better off looking at the Pentair Intellitrol or a third-party system like the Jandy Aqualink with an integrated chlorine sensor. The SM-20-2 is a workhorse for straightforward residential applications where someone is willing to do basic weekly tests and adjust settings manually. It will last five to eight years with proper care, assuming you replace the cell when the electrodes show significant wear rather than pushing it until output becomes unusable. For anyone actually going through the manual while working on one of these, pay close attention to the torque specifications on the cell end caps. Over-tightening them is the fastest way to crack the housing or strip the threads. snug is enough. The o-ring seal does the work, not brute force. I've replaced more cracked cell bodies from overtightened caps than from any other single cause.