Pentair FNS Plus Leaking at the Tank Lid? How to Replace the Lid Assembly (170019, 170020, 170021, 170022)
A puddle under the filter is one of those things pool owners tend to watch for a few weeks before doing anything about it. That is usually fine at first. But when the water is coming from the seam at the top of a Pentair FNS Plus D.E. filter, the clock is ticking, because the same pressure that pushes water out can eventually split the tank the rest of the way.
Here is how we walk customers through it at the counter.
Why is my FNS Plus filter leaking at the top?
Nine times out of ten it is one of three things: a worn clamp O-ring, a loose or corroded tank clamp, or a cracked tank lid. The lid is the plastic upper half of the tank, and on an FNS Plus that is the part sold as 170019, 170020, 170021, or 170022 depending on your filter size.
Plastic gets brittle after ten or fifteen seasons of sun, chlorine, and freeze cycles. Once it goes, water finds the crack fast.
- Old or flattened clamp O-ring that no longer seals
- Clamp band tightened unevenly, or tightened while the O-ring was pinched
- Hairline crack in the lid, often starting near the air relief valve or a mounting boss
- Overpressure from a clogged filter or closed valve downstream
- Water left in the tank over winter that froze and expanded
How do I know if it is the lid and not just the O-ring?
Shut the pump off, release the air relief, open the clamp, and lift the lid off. If the O-ring looks flat, cracked, or squared off instead of round, replace that first. It is cheap and it fixes plenty of leaks.
If the O-ring looks fine, dry the lid off completely and look at it in good light. Run your fingernail along the sealing lip and around the top. A crack shows up as a thin dark line, and it will often weep again within a minute of the pump starting back up. Also check whether the lid still sits flat on the tank bottom. A warped lid rocks slightly and will never seal, no matter how hard you crank the clamp.
One more clue: if you have already replaced the O-ring once and the leak came back within a season, the lid is almost always the real problem.
Which FNS Plus tank lid do I need?
Match the lid to your filter's square footage, which is printed on the label on the tank. The FNS Plus lids are not interchangeable between sizes.
- FNS Plus 24 sq ft uses part 170019
- FNS Plus 36 sq ft uses part 170020
- FNS Plus 48 sq ft uses part 170021
- FNS Plus 60 sq ft uses part 170022
If the label has faded off, count your grids or measure the tank height and call us before ordering. Getting the wrong size back is a hassle nobody needs in July.
How to replace the FNS Plus tank lid, step by step
This is a straightforward job. Give yourself an hour and have a bucket handy.
- Turn the pump off at the breaker, not just at the timer.
- Open the air relief valve on top and let all the pressure out. Wait until the gauge reads zero.
- Open the drain plug at the bottom of the tank and let the water out.
- Loosen and remove the tank clamp. Set the hardware somewhere you will not lose it.
- Lift the old lid straight up. If the grid assembly wants to come with it, wiggle it loose gently.
- Wipe the sealing surface on the tank bottom clean. Old D.E. grit here is a common cause of repeat leaks.
- Fit a fresh O-ring, lightly lubricated with silicone pool lube. Never use petroleum grease.
- Set the new lid on, line up the ports, and reinstall the clamp. Tighten evenly, working around the band rather than cranking one side down.
- Close the drain plug, open the air relief, and start the pump. Let the air burp out, then close the relief valve.
- Recharge the filter with D.E. through the skimmer.
What does an FNS Plus tank lid replacement cost?
The lid assembly itself typically runs a few hundred dollars depending on size, with the 170020 for the 36 sq ft filter sitting in the middle of the range. Add an O-ring and a bag of D.E. and you are still well under what a complete new D.E. filter costs.
If a pool company does the swap for you, expect labor on top of parts. It is an easy enough job that a lot of owners handle it themselves.
Should I repair the filter or replace the whole thing?
Replace just the lid if the tank bottom is solid, the clamp is in decent shape, and the grids still hold D.E. without blowing it back into the pool. That describes most FNS Plus filters with a leaky top.
Look at a full replacement instead if the tank bottom is also cracked, the grids are torn, the manifold is broken, or the filter is undersized for your pump. At that point you are buying most of a new filter one piece at a time, and a complete unit makes more sense.
Parts worth checking while the filter is open
Since you already have it apart, take two minutes and look at the rest. Replacing a five dollar part now beats draining the tank again in a month.
- Tank clamp and clamp nut, especially if the threads look rusty
- Air relief valve, which gets stiff and leaks with age
- Drain plug and its O-ring
- D.E. grids and the manifold, checked for tears or split seams
- Pressure gauge, if the needle does not return to zero
A leaking FNS Plus lid is not an emergency, but it does not fix itself either. Match your filter size to the right part number (170019, 170020, 170021, or 170022), put a fresh O-ring on it, and you should be good for another long stretch.
Not sure this is the right part for your problem? Reach out and we will help you figure out exactly what you need before you spend money on the wrong thing.