John Faulkner
Member
Every time I backwash, the pressure looks fine for a few hours, then it suddenly drops close to zero. Pump is still running, but barely any water returns. Did I break something or is it a clog?
Filter pressure drops suddenly after backwash?
- Thread starter John Faulkner
- Start date
Lily Allen
Member
I’ve had this happen too! It’s usually debris stuck in the impeller. I checked mine after backwashing, and sure enough, a few leaves had gotten stuck. I cleaned it out and the pressure went back to normal. Anyone else check the impeller after each backwash?
Henry Thompson
Member
I ran into something similar last season, and it turned out that the multiport valve wasn’t sealing properly after the backwash cycle. The gasket was a little worn, so water was seeping through, and the return pressure would eventually drop off.
Swapped it out, and the problem disappeared. It might be worth giving the valve and seals a quick inspection if the impeller and O-rings check out fine.
Swapped it out, and the problem disappeared. It might be worth giving the valve and seals a quick inspection if the impeller and O-rings check out fine.
Benjamin Tate
Member
That sudden drop after a few hours usually means something is getting pulled into place once full flow is restored, not that you broke anything during the backwash.
One thing I don’t see called out clearly yet is debris migrating from the skimmer or main drain line after the backwash resets flow. When you backwash, you change flow patterns and sometimes loosen stuff sitting upstream. Once you go back to filter and the pump runs for a bit, that debris can finally make its way to the pump basket or impeller and partially block it. Pressure looks normal at first, then falls off fast once the blockage shifts into a worse position. I had this exact behavior with a wad of pine needles that only choked the impeller after an hour or two.
Another possibility is the sand bed collapsing unevenly. After backwash, the sand is fluffy. If the laterals or standpipe have any restriction, the sand can settle in a way that starves flow, especially if there’s air trapped in the filter. Cracking the air relief after restart and making sure the pump is fully primed can help rule that out.
If it were mine, I’d check in this order: pump basket for partial collapse of debris, impeller for anything stringy, then confirm you’re doing a full rinse cycle before going back to filter. A near zero gauge with the pump running almost always points to a suction-side restriction showing up late, not a pressure-side failure. Does it recover instantly if you shut the pump off and restart, or does it stay dead until you clean something out?
One thing I don’t see called out clearly yet is debris migrating from the skimmer or main drain line after the backwash resets flow. When you backwash, you change flow patterns and sometimes loosen stuff sitting upstream. Once you go back to filter and the pump runs for a bit, that debris can finally make its way to the pump basket or impeller and partially block it. Pressure looks normal at first, then falls off fast once the blockage shifts into a worse position. I had this exact behavior with a wad of pine needles that only choked the impeller after an hour or two.
Another possibility is the sand bed collapsing unevenly. After backwash, the sand is fluffy. If the laterals or standpipe have any restriction, the sand can settle in a way that starves flow, especially if there’s air trapped in the filter. Cracking the air relief after restart and making sure the pump is fully primed can help rule that out.
If it were mine, I’d check in this order: pump basket for partial collapse of debris, impeller for anything stringy, then confirm you’re doing a full rinse cycle before going back to filter. A near zero gauge with the pump running almost always points to a suction-side restriction showing up late, not a pressure-side failure. Does it recover instantly if you shut the pump off and restart, or does it stay dead until you clean something out?
Brandon Hayes
Member
One more scenario that can cause that “fine for a bit, then suddenly nothing” behavior is the pump losing prime slowly after the backwash, even though it looks okay at first.
I ran into this once where the pump basket was full right after restarting, pressure looked normal, returns were moving water. Then an hour or two later the pressure tanked and flow almost stopped. Turned out a tiny suction-side air leak only showed up once the system warmed up and the plumbing expanded a bit. The pump didn’t fully suck air, it just lost enough prime that flow collapsed. Gauge dropped close to zero even though the motor kept running.
What tipped me off was opening the pump lid after shutdown and hearing a big hiss of air. Also noticed the water level in the pump basket was lower than normal right before things went bad.
If you haven’t already, watch the pump basket while it’s running after a backwash. If the water level slowly drops or you see a growing air pocket, check the lid o-ring, drain plugs, and any unions on the suction side. Sudden pressure loss without a loud noise or leak is often air sneaking in, not something you broke inside the filter. Anyone else ever have a leak that only showed up after the system ran for a while?
I ran into this once where the pump basket was full right after restarting, pressure looked normal, returns were moving water. Then an hour or two later the pressure tanked and flow almost stopped. Turned out a tiny suction-side air leak only showed up once the system warmed up and the plumbing expanded a bit. The pump didn’t fully suck air, it just lost enough prime that flow collapsed. Gauge dropped close to zero even though the motor kept running.
What tipped me off was opening the pump lid after shutdown and hearing a big hiss of air. Also noticed the water level in the pump basket was lower than normal right before things went bad.
If you haven’t already, watch the pump basket while it’s running after a backwash. If the water level slowly drops or you see a growing air pocket, check the lid o-ring, drain plugs, and any unions on the suction side. Sudden pressure loss without a loud noise or leak is often air sneaking in, not something you broke inside the filter. Anyone else ever have a leak that only showed up after the system ran for a while?