Filter pressure keeps dropping after backwash?

Manikshaw

Manikshaw

Member
So I backwash my sand filter and the pressure looks fine for a few hours, but then it slowly drops way below normal. Water flow to the returns also weakens. I thought pressure usually rises when the filter clogs, not drops. Am I missing something here?
 
That’s not typical. A pressure drop usually means your pump is sucking air somewhere or there’s a blockage before the filter, not after. Check pump lid gasket and skimmer line for leaks.
 
I had the same issue last season it turned out to be a collapsing flex hose between my pump and filter. Looked fine on the outside but was folding in on itself under suction. Worth checking your plumbing too.
 
I’d look at the internals of the filter too. When sand channels or gets worn out, it won’t hold pressure the way it should and you’ll see odd behavior like you’re describing. Another thing to check is your multiport valve, if it’s leaking internally, water can partially bypass the filter and that’ll drop your gauge reading and flow.

Since it happens after a backwash, it makes me think something’s not reseating right in the valve or the sand bed is shifting. If your pump basket and lines are all clear, I’d open the filter and inspect the sand and laterals just to rule that out.
 
Had the same thing happen! It turned out my flexible pipe was collapsing under the pump, not obvious from the outside but was folding in on itself. Once I replaced it, the pressure stayed steady. Anyone else ever deal with a collapsed flex pipe?
 
A pressure drop usually means there's a leak in the suction line, check the flexible hose between the pump and filter. Sometimes it folds in on itself even if it looks fine from the outside.
 
Yeah, you are right, a clogged filter usually makes pressure rise. If the pressure slowly drops after a backwash and the return flow gets weaker, I usually suspect something on the suction side: air getting in at the pump lid O-ring, a slightly loose suction connection, or a flexible hose collapsing when the pump pulls (it can look fine from the outside). I would start by checking the pump lid gasket, making sure the basket is clean and the pump stays full of water, and watching for bubbles in the pump basket. If that looks good, then I would check the hose and unions from the skimmer to the pump and over to the filter.
 
You are right, a clogged filter usually makes pressure rise. If your pressure drops way below normal and your return flow gets weaker too, that usually points to a problem before the filter or the pump losing prime. The most common things I see are air getting sucked in at the pump lid (a dry/dirty O-ring or gasket), the water level being a bit low so the skimmer pulls air, a blockage in the skimmer line or pump basket, or a flexible hose that looks fine but actually collapses/kinks under suction.
 
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