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Question DetailsAsked on 6/21/2016

Why won't my pool filter hold pressure?

My filter and pump were running great for 3 days after I opened my pool. I was doing a lot of vacuuming trying to get the algae out Everything was going smooth until the vacuum lost pressure. I thought I had clogged the line with something I'd sucked up so I tried everything to free up the flow. Eventually I was able to get water going through but not steadily. Since last night the pump xxxx water in until the basket before the filter is nearly full then the level suddenly drops. No matter what it continues to do this. Please help me. I can't think of anything else and I can't afford a pro this year!!!.

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Not knowing of course the exact configuration of your system (you can google for images and photos of typical setups), but assuming not antique so I will assume the filter is a pressure side filter, not a suction side filter - if pressure side almost certainly has a pressure gauge on it. Possible causes (and don't run the pump very long at a time when testing until you are sure water is passing through it or you can potentially burn it up), here are some possible causes:

From your description, my gut feeling is your problem is on the suction side of the pump, or possibly in the pump itself - both because you said the vacuum plugged up and then sort of started flowing but not well, and also because the filter basket is not filling all the way:

1) seal on the basket case is leaking air - but only if located on suction side. If on pressure side and basket has a clear casing, if you see air bubbling into the housing in profusion then you have a blockage or air leak on the suction side of the pump or pump has a problem.

2) you left the plug-port cover for the vacuum open or open hose connected or the sweeper/robot cleaner is connected and out of the water, so it is pulling air there

3) something is caught up in the main or skimmer pipe shutoff valves, or you shut one or both off during the cleaning and forgot to reopen them. You could close and reopen the valves in case something got stuck in them - if leavers or such, opening and closing the valves with pump running might chop up the debris enough to let it through

4) something got caught in the backflow checkvalve at the pump and is partly clogging that - no real way to clean that without disconnecting the pipe at the checkvalve

5) flush valve on the filter is open so pump is pumping water but dumping it into the drain after it passes through the filter

6) on some systems, there is an overpressure valve or controller - either directly in the line or as part of the filter but downstream of the pump or occasionally integral to the pump housing, which prevents overpressuring the filter and chlorinator - so it either turns the pump off electrically, or releases automatically and lets the pressure "blow down" through a drain pipe (usually the filter bypass pipe) - should hear this when it happens, and feel/hear (with ear to pipe) flow through the drain pipe that the flush water goes out.

A few DIY diagnosis things you can do - in order down the list till you find the problem source:

First check pool water level is high enough that you are not pulling just air through the skimmer trough pipe to the pump, and of course that there is nothing clogging either the skimmer or the main (pool bottom) drain inlets

I would then start by checking if there is water circulating through the pump and back to the pool - you should be able to feel suction at the inlet ports to the pool or hear (with ear to the piping) the flow through the pipes, so is there flow coming out the inlet jets into the pool ? If not, assuming the pump is running OK, you have a blockage (partial if low flow rate rather than a firm jet) in the lines from the pool to the pump, are pulling in air on the suction side of the pump, or have a blockage on the outbound side. From your description of the water level at the debris strainer basket almost filling then dropping (which I infer to mean it has a clear housing), my guess would be that you do not have intake water - something is blocking the flow from the pool to the pump.

Then if there is return flow but not full strength, check the skimmer trough inlet and main (bottom of pool) drains - is there a good suction there ? IF not, either a shutoff valve from them is closed, there is a partial lockage in the suction pipes, or the pump is not pumping right. If so, then problem is likely on the pressure side of the pump - see below.

If you have a good suction but no return flow to the pool, then either the filter is releasing it through the flush valve or you have a substantial leak on the pressure (return to pool) side. Check the filter to see if the water is flowing out the filter flush pipe - if so, your flush valve is open - either jammed, manually opened, or the backflush timer has failed to close it (or just happened to be operating the flush cycle last night while you are looking at it). If you have a filter bypass valve try that and see if the water then starts returning to the pool. If does flow with bypass open, then the problem is in the filter or a filter valve is closed. If no significant flow to pool but there is suction, then you have a leak somewhere and the water from the pump is escaping somewhere. If no suction because pump is stalled or unable to pass any water, then a blockage or closed valve in the system somewhere in the lines from the filter bypass to the pool.

One other thing to check - if you have a chlorinator or ozone unit, sometimes they automatically close a water shutoff valve if you run out of hypochlorate or whatever chemical it doses the water with.

IF you are into DIY light plumbing - here are some things you can do yourself to narrow down the list of potential problem sources:

Push comes to shove, start disassembling (if threaded connections) the piping if you are confident doing that - sequentially (one at a time, then test) on the filter side at the outlet of the pump, on the pool side of the filter, and on the pool side of the backflow valve. Be sure the water coming out does not hit electrical equipment/wiring.


First, run the pump with the piping at the pump outlet unhooked - if water comes out at a good flow, then suction side and pump are likely OK. Ditto if the pressure valve at the filter reads good pressure (typically 10-20 psi) for your system, so go to PRESSURE SIDE below. If little or no flow out of pump, problem is on the suction side or at the pump and don't bother disconnecting any more connections on the pressure side.

If there was no water from pool to the pump in above test, after checking valves are open on those pool-to-pump lines, disconnect piping on the pool (inflow) side of the backflow preventer near the pump. Then with the hose, using a rag wrapped around the hose end to tightly fit it into the pipe, run water in the reverse direction into the suction (intake lines) coming from the pool to the pump - if no flow out of the bottom drain or trough skimmer port at the pool, then those line(s) are clogged and probably need to a snaked. (Note this backflushing and almost certainly the snaking may eject a bit of debris back into the pool from the line). Sometimes a wet-dry vac, using the same rag-wrap seal technique, will pull the clog out of the line at the pool end. For the bottom drain of course it takes some jury-rigging with a piece of PVC pipe or two to reach down to the drain from the vac - they don't seem to work so well in the bottom of the pool. And of course be sure it is tied down to a table or something so you don't accidentally pull it into the water by pulling the hose - and it must be plugged into a GFCI/AFCI outlet for safety.


Then, if water flowed freely to the pool reconnect the suction pipe(s), but being careful to direct the water so it does not hit anything electrical, run the pump to see if it puts out a good flow with the pipe on its outlet side disconnected. If so, you are getting water on the suction side from the pool,, and the blockage is in the filter or the return piping to the pool (including at the heater unit if you have one). Reconnect the pump fitting and test the bypass on the filter if you have one and you can confirm or rule out the filter as the source of the problem.

If pump does not put out water but the suction lines flowed well with the hose, then blockage is in the backflow preventer before the pump or in the pump itself, or the pump ate itself up on some debris so there are no vanes on the pump to move the water any more.

From here on, assumes suction lines and pump are good and it puts out water when the outlet pipe from it is disconnected:

Open a pipe on the outflow side of the filter heading to the chlorinator and heater and see if water flows through (with pump on of course) - if not then either it is dumping the water ina flush cycle (meaning flush valve or timer problem maybe). Water should flow well if you open the filter bypass if you have one.

If water flows through the filter fine, then reconnect that fitting and try disconnecting (if you can safely without getting the heater wet) the pipe on the pool side of the heater - if flows well there, blockage is in return line to the pool.

If no flow out of heater, try disconnecting between the heater and chlorinator (or filter if no chlorinator) - if no flow there then problem is in line between filter and chlorinator or at the chlorinator. If flow there, then blockage is at heater. With some fancier models, there is a regulating valve that controls the flow so the water to the pool is at a set temperature, rather than variable temp depending on inflow temp. If this valve goes bad it can shut off the flow.

By the time you get here you should have found the problem source - and removed the blockage or replace the bad element.

Only other solution other than a pro is start reading web blogs of how to fix your sort of problem.

Answered 4 years ago by LCD

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