My Pool Filter Keeps Losing Pressure, Why?

There is nothing more frustrating than your swimming pool pump losing water pressure and you cannot figure out why. In the last decade and a half or so as a self-employed swimming pool service owner, I cannot recount how many times I’ve received this phone call. It is a common problem. In this article I will simplify the troubleshooting steps and potential issues to just the main problems you might find.

The main 3 potential causes for your pool pump to keep losing pressure are generally going to fall into three main categories.

1. Low water levels in the swimming pool
2. Plumbing blockage, either on the suction side or the return side
3. Mechanical problems, typically a seal or a gasket or internal failure to the impeller

Let’s explore each of these 3 main categories and determine why this will cause your swimming pool pump to lose its pressure, more commonly known as losing prime, and more importantly how to fix the problem. I will also slip in a few pro-tips in each section that make the fix a little easier to handle. This may seem obvious to some of you, but not everyone completely understands how swimming pools operate. Do not worry this is a safe place, no judgment here. To clearly answer this little problem, we need to make sure to understand how the water in your swimming pool is circulated.

Low water levels will cause your swimming pool pump to lose pressure

Many are under the impression that gravity is what moves the water from the swimming pool to the pump. This is only accurate if the pump is lower in elevation than the surface of the swimming pool’s water. Then there will be some movement from gravity. Gravity does not work as fast as a swimming pool pump when it’s on. When a swimming pool pump turns on, the water inside the pump pot is pulled into the impeller and forced up and out of the pump housing. The rapid movement of water creates a siphoning effect that pulls the water from the swimming pool into the pump to replace the water being pushed out, also known as equilibrium.

If you logically follow the path of the water back to the source, you will see the first point of entry for the plumbing is either a main drain at the bottom of the deep end or a skimmer at the waterline, or more commonly a combination of the two.

If you hear a gurgling sound coming from the skimmer at the waterline it’s a pretty sure bet that your water level is too low. Quite often when this is the case the gurgling sound will be followed closely by the pump pot becoming empty and then losing pressure.

How Full Should You Fill Your Swimming Pool?

The fix for this problem is not rocket science. Keep the pool water level up. But the follow-up question is; how high do you fill the swimming pool water? Good question, I’m glad you asked. For the most part about an inch above the center of the skimmer opening is about right. If the skimmer still gurgles, add more water until it stops. The idea with the skimmer is that it will pull leaves and grass and other floating debris off the top of the water when the pump is on. If the water level is too high there won’t be any skimming action.

So naturally there is a balancing act of keeping enough water in the pool so you don’t lose pressure, and not overfilling the pool so you don’t lose skimming. Here’s your pro-tip for this section: many newer pools built in the last decade or so have built-in fill valve that are set on either a timer or a float valve that ensure water levels are maintained. If you don’t have such a luxury, you can purchase an over deck fill valve. It will keep your pool pump from ever losing prime again by keeping your pool water level at the exact right level all the time.

A Plumbing Blockage Will Cause A Pool Pump To Lose Pressure

A more serious cause of the problem of your pool pump losing pressure is if there is a blockage in the line. This will usually happen for a couple of reasons;

1. you vacuumed the pool using the pools pump as your vacuum and you sucked up something just a little too big,
2. the skimmer basket was missing and a foreign object found its way to the skimmer and in turn the plumbing.

Often this is easy to determine because the pump will continue running steady, doesn’t make any new noises, and doesn’t act any differently, but just won’t fill the pump pot or build up pressure.

If you’re lucky the blockage will be right near the opening of the skimmer and you can extract it with a screwdriver. I have only been so lucky once. The fix for this is hopefully a simple one for you. You need to run the water in the opposite direction to dislodge the item. To do this you need a drain-jet. Attach it to a garden hose. Open the pool pump lid and insert the drain-jet into the pipe that’s attached to the front of the pump until you only see the garden hose. Then turn on the garden hose full blast. Usually the foreign object will dislodge immediately, but to be sure leave the drain-jet running for a good three to five minutes.

Here’s your next pro-tip for this section; if you don’t have a drain-jet you can drill holes in a tennis ball and then hold the ball in the opening of the pipe. Be prepared to get wet, but in a pinch, it will work. Or you can pick one up here. Bonus pro-tip; sometimes you have to run the drain-jet in the same direction as the water flow so stick it in the skimmer opening as well.

Typically there won’t ever be a blockage on the return side of the pump since your filter has caught any large objects. However, sometimes something gets dropped into the line when a filter is torn down. If this ever happens you will have to use a drain-jet one by one in all the returns until you find which line the foreign object went down. It’s most likely it got stuck somewhere inside the filter itself.

A Clogged Filter Will Cause A Pool Pump To Lose Pressure

One type of clog that is probably the first thing you should check into, is the filter. Many of troubleshooting journeys would be saved if I just started with cleaning the filter. Try as I might, you can never out-chlorinate a full filter.

Usually a clogged filter doesn’t cause pump issues, but if it has been a long time, it’s a good place to start. If you need a little assistance on cleaning your filter, check out this article I did on filter teardowns.

A Mechanical Breakdown Can Cause your Swimming Pool Pump To Lose Pressure.

A swimming pool pump can last for a very long time. It’s not uncommon for us to have a thirty-year-old pool pump. Don’t get it wrong, these are not efficient pool pumps and should be replaced, but that’s a topic for another time. It does happen from time to time that a pump, or its motor, will fail. I won’t go into all the explicit possibilities, but I will hit on two specific mechanical issues.

The first thing to check on the pool pump if it loses pressure is the pump lid and the pump lid O-ring. There is an incredible amount of suction on the pump pot and lid when the pump is running. Over time with sunlight exposure and various other factors related to pool chemicals, a plastic pump lid can develop cracks that could create suction leaks. If these cracks get big enough to pull in enough air, the pump will lose pressure on the output side. The only repair for this it to replace the lid, don’t try to repair it with superglue or anything like that, it simply won’t work.

Part of the pump lid is the pump lid o-ring. If the pump lid o-ring os cracked and dry, replace it. Even if it isn’t the cause, this will eliminate the o-ring as a potential failure point. Make sure to keep a tube of silicon or Teflon based lubricant on hand and keep all O-rings lubed up. It only takes a minute and it will make your O-rings last forever.

The second possible mechanical failure that could cause a pool pump to lose pressure is closely related to getting a blockage. That is the impeller being clocked. If you have a missing or cracked basket in your skimmer or pool pump, debris will work its way into the pumps impeller and eventually clog it. Here’s how to check (with most pump models). Turn off the pump at the breaker and confirm its off by trying to turn the pump on at the timer. Remove the pump basket and reach your fingers into the opening toward the motor side of the pump. If you feel anything sharp or furry, you know you have a foreign object lodged in the impeller.

Some pump models are easier than others, but you can usually clean the impeller without taking apart the pump by rotating the impeller by hand and pulling out the clogs. If not, you’ll need to disassemble the pump at where the case comes together. This is different for each model and I will eventually cover a number of them on my youtube channel, but for now you’ll have to settle for the instruction manuals found on the manufacturers website.

If you approach these fixes in the right way they can be pretty quick and straightforward, so I hope this helps you do that.