Solved: Under Sink Water Filter Slow Flow and Low Pressure

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Is it taking you ages to fill up a pitcher of water using your under sink water filter? Don’t worry; we got you covered!

There could be multiple reasons for low water flow and pressure from your under-the-sink filtration system. The top ones being clogged filters and changes in your water quality.

Stay with us as we explore all the possible problems and solutions to make that water running!

Key Takeaways

These are the common reasons for why your under sink water filter causes low water flow/pressure:

  • Clogged filters – Inspect the filter elements. Replace if needed.
  • Water quality – Have your water tested for contaminants. You might need to upgrade to a more extensive setup or one customized to your current water quality.
  • Water line kink – Ensure there are no sharp turns or bends in the tubing.
  • Low/fluctuating incoming water pressure
  • Closed valves – Ensure all water valves are open to full capacity.
  • Clogged faucet aerator – Take the faucet apart and clean any debris or buildup lodged in the aerator screen and internal parts.

What Could Cause Low Pressure/Flow with Your Under Sink Water Filter?

Here are a few reasons why water from your under sink water filter cannot maintain pressure and has gone down to barely a trickle.

Clogged Filters

The key to optimum performance of any water filter is replacing cartridges at the recommended time. Are your cartridge replacements strict as clockwork?

Even if you are consistent with changing under sink water filters, you may still end up with cartridges full of gunk due to an increase in usage. There are days when we need more water for cooking or drinking. It could also be subject to a leaky faucet or maybe the little one in your house left the water running too long.


Some filtration systems have a light that signals when filters need replacement, so it is easy to rule out this problem. If not, examine the filter elements to see if they appear dirty and replace them at your soonest.

Water Quality

Changes in your water quality, i.e., more sediment, higher amount of minerals, or presence of bacteria, means your under sink filter system is working overtime and extra hard. This situation results in slower filtration.


You might need to upgrade to a more extensive setup or one that is customized to your current water quality.

clean and dirty sediment filters

Water Line Kink

A kink in the water line can reduce water pressure as it limits the flow of the water.


Ensure there are no sharp turns or bends in the tubing that connect your filter system to the faucet.

Low/Fluctuating Incoming Water Pressure

The higher the incoming water pressure to your house, the better the flow rate for your under sink water filter will be. At Least 60 psi is required for most of these systems to work efficiently.

Note: If you are experiencing the issue only in the filtered water faucet, the problem lies within your system, not the incoming water pressure to your entire home. Some filter systems have a pressure limiting valve since they cannot operate at high pressures. As a result, these units tend to have slower water flow rates.


Check with your local water supplier why the pressure has changed. If the problem is isolated to your house only, it could easily be fixed by cleaning out the debris in water distribution lines. Nevertheless, your water utility should be able to take care of it.

If nothing works, installing a water pressure pump at the main water line should take care of the problem. You will need to hire a pro for it, though.

Closed Valves

You are possibly dealing with multiple valves in the case of an under sink water filter. The most commonly used valves are self-piercing saddle valves, dual check valves, and pressure limiting valves.

If any of the valves are faulty, partially plugged, or only partially pierced (applicable to self-piercing one), you will experience low water pressure as a result.


Ensure all valves are open to full capacity and are not clogged with debris.

Clogged Faucet Aerator

Dirt or debris inside the faucet aerator can also obstruct water flow.


Take the faucet apart and clean any debris or buildup lodged in the aerator screen and internal parts. You may need to use a small brush or stick. Make sure you fix it back tightly to avoid leaks.

Problems Specific to Under Sink RO Systems

Since RO systems require more time filtering water, they feature a storage tank that stores clean water to be dispensed at demand. It typically takes 2-4 hours for the tank to completely fill up. Without a tank, it would take you 5 minutes to fill a single glass of water.

The water tank may need a replacement if:

  • It has reached the end of its life.
  • It is severely eroded.

In both cases, a replacement is imminent.

Here are a few other problems associated with under sink RO water filter systems.

Low Tank Pressure

Storage tanks function through air pressure. If your RO tank has low pressure, the flow rate will be affected. An empty tank should have 5 to 8 psi for ideal performance.


Empty out the tank by closing the feed water valve and open the RO faucet to drain the water. Next, locate a blue cap covering the Schrader valve on the side of the RO tank. Then, use a pressure gauge to check the tank air pressure.

If it’s too low, use a bicycle pump to add pressure up to 8 psi. Keep a steady hand since over-pressurizing the tank can rupture the air bladder inside.

Under Sink Reverse Osmosis System with water storage tank

High Tank Pressure

Similarly, high tank pressure can also limit water flow since it will restrict the amount of water flowing into the tank.


Repressurize the tank using the method mentioned above.

Ruptured Air Bladder

If your RO faucet can deliver only one cup of water with good flow and then slows down to a trickle, chances are the air bladder is ruptured.


Sadly, a ruptured bladder can’t be fixed. Enlist the help of a water treatment technician to replace the storage tank.

Empty Tank

No water in the RO storage tank?


Ensure water inflow valves and ball valves are open. Moreover, repressurize the tank to bring the pressure down to a max of 8 psi. Higher air pressure inside the tank stops the water from entering it.

One or More Valves Not Turned On

If the check valve or restrictor valve is clogged or accidentally turned off, there may be little to no water from the faucet.


Ensure all valves are open and functional. You may need to clean them out if they are ridden with debris.

If you have any questions about under sink water filters causing low pressure or slow flow please don’t hesitate to leave a comment below!

About the Author Gene Fitzgerald

Gene Fitzgerald is one of the founders of BOS and currently head of content creation. She has 8+ years of experience as a water treatment specialist under her belt making her our senior scientist. Outside of BOS, Gene loves reading books on philosophy & social issues, making music, and hiking.
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