Solution: Under Sink Water Filter Low Pressure + Slow Flow

Author: Jason Hollow - Published: 2021/09/22 - Updated: 2023/01/05

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Has your under sink water filter become painstakingly slow? Yeah, we’ve all been there, once at least!

Therefore, we understand the agony of dealing with a system that works at a glacial pace. As it turns out, clogged filters and fluctuations in the quality of your source water are the two most significant reasons for restricted water flow from an under sink unit.

Nevertheless, the real question is not why it’s slow, rather how to fix a troublesome filtration system.

Stay with us as we discuss all the probable causes and solutions to fix a remarkably slow under sink water filter. (Spoiler alert: It’s not too complicated.)

So, here is our guide on how to fix low pressure and slow flow with an under sink water filter!


Your under sink water filter may cause low water flow or low water pressure due to these common reasons:

  • Filters are clogged – Check filter elements and replace if they are not in good condition.
  • Quality of your water – Get your water laboratory-tested for minerals and contaminants. Depending on your water quality you might have to upgrade or customize your system to adequately address your needs.
  • Kink in the water line – Check and make sure that there are no bended water lines.

Fluctuating or low incoming water pressure may be caused by these:

  • Valves are closed – Check and make sure that all water valves are fully open.
  • Aerator is clogged – Disassemble the faucet and remove any build up or debris lodged in it.

Possible Reasons and Solutions for Low Pressure and/or Flow in Under Sink Water Filters

under sink water filter low pressure slow flow thumbnail

While most under sink water filtration systems will work for years and years on end without a glitch, some might require a little more of your precious time.

However, once you get a grip on the most common problems associated with these units, you will be able to maintain them in the best possible way. Thus, ensuring optimum performance and decreased repair costs.


A jammed under sink filter cartridge is, in reality, the most common reason causing a drop in outflowing water pressure. Here are two scenarios that can clog filters sooner than expected.[1]

Increased Usage

Most people rely on the timeframe prescribed by the manufacturer to replace their under sink water filters. They don’t take into account any changes in the amount of water used over time.

Similarly, a leaky kitchen faucet could also deplete your cartridge’s life before you know it.

How to Fix

Remove the filter cartridge from the housing and examine it. If it is full of gunk, replace it with a new one. If not, move on to other solutions.

Changes in Quality of Source Water

In some cases, a drastic change in the water quality clogs your under sink water filters sooner than anticipated.

How to Fix

Try contacting your community water system to find out why there was a sudden change in the water chemistry and when it can go back to normal.

Alternatively, you can experiment with different filter media better suited to deal with a higher concentration of certain contaminants in your water supply.

Low Feed Water Pressure

The ideal incoming water pressure for residential usage is on average 60 psi. If you are subject to poor incoming water pressure, your filtration system will fail to work adequately.

Please note that if you are experiencing standard water pressure all around the house except for your kitchen sink, the problem lies within that very water line.

How to Fix

First, measure the incoming water pressure on a faucet near the point where water enters your house. If it’s below 40 psi, you may need to contact your local water supply and request an inspection.

Another option is to install a pressure pump at the main water supply line to manage fluctuating water pressure.

Choked or Faulty Water Line

The accumulation of dirt or debris inside the water line can also reduce water pressure significantly. It could also be damaged and non-functional.

How to Fix

Try to clean the incoming water line as good as you can. First, disconnect all removable components and wash them thoroughly. Next, use a small stick to dislodge any stuck-up debris. Finally, replace and tighten it.

If the water line is damaged, you may need a replacement.

Distorted Water Line

Due to fluctuations in incoming water pressure plastic tubings can bend or twist. Naturally, any sharp turns or bends in the tubings will lead to a considerable drop in outflowing water pressure.

How to Fix

Check all pipes/connections and fix any bends or kinks in the water lines.

Closed/Faulty Valves

Plumbing units like water treatment systems are riddled with multiple valves. Unfortunately, most homeowners have no idea about the various components limiting the amount of water flowing to their under sink water filter.

How to Fix

Go through your water filter’s instruction manual and identify the control valves associated with your system.

Turn on the valves fully to ensure uninterrupted supply to your filtration unit. If any of the control valves are damaged or faulty, fix them at your earliest convenience.

Why is My RO Under Sink Filter System Experiencing Slow Flow and Low Pressure?

under sink reverse osmosis system with storage tank

Under sink reverse osmosis systems are more complex and require additional components to function.

For example, since RO filters need more time to filter water, a functional water storage tank is an integral part of each system. Similarly, it requires a drain line connection to complete the wastewater draining process.

Here are a few problems specific to RO under sink systems.

Storage Tank Pressure

RO storage tanks operate through air pressure. The ideal pressure for an empty storage tank is 6-8 psi. If your tank has low air pressure or has been overly pressurized accidentally, you can expect a poor outflowing water pressure.

How to Fix

Close the feed water valve and drain out all the water from the storage tank. Once the tank is empty, use a pressure gauge to check the pressure inside. If it’s lower than 6 psi, use a bicycle pump to add a little air. If it’s over pressurized, allow a little air to bleed out.

Be careful not to go above the desired pressure, or you’ll damage the air bladder. Speaking of which…

Ruptured Air Bladder

You can tell that the air bladder in your RO storage tank has burst if water flow goes down to a bare minimum after working absolutely fine for one minute.

How to Fix

You’ll have to replace the tank since the thin bladder can’t be fixed.

No Water in Tank (One or More Valves Shut)

If the RO tank is not filling up, it might be over-pressurized, or perhaps any of the water control valves are shut.

How to Fix

Repressurize the tank using the method mentioned earlier and bring it to 8 psi. Simultaneously, check the tubing that sends water to the tank is not broken or leaking. Finally, ensure the feed water valve, check valve, restrictor valve and tank ball valve are on.

You may need to replace the valves if they are stuck.


In conclusion, there are multiple reasons for under sink water filter slow flow and low pressure problems.

It could be a fouled filter cartridge or changes in your water quality.

Other possible explanations are low incoming feed water pressure, a faulty water line or a closed valve among other things.

Low pressure from an under sink reverse osmosis system might have to do with a malfunctioning water storage tank.

The good news is, most fixes are quite simple and low-cost.

Further Reading


Meet Jason Hollow

Jason Hollow Jason is the founder of Water Masterz and head of content creation. After six years in the industry, he has tremendous knowledge and first-hand experience on all things related to water treatment.

His credo: Not a single American should have to drink unhealthy water at home.

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