Under Sink Water Filter Leaking? (How to Troubleshoot & Fix It)

You’ve opened your under-sink cabinet and discovered something alarming: a puddle of water underneath your water filter system.

Whether your under-sink water filter is dripping or gushing with water, this guide will help you diagnose and resolve the problem – and prevent it from ever happening again.

🪛 How to Fix a Leaking Under-Sink Water Filter: Step-by-Step

Once you’ve identified the source of a leaking under-sink water filter unit, you should be able to fix the leak yourself. Follow the steps outlined below to fix a leak in your filtration system.

Step 1: Gather Your Equipment

There isn’t much equipment that’s needed to identify a leak, and the exact items required depend on the cause of the leak. You should have most equipment to hand already, and the rest should be available in your local hardware store.

Potential equipment you may need to fix a leaking water filtration system are:

  • O-rings
  • Silicone-based lubricant
  • Teflon tape
  • Filter sump wrench
  • New filters
  • A bucket
  • A torch
Fixing a leaking under sink water filter

Step 2: Identify the Leak

You might know that your water filter leaks, but where is the leak located? The first step is to locate where the water is coming from.

Shine your phone’s flashlight around the system and look for the source of the water. You can also run your hand around the fittings and connections, feeling for water.

Some of the most common locations for leaks are connections, housings, storage tanks, pressure release buttons, and faucets or dispensers.

Leaks Vs Condensation

Before you panic that a couple of drops of water are caused by a leak, consider the possibility that the leak is actually condensation.

Condensation on pipes and water-using appliances are caused by warm air meeting cold water. If you notice that your filtration system looks moist on a warm day, dry the filter with a towel and watch the area. If moisture reappears in a certain spot, check the area for a leak.

While condensation is likely to create a puddle underneath a whole house water filter, it’s less obvious under an under-sink filter.

Leaking vs condensation

Step 3: Troubleshooting

Once you’ve identified the source of the leak, you can figure out why the system is leaking.

Common causes of leaking include incorrect installation, filter housing that’s been over- or under-tightened, faulty o-rings, loose valves, drain line connection and incompatible adapters. Hot water or high water pressure can also damage under-sink water filters, causing leaking.

Step 4: Resolve the Issue

Finally, once you know the cause and the location of the water filter leaks, you can resolve the problem before it gets any worse.

Here, we’ve outlined some of the most common causes of leaking under-sink water filters and how to fix them.

Over-Tightened or Under-Tightened Filter Housing

When installing an under-sink water filter, it’s important to get the right balance between under-tightened and over-tightened filter housing. Both extremes will cause the filter housings to leak. Over-tightening is especially damaging because it can cause hairline cracks on the threads or in the filter sump, or crush the o-rings.

To resolve an issue caused by under-tightening, use a filter wrench or hand-tighten the filter housings until they’re firmly in place, without forcing them any further.

To resolve an over-tightening leak issue, check the threads and o-rings for damage, and replace them if necessary.

Tightening the filter housing with a wrench

Clogged Filter Cartridges

Filter cartridges only last for so long. Once these filters become saturated with contaminants, they’ll massively reduce the flow rate of your water supply. Water can’t pass through the clogged filters as quickly as it needs to, and it’ll end up spilling out of the filter housing.

Check your user manual to see how long each filter cartridge in your under-sink system should last.

Replace the filters on time, and you should never experience leaking due to a clogged filter cartridge.

Mineral deposits on the filter cartridge can clog the cartridge within a matter of weeks. Consider installing a point of entry water softener at your drinking water supply to prevent mineral clogging from increasing your frequency of filter replacements.

Loose Connections

A bump or a knock to the system, fluctuations in your water pressure, or wear and tear over time, can all lead to a loose connection in your under-sink water filtration system.

Work out whether the nut or the adapter is leaking, then tighten the connection using a basic wrench. Loose quick-connect fittings are easiest to resolve – just push the connection until it’s all the way in. If you’re still dealing with leaking, the fittings or adaptors may need to be replaced.

Tightening a loose plumbing connection on under sink water filtration system

Damage to the Storage Tank

If you have an under-sink reverse osmosis system with a storage tank, the system may leak as a result of tank damage. Common issues with an RO system water tank are a ruptured air bladder, a hole in the tank, and a malfunctioning valve.

Switch off your water supply using the under-sink shut-off lever and divert water away from the unit with a bypass valve, then drain the tank. Some issues are easy enough to resolve (for instance, a hole can simply be patched up). Others, like broken components, can be difficult to even identify, and you may need to buy a new tank.

🚫 How to Prevent Future Leaks in Under-Sink Water Filters

Ultimately, you can’t entirely eliminate the possibility of a water leak in your under-kitchen-sink filter unit, but you can greatly reduce the likelihood of leaking by doing the following:

Use High-Quality Filters

When you replace the water filters in the system, make sure to choose high-quality filters with long lifespans. Poor-quality filters will clog quickly, reducing the water pressure inside the system and causing a slow leak around the edges of the entire unit. You don’t have to stick with the brand you bought the system from – most under-sink filter cartridges are universally-sized.

Clean the O-Rings Regularly

Every o-ring in your unit should be cleaned, lubricated, and replaced as necessary. A faulty o-ring is the cause of most leaks in an under-sink filtration system, allowing feed water to escape from the housing. Make sure the o-ring is seated properly in the housing, too, as an improperly placed o-ring is another cause of leaks.

You should only ever use silicone-based lubricants to lubricate an o-ring; never petroleum jelly, as this will eventually compromise the seal.

Rubber-o-rings in water filter housing

Don’t Over-Tighten the Filter Housings

You shouldn’t apply excessive force when you’re tightening the filter housings. Only use your hand or the plastic wrench provided to do the tightening. This will prevent issues like spring leaks caused by cracked sump housing.

Check your System Regularly

Finally, make sure to check your under-sink filtration unit at least once a week to make sure everything looks normal. If you have an RO system, check that the water storage tank looks in good condition and double check the connection to the drain line. If your filtered water starts to taste different, replace the filters before leaking can occur.

🧠 Under Sink Water Filter Leaking FAQs

When do I need a professional to fix my leak?

If you’re dealing with a catastrophic leak that you can’t resolve, or you can’t identify the source of the leak in your under-sink water supply, you should consider hiring a professional plumber or handyman to assess the situation for you. However, in most cases, you’ll be dealing with a basic leak that you’ll be able to resolve yourself.

Why do water filters leak?

The most common causes of leaking filters are loose connections, a faulty o-ring, an incompatible adapter, loose filter housing, and clogged filters.

Why is water coming out of my water filter?

If water is trickling out of your filter housing, it’s likely that there’s a cracked or loose o-ring, or a worn fitting. Check the fittings and replace the o-ring if necessary. This should stop the leaks.

Why is my reverse osmosis filter leaking?

A leaking reverse osmosis system is usually caused by accidental bumping or tugging of the water lines. Check that all your connections are still in place and tighten the housing using a filter wrench if necessary.

Tightening filter housing on under sink pou ro system with wrench

Why is my whole house filter housing leaking?

A leaking whole house water filter is caused by all the same reasons as a leaking under-sink water filter: worn o-rings, cracked fittings, loose connections, and so on. The difference is that a whole-house water filter has a bigger potential to cause major flooding and property damage because it’s in an area where water pressure is much higher. You can fix a leaking whole house water filter following these instructions.

How do I replace a rubber o-ring?

If your leaking filter is caused by a dirty, damaged or cracked o-ring, cleaning or replacing the o-ring should resolve the issue. Switch off your water supply, press the pressure release button to release trapped air from the unit, and remove the filters from their housing. Check the o-ring for damage and rub it between your fingers to remove dirt. If the o-ring is damaged, replace it with a new one. Lubricate the o-ring with silicone lubricant to prevent it from drying out. Check every o-ring in your system and follow the same instructions for each.

How do I make a filter replacement in my under-sink drinking water system?

If your filtration system is leaking due to compromised water flow caused by old water filters, replacing the filters should prevent leaks going forward. To replace the water filters in your under-sink unit, switch off your water supply and press the pressure release button to let trapped air escape. Then, carefully remove the filter housing and tip the worn-out filter cartridge out. Wash out the housing and slot the new filter cartridge into the housing.