Before you shop for an under-sink reverse osmosis system, spend a few minutes with the cabinet doors open and a flashlight. Many St. Louis kitchens were built with the under-sink space shared by the drain trap, the disposal, a dishwasher hose, and a shutoff valve or two. Knowing what is already there will tell you what you can fit, what needs to move, and what questions to ask before anyone starts work.
Start With What Is Already in the Cabinet
Take everything out from under the sink and photograph the space from the front and from the sides. Note where the cold supply line comes out of the wall, where the hot line runs, and whether the drain pipe sits toward the back or the middle. Pay attention to any pipe that runs across the back wall, because it can block a storage tank or a bracket.
Also note what else shares the space. A garbage disposal, a dishwasher drain connection, a soap dispenser pump, or a trash pull-out can all take up room that you were hoping to use for filter housings and a tank.
Things to Look For
- Depth of the cabinet from the front door to the back wall
- Whether the back wall is solid or has an open chase for pipes
- Height from the cabinet floor to the bottom of the sink basin
- Location of the cold water shutoff valve and whether it turns freely
- Whether the drain trap sits low enough to leave room for a second drain fitting
- Any water stains, soft wood, or rust marks on the cabinet floor
Check the Drain Side Carefully
An under-sink system produces wastewater during its normal cycle, so the drain side matters as much as the supply side. Look at the existing drain trap and see whether the space allows a separate fitting to be added without crowding the disposal or the dishwasher hose. A drain line that is already tight or slopes the wrong way can make a clean connection harder.
If you have a disposal, check whether its drain outlet is easy to reach. Some setups can be routed around the trap, but this is a question for whoever will do the work, and it is worth asking before you buy anything.
Look at the Supply Line and Valve
Turn the cold shutoff valve gently and watch for stiffness, drips, or grit when it moves. An old valve that is hard to turn may not seal well when you need it to, and a system install is a poor time to find that out. In older St. Louis homes, you may also see a saddle valve or a compression fitting that was added years ago. Note its type, but do not try to remove or adjust it yourself.
Check for a dedicated cold supply that is not shared with a refrigerator line or an ice maker before you assume the space is ready. Shared lines are common in older kitchens, and they can change where a tap or fitting needs to go.
Plan for the Storage Tank and Faucet
Most under-sink systems use a small storage tank that sits on the cabinet floor or against the back wall. Measure the free floor space from front to back and side to side, then compare it with the tank size listed on the product you are considering. Leave room to reach the tank valve, the filter housings, and the drain connection without bending the tubing sharply.
The faucet is usually mounted on the countertop, so check the sink deck as well. Look for an existing hole that matches the faucet you want, or a spare hole you can use. If the deck is crowded, a second faucet may need a different mounting spot.
Electrical Checks
Some systems include a booster pump that needs power. If you think you might need one, look at the nearest outlet and the cord route. Make sure the outlet is accessible, grounded, and not shared with a disposal or another heavy appliance. Keep cords away from the drain trap and any spots that get wet.
Questions to Ask Before Work Starts
- Where will the tank sit, and how will the tubing reach it without kinks?
- Which fitting will be used for the drain, and will it clear the disposal?
- Does the existing shutoff valve need replacing before the install?
- Will the work require any change to the sink deck or faucet hole?
- What will be done about the cabinet floor if there are signs of old water damage?
Write down the answers and keep them with your photos. If you later need service, the notes will help whoever comes out understand the setup quickly.
When the Space Does Not Work
Some St. Louis kitchens simply do not have enough room under the sink for a full system. In that case, you may want to look at a different location for the tank, a smaller footprint, or a different type of filtration. Talk through the options with a licensed plumber or a qualified installer who can see your cabinet in person. Ask for a written plan that shows where each part will go.
Taking time to check the cabinet first is one of the simplest ways to avoid surprises. A clear picture of the space, the drain, the valve, and the outlet will make any later decision easier, whether you go ahead with under-sink reverse osmosis or choose something else for your St. Louis home.