If your St. Louis home already has a water softener, that changes how an under-sink reverse osmosis unit should connect to your plumbing. Before you buy or schedule anything, spend a little time figuring out where the softener sits, how its bypass valve works, and which line feeds the kitchen cold tap. This guide covers the checks a homeowner can do safely with a flashlight and a notepad.
Find the softener and its bypass valve
In many St. Louis basements and utility rooms, the softener sits on the main cold water line near where the water enters the house. Look for a tank with a control head on top, a brine tank beside it, and a short run of pipe with a few valves in a row. That row usually includes a bypass valve, which lets water skip the softener while service is done.
Bypass valves come in a few styles. Some are a single lever, some are a pair of handles labeled in and out, and some are a multi-valve arrangement. Take a photo of the valve position as it sits now, so you can return it to the same setting later.
Confirm where the kitchen cold line starts
Trace the cold line that feeds the kitchen sink. Figure out whether it comes off before the softener or after it. Plumbing in older houses sometimes has a tee that sends untreated water to an outdoor hose bib or an icemaker, while the softened line runs to the rest of the house. Mark each branch with painter's tape and a short note.
Knowing the feed source matters because a reverse osmosis unit draws its supply from a cold line. Whether that line is softened or not affects scale on the membrane and how the unit is serviced later. Ask any service provider which feed they plan to use, and get the answer in writing.
Check the softener's regeneration and salt
A softener that is regenerating may send brine and rinse water through the lines for a period. If you plan to add a reverse osmosis unit, note the regeneration schedule shown on the control head. Keep the salt level above the water line in the brine tank, and check the display for any error code.
A softener that is not working well can also leave scale in the wrong places, so before you change plumbing, look at its recent service record. If there is none, write that down in your own file.
Questions to answer before any work
- Where is the softener, and is its bypass valve easy to reach?
- Which cold line feeds the kitchen sink, and does it come before or after the softener?
- What does the softener manual say about running a bypass, and how long can it stay in bypass?
- Does the reverse osmosis unit need a dedicated shutoff, and where will it be mounted?
- Is the drain point near the sink arranged to meet local plumbing practice?
Run a simple bypass test
Only do this if you know how to return the valve to its original position. Place a bucket under the kitchen cold tap, open the tap, and note the flow. Then shift the bypass valve as the manual describes, which should send unsoftened water to the house. Compare the flow and feel at the tap, then restore the valve and confirm the softener resumes normal operation on its display.
If the valve is stiff, seeps, or does not seal, stop and call a licensed plumber rather than forcing it. A sticking bypass is a plumbing issue, not a filter issue.
Before you choose a filter
Reverse osmosis is one option for a St. Louis kitchen, not the only one. Read your water utility's most recent water quality report, and consider having a sample tested by a certified lab if you want to know what is in your tap. A test result gives you a basis for choosing equipment instead of guessing.
Keep a written record of the valve layout, the feed source, the softener settings, and any service visits. That file will save time whenever a plumber, installer, or inspector asks what is connected where.