If your St. Louis home has a water heater, there may be a small tank sitting on the cold water line nearby that most people never think about. That is an expansion tank. Before you add a reverse osmosis unit under the kitchen sink, it helps to know whether the tank is there, whether it still works, and how the cold line that feeds your unit connects to it.
What an expansion tank does
Water takes up more room as it heats. In a home where a check valve or pressure reducing valve stops water from flowing back out to the street, that extra pressure has nowhere to go. An expansion tank gives it somewhere to go. Most tanks are sealed steel shells with a rubber bladder inside. Water fills one side of the bladder, and a pocket of air fills the other side. When pressure rises, the bladder flexes and absorbs the extra volume.
Many older St. Louis homes have no check valve at all, so a tank may not be needed. Others have one and should have a tank. Finding out which situation applies to your house is the first step.
Why this matters for under-sink reverse osmosis
A reverse osmosis unit usually takes its feed from the cold line at a valve under the sink. Depending on the model, it may also have a storage tank, a pump, a flow restrictor, and a drain line. Any of these parts can respond to pressure changes in the supply. A pump that cycles on and off, a storage tank that fills, or a faucet that closes quickly can all change how pressure moves through the cold line. If the house already deals with pressure swings from the water heater, adding a unit without checking the system first can make a noisy or unpredictable problem harder to trace.
Checks to run before you install
- Find the water heater and locate the cold water line entering it.
- Look for a small round or cylindrical tank on that cold line, usually within a short distance of the heater.
- Check whether there is a check valve or pressure reducing valve on the main supply near the meter or where the line enters the house. These often look like a plain fitting with a spring cap or a bell shaped body.
- Look at the valve on top of the tank, which resembles a tire valve. A gauge can show whether the air side still holds a charge. Do this only with the water off, following the instructions on the tank's label.
- Tap the tank lightly. A heavy, solid sound generally means the bladder is full of water and may have failed. A hollow sound generally means air is present.
- Write down where the tank sits, what its label says, and any date or sticker on it.
Signs the tank may need attention
- Rust or moisture at the seams or fittings of the tank.
- Water dripping from the relief valve on the water heater or from the tank valve.
- Pipes that knock or rattle when the water heater runs or when a faucet closes.
- A tank that feels heavy when it should feel light and mostly empty.
- A noticeable change in water pressure after the heater cycles.
Safety and when to call a licensed plumber
Expansion tanks and water heaters carry pressure and heat. Shutting off the wrong valve or opening a relief valve without a plan can cause a leak or a scald. Turn off power or gas to the water heater before you work near it, and shut off the cold supply to the house if you need to disconnect anything. If you see corrosion, a leak, or a tank that does not hold air, have a licensed plumber in the St. Louis area inspect the system before you add anything to the cold line. Ask them to check the tank, any check valve, and any pressure regulation, and to explain what they found in plain terms.
If your home is a rental, the tank and water heater usually belong to the owner, so talk with them before you open any of these parts.
What to keep on file
Take a photo of the tank, its label, and the route of the cold line. Note the location of the main shutoff and the water heater. Keep a short record of any repairs or inspections. When someone comes to look at your system later, that record saves time and questions.