Next to the water heater in many Lombard basements is a small tank, usually blue, white or gray and about the size of a basketball. Most homeowners never think about it until the water heater starts dripping or a plumber points at it and says, “This one’s shot.” That little tank is the expansion tank, and when it fails, your water heater and the rest of your plumbing take the strain. Here is what it does, how to tell when it is failing, and what to do about it.
What Does an Expansion Tank Do?
Water expands when it is heated. A typical 40-gallon water heater can gain around half a gallon of extra volume as it heats up, and because water barely compresses, that extra water has to go somewhere.
In many homes, it cannot flow back into the city water main. Pressure reducing valves, check valves and backflow preventers, often installed at or near the water meter, let water into your home but stop it from going back out. That protects the public water supply, but it also creates what plumbers call a closed system. As the Woonsocket Water Division explains, homes with these devices are the ones at risk from thermal expansion.
The expansion tank gives that extra water a place to go. Inside is a rubber bladder or diaphragm with water on one side and pressurized air on the other. As the water heats and expands, it pushes into the tank and compresses the air. When you open a tap, the air pushes the water back into the system. Without it, pressure spikes every time your water heater runs.
Why It Matters in Lombard Homes
Water in Lombard comes from Lake Michigan through the DuPage Water Commission, and many local homes have a pressure reducing valve or a backflow device that makes the plumbing a closed system. Add the winter months, when very cold incoming water means your water heater works longer and harder from November through February, and the expansion tank is doing a lot of work for something so small.
Many expansion tanks were installed at the same time as the water heater, so if your heater is getting older, the tank probably is too. Lombard, Glen Ellyn and Villa Park are full of homes where the heater has been replaced but the expansion tank never was.
7 Signs Your Expansion Tank Is Failing
1. The Relief Valve on Your Water Heater Is Dripping
This is the most common clue. The temperature and pressure (T&P) relief valve opens when pressure gets too high. If the expansion tank cannot absorb the extra water, the T&P valve releases it instead, and you will see water at the end of its discharge pipe near the floor. If you have found water at the base of your heater, read why your water heater is leaking from the bottom for other possible causes.
2. The Tank Sounds Solid When You Tap It
Knock gently on the top half of the tank and then the bottom half. A healthy tank sounds hollow on the top, because that side is full of air. If it sounds dull and solid all the way around, the tank is likely waterlogged, meaning water has filled the air side and the tank can no longer absorb expansion.
3. Water Comes Out of the Air Valve
Most expansion tanks have a small air valve, like the one on a bike tire, on the end opposite the water connection. Pressing the pin briefly should release only air. If water spits out, the bladder inside has torn. A ruptured bladder cannot be repaired, so the tank needs to be replaced.
4. Faucets Spurt or Pressure Jumps Around
A failed tank lets pressure build up while the water heater runs. You might notice a hard burst of water when you first open a faucet, banging pipes, or toilet fill valves that hiss and run on their own.
5. Rust, Moisture or Leaks on the Tank
Expansion tanks are steel on the outside. Rust spots, weeping around the connection, or a damp tank, especially in a humid basement, mean the shell or fitting is breaking down.
6. Appliances and Fixtures Wear Out Early
High pressure is hard on everything connected to your plumbing. Repeated pressure spikes wear out seals and valves in washing machines, dishwashers, faucets and toilets, and can shorten the life of the water heater itself.
7. The Tank Is Older Than 5 to 10 Years
Expansion tanks do not last forever. Many fail somewhere in the 5 to 10 year range, depending on water pressure and how hard they work. If you do not know how old yours is, it is worth having it checked.
What You Can Check at Home
- Do the tap test. Hollow on top is good. Solid all over is a warning sign.
- Look at the T&P discharge pipe. A wet floor or mineral crust at the end of the pipe means it has been releasing water.
- Check your house water pressure. A simple gauge on an outdoor spigot or laundry faucet shows your static pressure. Most homes do best around 40 to 60 psi. Readings above 80 psi are too high and should be looked at.
- Never cap the relief valve pipe. It is a safety device. Plugging it to stop a drip can turn a pressure problem into a dangerous one.
Checking or adjusting the tank’s air charge is best left to a plumber. The air pressure has to match your home’s water pressure, and it should be checked with the water supply shut off and pressure relieved first.
What to Do If Your Expansion Tank Has Failed
If the tank is waterlogged, leaking or spitting water from the air valve, the fix is replacement. A plumber will:
- Size the new tank for your water heater and your home’s water pressure
- Set the tank’s air charge to match your incoming pressure
- Mount and support it properly on the cold water line to the heater, so its weight does not stress the pipe
- Check the T&P valve and, if needed, the pressure reducing valve
Replacing an expansion tank is usually a quick job, and it is much less disruptive than dealing with a failed water heater or a leak caused by high pressure.
Is It Time to Look at the Water Heater, Too?
A failing expansion tank often shows up alongside other water heater problems. If your heater is popping, rumbling or running out of hot water quickly, see why your water heater makes popping noises and our list of 7 signs your water heater is about to fail. If you are planning a replacement, compare options in tankless vs tank water heaters.
Get Your Expansion Tank Checked
An expansion tank is small, but it protects your water heater, your pipes and every fixture in your home. If your relief valve is dripping, your tank sounds solid, or you are not sure the tank is doing its job, call Plumbing Lombard at (630) 403-0888. We will get a local plumber to you who can test the tank, check your water pressure and explain your options. Learn more on our water heater repair page. If water is leaking now, our line is answered 24/7 for emergency plumbing.
