When your water heater warms water, the water expands, and in a closed plumbing system the extra volume has nowhere to go. An expansion tank gives it somewhere to go. When the tank is waterlogged or missing, pressure spikes every heating cycle, and the first sign is usually a dripping relief valve on your water heater. A replacement is a small job, typically a few hundred dollars, and it protects the heater and every fixture in the house.
What does a water heater expansion tank do?
It soaks up the extra water created when the heater warms. Inside the tank a rubber diaphragm separates water from a cushion of compressed air. As the water expands, it pushes into the tank and compresses the air. When the heater cools or hot water is used, the air pushes the water back out.
Without it, water expansion raises the pressure in the whole house. That is only a problem when the system is closed, meaning something on the supply line stops water flowing back toward the street. A pressure regulator, a backflow preventer or a check valve closes the system. Many Los Angeles homes now have one of those, because the supply pressure can be high.
How do I know my expansion tank has failed?
Look for a dripping temperature and pressure relief valve, banging or hammering pipes, a faucet that spurts, or a pressure gauge that reads high after the heater runs. The tank itself gives clues: tap it, and it should sound hollow at the top and solid at the bottom. A tank that sounds solid all over is waterlogged.
A more reliable check is the air valve on the tank, which looks like a tire valve. With the water shut off and the tank drained of pressure, press the valve pin. Air should come out. Water spraying out means the diaphragm has failed, and the tank must be replaced. Pressure is the other test: a tire gauge on the air valve should read about the same as the house water pressure.
Do not ignore a dripping relief valve. It is the heater's safety device, and replacing it will not stop the drip if the expansion problem is still there. See our page on water heater release valve replacement for the valve side of the story.
Do I need an expansion tank in Los Angeles?
If your system is closed, yes, and a new or replacement heater installation is when it gets checked. Plumbing guidance and the Wikipedia safety summary both say an expansion tank or relief arrangement is needed to prevent pressure buildup. Your plumber will test whether your main line has a check valve or a regulator, and the inspector may ask.
LADWP notes that new residential plumbing requires a pressure regulator when main pressure can exceed 80 psi, and that older homes may lack one because they predate the current code. A home that gained a regulator in a later remodel has become a closed system without anyone noticing. Our page on the water pressure regulator explains that part.
If you are replacing the heater, the inspector for the permit may look for the tank. See our guide to the plumbing permit for a water heater for what is checked.
How much does an expansion tank replacement cost?
Roughly $300 to $800 for materials and labor nationally, and $200 to $500 for Los Angeles, according to figures published by HomeGuide and Angi. The tank is the smaller part of that bill. Labor, draining the heater, soldering or fitting a new connection and any code corrections account for the rest, so a quote should show each.
Things that change the price:
- Where the tank mounts. A tank that hangs from a pipe over the heater is simpler than one that needs a new support.
- Tank size. Bigger tanks and heaters need a larger unit.
- Pipe condition. Corroded fittings add time.
- Extra items. A pressure regulator or relief valve replaced at the same visit adds its own cost.
Ask for the price as a guide figure first. A plumber confirms the real number after testing pressure and looking at the line. If the heater is old as well, ask for a second price for replacement, using our water heater replacement cost page as a reference.
Can I replace an expansion tank myself?
You can if you are comfortable shutting off water, draining a line and sweating or threading a fitting. The risk is in the set-up. The tank must be precharged to match your house pressure, which is measured with a gauge at a hose bib, and set with a tire gauge before the tank goes on. Many tanks leave the factory at 40 psi.
A tank set for 40 psi on a house running 70 psi gives you little expansion room, and the symptom you were trying to fix returns. Support the tank so its weight does not hang from the copper. If your pipe is old and brittle, a plumber should do it, because a leak from a disturbed fitting is the usual way this job goes wrong. If the heater is gas and venting or gas work is involved, call a plumber.
When should I call a plumber for an expansion tank?
Call when the relief valve is dripping and you are not sure why, when you do not know whether your system is closed, when pressure is above normal, or when the tank is on an old heater. A plumber can test static pressure, set the precharge and tell you whether the real fault is the regulator, the valve or the tank.
For high or unstable pressure, see our page on water pressure and regulators too. Book a pressure test and tank swap through a quote request. The rest of our plumbing work is listed on the home page.
Common questions
How long does expansion tank replacement take?
Typically one to two hours, since the plumber must shut off the water, drain part of the system, fit the new tank, set the precharge and check for leaks. If a pressure regulator or relief valve also needs work, add time. Ask for the estimate after pressure has been tested, because that is when the plumber knows what is needed.
What should I ask before I hire someone to replace an expansion tank?
Ask whether they will test your static pressure and set the tank's precharge to match, and whether the price includes the tank, fittings and support. Ask whether the problem might be a regulator or relief valve instead. Ask what they will do if corroded pipe turns up, and get the answer in writing.
How long does a water heater expansion tank last?
There is no fixed figure, and I would not trust a published number without a source. The tank lasts until the diaphragm or shell fails, and conditions like high pressure and heat shorten that life. Plumbers recommend checking the air pressure about once a year, which catches a failing tank early.
Does someone need to be at the property?
Yes, someone should be there to give access, to agree any change to the price and to run the taps afterward. The water will be off for a short time. If you rent, tell the landlord, who is responsible for the heater and its parts, and who will usually need to approve a visit.