Is the system closed?
A pressure-reducing valve, a check valve, or a backflow preventer on the main line stops heated water from pushing back toward the street. With nowhere to go, the expanding water raises the pressure in the whole house.
Room for water to expand, so the relief valve does not have to
Water Heater Expansion Tank Installation and Replacement
Check, recharge, install, or replace the expansion tank on your water heater when the plumbing is a closed system, and find out whether a dripping relief valve or banging pipes come from thermal expansion.
Watch for these signs
Signs of a thermal expansion problem
- The water heater's relief valve drips after the heater runs
- Water pressure is higher after a long shower or hot-water use
- The expansion tank is full of water or knocks when tapped
- A pressure-reducing valve or check valve was installed on the main line
- Toilet fill valves and faucet cartridges keep failing
- A new water heater installation calls for one
Our services
What we handle
- Testing the tank's air charge against the house pressure
- Recharging a tank that has lost air
- Replacing a tank with a failed bladder
- New expansion tank installation and support
- Checking the relief valve and its discharge line
- Measuring static water pressure and the pressure regulator
Your options
Does your water heater need an expansion tank?
It comes down to whether the plumbing is a closed system.
What the code asks
The 2024 International Plumbing Code that Mesa reviews against calls for thermal expansion control (section 607.3) and for a pressure-reducing valve where a building's pressure is over 80 psi (section 604.8).
Is the tank still working?
A tank lasts only while its air side holds a charge. If it is waterlogged, the relief valve goes back to doing the expansion tank's job, and that is when it starts to drip.
Around the Valley
Phoenix-area pressure and closed systems
The City of Phoenix says acceptable pressure in its system is between 40 and 100 psi, that it does not regulate pressure beyond the meter, and that private pressure regulators are the owner's responsibility. A home with a regulator is usually a closed system, and that is when an expansion tank earns its place.
Planning the work
What affects the price
- Tank size for the heater and house pressure
- Whether the existing tank can be recharged
- Mounting and support for a new tank
- Relief valve or discharge line replacement
- Pressure regulator work found during the visit
Boiler expansion tanks
Heating and hot-water boilers have their own expansion tanks, sized and charged for the boiler system rather than the house water pressure. If a boiler's pressure climbs as it heats or its relief valve discharges, One Call Plumbing Services, part of the same local service network, repairs boilers and replaces their expansion tanks.
What happens next
How an expansion tank visit works
- 1
We measure the house pressure and check the regulator and any check valves.
- 2
The expansion tank's air charge and bladder are tested.
- 3
You get the recommendation and the price before work begins.
- 4
The tank is recharged or replaced, set to the house pressure, and the relief valve is checked.
Common questions
Before you schedule
Why is my water heater relief valve dripping?
In a closed system, the most common cause is thermal expansion with no working expansion tank to absorb it. High street pressure and a worn relief valve are the other usual causes. Never cap or plug the valve.
How do I know if my expansion tank is bad?
Tap the tank: a working tank sounds hollow at the top, and a waterlogged one sounds solid. A failed tank can also let water out of the air valve when its stem is pressed, which means the bladder has failed.
Does a tankless water heater need an expansion tank?
Tankless heaters store little water, so the thermal expansion problem is much smaller. Follow the manufacturer's instructions and the code your city enforces for the installation.
How long does an expansion tank last?
It varies with water quality and how often it cycles. Check its charge when the water heater is serviced, and replace it when it will not hold air.
Do you also replace boiler expansion tanks?
Boiler work is handled by One Call Plumbing Services, part of the same local service network. Its boiler page covers expansion tanks, relief valves, and pressure problems on boiler systems.
Related services
Related services and guides
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