Do You Need an Expansion Tank on Your Water Heater?

You need an expansion tank on a tank water heater when your plumbing is a closed system, meaning a check valve, backflow preventer, or pressure-reducing valve stops heated water from pushing back into the city main. The International Residential Code's thermal expansion section (P2903.4) requires a pressure control device in exactly those situations, and a thermal expansion tank is the common way to meet it. In an open system with none of those devices, expanding water has somewhere to go and a tank may not be required.

The clearest sign you need one is a dripping relief valve. Water heater manufacturer A.O. Smith says temperature and pressure (T&P) relief valves drip when water pressure is too high or because of thermal expansion in a closed plumbing system, and it tells homeowners to control water pressure and install a thermal expansion tank before replacing a dripping valve. Replacing the valve alone treats the symptom. In Allegheny County, adding an expansion tank is plumbing work, and the Allegheny County Health Department requires a permit for any plumbing that is installed or altered. This post explains how thermal expansion works, when the code requires control, how to tell if your home has a closed system, and what installation involves.

What Is a Water Heater Expansion Tank?

A thermal expansion tank is a small tank connected to the water heater's cold water supply that absorbs the extra volume water takes up when it is heated. The home inspection association InterNACHI explains that as expanding water is forced into the tank, it compresses air held inside a rubber bladder, which works as a cushion because air compresses and water does not.

InterNACHI notes that water expands as it gets hot and loses volume as it cools. In a closed system that added volume has nowhere to go, which is the pressure rise the code's thermal expansion section addresses. InterNACHI also stresses that an expansion tank is not a substitute for the T&P relief valve, which stays in place as the heater's safety device.

When Does the Code Require Thermal Expansion Control?

The International Residential Code sets two triggers in Section P2903.4:

• Pressure-reducing valve (P2903.4.1): For water services up to 2 inches, a pressure control device is required where thermal expansion pushes pressure on the downstream side of a pressure-reducing valve above the valve's setting.

• Backflow preventer or check valve (P2903.4.2): Where a backflow prevention device, check valve, or similar device is installed on a system with storage water heating equipment and thermal expansion raises the pressure, a pressure control device is required.

Pennsylvania plumbing in Allegheny County is permitted and inspected through the Allegheny County Health Department, so the local inspector has the final word on what your installation needs.

How Do You Know If Your Home Has a Closed System?

A closed system has a one-way device between your home and the water main. Look near where the water service enters the house and near the meter for:

• A pressure-reducing valve on the main line

• A backflow preventer or check valve

• A meter setup that includes a check valve

If you are not sure, ask a plumber to look, or ask your water utility whether your meter or service includes a check valve. A plumber can also measure your static water pressure and compare it with the code limit.

What Are the Signs You Need an Expansion Tank?

Your plumbing often tells you before a code inspector does. Watch for:

• A dripping T&P relief valve, which A.O. Smith links to high pressure or thermal expansion in a closed system

• Water running from the relief valve discharge pipe after the heater completes a cycle

• Leaks at pipe connections, which A.O. Smith says can loosen over time from thermal expansion and wear

• High static pressure, since the International Residential Code (P2903.3.1) caps static water pressure at 80 psi and requires an approved pressure-reducing valve where the main pressure is higher

A.O. Smith also lists excessive water pressure as a cause of tank and connection leaks, and says installing a pressure-reducing valve or an expansion tank can resolve it. If you have seen any of these, our guide on when to schedule water heater repair in Pittsburgh covers other warning signs worth checking.

How Is an Expansion Tank Installed?

The International Plumbing Code's version of this rule (Section 607.3) says the expansion control device connects to the water heater's cold water supply pipe downstream of all check valves and pressure-reducing valves, and that thermal expansion tanks are sized according to the tank manufacturer's instructions. InterNACHI adds that inspectors look for proper location and support, corrosion, and leakage. A.O. Smith's troubleshooting steps refer to a "correctly charged expansion tank," so the tank's air charge is part of the job too.

In Allegheny County, the Health Department's permitting page says any plumbing that is installed or altered requires a permit, that a plumbing plan must be approved first, and that a final inspection is required. For more on local rules, see do you need a permit for plumbing work in Pittsburgh.

Do Expansion Tanks Wear Out?

They can lose effectiveness. InterNACHI says the bladder can leak air over time and that a Schrader valve, like the fill valve on a bike tire, can be used to add air. If a relief valve starts dripping again on a home that already has an expansion tank, have the tank's charge and condition checked before replacing other parts.

Do Tankless Water Heaters Need Expansion Tanks?

The code's check valve trigger in P2903.4.2 refers to systems using storage water heating equipment, which is a tank. A tankless unit does not store a large volume of heated water, so the thermal expansion issue is different. Ask your installer and your local inspector what your specific setup requires, and check the tankless manufacturer's installation instructions.

If you are comparing systems, our guide to tank vs tankless water heaters for Pittsburgh homes covers the main differences. Pressure problems in the other direction, where water barely trickles, are covered in why your home's water pressure is low.

FAQ

Is an expansion tank required by code?

The International Residential Code requires thermal expansion control where a pressure-reducing valve, backflow preventer, or check valve creates a closed system with a storage water heater. An expansion tank is a common way to meet that requirement.

Why is my water heater relief valve dripping?

A.O. Smith says T&P valves drip when water pressure is too high or because of thermal expansion in a closed system. It recommends controlling pressure and installing an expansion tank before replacing the valve.

Where does an expansion tank go?

On the water heater's cold water supply, downstream of any check valves and pressure-reducing valves, per the International Plumbing Code's Section 607.3, and sized to the tank manufacturer's instructions.

Can an expansion tank fix high water pressure?

It absorbs pressure spikes from heating water. It does not lower the pressure coming from the street. If static pressure is above 80 psi, the code calls for a pressure-reducing valve.

How long does an expansion tank last?

There is no single number. InterNACHI notes the bladder can leak air over time, so have the tank checked when a relief valve starts dripping or during water heater service.

Do I need a permit to add an expansion tank in Allegheny County?

The Allegheny County Health Department requires a permit for plumbing that is installed or altered. Ask the plumber pulling the permit or call the Health Department's plumbing office at 412-578-8036.

Water Heater and Pressure Problems Solved in the South Hills

All Pro Plumbing Heating & Cooling repairs and replaces tank and tankless water heaters from all major brands, including pressure relief valve problems, for homes across Bethel Park, the South Hills, and Allegheny and Washington counties. Registered Master Plumber John Oakes founded the company in 2006 as a home-based family business, and it now works from its own office on Library Road with 10 local employees. You get clear pricing and options before any work starts. See water heater repair and replacement and tank and tankless water heater service.

Visit All Pro Plumbing or call 412-595-8204 today. Contact us for more information.