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Does a new water heater need an expansion tank?

Not in every home: it depends on the plumbing code your building department adopted. The 2024 Uniform Plumbing Code calls for an expansion tank or a device with a similar function on a water system with a check valve or backflow preventer; the 2021 International Residential Code calls for a device for controlling pressure in two listed cases.

Source: IAPMO, 2024 Uniform Plumbing Code (online read-only edition), Sections 608.2 and 608.3 and the sample adoption page, read 22 October 2026.
Source: International Code Council, 2021 International Residential Code, Chapter 29 Water Supply and Distribution (P2903), read 22 October 2026 (our saved browser copy, as read on 2026-10-22).
Source: International Code Council, 2021 International Residential Code, Chapter 1 Scope and Administration, read 22 October 2026 (our saved browser copy, as read on 2026-10-22).

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By InstaBids editorial team with AI research agents. Sources read: 22 October 2026 Updated 2026-10-22

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Key facts

What does the Uniform Plumbing Code say about expansion tanks?

It names the expansion tank outright, in two places: Section 608.3 of the 2024 Uniform Plumbing Code, published by IAPMO, ties it to a check valve or backflow preventer, with an exception for an instantaneous non-storage (tankless) water heater, and Section 608.2 ties it to a pressure regulator. Both are quoted below in their own words.

Source: IAPMO, 2024 Uniform Plumbing Code (online read-only edition), Sections 608.2 and 608.3 and the sample adoption page, read 22 October 2026.

Section 608.3 of the 2024 Uniform Plumbing Code, read 22 October 2026, covers a water system provided "with a check valve, backflow preventer, or other normally closed device that prevents dissipation of building pressure back into the water main, independent of the type of water heater used, shall be provided with an approved, listed, and adequately sized expansion tank or other approved device having a similar function to control thermal expansion."

Source: IAPMO, 2024 Uniform Plumbing Code (online read-only edition), Sections 608.2 and 608.3 and the sample adoption page, read 22 October 2026.

On where it goes, the same section says: "Such expansion tank or other approved device shall be installed on the building side of the check valve, backflow preventer, or other device and shall be sized, securely fastened to the structure, and installed in accordance with the manufacturer’s installation instructions."

Source: IAPMO, 2024 Uniform Plumbing Code (online read-only edition), Sections 608.2 and 608.3 and the sample adoption page, read 22 October 2026.

Section 608.3 then gives one exception: "Exception: An expansion tank shall not be required for an instantaneous non-storage water heater."

Source: IAPMO, 2024 Uniform Plumbing Code (online read-only edition), Sections 608.2 and 608.3 and the sample adoption page, read 22 October 2026.

Section 608.2 opens on supply pressure: "Where static water pressure in the water supply piping exceeds 80 psi (552 kPa), an approved-type pressure regulator preceded by an adequate strainer shall be installed and the static pressure reduced to 80 psi (552 kPa) or less." It then says: "An approved expansion tank shall be installed in the cold water distribution piping downstream of each such regulator to prevent pressure exceeding 80 psi from developing due to thermal expansion."

Source: IAPMO, 2024 Uniform Plumbing Code (online read-only edition), Sections 608.2 and 608.3 and the sample adoption page, read 22 October 2026.

In our reading, the 608.3 duty starts with the water system (a check valve, backflow preventer or other normally closed device), but the kind of heater still matters: its exception says an expansion tank is not required for an instantaneous non-storage water heater. Section 608.2 also has a design exception that this page does not quote, so read the full section at the source if a different device is offered.

Source: IAPMO, 2024 Uniform Plumbing Code (online read-only edition), Sections 608.2 and 608.3 and the sample adoption page, read 22 October 2026.

What does the International Residential Code require for thermal expansion?

A device for controlling pressure, in the two cases its Section P2903.4 lists: the 2021 International Residential Code, published by the International Code Council, is quoted below, and the sections quoted here do not use the words expansion tank.

Source: International Code Council, 2021 International Residential Code, Chapter 29 Water Supply and Distribution (P2903), read 22 October 2026 (our saved browser copy, as read on 2026-10-22).

Section P2903.4 of Chapter 29 of the 2021 International Residential Code, read 22 October 2026, says: "A means for controlling increased pressure caused by thermal expansion shall be installed where required in accordance with Sections P2903.4.1 and P2903.4.2."

Source: International Code Council, 2021 International Residential Code, Chapter 29 Water Supply and Distribution (P2903), read 22 October 2026 (our saved browser copy, as read on 2026-10-22).

The first case, Section P2903.4.1, reads: "For water service system sizes up to and including 2 inches (51 mm), a device for controlling pressure shall be installed where, because of thermal expansion, the pressure on the downstream side of a pressure-reducing valve exceeds the pressure-reducing valve setting."

Source: International Code Council, 2021 International Residential Code, Chapter 29 Water Supply and Distribution (P2903), read 22 October 2026 (our saved browser copy, as read on 2026-10-22).

The second case, Section P2903.4.2, reads: "Where a backflow prevention device, check valve or other device is installed on a water supply system using storage water heating equipment such that thermal expansion causes an increase in pressure, a device for controlling pressure shall be installed."

Source: International Code Council, 2021 International Residential Code, Chapter 29 Water Supply and Distribution (P2903), read 22 October 2026 (our saved browser copy, as read on 2026-10-22).

The same chapter sets a pressure ceiling in Section P2903.3.2: "The static water pressure shall be not greater than 80 psi (551 kPa)."

Source: International Code Council, 2021 International Residential Code, Chapter 29 Water Supply and Distribution (P2903), read 22 October 2026 (our saved browser copy, as read on 2026-10-22).

In our reading, the IRC leaves the choice of device open, while the UPC names a tank first; both texts tie the duty to what is installed on the water supply.

Is the relief valve on the tank a separate rule?

Yes: the IRC puts relief valves in their own section of its water heater chapter, Section P2804, and the parts that matter here are quoted below.

Source: International Code Council, 2021 International Residential Code, Chapter 28 Water Heaters (P2804), read 22 October 2026 (our saved browser copy, as read on 2026-10-22).

Section P2804.1 of Chapter 28 of the 2021 International Residential Code, read 22 October 2026, says: "Appliances and equipment used for heating water or storing hot water shall be protected by one of the following:" Its two choices are "A separate pressure-relief valve and a separate temperature-relief valve." and "A combination pressure-and-temperature relief valve."

Source: International Code Council, 2021 International Residential Code, Chapter 28 Water Heaters (P2804), read 22 October 2026 (our saved browser copy, as read on 2026-10-22).

On the setting, Section P2804.3 says: "They shall be set to open at not less than 25 psi (172 kPa) above the system pressure and not greater than 150 psi (1034 kPa)."

Source: International Code Council, 2021 International Residential Code, Chapter 28 Water Heaters (P2804), read 22 October 2026 (our saved browser copy, as read on 2026-10-22).

Section P2804.6 adds: "A check or shutoff valve shall not be installed in any of the following locations:" and one of its listed places is "Between a relief valve and a tank."

Source: International Code Council, 2021 International Residential Code, Chapter 28 Water Heaters (P2804), read 22 October 2026 (our saved browser copy, as read on 2026-10-22).

In our reading, a relief valve and a thermal expansion device answer separate sections, so a bid that lists one does not tell you whether it covers the other.

Which plumbing code covers your water heater replacement?

The one your state or city adopted, if any: both of these are model codes written to be adopted, as their own words quoted below say.

Source: IAPMO, 2024 Uniform Plumbing Code (online read-only edition), Sections 608.2 and 608.3 and the sample adoption page, read 22 October 2026.
Source: International Code Council, 2021 International Residential Code, Chapter 1 Scope and Administration, read 22 October 2026 (our saved browser copy, as read on 2026-10-22).

The 2024 Uniform Plumbing Code's sample adoption page says: "The Uniform Codes are designed to be adopted by jurisdictions through an ordinance."

Source: IAPMO, 2024 Uniform Plumbing Code (online read-only edition), Sections 608.2 and 608.3 and the sample adoption page, read 22 October 2026.

Chapter 1 of the 2021 International Residential Code, read 22 October 2026, says in its user note: "The one- and two-family dwelling code is intended to be adopted as a legally enforceable document, and it cannot be effective without adequate provisions for its administration and enforcement."

Source: International Code Council, 2021 International Residential Code, Chapter 1 Scope and Administration, read 22 October 2026 (our saved browser copy, as read on 2026-10-22).

Its permit section, R105.1, lists plumbing work among the jobs for which a person "shall first make application to the building official and obtain the required permit." The full sentence, with every kind of work it covers, is at the source.

Source: International Code Council, 2021 International Residential Code, Chapter 1 Scope and Administration, read 22 October 2026 (our saved browser copy, as read on 2026-10-22).

In our reading, the building department that issues the permit for your replacement is the place to ask which code and edition it enforces, since that answer decides which of the texts above applies to your home.

What should you ask before your water heater is replaced?

Ask each bidder to put the pressure question in writing, so you can compare bids that cover the same work. This is practical reasoning, not legal advice and not a finding from any agency.

  1. Ask the building department, or the bidder, which plumbing code and edition applies to your home.
  2. Ask whether your water service has a check valve, backflow preventer or pressure regulator. The IRC case in P2903.4.2 starts: "Where a backflow prevention device, check valve or other device is installed on a water supply system using storage water heating equipment such that thermal expansion causes an increase in pressure, a device for controlling pressure shall be installed."

    Source: International Code Council, 2021 International Residential Code, Chapter 29 Water Supply and Distribution (P2903), read 22 October 2026 (our saved browser copy, as read on 2026-10-22).

  3. Ask what static pressure the bidder measured at your home, and compare it with the code line: "The static water pressure shall be not greater than 80 psi (551 kPa)."

    Source: International Code Council, 2021 International Residential Code, Chapter 29 Water Supply and Distribution (P2903), read 22 October 2026 (our saved browser copy, as read on 2026-10-22).

  4. Ask each bid to name the thermal expansion device it includes, or to say in writing why none is needed for your system.
  5. Ask whether the bid includes the permit. The IRC's R105.1 says the applicant "shall first make application to the building official and obtain the required permit."

    Source: International Code Council, 2021 International Residential Code, Chapter 1 Scope and Administration, read 22 October 2026 (our saved browser copy, as read on 2026-10-22).

  6. Keep the bids, the permit and any inspection record with the water heater's own papers.

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Contact details stay hidden until the homeowner picks a contractor and they confirm the connection.

Source: InstaBids process and privacy, live-page excerpts, read 22 October 2026.

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