A 50-gallon tank of water takes up more room hot than cold. In an open system, the extra volume pushes back into the water main. In a closed system, it has nowhere to go, so the pressure in the house climbs every time the heater runs.
When the code requires one
Denver's 2025 building and fire codes took effect December 31, 2025, and are based on the 2024 International Codes. Section 607.3 of the 2024 International Plumbing Code and section P2903.4 of the 2024 International Residential Code both say that when a storage water heater's cold-water supply passes through a check valve, a pressure-reducing valve, or a backflow preventer, there has to be a thermal expansion control device downstream of it. In practice, that's usually an expansion tank on the cold-water line near the heater. Bradford White's installation manual says the same thing in plain terms: closed systems need "a properly sized and properly installed thermal expansion tank and/or other device(s)."
Other cities adopt their own code editions and amendments. If you're outside Denver, your city's building department can tell you which edition applies.
How to tell if your house has a closed system
- A pressure-reducing valve: a bell-shaped brass valve on the main water line, usually just after the main shutoff
- A check valve or backflow preventer on the main line inside the house
- An existing expansion tank: a small tank, about the size of a basketball, on the cold-water pipe above or near the heater
Signs you need one, or yours has failed
- The temperature and pressure relief valve drips, especially after the heater has been running (what that drip means)
- Faucets spit or pipes bang when you open them after a long heating cycle
- Toilet fill valves and faucet cartridges wear out faster than they should
- On an existing tank: the top half sounds solid instead of hollow when you tap it, or water comes out of the air valve on the end
Getting it right
An expansion tank has to be sized for the water heater and the house's water pressure, and its air charge has to be set to match the house pressure before it's installed; a tank that's undercharged or too small doesn't do much. It also needs support, because it's heavy when full. When we replace a water heater in a house with a closed system, the expansion tank is part of the job. Water heater repair and replacement →
Sources: Denver building codes · 2024 International Plumbing Code, chapter 6 · 2024 International Residential Code, chapter 29 · Bradford White installation manual. Checked October 22, 2026.