Thermal Expansion: Why a Closed System Needs an Expansion Tank

How much bigger the water gets

Water is densest just above freezing and becomes less dense as it warms. The U.S. Geological Survey publishes the figures: 0.99802 grams per cubic centimetre at 70°F, 0.98870 at 120°F, 0.98338 at 140°F [4]. The same weight of water therefore needs about 0.9 percent more room at 120°F than at 70°F, and about 1.5 percent more at 140°F.

On a 50-gallon tank that is roughly 0.47 gallons heated to 120°F, and about 0.74 gallons heated to 140°F. That is our own arithmetic on the USGS table, not a measured figure for any particular heater — but it is the order of magnitude that matters. Liquid water barely compresses, so that half-gallon has to go somewhere every time the tank heats up.

Open and closed systems

Rheem's use and care manual for its residential electric water heaters draws the line clearly. If nothing on the cold supply stops water flowing backwards, the system is open: the expanding water is pushed back towards the city main, where the pressure dissipates [1].

If there is a check valve on the cold supply, the system is closed. The manual lists where that check valve may be hiding: a separate backflow preventer, or built into a pressure-reducing valve, the water meter or a water softener [1]. In a closed system the expanding water cannot leave, and the manual describes the result as a rapid and dangerous pressure rise in the heater and the piping, which can reach the relief valve's setting and open it on every heating cycle [1].

It also says what that does over time: the repeated expansion and contraction can wear out the relief valve early, and possibly the heater itself — and replacing the relief valve will not correct the problem [1].

Why the relief valve is not the answer

Every storage water heater carries a temperature and pressure relief valve. It is a safety device for when something has gone wrong. The Florida Building Code, Plumbing, is explicit: the relief valve shall not be used as a means of controlling thermal expansion [2]. Its pressure setting can be as high as the heater's rated working pressure or 150 psi, whichever is less [2] — far above the 80 psi the same code allows in the house piping [2].

So a relief valve that drips a little after every shower is not a valve doing its job. It is a sign that the system is closed and nothing is absorbing the expansion. Rheem's troubleshooting table puts a relief valve that pops or drains down to exactly this cause, calls it an unacceptable condition that must be corrected, and says never to plug the relief valve outlet [1].

What an expansion tank does

A thermal expansion tank is a small steel vessel split in two by a flexible diaphragm. One side holds a charge of air; the other side is open to the house water. When the heater warms the water and pressure starts to rise, the air cushion compresses and the extra water moves into the tank, instead of forcing the relief valve open [1]. When hot water is drawn, the air pushes it back out.

The Florida code puts it on the water heater's cold water supply pipe, downstream of every check valve, pressure-reducing valve and backflow preventer, and requires it to be sized to the manufacturer's instructions so that the pressure in the house stays within the 80 psi limit [2]. Rheem's drawing shows the same place: in the cold line between the heater and the check valve [1].

The air charge has to match the house

A tank only works if its air side is charged to the right pressure. The installation sheet for the Eastman thermal expansion tank — the tank in the photograph above — gives the sequence: find the system pressure first, by opening a faucet, closing it and reading a pressure gauge, then set the tank's air charge to that same pressure [3].

The same sheet says the air charge must only be adjusted when the tank is at zero system pressure, with a tire gauge and a hand pump, and never above 80 psi [3]. It also says plainly that any expansion tank may leak over time, and not to install one without a way for that water to drain [3].

• Measure the house pressure first
• Set the tank's air charge to match it, with the system at zero pressure
• Never above 80 psi of air
• Mount it with the piping braced to carry its weight when full [3]

Signs of a closed system with no working tank

The relief valve drips or discharges now and then, often after hot water has been used and the heater refills and reheats. Rheem's manual names thermal expansion in a closed system as the likely cause [1].

A popping noise from the relief valve — listed by Rheem with the same cause [1].

An expansion tank is there, but the problem is back. In our experience the usual reason is a tank that has lost its air charge or whose diaphragm has failed, so it is full of water and absorbs nothing. That is trade experience rather than a figure from a source; the check is the one in the installation sheet — isolate and drain down, then read the air charge with a gauge [3].

What this means in Tampa Bay

The Florida Building Code applies statewide, so the rule is the same in every city we work in: a storage water heater fed through a check valve, a pressure-reducing valve or a backflow preventer needs thermal expansion control on its cold supply [2]. Whether your supply has one of those is not always obvious — Rheem's list includes the water meter and the water softener [1], and softeners are common on this coast. Your utility can tell you what is on its side of the meter [1].

When we replace a tank heater, we look for a closed system before we decide on the expansion tank, and we set the tank's air charge to the pressure we measure at the house, not to the number it left the factory with [3]. The relief line goes to a pan or another place the code allows, where you can see it if it ever discharges [2].

FAQ

Sources

We review and cite. We do not reproduce a paper or a manual — follow the links for the originals.

1. [1]Water Heaters, Electric Residential — Use & Care Manual (AP23124 Rev 01) — Rheem Manufacturing CompanyManufacturer's manual · Thermal Expansion section: open vs closed system; check valve may be a backflow preventer or part of a PRV, water meter or softener; rapid pressure rise can reach the relief valve setting each heating cycle; premature failure of the relief valve and possibly the heater; replacing the relief valve will not correct it; expansion tank in the cold line between heater and check valve, air cushion compresses; troubleshooting: relief valve popping or draining = thermal expansion in a closed system, do not plug the outlet
2. [2]2023 Florida Building Code, Plumbing, 8th Edition — Chapter 5 (Water Heaters) and Chapter 6 (Water Supply and Distribution) — Florida Building Commission / International Code Council · 2023Law / code · §607.3 thermal expansion control (storage heater fed through check valve, PRV or backflow preventer; device on the cold supply downstream of all of them; sized so pressure stays within §604.8); §604.8 PRV where static pressure exceeds 80 psi; §504.4 relief valve shall not be used to control thermal expansion; §504.5 setting not above rated working pressure or 150 psi; §504.6 discharge piping (chapter 5: https://codes.iccsafe.org/content/FLPC2023P1/chapter-5-water-heaters)
3. [3]Thermal Expansion Tank — Installation Instructions — Eastman (instruction sheet, copy hosted by The Home Depot)Manufacturer's manual · determine system pressure with a gauge; pre-charge tank to the same pressure; adjust only at zero system pressure; do not exceed 80 psi air charge; piping must be braced to carry the tank; all expansion tanks may leak over time — provide drainage; tank rated 150 psi / 200°F
4. [4]Water Density — U.S. Geological Survey, Water Science SchoolGovernment or standards body · density table: 70°F 0.99802, 120°F 0.98870, 140°F 0.98338 g/cm³ (read on the live page 2026-09-29)