Thermal Expansion Tank Sizing for Closed Hot Water Storage Systems

The requirement is triggered by a valve, not by storage volume

Section 607.3 of the International Plumbing Code applies where a storage water heater is supplied with cold water that passes through a check valve, pressure-reducing valve or backflow preventer. A thermal expansion control device must connect to the heater's cold water supply pipe downstream of all such devices. Thermal expansion tanks must be sized in accordance with the tank manufacturer's instructions so that water distribution pressure does not exceed the limit set by Section 604.8.

The North Carolina Office of the State Fire Marshal's 2019 interpretation states that the requirement applies to residential and commercial buildings alike. It applies when an existing heater is replaced on a line that already carries one of those devices, and does not apply to a tankless heater unless it serves a closed recirculation loop. Colorado's Division of Housing issued a 2020 memorandum requiring the control to appear on plan submittals and be verified in the field.

Nothing in the section refers to volume. A 300-gallon tank behind a dual check at the meter is governed by the same sentence as a 40-gallon heater. Volume changes the size of the control, not the obligation.

The pressure limit is the 80 psi figure that appears throughout state and local code. The Pacific Northwest National Laboratory's Building America Solution Center notes that most plumbing codes require a pressure-reducing valve where service pressure exceeds 80 psi. The valve itself creates a closed system in which thermal expansion raises pressure.

Bonney Lake, Washington cites Section 608.2 of the 2018 Uniform Plumbing Code for the same 80 psi threshold, the UPC equivalent of IPC 604.8. The Town of Bel Air, Maryland writes the limit into its plumbing amendments as 80 psi under no-flow conditions where mechanical devices provide thermal expansion relief.

The Palm Beach County Building Code Advisory Board, interpreting the Florida Plumbing Code's Section 607.3, names two accepted controls: an expansion tank at the heater or an auxiliary relief valve on the service set to 80 psi. It recommends the tank because the valves fail to reseat after discharging.

The water purveyor is usually the party that closes the system. The Washington State Department of Health explains that a purveyor may require a backflow preventer downstream of the meter. Once installed, a properly installed temperature and pressure relief valve and an expansion tank are both required. A contractor who inherits a cross-connection control order inherits a thermal expansion problem with it.

Why the relief valve is not the answer at any scale

Every storage heater and storage tank carries a temperature and pressure relief valve. The Washington DOH guide gives typical settings: the temperature element opens at approximately 210 °F, and the pressure element is normally preset at 125 psi or 150 psi. Those settings sit 45 to 70 psi above the 80 psi distribution limit.

A system that relies on the T&P valve to relieve expansion cycles its piping, fixtures and tank to 125 or 150 psi on every recovery. That is the mechanism behind the leaking fill valves and burst appliance hoses described in utility notices. The City of Kingsport, Tennessee states that the T&P valve is an emergency device not intended to handle the pressure increase from thermal expansion.

At commercial scale, a 700-gallon vessel relieving 11 gallons of 140 °F water on every recovery is also a discharge routing problem. Expansion control prevents that event; the relief valve prevents a rupture when everything else has failed.

Pressure thresholds a closed commercial hot water system has to respect

• Static inlet pressure, regulated side of PRV: 40 to 80 psi. Measured on site; sets expansion tank precharge.
• Maximum distribution pressure: 80 psi. IPC 604.8 and UPC 608.2; the ceiling the expansion tank must hold the system below.
• Auxiliary relief valve setting, where used in lieu of a tank: 80 psi. Palm Beach County BCAB; the alternative control where a tank is not used.
• T&P relief valve, pressure element: 125 or 150 psi. Washington DOH; emergency relief, not expansion control.
• T&P relief valve, temperature element: approximately 210 °F. Washington DOH; emergency relief.

How much 140 °F water expands, from published density

Expansion volume is a physical quantity independent of who manufactures the tank. The United States Geological Survey publishes water density by temperature, citing Bureau of Reclamation data: 62.408 lb per cubic foot at 50 °F, 62.366 lb per cubic foot at 60 °F, 61.386 lb per cubic foot at 140 °F and 61.006 lb per cubic foot at 160 °F. Mass is conserved on heating, so the fractional volume increase is the cold-to-hot density ratio minus one.

Fractional expansion, cold fill to storage temperature e = (rho_cold / rho_hot) - 1 e_140 = (62.366 / 61.386) - 1 = 0.01596 (1.60 percent, 60 °F to 140 °F) e_160 = (62.366 / 61.006) - 1 = 0.02229 (2.23 percent, 60 °F to 160 °F)

Using the 50 °F fill density instead raises each figure by about 0.07 percentage points, to 1.66 percent and 2.30 percent. The calculations below assume a 60 °F fill temperature. A project in a cold-inlet climate should substitute the measured winter inlet temperature.

Expansion volume V_exp = V_stored × e V_stored = 300 gal: V_exp = 300 gal × 0.01596 = 4.79 gal at 140 °F V_exp = 300 gal × 0.02229 = 6.69 gal at 160 °F V_stored = 700 gal: V_exp = 700 gal × 0.01596 = 11.2 gal at 140 °F V_exp = 700 gal × 0.02229 = 15.6 gal at 160 °F V_stored = 1,400 gal: V_exp = 1,400 gal × 0.01596 = 22.3 gal at 140 °F V_exp = 1,400 gal × 0.02229 = 31.2 gal at 160 °F V_stored = 2,000 gal: V_exp = 2,000 gal × 0.01596 = 31.9 gal at 140 °F V_exp = 2,000 gal × 0.02229 = 44.6 gal at 160 °F

The 1,400-gallon case is two 700-gallon modules piped as one volume; the 2,000-gallon case marks the upper edge of most heat pump hot water plants. These volumes are what the system must absorb. They are not the tank size.

The 140 °F case is the normal design point, not an upper bound. CDC's potable water systems module directs that hot water be stored above 140 °F and circulated at or above 120 °F because Legionella grows most readily between 77 °F and 113 °F. A plant storing at 140 °F for water safety runs at the 1.6 percent point every day; one storing at 160 °F to stretch usable capacity runs at 2.2 percent.

From expansion volume to tank size: precharge and the 80 psi ceiling

A diaphragm tank does not accept its full shell volume. Its air cushion is precharged to static inlet pressure so the tank holds no water at cold no-flow conditions. As water expands, it compresses the air until system pressure reaches the allowed maximum. The water accepted between those states is the acceptance volume, given by isothermal compression of the air charge.

Required tank volume: diaphragm type, isothermal air charge V_tank = V_exp / [1 - (P_pre + 14.7) / (P_max + 14.7)] P_pre = precharge = static inlet pressure, in psig. P_max = 80 psig distribution limit. Adding 14.7 psi converts gauge pressure to absolute pressure.

The manufacturer document used for the chart method covers 30 to 120 gallons of heater capacity and 140 to 180 °F, with a largest potable tank selection of 10 gallons. Its 86-gallon vessel is selected only for space heating. The document specifies a 38 psi factory charge for potable tanks and instructs the installer to raise the charge to match inlet pressure, but not above 80 psi.

For the following illustration, assume a measured static inlet pressure of 60 psi and an 80 psi distribution limit.

Acceptance factor at 60 psi inlet f = 1 - (P_pre + 14.7) / (P_max + 14.7) = 1 - (60 + 14.7) / (80 + 14.7) = 1 - 74.7 / 94.7 = 1 - 0.789 = 0.211 (dimensionless) Required tank volume, 140 °F storage V_tank = V_exp / f V_tank (300 gal) = 4.79 gal / 0.211 = 22.7 gal V_tank (700 gal) = 11.2 gal / 0.211 = 52.9 gal V_tank (1,400 gal) = 22.3 gal / 0.211 = 106 gal Required tank volume, 160 °F storage V_tank (700 gal) = 15.6 gal / 0.211 = 73.9 gal V_tank (1,400 gal) = 31.2 gal / 0.211 = 148 gal

At a 60 psi inlet, the tank must be 4.7 times the raw expansion volume. The multiplier is sensitive to inlet pressure: 3.2 at 50 psi and 9.5 at 70 psi. At 70 psi, the air cushion has only 10 psi of travel before the 80 psi ceiling. That sensitivity is why designers often set the PRV lower rather than buy the tank. Where the volume exceeds one vessel, tanks are installed in parallel and their acceptance volumes add.

Two caveats attach to every figure. The expansion volume should include the heater, recirculation loop and distribution piping between the backflow preventer and the fixtures, not the storage tank alone. The code requires sizing per the tank manufacturer's instructions, so the manufacturer's calculator governs final selection. The arithmetic shows what that selection must achieve and why a chart that stops at a 10-gallon tank does not reach it.

Expansion tank comparison: 140 °F storage, assumed 60 °F fill, 80 psi maximum Tank volumes below use acceptance factors of 0.317 at a 50 psi inlet, 0.211 at a 60 psi inlet and 0.106 at a 70 psi inlet.

• 300 gal stored: 4.79 gal expansion; 15.1 gal tank at 50 psi inlet; 22.7 gal tank at 60 psi inlet; 45.2 gal tank at 70 psi inlet.
• 700 gal stored: 11.2 gal expansion; 35.3 gal tank at 50 psi inlet; 52.9 gal tank at 60 psi inlet; 106 gal tank at 70 psi inlet.
• 1,400 gal stored: 22.3 gal expansion; 70.3 gal tank at 50 psi inlet; 106 gal tank at 60 psi inlet; 211 gal tank at 70 psi inlet.
• 2,000 gal stored: 31.9 gal expansion; 101 gal tank at 50 psi inlet; 151 gal tank at 60 psi inlet; 301 gal tank at 70 psi inlet.

Coil-type thermal storage splits the expansion problem in two

Everything above assumes the stored gallons are potable. In a coil-type thermal store they are not. The TEHQ Thermal Tank holds its stored volume as a closed hydronic charge. Potable water passes through a dual-coil heat exchanger of 32 mm DN corrugated stainless steel tubing, 40 to 80 metres long, with all connections through the top head plate. The tank is not the potable vessel, so the expansion problem lands in two places, each sized separately.

The estimated coil water content is roughly 8.5 to 17 gallons. This estimate uses π × (16 mm)² × 40 to 80 m, treating 32 mm DN as the inside diameter. Actual corrugated tube water content differs; the estimate is not a published coil water-content specification.

On the potable side, the expansion volume includes the coil content, any potable storage downstream of the coil, and the piping. For a 700-gallon module with an 80-metre coil, roughly 17 gallons, and an assumed 119-gallon potable finishing tank, the potable volume heated to 140 °F is roughly 136 gallons rather than 819. The expansion is about 2.2 gallons, and the acceptance required at an assumed 60 psi inlet is roughly 10 gallons.

The potable device is therefore sized on the order of a residential tank. The 607.3 requirement is met on the cold supply downstream of the backflow preventer exactly as for any storage heater. The 119-gallon finishing tank is an illustration; project sizing must use the actual downstream potable volume.

On the hydronic side, the 700-gallon charge still expands by 11.2 gallons at 140 °F and 15.6 gallons at 160 °F. A hydronic expansion tank must absorb that volume, sized on the charge volume under the mechanical code, not the plumbing code.

The International Residential Code's expansion tank table, as adopted in Seattle's residential code, sizes hydronic tanks on system volume at a basis of 12 psig fill and 30 psig maximum, with a 195 °F average temperature. Commercial mechanical codes require the calculation from the actual loop pressures. The Boyle's law expression is identical in form, with loop fill pressure as the precharge and the loop relief setting as the maximum.

TEHQ does not publish a Thermal Tank pressure rating, tank material, certification or operating temperature range; none is specified here. The 12 psig fill and 30 psig maximum used below are illustrative assumptions drawn from the IRC table basis, not Thermal Tank specifications. The engineer sets these pressures from the loop equipment.

Hydronic expansion tank: 700-gal charge at 160 °F, assumed 12 psig fill and 30 psig maximum f_hyd = 1 - (P_fill + 14.7) / (P_max + 14.7) = 1 - (12 + 14.7) / (30 + 14.7) = 1 - 0.597 = 0.403 (dimensionless) V_tank,hyd = V_exp / f_hyd = 15.6 gal / 0.403 = 38.7 gal

A coil-type store therefore carries two expansion tanks: a small potable tank on the cold supply and a hydronic tank on the charge loop. A direct storage tank of the same gallons carries one large potable tank.

For a two-module array, the hydronic tank is sized on 1,400 gallons of charge, or 31.2 gallons of expansion at 160 °F, while the potable tank changes only if potable storage downstream of the coils changes. How commercial hot water storage tanks are sized, specified and costed covers the direct-storage alternative.

Which side expands: direct potable storage versus coil-type thermal storage

• Stored volume, 700 gal: In direct potable storage, the volume is potable and counts in full toward the IPC 607.3 potable expansion calculation. In a coil-type TEHQ Thermal Tank, it is a closed hydronic charge, excluded from the potable calculation and sized under mechanical-code hydronic expansion rules.
• Potable water inside the heat exchanger: Not applicable to direct potable storage. In the coil-type store, 32 mm tubing, 40 to 80 m long, holds roughly 8.5 to 17 gal by the internal cross-section approximation. This volume counts toward the potable calculation.
• Finishing heater or potable buffer tank downstream of the coil: Potable; counts in full for both arrangements.
• Distribution and recirculation piping: Potable; counts for both arrangements.
• Governing pressure limit: Direct potable storage uses the 80 psi distribution limit under IPC 604.8. A coil-type store uses 80 psi on the potable side and the loop relief setting chosen by the engineer on the hydronic side.

What the split is worth to the project

• Smaller potable expansion hardware. In the illustrated 700-gallon coil-type store with an 80-metre coil, an assumed 119-gallon finishing tank and a 60 psi inlet, roughly 10 gallons of potable acceptance is required. A 700-gallon direct potable tank requires roughly 53 gallons at 140 °F or 74 gallons at 160 °F.
• Conventional potable code scope. The 607.3 device is a standard potable expansion tank on the cold supply, and the water management program treats the coil as a heat exchanger rather than a storage vessel.
• Hydronic design freedom. The engineer sets loop fill and relief pressures from the loop equipment, not from the 80 psi potable limit. The hydronic tank follows the method the trade already applies to boilers and buffer tanks. Heat pump buffer tank sizing for commercial systems covers the loop-volume side.
• Scalable arrays. A second 700-gallon module adds 11.2 gallons of hydronic expansion at 140 °F and no change to the potable device unless potable storage downstream of the coils changes. Designing thermal storage for existing buildings covers the sequence for adding modules.
• Storage capacity is unaffected. The 700-gallon module's published 108 kWh thermal rating is at a 35 °C (63 °F) delta-T. Expansion control changes no thermal figure. Thermal kWh is not electrical kWh; divide thermal kWh by the heat pump COP at the design lift to convert to electrical kWh.

Regulatory and market timing, as of the publish date

As of October 2026, IPC Section 607.3 and its Section 604.8 pressure limit remain in the 2021 and 2024 editions, and state adoptions range from the 2015 through 2024 editions. The UPC carries the parallel requirement in Chapter 6, with the 80 psi limit in Section 608.2 and expansion tank provisions in Section 608.3.

The adopted edition, any local amendment such as Bel Air's no-flow language, and the AHJ's interpretation govern. Where a purveyor's cross-connection program is the reason a backflow assembly is being added, obtain the thermal expansion requirement in writing before pricing the work.

No statement here is a code determination for any project. The engineer of record and the AHJ make that determination.

Eight checks before you specify expansion control

• Confirm whether a check valve, pressure-reducing valve or backflow preventer exists or is being added on the service. If so, 607.3 applies regardless of storage gallons.
• Measure static inlet pressure on the regulated side over 24 hours; the high reading is the precharge. If it exceeds 80 psi, a PRV comes first.
• Set the design storage temperature from the water safety requirement, not the heater setpoint. CDC guidance places storage above 140 °F; use 160 °F if the plant will run there.
• Total the potable volume heated: storage or finishing tank, heater, recirculation loop and distribution piping between the backflow preventer and the fixtures.
• For a coil-type store, separate the hydronic charge from the potable volume and size two tanks: the potable tank under the plumbing code at 80 psi and the hydronic tank under the mechanical code at loop pressures.
• Run the density and acceptance arithmetic, then confirm the selection against the expansion tank manufacturer's instructions, because the code requires it.
• Verify the T&P relief valve is present, rated for the vessel and piped to an approved discharge. Nothing in the design may rely on it to relieve expansion.
• Submit the expansion control on the plumbing plan set with the calculation attached, the documentation Colorado's memorandum and most plan reviewers expect. The commercial kitchen hot water storage article shows how a health department worksheet treats the same storage volume.