Sources and simplifications
Every reference cited on this site, and every place a figure idealizes the real system.
References§
- (2016). University Physics Volume 2, §1.3 Thermal Expansion (Table 1.2, thermal expansion coefficients). OpenStax, Rice University. textbook ΔV = βVΔT; steel α = 12 × 10⁻⁶ /°C, β = 35 × 10⁻⁶ /°C; water β = 210 × 10⁻⁶ /°C (near room temperature); water is densest at 4 °C. (How the tank works)
- (2025). NIST Chemistry WebBook, SRD 69: Thermophysical Properties of Fluid Systems (water, isobaric data at 0.101325 and 1.101325 MPa, 1–99 °C). National Institute of Standards and Technology. reference data Density of water from the IAPWS-95 formulation, retrieved 2026-11-10 in 1 °C steps: 999.975 kg/m³ at 4 °C, 998.207 at 20 °C, 983.196 at 60 °C. The same query at 1.101325 MPa gives the compressibility used here (about 3.0–3.3 × 10⁻⁶ per psi between 10 and 60 °C). (How the tank works)
- (2019). Sizing the Therm-X-Trol: For Pressurization and Expansion Control of Hot Water Systems, MC 10007, 9017-112 (03/19). Amtrol, West Warwick, RI. manufacturer sizing guide Check valves, backflow preventers and PRVs make a system that needs expansion control; tank volume = design pressure factor × expanded water; Table 1 expansion factors from 40 °F (120 °F: .0100, 140 °F: .0150, 150 °F: .0179, 180 °F: .0276); Table 2 DPF and the equation DPF = (max + 14.7)/(max − line); worked example 240 gal, 150 °F, 60 psi line, 125 psi max → 4.3 gal × 2.1 = 9.0 gal; precharge equal to static line pressure; a tank left at 40 psi on a 70 psi supply loses room before heating; tank on the cold supply, not the hot outlet. (How the tank works)
- (2026). Expansion Tank Sizing Calculator (Potable). Watts, North Andover, MA. manufacturer sizing tool Inputs: heater gallons, initial (building cold water) pressure, which typically does not exceed 80 psi per the UPC; air precharge should match initial pressure; final pressure should never exceed 85% of the T&P relief valve rating; install a PRV where system pressure exceeds 80 psi; tank on the cold supply upstream of the heater; PLT-5 2.1 gal, PLT-12 4.5 gal; 150 psi maximum; a general guide, not a guarantee. (How the tank works)
- (2014). Series 1L, 1XL, 10L and 100XL Temperature and Pressure Relief Valves, specification sheet ES-10L/100XL. Watts, North Andover, MA. manufacturer data sheet Temperature relief 210 °F; standard pressure settings 75, 100, 125, 150 psi; ANSI Z21.22 certified; maximum system operating pressure must not exceed 75% of valve set pressure; lever operated at least once a year; if no water flows the valve is inoperative: turn off the heater and call a plumber; 'designed for emergency safety relief and shall not be used as an operating control'. (How the tank works)
- (2018). Water Heater Thermal Expansion Tanks, Owner's Manual, Models 2–5 Gallon Capacity, 100306555 200055907 Rev. A. A. O. Smith Corporation (copy hosted at aosmithatlowes.com). manufacturer manual Backflow preventers seal off the system, so an expansion tank is required or the relief valve will discharge on every heating cycle; the tank stores expanded water and returns it when the temperature drops; 150 psi, 200 °F; above 80 psi incoming a PRV is required, 60 psi or less recommended; air charge matched to incoming pressure, read with the system cold; a weeping T&P may be a worn valve, inlet pressure above 60 psi or an air charge not matched to incoming pressure; 'Do not plug the Temperature-Pressure Relief Valve opening.' (How the tank works)
- (2022). CodeNotes: Water Heater Safety in the I-Codes (based on IPC 2021 §504.6 and IRC §P2804.6.1). International Code Council. standards body guidance Any discharge from the relief valve indicates a distribution-system problem, a water heater problem or a malfunctioning valve; discharge piping must have no threaded end (so it cannot be capped) and no valves or tees; improper discharge piping can create a dangerous situation that could result in explosion. (How the tank works)
- (2024). Therm-X-Trol Thermal Expansion Absorbers, Installation & Operation Instructions, Models ST-5C(DD) through ST-210VC, 9017-091 (04/24). Amtrol, West Warwick, RI. manufacturer manual Must be installed by a licensed professional; precharge set to match cold water supply pressure with the tank empty of water; installed between the water heater and the backflow preventer, check valve or PRV; the air charge keeps water out until system pressure exceeds the precharge, absorbs the expanded water and keeps pressure below the relief valve setting; a licensed contractor should inspect the T&P valve at least every 3 years or if discharge is present; maximum 200 °F. (How the tank works)
- (2019). Chemistry 2e, §9.2 Relating Pressure, Volume, Amount, and Temperature: The Ideal Gas Law. OpenStax, Rice University. textbook Boyle's law: at constant temperature and amount, the volume of a gas is inversely proportional to its pressure (PV = constant); ideal-gas assumptions. (How the tank works)
- (2008). Guide for the Use of the International System of Units (SI), NIST SP 811, Appendix B.8 Factors for units listed alphabetically. National Institute of Standards and Technology. standard Standard atmosphere = 1.01325 × 10⁵ Pa; 1 psi = 6.894757 × 10³ Pa; so 1 atm = 14.696 psi. (How the tank works)
- (2025). Therm-X-Trol Water Heater Expansion Tanks brochure, MC4090 (09/25). Amtrol, West Warwick, RI. manufacturer data sheet ST-5 2.0 gal and ST-12 4.4 gal in-line models, 150 psig maximum working pressure, 200 °F, 50 psig factory precharge; vertical models 40 psig factory precharge; quick sizing charts based on 40 °F incoming water, 150 psi T&P, precharge equal to static supply pressure (e.g. 50 gal heater, 140 °F, 40–80 psi: ST-5). (How the tank works)
- (2026). No. LF127W Pressure Reducing Valve, installation and maintenance instructions (IS-127W). Watts, North Andover, MA. manufacturer manual 'When a reducing valve is used, it makes a closed system; therefore, a pressure relief valve must be installed in the downstream side of the regulator to protect equipment against excess pressure which is built up by the expansion of water from the domestic hot water supply tank or heater.' (Why Your Water Heater Needs an Expansion Tank)
- (2013). Model PRH (36H and Lead Free 36H Series) High Capacity Water Pressure Reducing Valve, installation, operation and maintenance, Rev. M. Apollo Valves, Matthews, NC (copy hosted by Cooney Brothers). manufacturer manual Factory set 50 psig static, adjustable 25–75; at no flow the diaphragm force from downstream pressure exceeds the spring and the valve is closed; under flow the downstream pressure falls below set and the spring opens the valve; fall-off chart (e.g. ¾" unit 23 gpm at 10 psi fall-off from 100 psi inlet, 50 psi set); troubleshooting: outlet pressure equalizes with inlet at no flow (seat disc not sealing, cut O-ring, damaged diaphragm, by-pass leaking); outlet pressure rises above inlet (by-pass not opening, thermal expansion downstream); excessive noise (high water velocity). (Why Your Water Heater Needs an Expansion Tank)
- (2026). LF15M2 Lead Free Water Hammer Arrestors, product page. Watts, North Andover, MA. manufacturer product page Added by the verifier for the banging/water hammer claim: 'The noise from banging pipes is caused by shocks of water flowing at high speed in the piping system' when a fixture is suddenly closed; 'Sudden stoppage of water (a non-compressible liquid) flowing at a given pressure and velocity causes a surge'; a pressure wave travels back through the piping until it finds a point of relief. Retrieved 2026-11-10. (Why Your Water Heater Needs an Expansion Tank)