Heated water grows by about 1 %, and water barely compresses, so it has to go somewhere. Here is a water heater and its supply, simulated and running. Drag the hot tap, or pick your symptom.
A water heater’s supply in section, simulated (not a recording; states are sized to make each pattern visible). Every simulated hour a 16-minute shower draws hot water, then the burner reheats the heater. Drag the tap’s red-dotted handle to draw water yourself. The trace shows pressure (solid) and water temperature (dotted); amber dashes mark the PRV setting, orange the 150 psi T&P relief setting.
A water heater warms about 50 gallons of water, and warm water takes up more room: heated from 50 °F to 120 °F it grows by a little over 1 %, more than half a gallon [1]. Water barely compresses, so that half gallon has to go somewhere. In an open system it pushes back into the street main and nobody notices. Once a pressure-reducing valve (PRV), check valve or backflow preventer closes the supply, it can only squeeze an expansion tank’s air cushion or leave through the T&P relief valve[2][3]. A dripping relief valve, a gauge that climbs after the heater runs, and a tank that “stopped working” are all the same physics.
Figure 1 The same simulated supply, pinned while you read. Each step switches it to one state; drag the tap or change anything to take over, and use Return to the story to hand back. Simulated, sized to make each pattern visible; not a recording.#
1. An open system: expansion goes back to the main§
On the gauge: 60 psi, flat, whether the shower runs or the burner reheats. At the relief valve: nothing.
Why: with no PRV, check valve or backflow preventer on the supply, the house is connected to the street main both ways. As the heater warms after the shower, the water it pushes out (about a quart per cycle here) flows back up the cold pipe into the main.
2. Closed, with no expansion tank: the relief valve drips§
On the gauge: once the shower stops, the pressure leaps from 60 to 150 psi within a minute or two, sits there while the burner reheats the water, and falls back the moment hot water runs. At the relief valve: a drip or trickle from the T&P drain line through every recovery, about a quart here.
Why: the PRV lets water in but not back out, so the expansion has nowhere to go [4]. In a rigid system with no air, heating water near 110 °F by one degree raises the pressure by about 74 psi [1]; real tanks and pipes stretch somewhat, which slows that, but the rise still takes degrees, not tens of degrees. The T&P valve opens at its 150 psi setting [5] and discharges the expansion that went back to the main in step 1.
Also: a T&P valve that drips can also be worn or scaled, or the house pressure can be too high [3]. The valve is an emergency device, not a pressure control [5]: a valve that relieves every cycle is a sign of a system problem, and the drain line must never be capped or plugged [6].
On the gauge: 60 psi while water runs; a gentle climb to about 72 psi while the heater reheats; back down when the next shower starts. At the relief valve: nothing.
Why: the expanding water now squeezes a cushion of air. Air obeys Boyle’s law [7]: pushing 0.28 gallons into this 2-gallon tank shrinks the air from 2 to 1.72 gallons, and its absolute pressure rises in the same ratio, from 74.7 to about 87 psia. The air charge is set to the house pressure, with the tank empty of water, so all of the tank’s air is still there when heating starts [8]. A rise like this while the heater recovers is normal for a closed system with a tank; how far it goes depends on the tank’s size and air charge and on how much water was reheated. Reheating a whole tankful from cold pushes in more water, and a tank sized to the makers’ rules may then let the pressure climb several times further, while staying well below the relief setting [5].
On the gauge: the same leap to 150 psi as step 2. At the tank: the cutaway is nearly all water. At the relief valve: a drip through every recovery.
Why: the tank has lost its air, through a ruptured diaphragm or a slow leak at the air valve; Amtrol warns that an unsealed valve cap loses the precharge and the tank fails [8]. With almost no air, Boyle’s law has nothing to squeeze, and the system behaves as if there were no tank.
Also: a tank that is too small for the heater looks the same once its air is used up. A tank that has lost air may be corroded; tank makers say not to re-pressurize a corroded or damaged tank and leave service to licensed professionals [8].
On the gauge: a climb to about 83 psi instead of 72 on each recovery. Not dramatic, but the tank is doing less than it could, and with a larger heater or a hotter setting it would not be enough.
Why: this tank was charged to 30 psi but the house is at 60. Before any heating, the house pressure pushes water into the tank until its air reaches 60 psi, using up 0.8 of its 2 gallons. Amtrol shows the same thing with a 40 psi tank on a 70 psi supply: the incoming water “takes up valuable space before the water has even begun to heat”, which is why the precharge is matched to the incoming pressure [2]. A precharge above the house pressure has the opposite fault: the tank takes nothing until the pressure reaches it.
Also: factory precharges are often 40 or 50 psi [9], and A. O. Smith notes that a mismatched air charge is one reason a T&P valve weeps [3]. The precharge is set with the tank empty of water and the system cold; makers leave that to a licensed installer [8]. A new tank can also see high pressure for other reasons: it may be too small for the heater and its temperature setting (the estimator checks this), it may be losing air (step 4), or a failing PRV may be letting the house pressure creep up before the heating even starts (step 6).
6. A failing PRV: pressure creeps up between uses§
On the gauge: 60 psi while water runs, then, with every tap shut, a slow climb toward the street’s 110 psi, with the heater’s expansion on top: about 115 psi here. Typically noticed as high pressure first thing in the morning.
Why: a PRV’s seat should seal when no water flows. If the seat disc, an O-ring or the diaphragm is damaged, street pressure leaks past until the outlet matches the inlet; Apollo’s troubleshooting lists exactly this, “inlet pressure equalizes outlet pressure at no flow” [10].
Also, and this is the common mix-up: pressure above the PRV setting after the heater runs is what thermal expansion does in any closed system (steps 2, 4 and 5). Apollo lists “thermal expansion downstream” as a cause of outlet pressure rising above inlet [10]. Check the expansion tank before blaming the PRV, and let a licensed plumber confirm which it is.
The tour simulates one mechanism for each symptom. Real faults overlap, so here is the longer list. It is a guide to describing the problem, not a diagnosis; a licensed plumber confirms the cause before parts are replaced.
The T&P valve drips or runs, especially while the heater reheats. Thermal expansion in a closed system with no tank, a waterlogged tank or one with the wrong precharge; house pressure above the recommended 60 psi; or a worn or scaled valve [3]. Any discharge indicates a problem in the system, the heater or the valve [6]. A valve that keeps running after its test lever is released, or that lets no water out when the lever is lifted, needs attention at once: makers say to shut off the heater’s cold supply or turn the heater off, and call a plumber [3][5].
The gauge climbs toward the relief setting after hot water use. Thermal expansion with a missing, waterlogged, undersized or mis-charged tank. Some rise is normal with a working tank, and how much depends on the tank’s size and air charge and on how much water was reheated: about 12 psi in step 3’s partial reheat, but tanks are sized so that a full heat-up from cold may climb much further, as long as it stays well below the relief setting; Watts asks that operating pressure stay below 75 % of it, about 112 psi for a 150 psi valve [5]. The estimator shows the peak for your heater and tank.
High pressure in the morning or after a quiet night. A PRV whose seat leaks (step 6), or thermal expansion if the heater reheated overnight with nowhere for the water to go.
Banging or knocking. The pressure rise from thermal expansion makes no sound by itself: it climbs over minutes as the heater reheats (step 3). A bang when a tap or appliance valve shuts suddenly is more likely water hammer: water moving at speed is stopped at once, and the surge travels back through the pipes [11]. Other noises have other causes; PRV makers, for example, list high water velocity as a cause of a whistle or hum at the valve [10]. If the gauge also reads high, mention both to the plumber.
Low pressure at every tap after a PRV. A PRV set low, a clogged inlet strainer, or a damaged diaphragm. A PRV also lets its outlet fall below the setting when water flows, which is how it opens: Apollo’s chart shows a ¾-inch valve 10 psi below its setting at 23 gpm [10]. Some fall-off at a big flow is normal; a large drop at one tap is not.
The T&P relief valve is a life-safety device. It opens if the pressure or the water temperature (210 °F) gets dangerously high [5]. Never cap, plug or valve off it or its drain line: codes require the drain to have no threaded end and no valve precisely so that nobody can, because a blocked relief valve can let a heater explode [6][3]. If yours drips, find out why. Gas water heaters, expansion tanks and PRVs are licensed-professional work in most places: Amtrol requires its tanks to be installed by a licensed professional [8]. Codes and licensing vary by state and city. This page explains how the system works so you can describe the problem accurately; it is not a repair procedure.
An expansion tank is a small steel tank, usually 2 to 5 gallons for a house, with a rubber diaphragm or bladder inside. On one side is a sealed charge of air; the other side is open to the cold water pipe feeding the heater. The air charge keeps water out until the system pressure exceeds it. As the heater warms, the expanded water enters the tank and squeezes the air; when hot water is used, the air pushes that water back out [8]. It belongs on the cold supply between the heater and the PRV or check valve, so that the water it stores is cold and is returned to the heater, not sent cooled to a tap [2].
It is the same device as a well pressure tank, doing a different job: there the air pushes stored water to the taps between pump starts; here it gives the heater’s expansion somewhere to go. The physics is identical, and so is the drawing: the sizing estimator uses the same Boyle’s-law engine as the well pressure tank page.
Why is it needed at all? Many local codes require backflow preventers, and a backflow preventer, check valve or PRV makes the house a closed system; A. O. Smith puts it plainly: an expansion tank is then required “or the relief valve will discharge on every heating cycle” [3]. Amtrol’s brochure says expansion control is required by most building codes and all water heater warranties [9].
The T&P relief valve: an emergency valve, not a control§
The combination temperature-and-pressure relief valve on the heater opens at a set pressure, commonly 150 psi on residential heaters [9], or when the water at the top of the tank reaches 210 °F [5]. Watts states that it is “designed for emergency safety relief and shall not be used as an operating control”, and that normal system pressure should stay below 75 % of its setting, about 112 psi for a 150 psi valve [5]. A system that sends its thermal expansion out through the T&P every day is using it as an operating control: it wastes water, and the valve itself can wear or clog with mineral deposits [3].
The International Code Council’s guidance is that any discharge from the relief valve indicates a problem with the distribution system, the heater or the valve, and that the drain line must end without threads and without any valve so that it cannot be capped [6]. Watts asks that the valve be operated once a year and inspected every three years by a licensed contractor [5].
The pressure-reducing valve: a spring against a diaphragm§
Many city mains run well above what household fixtures want. A PRV holds the house below the street: A. O. Smith calls for one where incoming pressure exceeds 80 psi and recommends 60 psi or less [3], and Watts notes that building pressure typically does not exceed 80 psi under the plumbing code [12].
Inside, a spring pushes the valve open and the house-side pressure, acting on a diaphragm, pushes it shut. With no water flowing, the house pressure rises until its force on the diaphragm beats the spring and the valve closes: that balance point is the set pressure, 50 psi from the factory on Apollo’s PRH series, for example, adjustable from 25 to 75 [10]. When a tap opens, the house pressure dips, the spring wins, and the valve opens just enough to feed the flow; the bigger the flow, the further the outlet sits below the setting (its fall-off). Turning the adjusting screw changes the spring’s force, and so the pressure at which the diaphragm balances it; that is all an “adjustment” does [10]. Changing the house pressure also changes what the expansion tank’s precharge should be.
A PRV without a working bypass lets water through in one direction only, so installing one creates a closed system that must be protected against thermal expansion [4]. Some PRVs have a bypass that lets water flow back to the street when house pressure exceeds street pressure, but that only helps when the street pressure is lower than the house pressure, which is usually not the case where a PRV is needed.
Adjusting a PRV. Makers say a pressure gauge must be used to verify any change [10], and a change of house pressure means the expansion tank’s precharge must be reset with the tank empty. This is work for a licensed plumber. Nothing here guarantees that a setting, tank or valve suits your system or meets your local code.
Physics of Buildings explains the systems inside homes and commercial buildings through their physics. Every model is checked by an independent verifier against the sources cited; see about and the sources.
Amtrol (Worthington Industries) (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 guideCheck 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.
A. O. Smith (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 manualBackflow 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.'
Watts Water Technologies (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.'
Watts Water Technologies (2014). Series 1L, 1XL, 10L and 100XL Temperature and Pressure Relief Valves, specification sheet ES-10L/100XL. Watts, North Andover, MA. manufacturer data sheetTemperature 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'.
International Code Council, PMG Membership Council (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 guidanceAny 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.
Amtrol (Worthington Enterprises) (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 manualMust 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.
Amtrol (Worthington Enterprises) (2025). Therm-X-Trol Water Heater Expansion Tanks brochure, MC4090 (09/25). Amtrol, West Warwick, RI. manufacturer data sheetST-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).
Apollo Valves (Conbraco Industries) (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 manualFactory 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).
Watts Water Technologies (2026). LF15M2 Lead Free Water Hammer Arrestors, product page. Watts, North Andover, MA. manufacturer product pageAdded 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.
Watts Water Technologies (2026). Expansion Tank Sizing Calculator (Potable). Watts, North Andover, MA. manufacturer sizing toolInputs: 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.