Physics of Buildings

Physics of Buildings · Water

Popping, Rumbling, Running Out

What is happening inside your water heater. Here is a tank cut open and running, gas or electric. Drag the shower handle, or pick your symptom.

A 40-gallon tank cut open, simulated (not a recording; faults are sized to make each pattern visible). Colour is temperature: pale blue cold, vermilion hot. Drag the shower handle’s red-dotted knob to run water and watch cold water rise from the bottom. The amber line beside the tank measures the water still hot enough for a 105 °F shower. Below: the last hour of temperatures.

A tank water heater is a column of water that sorts itself by temperature: hot water floats on top, cold water sinks to the bottom. A dip tube carries the incoming cold water down to the bottom so it does not stir the hot water above, a gas burner under the tank or two electric elements put heat back in, and a sacrificial anode corrodes so the steel tank does not [1][2]. Running out, lukewarm showers, popping and leaks are, most of the time, one of those parts going wrong.

Figure 1 The same simulated tank, pinned while you read. Each step switches it to one state and starts a fresh, fully heated tank; drag the shower handle 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. A healthy gas heater: draw and recovery

At the tap: a 10-minute shower at 2 gallons a minute stays at 105 °F from start to finish. In the tank: a band of cold water climbs from the bottom while the hot water above it leaves through the top, and the two hardly mix.

Why: water at 120 °F is about 1 percent less dense than water at 60 °F [3], enough for hot water to float on cold. The shower mixes 120 °F water from the tank with 50 °F water from the cold pipe, so only about 79 percent of its flow comes from the tank. About two minutes in, the cold water reaches the thermostat low in the tank and the burner lights; after the shower stops, it runs until the tank is back at 120 °F, about 20 minutes here. The amber line beside the tank measures the water still at 105 °F or hotter.

Question 1 Prediction: can the burner keep up with a long shower?#

2. Running out: the shower outruns the tank

At the tap: full heat for about 18 minutes, then a fall to lukewarm over a few minutes. In the tank: the boundary between hot and cold climbs until it reaches the outlet.

Why: at 2.5 gpm the shower draws about 2 gallons of tank water a minute. The burner’s 30,800 Btu an hour, spread over 2 gallons a minute of 50 °F water, can only warm it by about 30 °F, so the water it heats during the shower is too cool to use. The shower lasts about as long as the stored hot water does: in the simulation about 36 of the 40 gallons reach the shower before it cools. To keep up indefinitely at 105 °F, the burner could carry a shower of only about 1.1 gpm. The shower-time calculator works this out for your own tank.

Tank size is not the whole story. The number that measures “how much hot water” is the first-hour rating: the most hot water a heater can supply in an hour starting full, in a federal test that draws at 3 gpm [4]. A 40-gallon gas model rated at 40,000 Btu/h lists 68 gallons [5].

Also: A. O. Smith lists these other causes of too little hot water: a heater too small for the household, a thermostat set low, a faulty mixing valve in a shower control, reversed connections or a melted dip tube after an installation, and a leak [6]. Steps 3, 6 and 7 show three of them.

3. Broken or short dip tube: lukewarm within minutes

At the tap: hot at first, then lukewarm much sooner than it used to be, with the tank still warm to the touch. In the tank: the whole column cools together instead of a clean cold band rising from the bottom.

Why: the dip tube exists to deliver cold water to the bottom of the tank so it does not mix with the heated water [7]. Broken off near the top, it releases cold water right under the outlet. Cold water is denser, so it sinks and mixes through the hot water on the way down. Here the shower falls below 105 °F after about 12 minutes, against about 23 for a healthy tank at the same 2 gpm.

Also: soldering copper pipes while they are attached to the heater can melt the plastic dip tube, and hot and cold lines connected the wrong way round give the same symptom; both usually show up soon after an installation [6]. A failing mixing valve in a shower control can also deliver lukewarm water from a tank that is fine: check several taps [6].

4. Sediment: popping, rumbling and slow recovery

At the tank: popping, crackling or a low rumble whenever the burner runs. On the trace: the same 10-minute shower as step 1, but the burner now runs until about 35 minutes instead of 20.

Why: minerals in hard water come out of solution at the hottest surfaces, which in a gas heater is the bottom of the tank, and build up as scale and sediment [8]. The burner now heats the water through that crust. Water trapped in and under it overheats, turns to steam, and the bubbles collapse in the cooler water above, which is the popping. The crust also insulates the bottom: less of the burner’s heat reaches the water and more leaves up the flue, and the steel underneath runs hotter, which weakens the tank bottom over time [8]. The simulation removes 45 percent of the burner’s heat, an illustrative figure.

Also, and normal: water condensing on a cold tank can drip onto the burner and sizzle or pop during heating, and metal parts tick as they expand and contract; A. O. Smith treats both as normal [1]. On an electric heater, crackling usually means scale on the elements [8]. An electric tank must never be drained or refilled with its power on: powering an element that is not covered by water, “dry firing”, can burn it out [6], so draining one is a job for a licensed plumber. Manufacturers schedule draining and flushing to limit sediment [1]; on a gas heater, have a plumber or the manufacturer’s service agency do it if you are unsure.

5. A healthy electric heater: two elements, one at a time

At the tap: the same 10-minute shower, just as hot. In the tank: an upper and a lower element, each with its own thermostat; the lower one does nearly all of the reheating, and it takes until about 29 minutes.

Why: an element heats only the water at and above it, because heated water rises. The control energizes one element at a time and gives the upper element priority [6]: when hot water runs low near the top, the upper element heats the top of the tank first, so some hot water comes back quickly; then the lower element heats the rest. A 4.5 kW element puts about 15,400 Btu an hour into the water, half the gas burner’s 30,800, which is why electric tanks recover more slowly.

6. Electric, lower element dead: some hot water, much less

At the tap: a few minutes of hot water, then lukewarm. In the tank: only the water above the upper element is hot; everything below it stays at inlet temperature.

Why: with the lower element dead, only the upper one heats, and it can heat only the water above it, here about 8 gallons, five minutes of shower. A. O. Smith describes exactly this symptom, “some hot water but not as much as before”, and notes that the lower element usually wears out first because it does most of the work [6].

Also: when the upper element fails, the result depends on the controls. Some electronic controls let the lower element carry on alone [6]; with others there may be no hot water at all. A tripped high-limit cutoff also stops the heating. Testing elements and thermostats means working at 240 volts: turn the heater off at the breaker and have a licensed electrician or plumber do it.

7. Thermostat set low: never quite hot enough

At the tap: the shower reaches temperature only with the handle at full hot, and runs cold sooner. In the tank: the same 40 gallons, stored at 108 °F instead of 120 °F.

Why: to make a 105 °F shower from 108 °F water, about 95 percent of the flow must come from the tank, against 79 percent at 120 °F. The tank empties faster: the shower turns cool after about 16 minutes instead of 23. Each gallon stores less heat above the cold water, too.

Also, the trade-off: the U.S. Consumer Product Safety Commission urges 120 °F because hotter water scalds fast: at 130 °F a 30-second exposure can cause a third-degree burn, at 140 °F six seconds [9]. OSHA notes that water heaters kept below 140 °F with scale and sediment may foster Legionella bacteria [10]. Storing hotter and fitting thermostatic mixing valves at the taps, as manufacturers recommend for homes with children or older adults, addresses both [1]. Thermostats also overshoot: short, repeated draws can push the water up to 30 °F above the setting [1].

8. The anode used up: rust and leaks

At the tap: often nothing at first; later, rusty hot water, and in the end a leak at the bottom of the tank. In the tank: the anode rod is down to its steel core wire, and the steel shell has begun to corrode at the gaps in its glass lining.

Why: the tank is steel, and steel in water corrodes. Connect it to a more active metal, such as magnesium or aluminum, and that metal corrodes in its place: this is cathodic protection, and textbooks cite water heaters as an example [2]. The anode is meant to be used up. Every tank heater has at least one, gas or electric [1]. The anode’s age in this figure is illustrative: how fast one wears depends on the water, and softened water shortens its life [11].

Also: a rotten-egg smell in the hot water is hydrogen sulfide. It needs four things at once: sulfate in the water, little dissolved oxygen, sulfate-reducing bacteria, and hydrogen produced by the anode’s protective reaction. A less active anode and a chlorine treatment by a service agency can cure it; removing the anode is not the answer, because the tank then corrodes and the warranty is void [1].

Common causes, by symptom

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, gas fitter or electrician confirms the cause before parts are replaced.

  • Hot water runs out fast. A shower or a household that needs more than the tank and its heater can supply; a thermostat set low; a broken or melted dip tube; on electric heaters a dead lower element; a faulty mixing valve in a shower control; a leak on the hot side [6].
  • Lukewarm almost from the start. Broken dip tube or reversed connections, a dead element, a tripped high-limit cutoff, a failing shower mixing valve [6][1]. Check several taps: if only one is lukewarm, the cause is in that fixture.
  • Popping, rumbling or crackling. Sediment or scale on the tank bottom (gas) or on the elements (electric) [8]. Sizzling from condensation on the burner during heating, and ticking as the tank warms, are normal [1].
  • Not hot enough. Thermostat set low, a dead element or failing burner control, too small a heater for the use [6][1].
  • Rotten-egg smell from the hot water only. Hydrogen sulfide from the anode’s reaction with sulfate and bacteria in the water [1].
  • Rusty hot water, or water under the tank. A used-up anode and a corroding tank; but water on the floor can also be condensation, a loose connection or the relief valve, so find the source first [1].

Safety first. Gas burners, gas valves and venting are work for a licensed gas fitter or plumber: a fault there can mean a fire or carbon monoxide. Electric elements and thermostats sit on a 240-volt circuit: turn the heater off at the breaker and leave testing and replacement to a licensed electrician or plumber. Water hotter than 120 °F scalds in seconds [9]. This page explains how a water heater works so you can describe the problem accurately; it is not a repair procedure, and codes and licensing rules vary by state and city.

The T&P relief valve: never plug it

Near the top of every tank is a temperature and pressure relief valve with a pipe running down toward the floor. If the controls fail and the water overheats, or the pressure climbs too high, it opens and dumps water before the tank can burst. It must never be plugged, capped or removed: A. O. Smith’s manual warns that doing so can cause property damage, serious injury or death [1].

A valve that drips now and then is telling you something. The most common cause, in the same manual, is excessive water pressure from thermal expansion: heated water expands, and in a “closed” system with a check valve or pressure reducer upstream it has nowhere to go, so the pressure rises until the valve lets some out. The fix is a correctly sized thermal expansion tank, not a plug. A valve that releases large amounts of very hot water points instead to a heater that is overheating, such as a shorted element or a failed thermostat [6]: turn the heater off and call a licensed professional.

Anode rods: magnesium, aluminum and powered

The anode is a rod of magnesium, aluminum or zinc alloy screwed into the top of the tank [11]. Magnesium is the most active of the three: on the ladder of standard electrode potentials it sits furthest below iron [12], so it protects strongly and is used up sooner. Aluminum, and aluminum-zinc rods, are less active; A. O. Smith suggests a less active anode where the hot water smells of hydrogen sulfide [1]. A powered anode has no metal to use up: a titanium rod is connected to a small power supply that drives a direct current into the tank, doing the anode’s job electrically [13]. The essay on how the tank works sets out the electrochemistry.

How often to check one depends on the water. One manufacturer’s manual says to inspect it after the first six months and then at least annually, more often with hard water [6], and softened water wears anodes faster [11]. A plumber checks it by taking it out; a rod worn down to its core wire is replaced. Running a tank without its anode shortens its life and voids the warranty [1].

Heat pump water heaters

A heat pump water heater uses electricity to move heat instead of making it: like a refrigerator working in reverse, it draws heat from the air around it and puts it into the water in its tank [14]. The tank, the stratification, the dip tube and the anode are the same as in an electric heater. Most also have resistance elements, and in their hybrid mode they switch to them automatically when demand is high [14], so a long shower on a heat pump tank behaves much like step 5.

Go deeper

The essay Inside the tank: why hot water runs out and how the anode saves the steel works through stratification and the dip tube, the energy balance of a shower, the first-hour rating, and the electrochemistry of the anode. The shower-time calculator runs the same tank model for your own heater and shower.

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 this site and the sources. The publication’s first piece explains why a well pump keeps cycling.

Sources

  1. A. O. Smith Corporation (2021). Installation Instructions and Use & Care Guide: Residential Gas Water Heaters, 100341468_2000598643 Rev A (March 2021). A. O. Smith, sold through Lowe's (AOSmithAtLowes.com). manufacturer manual Component diagram (anode, inlet dip tube, T&P valve); every heater has at least one anode rod; rotten-egg odor needs sulfate, little oxygen, sulfate-reducing bacteria and active hydrogen from the anode; less active anode plus chlorination by a service agency; do not remove the anode; condensation sizzling and popping in the burner area is normal; expansion noises are normal; T&P valve must not be plugged; most T&P leakage is thermal expansion in a closed system; thermostatic mixing valves; short repeated heating cycles can raise temperature up to 30 °F above the setting; drain and flush every 6 months.
  2. Flowers, P., Theopold, K., Langley, R. & Robinson, W. R. (2019). Chemistry 2e, §17.6 Corrosion. OpenStax, Rice University. textbook Rusting of iron as a galvanic cell; cathodic protection: connecting iron to a more active metal such as zinc or magnesium; 'also used to protect the metal parts in water heaters'; sacrificial anodes are used up and must be monitored and replaced.
  3. U.S. Geological Survey. Water Density (Water Science School). USGS. government reference Density table: 0.99907 g/cm³ at 60 °F (15.6 °C), 0.98870 at 120 °F (48.9 °C), 0.98338 at 140 °F (60 °C).
  4. U.S. Department of Energy. 10 CFR Part 430, Subpart B, Appendix E: Uniform Test Method for Measuring the Energy Consumption of Water Heaters. Electronic Code of Federal Regulations (eCFR), current as of 2026. federal regulation §1.5 first-hour rating: estimate of the maximum volume of hot water a storage heater can supply within an hour that begins fully heated; §2.3 supply water 58 °F; §2.4 outlet 125 °F; §5.3.3 draws at 3.0 gpm (tanks ≥ 20 gal), each ended when the outlet falls 15 °F below its maximum; electric heaters with two or more elements that do not operate simultaneously; §1.11 recovery efficiency.
  5. A. O. Smith Corporation. ProLine 40-Gallon Atmospheric Vent Short Natural Gas Water Heater, model GCBL-40 400 (product page). hotwater.com. manufacturer data sheet 40-gallon nominal (38 gal rated storage), 40,000 Btu/h input, 68-gallon first-hour rating, recovery 41 gallons per hour, UEF 0.57.
  6. A. O. Smith Corporation (2021). Installation Instructions and Use & Care Guide: Residential Electronic Thermostat Electric Water Heater (printed 10/21/2021). A. O. Smith, sold through Lowe's (AOSmithAtLowes.com). manufacturer manual Upper and lower element; the control energizes one element at a time and the upper has priority; limp mode with one failed element; troubleshooting 'insufficient hot water': undersized heater, temperature set low, faulty thermostatic mixing valve in a shower control, reversed connections or melted dip tube (soon after installation), leak, failed lower element ('some hot water but not as much as before'; the lower element does most of the work and usually wears out first); 4500/5500 W elements; inspect the anode after six months, then at least annually depending on hardness.
  7. McClellan, W. T. (2011). High-efficiency water heater dip tube and method for reducing turbulence in water heaters, US 7,861,678 B2 (granted 2011). U.S. Patent and Trademark Office. patent The dip tube brings cold inflowing water down to the bottom of the heating chamber; preserving natural stratification is highly desirable; mixing of heated and incoming water impairs the heater's useful output.
  8. Friedman, D. (InspectAPedia). Hot Water Heater Noise Diagnosis. InspectAPedia. reference website Scale forms as minerals precipitate at the hottest points, often the tank bottom; thicker scale means worse banging and popping; scale on the bottom insulates it, raising its temperature and weakening the tank bottom; crackling on electric heaters can be scale on the elements.
  9. U.S. Consumer Product Safety Commission. Avoiding Tap Water Scalds (CPSC Document #5098). U.S. Consumer Product Safety Commission, Bethesda, MD. government safety alert Urges all users to lower water heaters to 120 °F; most adults suffer third-degree burns from 150 °F water in 2 s, 140 °F in 6 s, 130 °F in 30 s, and even 120 °F in 5 min; most injuries are to the elderly and children under five; measure at the tap with a thermometer.
  10. U.S. Occupational Safety and Health Administration. Legionnaires' Disease: Control and Prevention. OSHA. government reference Water heaters maintained below 140 °F that contain scale and sediment may foster Legionella growth; storing domestic hot water at a minimum of 140 °F and delivering at a minimum of 122 °F to outlets; high temperatures carry a serious burn risk.
  11. Leonard, J. and Content Creation Team. Corrosion Protection (Hot Water University guide). Hot Water University (hotwateruniversity.dozuki.com). trade training guide Anode rods are typically made from magnesium, aluminum or zinc alloys; have a plumber check the anode after the first six months, then annually; water softeners reduce anode life.
  12. Flowers, P., Theopold, K., Langley, R. & Robinson, W. R. (2019). Chemistry 2e, Appendix L: Standard Electrode (Half-Cell) Potentials. OpenStax, Rice University. textbook E° (V): Mg²⁺/Mg −2.372; Al³⁺/Al −1.662; Zn²⁺/Zn −0.7618; Fe²⁺/Fe −0.447.
  13. Veilleux, V. (Corro-Protec) (2023). Decoding the Science: The Functioning of Powered Anode Rods (1 May 2023). Corro-Protec (manufacturer of powered anodes). manufacturer article Powered (impressed-current) anodes: a titanium or similarly inert rod connected to an external power supply that drives a direct current from the rod to the tank; the rod is not consumed.
  14. ENERGY STAR (U.S. EPA and DOE). Heat Pump Water Heaters: How It Works. ENERGY STAR. government reference Uses electricity to move heat rather than make it, like a refrigerator in reverse, drawing heat from the surrounding air into the tank; operating modes; during high demand switches automatically to standard electric resistance heating ('hybrid').