Plumbing · Lesson 5 of 13Intermediate13 min read

Water Heaters

Water heating is the second-largest energy expense in the average American home, according to the Department of Energy. A tank heater is simple inside, but it holds a lot of hot water under pressure, so it has one safety part everyone should understand: the temperature-and-pressure relief valve.

How a tank water heater works

The Department of Energy describes a conventional storage water heater as a tank holding 20 to 80 gallons for a single-family home, fueled by natural gas, propane, fuel oil or electricity. When you open a hot tap, hot water leaves from the top of the tank. Cold water enters at the bottom through the dip tube, a pipe that carries it down past the hot water, and gets heated there, so the tank always stays full.

Hot water is less dense than cold, so it floats on top. A good tank keeps that layering: hot at the top, where it leaves, and incoming cold at the bottom, where the heat goes in. A gas heater has a burner under the tank and a flue pipe up through its middle; the hot exhaust gives up heat to the water on its way out and then leaves through a draft hood and a vent. An electric heater has heating elements reaching into the water, usually an upper and a lower one, each with its own thermostat; CPSC’s scald guide mentions both behind their access panels.

A tank loses heat to the room all day, even when nobody uses hot water. That’s called standby loss, and gas and oil heaters also lose heat up the vent. Heat traps on the pipes keep warm water from rising out of the tank when it isn’t in use, according to the University of Maryland Extension.

Run a hot tap and watch cold water come in through the dip tube. Pick gas or electric, try each thermostat setting, and switch to the tankless calculator to see how much power instant hot water takes.

Tank water heater parts
Part	What it does	Signs of trouble
Tank	Holds the water; insulation around it slows the heat loss.	Water around the base from the tank itself: HUD says a leaking tank usually can’t be repaired.
Dip tube	Carries incoming cold water to the bottom of the tank.	If it breaks, cold water mixes in near the top and hot water runs out fast.
Burner or elements	Heat the water (gas burner below; electric elements inside).	No hot water or not enough.
Thermostat	Turns the heat on and off to hold the set temperature.	Water too hot or not hot enough.
Anode rod	A metal rod that corrodes so the tank doesn’t; DOE suggests inspecting it every three to four years.	Once it’s used up, the tank itself starts to corrode.
T&P relief valve	Opens to release water if the temperature or pressure gets too high.	Water dripping from its discharge pipe; never cap or plug it.
Flue, draft hood and vent (gas)	Carry exhaust, including carbon monoxide, outside.	Soot on top of the tank, a blocked or disconnected vent: stop and call a pro.
Drain valve	Near the bottom, for draining the tank.	Drips; sediment can keep an old valve from sealing.

The temperature-and-pressure relief valve

Water expands when it’s heated. OpenStax lists its coefficient of volume expansion as 210 × 10⁻⁶ per degree Celsius near room temperature, so 40 gallons heated by 70°F (39°C) grow by about a third of a gallon, and more than that in practice, because hot water expands faster. Water barely compresses, so in a closed tank that extra volume has to go somewhere, and the pressure climbs. If a thermostat failed and the heat stayed on, the temperature and pressure would keep rising.

The temperature-and-pressure relief valve, or T&P valve, is the last line of defense. The University of Maryland Extension calls it a critical safety feature that releases hot water or steam if pressure or temperature builds up too far. HUD’s inspection guide says it belongs on top of the tank or on the hot water line, not the cold. Its discharge pipe runs down to a few inches above the floor, to a floor drain, or outside, depending on local code, so the scalding water goes where it can’t hurt anyone.

HUD’s inspection standard for the housing it oversees treats a problem with this valve as a severe deficiency to fix within 24 hours: a valve that’s blocked so it can’t open fully, or a discharge pipe that is damaged, capped, slopes upward or is made of an unsuitable material. HUD’s reason is the risk of a rupturing water heater. A missing or misplaced discharge pipe is flagged too, for the risk of burns.

StopNever cap, plug or block the T&P valve or its pipe

Water dripping from the discharge pipe is a symptom, not a nuisance to stop. It means the valve opened because of high temperature or pressure, or that the valve is worn. Have a plumber find out which. Testing the valve releases scalding water, and HUD’s guide warns that an old valve may stick open afterward, so have a replacement on hand or leave the test to a pro. EPA suggests checking the valve at least once a year and DOE every six months; follow your heater’s manual.

What temperature?

Here the agencies weigh two risks: scalding and germs.

Energy and scalding (DOE). Some manufacturers set thermostats at 140°F, but most households need only 120°F, which DOE calls safe for most of the population. A heater kept at 140°F can waste $36 to $61 a year in standby losses, and it significantly increases the risk of scalding. DOE adds that people with a suppressed immune system or chronic respiratory disease may consider keeping the tank at 140°F, with mixing valves on the taps used for washing and bathing.
Scalds (CPSC). The Consumer Product Safety Commission urges setting water heaters to 120°F. Most tap water scald injuries and deaths happen to older adults and children under five. At 140°F, burns can happen in six seconds; at 130°F, in thirty seconds; even at 120°F, five minutes can cause third-degree burns.
Germs (CDC). Legionella, the bacteria behind Legionnaires’ disease, grow best between 77 and 113°F and spread when people breathe in mist from showers and faucets. The CDC says to set water heaters at or above 120°F to minimize its growth. Water at 130 to 140°F kills many harmful germs but raises the scalding risk, so if a heater is set above 120°F, the CDC advises thermostatic mixing valves at faucets and showers.

To check your temperature, CPSC suggests holding a candy or meat thermometer under a hot faucet, first thing in the morning or at least two hours after hot water was last used. Thermostats on gas heaters vary, so CPSC says to follow the manufacturer’s instructions or call the gas company. Electric heaters have thermostats behind panels near live wiring, and CPSC’s instructions start with switching off the power at the breaker.

Source: U.S. Consumer Product Safety Commission, Avoiding Tap Water Scalds. Young children and older adults are most at risk.

Sediment and hard water

When hard water is heated, calcium carbonate comes out as solid deposits, the U.S. Geological Survey explains, and that scale lowers the efficiency of electric water heaters and raises the cost of heating water. Some sediment and rust collect at the bottom of every tank. A thick layer of it can make a heater rumble or pop as the water under it boils in little bursts.

Advice on flushing varies. DOE’s Energy Saver guide suggests draining a quart from the tank every three months; EPA’s checklist says to flush the heater once a year; the University of Maryland Extension says once or twice a year, especially where water is hard. Follow your heater’s manual. Drained water is scalding hot, and an old drain valve may not reseal, so many people have a plumber do it with the yearly checkup EPA suggests.

Tankless water heaters

A tankless, or on-demand, water heater has no tank. The Department of Energy describes the parts of a gas unit: a high-powered burner, a fan and a heat exchanger. When a tap opens, a sensor detects the flow and starts the burner, which raises the water to a set temperature; when the tap closes, it shuts off. With no tank, there’s no standby loss. DOE says high-efficiency models can typically deliver 2 to 5 gallons per minute and that tankless heaters are 24 to 34 percent more efficient for homes using 41 gallons or less of hot water a day, or 8 to 14 percent for homes using about 86 gallons a day.

The catch is power. Heating water as fast as it flows takes a lot of it. OpenStax gives the heat needed as Q = mcΔT, with water’s specific heat at 4,186 joules per kilogram per degree Celsius. Two gallons a minute is about 0.126 kilograms each second. Warming that by 60°F (33.3°C) takes about 17.6 kilowatts, or about 60,000 Btu per hour, steadily, for as long as the shower runs. That’s why the University of Maryland Extension notes that tankless heaters often need electrical or gas upgrades.

power = flow × specific heat × temperature rise (P = ṁ c ΔT)
0.126 kg/s × 4,186 J/(kg·°C) × 33.3°C ≈ 17,600 W ≈ 60,000 Btu/h

Heat pump water heaters

A heat pump water heater doesn’t make heat; it moves it. DOE compares it to a refrigerator running in reverse: it pulls heat from the surrounding air and pumps it into the water in a tank. That makes it two to three times more energy efficient than a conventional electric resistance water heater, and DOE’s guide says an ENERGY STAR model uses 70 percent less electricity for the same hot water.

It needs the right spot. DOE says the space should stay between 40 and 90°F year-round and have at least 1,000 cubic feet of air around the heater, and since it takes heat from the room, it tends to cool the space it’s in. The Science Lab explains how heat pumps move heat uphill (Engines, Refrigerators and Heat Pumps).

Water heaters compared (U.S. Department of Energy)
Type	How it heats	Typical life
Storage tank	Gas burner or electric elements keep a tank hot	10 to 15 years
Tankless (on demand)	Heats water only as it flows	About 20 years
Heat pump	Moves heat from the air into a tank	10 to 15 years

Sources: U.S. Department of Energy Energy Saver and Building Science Education pages. Heat pump efficiency is compared with standard electric resistance heaters.

StopLeave installation to a qualified pro

DOE recommends a qualified plumbing and heating contractor to install a storage water heater, because proper installation depends on fuel type, climate, local building codes and safety, especially the combustion safety of gas- and oil-fired units. A water heater usually needs a permit. A gas heater’s vent carries carbon monoxide outdoors, and HUD treats a blocked, misaligned or missing flue as life-threatening (Lesson 10).

Check yourself

Tap an answer to check it, or open “Show the answer.”

Where does cold water enter a tank water heater?
Show the answer
At the bottom, through the dip tube. The dip tube carries cold water to the bottom so hot water can leave from the top.
What should you never do to a T&P valve’s discharge pipe?
Show the answer
Cap or plug it. A capped or blocked pipe defeats the valve. HUD flags it because of the risk of a rupturing water heater.
According to CPSC, how fast can 140°F tap water cause a burn?
Show the answer
Six seconds. Six seconds. At 130°F, thirty seconds; at 120°F, five minutes can cause third-degree burns.
In what range does the CDC say Legionella grows best?
Show the answer
77 to 113°F. 77 to 113°F, which is why the CDC says to keep water heaters at or above 120°F.
Why do tankless heaters often need bigger gas lines or electrical service?
Show the answer
Heating water as fast as it flows takes a lot of power. About 17.6 kW for 2 gallons a minute warmed by 60°F.

Go deeper

U.S. Department of Energy, Energy Saver: Storage Water Heaters DOE on storage water heaters: sizing, efficiency, maintenance.
U.S. Department of Energy, Energy Saver: Heat Pump Water Heaters DOE on heat pump water heaters.
U.S. Consumer Product Safety Commission: Avoiding Tap Water Scalds (Publication 5098) CPSC: Avoiding Tap Water Scalds.
Science Lab: Energy and the First Law of Thermodynamics Science Lab: the energy in hot water and the First Law.
Science Lab: Engines, Refrigerators and Heat Pumps Science Lab: how heat pumps move heat.

Sources

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