How a tank water heater works

Draw from the top, fill from the bottom. That is the whole habit of a tank water heater, and every other part on it is there to heat the water, to keep the steel from rusting, or to keep heat and pressure inside safe limits. University of Florida extension gives the short version: water is heated in an insulated tank, and when a hot tap opens, hot water flows out of the top while cold water flows into the bottom to replace it.

Cold in, hot out

The cold line does not empty into the top of the tank. It ends in a dip tube, which A. O. Smith’s gas heater manual describes as a long plastic tube inside the tank attached to the cold water inlet, and which Howard County, Maryland’s utility bureau calls the extension of the cold water supply pipe. Bradford White’s service manual says what it is for: as hot water is drawn off, it is replaced with cold water delivered through the dip tube to the bottom of the tank. The hot water waiting at the top is what leaves first.

The arrangement has a cost. The Air-Conditioning, Heating, and Refrigeration Institute (AHRI) notes that to keep hot water always available the tank is heated even when no tap is running, which is standby heat loss. Insulation between the tank and its outer jacket slows that loss but cannot stop it.

How the water gets hot

University of Missouri Extension says gas and electric storage heaters work basically the same way, with two differences. A gas heater has a pilot light at the bottom to ignite the burner and a flue running up through the center of the tank to carry off the combustion gases. An electric heater generally has a lower and an upper heating element inside the tank.

On a gas heater with a standing pilot, A. O. Smith’s service handbook gives the sequence. The pilot stays lit. When incoming cold water activates the thermostat, gas flows to the main burner and the pilot flame lights it. The main flame burns until the tank reaches its set temperature, and then the thermostat shuts the main gas off. The pilot also heats the tip of a thermocouple, which makes the small electric current that holds the gas valve open; that current lasts only as long as the pilot flame is heating the thermocouple. A second safety sits in the tank water itself: a temperature probe with an energy cut off switch that opens if the water gets excessively hot and stops the gas.

On a two-element electric heater, Bradford White describes one arrangement, called non-simultaneous: only one element runs at a time. With a cold tank the upper element heats the top of the tank first. Only when the upper thermostat is satisfied does power pass to the lower thermostat and the lower element, which heats the rest.

The rod that corrodes so the tank does not

On the heaters A. O. Smith’s service handbook covers, the tank is steel with a glass lining bonded to the inside, and the handbook says the lining is what keeps water from touching metal. It cannot cover everything. The anode rod, in the handbook’s words, offers secondary protection against corrosion where glass is not possible, such as the threaded tank openings. A. O. Smith’s manual calls it a sacrificial metal rod and a consumable item, and adds that a water softener makes it deplete faster than normal.

How often to look at it depends on who you ask. A. O. Smith says to inspect it yearly; the Insurance Institute for Business and Home Safety (IBHS) says to have a professional inspect it at least every 2 years, and every year once the warranty has run out. Go by your heater’s manual.

Sediment

Missouri Extension: sediment and scale, meaning dirt and mineral deposits from the water, build up over time inside the tank and reduce both heating efficiency and the capacity of the heater. A. O. Smith’s handbook names the sound it makes. Rumbling, crackling and popping are the symptoms of mineral buildup, and the popping is water trapped under lime deposits.

The cure is to flush water out through the drain valve near the bottom of the tank. How often is where our sources part company, with intervals that run from monthly to yearly. Missouri Extension ties the answer to how hard the water is, and IBHS notes that sediment builds up faster where the water is hard. Your manual has the last word, and the February almanac page puts the job in the yearly round. Mind the heat and the power. A. O. Smith’s maintenance guide says to turn off the heat source before flushing: the dial on the gas control to OFF on a gas heater, the breaker or disconnect on an electric one. Its manual adds a warning to be sure the water runs cool before draining the tank, to reduce the risk of scalding. Before the heat goes back on, the tank must be full again. A. O. Smith’s manual says to make sure of that before the pilot is lit, and A. O. Smith’s guide says to relight a gas heater by the instructions on the heater itself (if you smell gas, light nothing; the last paragraph of this page has the manual’s steps); on an electric heater, the guide says, a tank that is not full will burn out the element.

The temperature and pressure relief valve

This is the part that matters most. A. O. Smith’s manual says the temperature and pressure relief valve is there to reduce the risk of explosion by discharging hot water. Missouri Extension describes it as the valve near the top of the tank that lets steam or hot water escape safely if a thermostat fails.

The service handbook gives the usual ratings. The valve normally has a temperature rating of 210 F, sensed by a probe that reaches into the top six inches of the tank, and a pressure rating no higher than the working pressure of the tank, generally 150 psi. Approaching the temperature rating it opens fully; at the pressure rating it opens slightly, which shows up as dribbling or weeping from the valve.

The valve empties into a discharge pipe, and the pipe has rules. The International Code Council’s 2019 summary of the 2018 International Plumbing Code and International Residential Code lists them, and among them the pipe must drain by gravity with no trap and no sags, have no valves or tees in it, have no threads at its end, discharge where it cannot injure anyone or damage the structure, and end no more than 6 inches above the floor, a floor drain or a waste receptor. That is the model code as that edition wrote it; the edition your town or state has adopted is the one that applies. A. O. Smith’s installation drawing shows the same 6 inch maximum, and prints the rule that outranks the others: do not cap or plug. The manual’s warning is that an explosion could occur if the relief valve or its discharge pipe is blocked.

The manual also asks for a test, on the heaters it covers. We look at the pipe before anything else. If there is no discharge pipe, or its end is capped or plugged, do not test anything and call a licensed plumber: the codes above require the pipe, and a capped pipe is the blocked pipe the manual warns could cause an explosion. If it is capped or plugged, turn the heat off as well, the gas control to OFF on a gas heater or the circuit breaker on an electric one, and leave it off until the plumber has been. With the pipe in place and open, the test is this. At least once a year, operate the valve by hand so its waterways stay clear and its mechanism moves freely. Before you do, check that it will discharge in a safe place and that no one is in front of or near the end of the pipe, because the water released can be extremely hot and cause severe burns. The manual also says to follow the instructions from the valve’s own maker, on the label attached to the valve, and to have a qualified person inspect the valve and its discharge pipe from time to time; it gives two different intervals in two places, so we leave the number to that label. Where the manual says a qualified person, it means a licensed plumber, gas company personnel or an authorized service technician; for anything on the relief valve or its pipe, we say a licensed plumber.

If water does not flow freely from the end of the discharge pipe, the manual, which is written for a gas heater, says to turn the gas control to OFF and call a qualified person to find the cause. On an electric heater, the way to stop it is the circuit breaker, which Missouri Extension names as the place an electric heater shuts off. The same manual warns not to operate the water heater without a functioning relief valve, because this could cause an explosion. So on any heater, a valve that will not discharge means the heat stays off until a licensed plumber has been, just as a capped pipe does.

Why a relief valve drips

Water expands when it is heated. The Ashley Valley Water and Sewer Improvement District explains what happens next in a closed system, one in which water has no way to leave the house plumbing: the added volume has nowhere to go and the pressure rises. A. O. Smith’s handbook puts a number on it. Water in a closed tank at 50 psi, heated just 10 degrees, will reach 250 psi.

The expansion tank is the answer. It goes on the incoming cold water supply line, before the heater, and when the water expands it flows into the expansion tank and compresses the air there instead of raising the pressure in the heater and the pipes. A. O. Smith’s manual says a dripping relief valve is usually caused by house water pressure that is too high or by the lack of an expansion tank. The drip is a message. Have the cause found, and never stop the drip by plugging the pipe. A large flow of very hot water from the pipe is a different matter. Whatever the fuel, keep clear of the end of the pipe while you shut the heater down. The manual says the tank may be overheating, and on a gas heater to shut the gas off at the manual valve on the heater’s supply line and at the gas control, and call a qualified person. That manual is written for gas heaters. On an electric heater, Missouri Extension notes, the heater shuts off at the circuit breaker; do that, and call a licensed plumber.

The temperature setting

University of Florida extension calls 120 F the optimum setting for both energy efficiency and function, and says higher temperatures raise energy costs and the risk of scalding. Missouri Extension adds that a lower setting slows mineral buildup and corrosion in the tank and pipes. The Consumer Product Safety Commission urges all users to lower their water heaters to 120 F. Both CPSC and A. O. Smith’s manual put a serious burn at about thirty seconds in 130 F water, and at 140 F it takes only seconds. Even 120 F is not harmless: CPSC says a five minute exposure at that temperature could result in third-degree burns.

There are two cautions the other way. Missouri Extension notes that most electric heaters are set at 140 F, a setting needed only for a dishwasher without a booster heater. And A. O. Smith’s manual says a higher tank temperature, 140 F, can reduce the levels of bacteria that cause water-borne diseases; OSHA’s guidance on Legionnaires’ disease, written for workplaces and larger buildings, notes that water heaters kept below 140 F that hold scale and sediment may foster Legionella growth. That is the trade between scalds and bacteria, and the manual’s answer is thermostatic mixing valves at each tap: with them properly adjusted, it says, the tank can be set higher without raising the risk of a scald, and above 120 F it calls them particularly important.

On a gas heater the dial is on the gas control valve near the bottom of the tank, as A. O. Smith’s manual draws it. On an electric heater, CPSC says, the thermostats sit behind upper and lower access panels; most heaters have two, and both must be set to the same temperature. CPSC’s instruction comes first: shut off the power to the heater at its circuit breaker or fuse before the panels come off, and University of Florida extension gives the same warning about the wires. A. O. Smith’s guide adds to take the panel off carefully and keep clear of the wiring and components behind it. Set the thermostats by the instructions that came with the heater, CPSC adds, and A. O. Smith’s maintenance guide puts the cover back on before the power goes back on. CPSC then suggests checking the result with a candy or meat thermometer under a tap, first thing in the morning. It also offers the easier road: your electric company may adjust the thermostat for you, and for a gas heater CPSC says to call the gas company for instructions, because thermostats differ.

How long it lasts

University of Florida extension says most water heaters last 10 to 15 years, and ENERGY STAR says that once yours is more than 10 years old it is time to consider replacing it, before a failure makes the choice for you. IBHS is less patient: in its study the chance that a water heater will leak or burst begins to rise dramatically at 5 years old, and three quarters fail before they are 12. Know the age of yours.

What is yours, and what is a plumber’s

Yours: knowing where the shutoff valve on the cold water line to the heater is, setting the temperature (with the breaker off before any electric heater panel comes off, and the panels back on before the breaker goes back on), the yearly lever test when the discharge pipe is in place and open, and draining sediment the way your manual describes.

A licensed plumber’s: replacing a relief valve or anything on its discharge pipe, a relief valve that keeps dripping, a tank that leaks, an expansion tank, and any repair or service on the gas line, gas control, burner or vent.

The anode rod sits between the two. A. O. Smith’s manual lets an owner inspect it and replace it, or have a qualified person do it, and only after the first steps of its draining procedure: gas off (on an electric heater, A. O. Smith’s guide turns off the electricity instead), hot water run until it is cool, the cold water supply shut off and the drain valve open. Afterward the guide has the tank refilled and the air run out of it at a faucet before the power goes back on or the pilot is relit.

A. O. Smith’s maintenance web page goes further than we do. It shows an owner how to replace the relief valve and how to work the lever over a bucket on a valve that has no discharge pipe, and its yearly checklist says to tighten electrical connections. We stop short of all three: A. O. Smith’s own manual sends a valve that still drips, once the other causes are ruled out, to a qualified person for replacement; the codes above require the pipe; and Bradford White’s service manual for its electric heaters, line voltage wiring included, is marked to be performed only by qualified service providers.

If you smell gas, the manual’s instructions are short. Do not try to light any appliance. Do not touch any electrical switch, and do not use any phone in the building. Immediately call your gas supplier from a neighbor’s phone and follow their instructions; if you cannot reach the supplier, call the fire department.

Related

• Almanac: February, the water heater’s annual check
• How a pressure-balance shower valve works, the other guard against a scald
• Glossary: dip tube, anode rod, temperature and pressure relief valve, expansion tank, thermocouple