Water Heaters

How a tank heater works

A storage water heater keeps 40 or 50 gallons hot and ready. Cold water enters at the top and runs down a dip tube to the bottom, so it doesn’t mix straight into the hot water leaving from the outlet at the top. A gas burner under the tank (with a flue up the middle) or electric elements inside heat the water, controlled by a thermostat. A sacrificial anode rod, usually magnesium or aluminum, corrodes in place of the steel tank’s weak spots; when it’s used up, the tank starts to rust.

Water heating is the second-largest energy cost in most homes, 14 to 18 percent of the utility bill ¹.

The relief valve

If a thermostat fails, a tank of water can heat past boiling under pressure. The temperature and pressure (T&P) relief valve is what keeps it from becoming a bomb. Wisconsin’s rules ²:

• Every pressurized storage heater has a combination T&P relief valve, rated at least as high as the heater’s BTU input.
• Its sensor reaches into the top 6 inches of the tank, where the water is hottest.
• No valve or other restriction may be installed between the tank and the relief valve.
• The discharge pipe: an approved water pipe material, at least the size of the valve outlet, not trapped, no valve in it, draining by gravity to a floor with a floor drain or to a receptor, and ending within 6 inches of the floor or receptor but no closer than twice its diameter.

Never cap or plug a dripping relief valve. A T&P that drips now and then often means excessive pressure, frequently from thermal expansion (below); one that opens hard means overheating, and the heater needs service now.

Expansion, drain, and temperature limits

• Thermal expansion: heated water expands. When a check valve, backflow preventer, or other one-way device makes the system closed, that expansion has nowhere to go, and Wisconsin requires an expansion tank or similar device on the heater ².
• Drain valve: at least 3/4 inch with a hose thread at the lowest point of the tank ³.
• Shutoff: a valve on the cold supply to the heater ².
• Temperature: instantaneous heaters for homes must deliver no more than 125 °F initially, and combination space-and-water heating systems that run hotter need a mixing valve to limit domestic hot water to 125 °F ³.
• Access: the heater must be accessible for inspection, service, and replacement ³.

Care

The University of Florida’s extension guidance ¹:

• Set the thermostat to about 120 °F: efficient, and less scald risk. Electric heaters have two thermostats; set both the same.
• Drain about a quart every 3 to 6 months to remove sediment, which collects at the bottom and slows heat transfer. Hard well water makes more of it.
• Plan to replace a heater over about 10 years old before it fails; a new one costs less to run, and a planned swap beats a flooded basement.

Other kinds

• Tankless (on-demand) heaters heat water as it flows, with no tank to keep hot, which avoids standby losses; hard water means they need periodic descaling ¹. Large gas units need a big gas line and venting; electric ones need a large electrical service.
• Heat pump water heaters use electricity to move heat from the surrounding air into the tank rather than making heat directly, using far less electricity ¹. They cool and dry the room they sit in, which suits a basement in summer.

What to take from this

A tank heater takes cold water down a dip tube, delivers hot from the top, and relies on an anode rod to protect the tank. The T&P valve senses the top 6 inches of the tank, with no valve before it or in its discharge pipe, which runs full size and untrapped to within 6 inches of a drain. Closed systems need an expansion tank. Set 120 °F, drain sediment every few months, and plan replacement past about 10 years.