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Water Heater

The typical water heater is usually a cylindrical tank with a heat source that heats water from the incoming water supply and also maintains and stores the heated water until it is used. Most water heaters have a 40 to 50 gallon glass-lined steel tank. The fuel types used to power the heat source can be either gas (natural or propane), oil or electricity. The life expectancy of the average water heater is 8 to 12 years. Other types of hot water systems used in the United States are: tank-less (coil) systems that uses a boiler to heat the water on demand; instant hot water heaters that are small (2 gallon), located near the tap design that provides hot water for only one tap; high efficiency gas systems; and combination systems that are tied into a gas warm air furnace or boiler to take advantage of the additional heat source.

Basic Components

Whether the water heater uses gas, oil or electric to fuel the heat source, all water heaters share the following basic components:

  • Cold Water Inlet Pipe
  • Cold Water Control Valve
  • Cold Water Dip Tube
  • Hot Water Outlet Pipe
  • Anti-Corrosion or Sacrificial Anode
  • Drain Valve
  • Temperature/Pressure Relief (TPR) Valve (TPRV)
  • TPRV Discharge Tube

Gas-fired Water Heaters

Gas-fired water heaters have a gas burner located below the tank, a thermostatic switch, a gas valve and a pilot light. The thermostat will open the gas valve when it senses the water temperature at the bottom of the tank has dropped below the present limit. Gas flows to the burner and is ignited by the pilot light, heating the tank from below. In addition, the hot exhaust gases are vented through a hollow core in the center of the tank to help heat the water. The exhaust is then passed into the draft hood and to the vent connector where it is discharged to a chimney flue.

Electric Water Heaters

The electric water heater uses two immersion heating elements, one near the top of the tank and one near the bottom to heat the water. The heating elements are electrical-resistant elements that operate in a similar fashion to an electric stove-heating element. The unit is powered by a 220-volt circuit.

Heating Elements and Controls

Each heating element has a thermostatic switch that turns the individual heating element on and off. Both thermostatic switches are usually set to the same temperature. When the water temperature (at the top or bottom) drops below the preset limit, a control switch closes to create an electrical circuit through the heating element. The electric current heats the element, thereby heating the water. When the water temperature reaches its preset limit again, the heating element will turn off. Thermostats are adjustable and are set between 120°F for increased energy savings and scald protection.

Lower and Upper Heating Elements

The lower element maintains the water temperature in the tank (when idle) because the water at the bottom of the tank will cool first (heat rises). In addition, the lower element heats the cold water entering the tank through the dip tube (when hot water is being drawn out of the top of the tank).

When the lower heating element cannot keep the water being drawn of the top of the tank hot (e.g. high hot water demand), the upper element activates and will try to heat the warmer water at the top tank as it is drawn. Note: most electric water heaters are designed so that both elements will NOT work at the same time. The lower element will shut off when the upper element is active.

Once the water temperature reaches the set limit of 120° F to 140° F, the gas valve is closed and the burner is turned off.

Note: Fuel-fired systems heat the water by heating the tank. As a result, the tank experiences more wear and tear than with an internal electric heat source. In addition, proper venting of the exhaust is always important to prevent carbon monoxide poisoning. Be sure that the vent/exhaust system is properly installed. See gas heating systems.

Oil-fired Water Heaters

Oil-fired water heaters use an oil burner and a combustion chamber (similar to an oil-fired furnace), which is located below the tank, as the heat source to heat the water. When the thermostat (at the bottom of the tank) senses that the water temperature has fallen below the preset limit, the oil burner will ignite. A jet-like flame is created in the combustion chamber to heat the bottom of the tank. In addition, the hot exhaust gases from the combustion chamber are vented either through a hollow core at the center of the tank (like a gas-fired system) or around the tank to help heat the water. The exhaust is then passed into the vent connector and directed to a chimney flue.

Once the water temperature reaches the set limit of 120° F to 140° F, the oil burner is turned off.

Note: Fuel-fired systems heat the water by heating the tank. As a result, the tank experiences more wear and tear than with an internal electric heat source. In addition, proper venting of the exhaust is always important to prevent carbon monoxide poisoning. Be sure that the vent/exhaust system is properly installed. See gas heating systems.

Tankless Water Heater

Coil Systems

Homes with a hydronic (hot water or steam) home heating system can use the tankless or coil system. A pipe coil built into the boiler and the water from the the cold water supply is heated as it passes through the coil. To accommodate periods of high demand, small storage tanks are sometimes connected to increase hot water availability. A cold water mixing valve is usually installed as a safety device to prevent scalding. Due to the extremely high temperature of a boiler, the hot water leaving the coil is beyond the safe limit of 120° F to 140° F. The mixing valve tempers the hot water by adding cold water to the hot water leaving the coil.

Generally, these systems are advantageous in regions where the furnace is on most of the year. There is no cost for maintaining and heating a large volume of stored hot water, and by sharing the boiler, no separate heat source and fuel is needed. However, during warm seasons, the boiler will have to generate the same amount of heat used to heat the house but only to heat the water. As a result, the the fuel saving is lost.

Instantaneous Hot Water Heaters

In addition to boiler-mounted tankless heaters, stand-alone units are available. Gas-fired instantaneous water heaters have a heat exchanger with a built-in coil. Like boiler-mounted systems, water from the the cold water supply is heated as it passes through the coil. Likewise, there is no cost for maintaining and heating a large, stored volume of hot water. However, stand-alone units typically have a lower flow rate than boiler-mounted systems and may fall short during periods of high demand.

TPR Valve

The temperature pressure relief (TPR) valve is a safety device that is designed to:

  • prevent the water in the tank from exceeding 212° F, and
  • prevent the water pressure in the tank from exceeding 150 pounds/square inch (psi).

Basic Physics (Temperature and Pressure of Water)

Water boils at 212° F when at sea level or under 14.7 psi of pressure, however, the boiling point increases when under pressure, which allows the water to superheat beyond 212° F. Typical residential water pressure ranges between 40 and 80 psi and at 50 psi, the water can reach 300° F.

Potential Problem

If the water heater’s thermostat, which controls the heat source, malfunctions, the pressurized water in the tank could continue to heat and superheat (beyond 212°F). This will cause two problems:

  • First, since water expands when heated, the water pressure in the tank will increase as the water is superheated. If the pressure exceeds the maximum pressure threshold (approximately 300 psi) the tank could rupture or even explode.
  • Second, the release of superheated water (now significantly above 212°F) would immediately return to atmospheric pressure (the boiling point of water is 212°F) and flash into steam, causing a sudden increase in volume and release of energy. The force of the flash steam would exceed the explosion of one pound of trinitrotoluene (TNT).


How it Works

If the water reaches a temperature of 210°F (before superheating occurs) or the water pressure exceeds 150 psi, the valve will open and release the overheated water. This will allow cold water to enter, lowering the temperature of the water in the tank and consequently the water pressure. The valve will remain open as long as the temperature or pressure exceeds the preset limit. The valve must be located on the tank at the top or on the side near the top, where the water is generally the hottest. TPR valves are not included with the water heater, so be sure that the TPR valve matches the BTU (heat rating) rating of the water heater. A Discharge Tube must be attached to all TPR valves.

TPR Valve Discharge Tube

Discharge Tube

A discharge tube is a tube or pipe that is attached to the TPR valve that directs the superheated water down to the floor and away from anyone in the discharge area to prevent scalding or burning.

The pipe itself must be made of a material that is rated for both high temperature and pressure, which includes most rigid wall copper, iron or even chlorinated polyvinylchloride (CPVC plastic) pipe. Also, the size of the pipe must match the opening size of the TPR valve discharge (usually ¾ inch).

The tube must terminate between 6 and 12 inches from the floor or be directed to the exterior of the home.If the discharge tube is routed to the exterior, the pipe must discharge 6 to 24 inches from grade, with a downward slope to prevent the pipe from clogging or forming a trap. Blocked discharge tubes will prevent the superheated water from discharging and will burst. Though counterintuitive, it is often recommended that the discharge tube terminate next to the water heater so that any malfunction of the water heater will be more readily noticed.

Safety and Efficiency


The following are general safety and installation requirements for a standard water heater.

  • Gas-fired water heaters placed in the garage must be 18 inches off the floor to prevent accidental combustion of gasoline fumes in the garage.
  • All water heaters in the garage must have a physical barrier to prevent impact damage from automobiles.
  • In some locations, water heaters must be mechanically secured (i.e. earthquake straps) to prevent the system from being dislodged.
  • Gas or oil-fired water heaters should not be located in sleeping areas.
  • Heaters are generally set to 120 ° F to help prevent accidental scalding. However, with some dishwashers that do not have a heating element to raise the temperature of the water, spotting of glassware and dishes may occur. Check the dishwasher’s manufacturer information for the recommended temperature setting.
  • Require properly installed temperature and pressure relief valve and discharge tube.

Recovery Rate

Recovery rate is a measure of the number of gallons water that can be heated (from approximately 50° F) to 100° F in one hour. The faster the recovery rate, the more water can be used without running out of hot water. Generally, oiled-fired systems create the most heat and have the fastest recovery rate followed by gas-fired systems and then electric systems. Since fuel-fired heaters have a higher recovery rate, a large tank may not always be needed. Conversely, larger tanks that hold more usable hot water, do not require a faster recovery rate and can be sufficient for intermittent use. Either way, when choosing a water heater, select one with an appropriate capacity and recovery rate to match the daily hot water demands of the home.


An external insulation blanket wraps around the sides of the water heater. Insulation blankets prevent heat from radiating from the tank and help reduce the fuel cost associated with marinating the 120° F to 140° F storage temperature. To determine if one is needed, feel the water heater jacket. If it is warm, heat is escaping and the tank should be insulated. If it is cool, no insulation is needed.

Newer heaters do not require and actually have warnings posted not to add an insulation blanket. With gas or oil-fired systems, the insulation should be kept away from the burner area. No insulation should be installed on the top of the heater

Pipe insulation of the hot water line will also help increase efficiency.

Peak Timers

Peak timers are installed on electric heaters and allow the water heater to turn on during specific times of the day. However, this requires that all activities using hot water (e.g. bathing, laundry, dishwashing, etc.) be accomplished at generally the same time of the day.