Part 4: Plumbing Systems

4.7 Water Heating

Conventional Water Heating Systems

The most common residential water heater has a 40 or 50 gallon storage tank capacity. They are available in electric, natural gas, and propane (LPG) models. The height and diameter of a water heater varies with the gallon capacity. When selecting water heaters, it is important to know the measurements of the space the appliance will occupy, and to check the water heater manufacturer specifications and codes for space requirements for ventilation, clearances, and other installation requirements. The shorter versions water heaters are known as a “low-boy”. that can fit in closet spaces or under stairs, or as a “squat” water heater that can fit under countertops. Squat water heaters are generally offered in electric configurations ranging from 5 to 40 gallons, and gas is not used due to ventilation and carbon monoxide concerns.

All water heaters must be installed per local codes and manufacturer instructions:

  • An isolation valve must be installed on the cold water piping near the inlet.
  • A typical residential water heater has 3/4” male or female threaded water supply connections.
  • Most codes dictate that any water heater located above a finished area must be installed in a safety pan.
  • Most relief valve connections of a residential water heater are 3/4” female and require a plumber to install a 3/4” male adapter.

Electric Water Heaters

An electric water heater only requires a plumber to connect the hot and cold water piping from the rough-in stub-outs to the designated inlet and outlet connection. They do not require venting or gas piping and are less expensive to purchase. Most codes allow the safety pan for an electric water heater to be plastic due to the lack of heat generated externally from the water heater in comparison to gas fired models.

A standard residential electric water heater is classified as a 240-volt, 4500-watt, non-simultaneous water heater. Models with faster recovery rates may have higher wattage elements. The higher the wattage rating, the faster a water heater can heat water. Although most electric water heaters have two elements, each rated at 4500 watts, a non-simultaneous heating cycle has only one of the elements operating at a time. Each wire providing electricity to a water heater is known as a leg. The two different wires (legs) that connect to the high-limit device are identified as line voltage one and two. One wire eventually provides 120 volts of electricity to one side of a heating element and the second wire eventually completes the circuit by providing an additional 120 volts to the same element.The screws that secure the wire connection to the electrical devices are known as terminations, poles, or posts.

No water heater should ever have the ignited gas or electricity energized before filling the system with water and removing all trapped air from the system.

Electric Water Heater Components and Their Functions

Temperature and Pressure (T & P)Relief Valve- Electric, gas & solar applications, relieves at150 psig and/or 210° F. (Temperature (only) relief valves are used on the rooftop collector panels of solar water systems).


Mythbusters- water heater

Dip Tube- As heat rises, and the outlet supply of hot water to the fixtures feeds from the hottest point at the top of the water heater, manufacturer install a device known as a dip tube into the cold water connection of a top-fed water heater to route the incoming cold water to the bottom of the heater. If a dip tube were not used, the incoming cold water would mix with the hot water located in the top of the water heater, cooling the exiting water during use.

Drain– Used to flush the water heater’s storage tank during maintenance.

Ball valve- Used to shut off the supply water to the tank.

Mixing Valve- Allows cold water to enter hot water stream to temper over-temp water to prevent scalding. Mixing valves are normally required to be installed on solar water heating systems as temperatures can potentially reach in excess of 160°.

Tank Anode Rod- A residential water heater storage tank is manufactured with carbon steel, with most common residential water heaters incorporating a very thin coat of porcelain enamel called a “glass” lining, designed to fill every internal crevice created by the manufacturing process of the tank, to prevent the carbon steel tank from corroding. However, over time, this lining breaks down and the steel tank begins to corrode. An anode rod is a sacrificial device that dissolves (corrodes) over a period of time. An anode rod is installed by the manufacturer to prevent from rusting and corrosion of the inside of the tank due to glass lining imperfections or minor damage during shipping and handling. Magnesium-based and aluminum rods are more vulnerable to rust and minerals that would normally attack the steel walls of a tank and attract corrosives to attack the anode rod instead of the tank.

New versus Expended Anode Rod by Cory Early is licensed under CC BY 4.0


Top Access Cover– The thermostat and the red button are located in here. The upper heating element is also found here.

Bottom Access Cover– The lower thermostat and heating element are located here.

Upper Thermostat– Limits the temperature that the element will heat the water up to. Both, upper and lower, thermostats are  secured in place and held against the surface of the tank with a retainer clip, keeping the thermostat in contact with the external portion of the storage tank. Most safety standards do not allow a plumber to set the temperature above 120 degrees Fahrenheit to prevent scalding. The upper thermostat of a seven-pole design only has three posts. This three-post design combined with the four posts of the high limit switch is how the seven-pole design is recognized. The upper thermostat has a temperature setting feature that is typically either identified alphabetically from A through D, or identified as warm, hot, and very hot.


Replacing Upper Thermostat


Lower Thermostat- The lower thermostat only has two posts and numbered 1 and 2. It also has a temperature setting feature that is the same design as the upper thermostat. Like the upper thermostat, lower thermostats can be adjustable and have similarly marked adjustment points.

Hi-Limit Device (Red Button Over Temperature Thermostat)– The high-limit device halts the electrical current to the thermostats and elements if an unsafe water temperature is present. Most high-limit devices have a reset button designed to pop out if the temperature reaches 190°. These devices can be reset once the temperature of the tank cools below the “pop-off” temperature.

Heating Element- Elements convert electrical energy to heat energy and transfer heat to the water in the tank.  They are rated in watts and selected for tanks according to tank size and recovery rate. Residential electric water heating elements are usually a screw-in type, but bolt-in types are used for certain water heater models. The elements can vary in length, with 12” being the typical length provided by a manufacturer in most water heaters. The electrical voltage and wattage ratings of the element are indicated by a manufacturer on the element so its identification of replacement parts specifications is available on the exterior.

Lined and Dielectric Pipe Nipples- Many water heaters are manufactured with pipe nipples that are lined with corrosive-resistant material such as PEX. These pipe nipples are manufactured to resist corrosion due to electrolysis from joining dissimilar metals to the iron tank (copper to iron = rust). Brass pipe fittings can also be used between iron pipe fittings and copper to resist electrolysis. A plumber must take precautions when connecting copper tube to a lined nipple as heating the lining of the pipe nipple directly with a torch or by connecting a fitting that has been soldered and not allowed to cool can melt the internal lining of the nipple. Brass to copper unions provide the same benefit in non-concealed applications.

Expansion Tank- Every water heating system must be protected against dangerous occurrences that exist when water is heated. As a heating cycle occurs, water expands and can cause the pressure relief valve to begin dripping. Expansion tanks are installed, mainly in solar water systems, to absorb the expansion of a system.  An expansion tank used for a potable hot water system has an internal rubber membrane known as a bladder. Most codes require that an expansion tank be installed near a water heater to protect the piping system from high pressure caused by the heating cycle.


Gas Water Heaters

Natural gas and propane (LPG) are the two types of gas used for water heating. A water heater designed for natural gas cannot be used with propane unless the particular water heater can be converted. The internal gas regulating orifice is different for each type of gas. The venting requirements of a gas water heater are dictated by code. The exhaust fumes from a gas water heater contain carbon monoxide. Carbon monoxide fumes are odorless and can kill occupants of a home or building. Conventional gas water heaters are vented atmospherically, that is, to the exterior of the building. Atmospheric water heater venting must terminate in specific locations through, and heights above a roof. Gas water heaters must have adequate space around the water heater and replacement air to create proper draft conditions that allow the fumes to be exhausted to the exterior of the building. If adequate air is not provided in the room where an atmospheric vented water heater is located, the exhaust fumes could enter the occupied space.

Selecting a gas water heater based on its capabilities requires manufacturer’s specifications for the specific appliance and knowledge of the load demand. The recovery rate is the most important aspect of determining if a certain gas water heater is capable of being installed for a specific home or use. Gas water heaters are rated by the gallons of hot water they can produce. One British Thermal Unit (BTU) is the amount of heat required to raise one pound of water by one degree Fahrenheit. One gallon of water weighs 8.33 pounds, and 8.33 btus are required to raise the temperature of one gallon of water by one degree Fahrenheit. Temperature rise is the difference between the incoming cold water and the desired temperature expected from a water heater. Temperature rise determines the capabilities of a certain Btu rating of a gas water heater. A length of time is used to clarify the recovery capabilities of a gas water heater, and gallons per hour (gph) is the most common method used in rating a hot water heater.

  • Codes vary pertaining to the gas supply connections and venting regulations.
  • A gas water heater uses a metal safety pan.
  • The gas supply configuration is fairly consistent with most residential water heaters: black iron pipe or approved polyurethane tubing, brass fittings, gas-cock, flex supply line, and regulator

Gas Regulators

A gas regulator is an automated device that controls the gas flow to a burner assembly. The gas supply pipe is connected to the gas regulator which regulates the flow of gas to the burner A thermocouple must sense a pilot flame in order to allow the gas to flow through a gas regulator. The design of a gas regulator is based on safety, and most codes do not allow anyone not certified to repair a gas regulator to disassemble one for repair. Another feature of the regulator is to control the gas flow to a pilot flame. The gas regulator for a residential gas water heater typically has a 1/2” female threaded connection.


How to Relight a Gas Water Heater Pilot Light


Tankless Water Heaters

A tankless heater design can be suitable for many residential applications.  Water flow is regulated to ensure that the desired temperature leaves the heater. They are becoming more desirable and are considered environmentally friendly. Tankless models are also known as an instantaneous water heaters.

The two basic types of tankless water heaters are interior or exterior, defining whether the installation location is inside or outside of the building structure. Tankless water heaters are available in electric models for interior applications. Tankless water heaters can be installed inside or outside a building, but units installed outside must be gas. The units are not interchangeable. Interior units require venting of the flue gas and exterior units may require freeze protection in certain climates.

Solar Water Heaters

Although a solar thermal water heating system can save a residence up to a 40% savings on utility bills, the initial investment to install a solar water heating system may deter many homeowners. Federal, and sometimes state, tax credits are available for installing alternate energy systems and utility savings over time are a good return on investment (ROI).

Solar water systems use solar collector panels, usually mounted on the roof to heat the system water. When the temperature of the water in the collector reaches a specific higher temperature than the water in the tank, a pump circulates the cooler water from the tank which pushes the hotter water in the roof panel back into the tank until the temperature of the panel and the temperature in the tank are equal.

The location and angle of the solar panel (orientation and tilt for optimal sun exposure) is crucial to obtaining optimal efficiency of the entire system. Most solar systems are directly connected to gas or a single electric heating element to provide adequate hot water during non-solar heating periods.

Diagnosing Common Water Heater Problems

Water heaters should be flushed annually to remove contaminates and scale from the tank. Anode rod should be inspected every 2 years and replaced approximately every 5 years, or sooner if conditions show significant corrosion of the rod. Troubleshooting gas and electric water heaters must be performed by qualified individuals.  Basic electrical knowledge must be known to safely troubleshoot an electric water heater, and a plumber must have an electrical voltage/amperage meter to diagnose and service an electric water heater. Always remember that a water heater must be always be filled with water before igniting the gas supply or electrically energizing the system and only a certified technician can repair gas regulators.

Electric Water Heater Diagnostics

Electric Water Heater Problem Diagnosis: Cause & Effect

A basic understanding the symptoms and component functions can lead to simple diagnosis of common water heater problems.

No hot water No electricity from
Check breaker
No hot water Electrical problems with
Test and replace appropriate thermostat/s
No hot water Failed heating
Test and replace appropriate element/s
Little hot water Dip tube failure Inspect dip tube and replace if necessary
Little hot water Lower thermostat or element failure Replace lower thermostat and/or element
Little hot water Thermostat/s failure Test and replace appropriate thermostat/s
Water too hot Thermostat/s failure Test and replace appropriate thermostat/s
Rest button tripped Water too hot Test and replace appropriate thermostat/s
Rotten egg smell of
Expended anode rod Inspect and replace
Popping noise when
Scale build-up on
Replace element(s)


Water Heater Not Heating? Thermostat Testing


Heating Element Diagnostic Procedure

  1. Switch off the power to the water heater at the main electrical panel.
  2. Locate the electrical access panels, one near the base of the tank and one nearer the top (solar water systems only have one back-up element and thermostat located in the upper access). Beginning with the top element, remove the panel’s screws with a screwdriver. Take off the panel and remove insulation behind it to uncover the heating element.
  3. Disconnect either of the two electrical wires screwed into the element terminals. It is not necessary to disconnect both wires to test the element.
  4. Set the multimeter or Ohmeter to read Ohms, and set the scale to RX1. Touch one probe to each of the two terminal screws. If the needle moves at all, or if there is any reading besides “infinity” on a digital readout, the element is good. If it doesn’t move, or displays “infinity,” no electricity is flowing through the element and it should be replaced.
  5. Complete replacement or repairs as necessary. Reconnect the wire to the element terminal. Replace the insulation and reinstall the access panel.
  6. Repeat the process on the bottom element. Restore power to the water heater from the main electrical panel.


How to Replace an Electric Water Heater Heating Element

Voltmeter Diagnosis of High Limit & Thermostat Devices for 240 Volt Water Heaters

Testing with live voltage can be extremely dangerous. Water heater installation and service technicians should be fully trained in the safe use of voltmeters with alternating current.

All Any Post Steel Tank 120 Volts Confirms that voltage is present at that post or terminal.
All Steel Tank Any Post 120 Volts Confirms that voltage is present at that post or terminal.
High Limit No. 1 Post No. 3 Post 240 Volts Confirms that correct voltage is coming from the breaker.
High Limit No. 2 Post No. 4 Post 240 Volts Confirms that the correct voltage is coming through the reset button.
Upper Thermostat / HL No. 1 Post No. 4 Post 240 Volts Indicates that at least one element is fully energized.
Upper Thermostat / HL No. 2 Post No. 4 Post 240 Volts Indicates that the upper element is fully energized.
Upper Thermostat / HL No. 2 Post No. 4 Post 120 Volts Indicates that the upper element is not fully energized.
Upper Thermostat / HL No. 4 Post No. 4 Post 240 Volts Indicates that the lower element is fully energized.
Lower Thermostat / HL No. 1 Post No. 4 Post 240 Volts Indicates that the lower element is
fully energized.
Lower Thermostat / HL No. 1 Post No. 4 Post 120 Volts Indicates that the lower element is not
fully energized.
Lower Thermostat / HL No. 2 Post No. 4 Post 240 Volts Indicates that the lower element is
fully energized.
Lower Thermostat / HL No. 2 Post No. 4 Post 120 Volts Indicates that the lower element is not
fully energized.

Gas Water Heater Diagnostics

Gas Water Heater Problem Diagnosis: Cause & Effect

Liquid propane gas (LPG) and natural gas water heating diagnosis, like electric, can be simplified by a basic understanding of the system's components and their functions.

No gas to pilot No gas from meter or gas cock Turned off at meter or gas cock
No gas to pilot No gas from regulator Debris in regulator. Clean or replace defective regulator
No gas to pilot Faulty regulator Replace regulator
No gas to pilot Crimped pilot tube Repair or replace tube
No gas to pilot Leak in pilot tube Replace tube
No pilot flame No gas flow from regulator Debris in regulator. Clean or replace defective regulator
No pilot flame Defective thermocouple Replace thermocouple
No pilot flame Air in gas piping Purge air from piping
No gas to burner Defective regulator Replace regulator
No gas to burner Crimped burner tube Repair or replace burner tube
No gas to burner Blockage in burner tube Remove tube and clean
No gas to burner Remove and clean Run water to cool tank
No gas to burner Defective high-limit device Replace device or regulator
Temperature and Pressure (T&P) relief valve leaking Water is too hot Run water to cool tank
Temperature and Pressure (T&P) relief valve leaking Defective relief valve Replace relief valve
Temperature and Pressure (T&P) relief valve leaking Excessive pressure Install expansion tank
Temperature and Pressure (T&P) relief valve leaking Excessive pressure Check pressure regulator valve to system supply
Low water temperature Thermostat set too low Adjust temperature setting on thermostat
Low water temperature Dip tube failure Inspect and replace if necessary
Low water temperature Defective thermostat Test and replace appropriate thermostat/s
Slow recovery time Sediment in tank Drain and flush tank
Slow recovery time Dirty burner assembly Clean burner assembly
Slow recovery time Poor flame Adjust burner air supply
Slow recovery time Poor flame Supply more combustion air
Not enough hot water Insufficient size heater Calculate demand load and replace with an appropriate sized water heating system
Not enough hot water See low water temperature and slow recovery symptoms See low water temperature and slow recovery symptoms
Popping / banging noises Calcium or sediment build up Drain and flush tank
Banging noise Check valve slamming when a faucet or valve is being opened or closed Install a shock absorbing device
Fume odor Poor draft on flue system Examine flue pipe installation and ensure termination location provides sufficient draft
Gas odor Leak on piping system Soap test all piping joints and supply lines for leaks and repair as required
Soot build up Poor draft on flue system See fume odor, install a draft hood fan
Soot build up Insufficient combustion air Install air supply ducts or vents
Soot build up Poor burner flame Clean and adjust burner
Flame back flash Negative air pressure Isolate heater air


Icon for the Creative Commons Attribution 4.0 International License

Building Maintenance & Construction: Tools and Maintenance Tasks (Interactive) Copyright © 2018 by University of Hawai’i Maui College and the Construction Technology Program is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.