Most tank electric water heaters of 30 gallons or more are dual element, meaning that they have a top and bottom heating element at the side of the tank. Some older mobile homes have a single heating element for a 20 or 30 gallon water heaters, but nowadays the only single-element water heaters we see are 19 gallons or less, and they are used for a single application such as a bar sink. Also, the single-element water heaters are usually 120-volt, (single breaker in the electric panel) whereas the larger water heaters are all 240-volt (double breaker). Dual element water heaters are recognizable by the two raised access panels, one over the other as in the photo above, at the side of the water heater for each element, whereas the single element water heaters only have one.
Although dual element water heaters have two heating elements, only one element operates at a time. The top element comes on first and, when the water at the top of the tank has reached the temperature setting for the top thermostat, then the bottom element kicks in until its thermostat is satisfied. Dual element water heaters are rated as more energy efficient and have a faster recovery time that single element ones.
There is a peculiarity of dual element water heaters than can cause a trouble-shooting problem if the water heater is no longer working: because the top thermostat has priority and must be satisfied before the bottom element can turn on, if the top element is dead, the water heater will not work at all even though the bottom element is still good—because the top thermostat will never be satisfied and transfer power to the bottom.
Conversely, if the bottom element is fried, then it will work but the water may turn cold before the end of your shower even though the water heater is functional, because the bottom half of a tall water heater tank never get fully heated in the recovery cycle. So, a water heater that is completely dead may just have a bad top element, and a water heater that seems to be doing a poor job of heating the water probably needs a new bottom element.
Also see our blog post Can you wire a 240-volt water heater with 120 volts?
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Here’s links to a collection of more blog posts about WATER HEATERS:
• Are water heaters required to be raised off the floor?
• Can a Temperature and Pressure Relief (TPR or T&P) valve be mounted to discharge horizontally?
• Where are gas water heaters not allowed to be installed?
• Is the minimum size water heater inlet pipe 1/2" or 3/4" according to the building code?
• Can you use a light switch for a water heater disconnect?
• How can I tell if a water heater is HUD-approved for mobile/manufactured homes?
• Is it alright to have a shut-off valve on both the hot and cold water pipes at a water heater?
• What is the minimum clearance to doors and windows for an outdoor tankless gas water heater?
• What is required clearance for access and working space in front of an electric water heater?
• Why is the water heater older than the house?
• Does a water heater need a shut-off valve?
• Why should a tankless water heater have an isolator/service valve kit installed?
• When was a gas water heater first required to be elevated 18 inches above a garage floor?
• Can the temperature and pressure relief (TPR) valve discharge pipe for a mobile/manufactured home water heater terminate under the home?
• What is the purpose of a thermostatic mixing valve above a water heater?
• Does a tankless gas water heater still work with no electricity during a power outage?
• Why do water heaters have a sacrificial anode?
• What is the difference between a manufactured/mobile home water heater and a regular water heater?
• Does a tankless water heater require a pressure relief valve?
• When is a water heater drain pan required?
• Why is there water in my water heater drain pan?
• What does it mean when a water heater Temperature/Pressure Relief (TPR) discharge pipe is "trapped"?
• Can I leave a gas water heater in place when remodeling a garage into a family room or bedroom?
• Where do I find the water heater in a mobile home?
• Does a tankless water heater in an attic require a drain pan?
• Does an electric water heater require a disconnect?
• Is a catch pan and drain piping required for a replacement water heater?
• What is an FVIR water heater?
• What is a heat pump water heater?
• What is a dielectric union?
• What's that powdery crust on the pipe connections at the water heater?
• What are the most common installation mistakes with water heater replacement?
• Why is my water heater making strange (rumbling, gurgling, knocking or banging) noises?
• What can I do to make my water heater last longer?
• How can I determine the age of a water heater if the serial number is missing or decoding it is impossible?
• How does a hydronic heating system work?
• What is the difference between a regular water heater and a direct vent water heater?
• What is the difference between a regular water heater and a power vent water heater?
• What is backdrafting at a gas water heater?
• How do I determine if a water heater is gas or electric?
• What does it mean when a gas appliance (water heater, furnace, or range) has been "red tagged"?
• What's the valve with the flip-up handle on the water heater for?
• Why is an older water heater an insurance problem?
Visit our WATER HEATERS page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.