Among the new types of water heaters available today, the conventional tank-type unit is still the most popular. Of these hot water storage units, approximately half of them are gas-fired and the other half are electric. Any plumber will tell you that electric water heaters are the easiest to install, but not everyone knows how to fix one when it breaks down.

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In America, we will replace about 1.5 million water heaters each year simply because they are not performing as expected. How many of these units could be repaired, thereby extending their useful life? In this article, we will identify the main components of an electric water heater, explain how they work together to provide hot water for a home, and describe what can be done to get better performance utilizing an electric water heater thermostat.

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Water Heater Elements

The device that converts electricity into heat inside of a water heater is the element. It&#;s basically an electrical resistor. When electricity flows through the element, it gets extremely hot. Since the element is immersed in the water, it transfers the heat to the water until it reaches a specific temperature (typically 120 degrees).

Elements come in different shapes to improve their performance and longevity and are sized according to wattage. The most common size is watts, with and watt versions also available. These sizes are not interchangeable, but are specified on the rating plate of every water heater.

Electrical Water Heater Thermostats

The upper thermostat is the main control component for most water heaters. It controls both the upper and lower elements. A thermostat is essentially a temperature-activated switch. When the thermostat senses a water temperature below its set point, it will energize one of the elements in response to this &#;call for heat.&#;

The way the thermostat senses the water temperature is unique since it never comes in direct contact with the water. The back of the thermostat senses the temperature of the outer layer of the tank. It&#;s pressed tightly against it and is held in place by a clip. This metal-to-metal contact transfers heat from the water to the thermostat by conduction through the shell of the tank. All other parts of the tank are covered with insulation except for the two spots where the thermostats are mounted.

Most electric water heaters above 20 gallons have two thermostats that sense the temperature of the water in both the top and bottom of the tank. The upper thermostat is the boss and always tells the lower thermostat what to do.

Sequence of Operation

When you wake up in the morning and begin to use hot water, you effectively draw water off the top of the tank. As the hot water is dispensed, the heater simultaneously fills with cold water. Since that water is directed to the bottom of the tank through the dip tube, the lower thermostat senses a drop in temperature first.

It calls out to the upper electric water heater thermostat, and as long as the upper half of the tank is still hot, it will energize the lower element. In most residential units, the elements are only energized one at a time. If you only use a few gallons of hot water, the lower thermostat alone can handle the job. However, if you have a hard time waking up and need a long shower, or if there is a line outside the bathroom door, you may end up emptying the tank of all its available hot water.

This could happen, but not if the upper element has anything to say about it. When the water in the top half of the tank begins to cool off, the upper thermostat will take over to maintain the temperature of the exiting water before it disappoints the next bather.

A storage-type water heater carries what is called a &#;first-hour rating.&#; This rating is typically about 30% greater than the storage capacity of the tank. Since the elements become energized, as soon as they sense a drop in temperature, the heater begins making more hot water. In the span of an hour, you can get more than 50 gallons of hot water out of a nominal 40-gallon tank.

Limit Controls, Grounding and Pressure Relief

Every water heater includes a few safety devices to keep you from getting scalded by hot water, electrocuted or injured by an exploding tank. These pressure vessels are not to be trifled with, but most plumbers have nerves made of hard drawn copper, so they are not easily intimidated.

The upper electric water heater thermostat includes a high-temperature limit device that will disconnect power to both elements if it senses a temperature above 150 degrees. Water at this temperature will burn your skin in a hot second.

Water heaters, like all electrical appliances, have an equipment ground lug inside the wiring compartment at the top of the tank. This ground lug provides a low resistance path for current to flow in case the jacket of the heater becomes energized.

The relief valve is arguably the most important safety device in any pressurized heating system. If the thermostat fails and the high-temperature limit doesn&#;t shut down the heater, the pressure relief valve will prevent the unit from causing any real damage. If it senses a pressure above 150 psi or a temperature above 210 degrees, just below the boiling point of water, it will open, allowing that pressure to be relieved in a controlled, rather than explosive manner.

All these safety devices should be checked periodically as part of routine maintenance to keep the heater running safely and trouble-free.

Improving Your Heater&#;s Performance

The biggest thief robbing a water heater of efficiency is sediment. Sediment is formed out of minerals like calcium and magnesium that are found in hard water. These undissolved minerals, along with any sand that might be suspended in the water, collect on the bottom of every tank-type water heater, reducing its capacity to store hot water.

This crusty layer of elements affects gas-fired and electric heaters in different ways. In the electric units, the sediment builds up until it buries the bottom element. It displaces the cool water that normally surrounds the element causing it to overheat.

A homeowner may not even notice that the element has failed, only that the water heater just doesn&#;t seem to keep up anymore.

Regular Maintenance of a Water Heater

Diagnosing and replacing a failed bottom element is a very common repair, but to prevent it from happening again, the sediment, or as much of it as possible, should be flushed out. Periodic flushing of sediment could have prevented the repair altogether and will keep the heater operating at its maximum capacity.

One recommendation a plumber might make would be to install a sediment filter if sand is present at the bottom of the unit. If minerals seem to be building up quickly, installing a water softener would also be a great idea. These measures will capture the sediment and remove the minerals from the water before the tank ever sees them.

Another common problem, that could be mistaken for a tank leak, is a dripping relief valve. While there is more than one cause for this, it&#;s commonly an issue with the valve itself. About half of the time, all one has to do is exercise the relief valve to stop the drip. If that doesn&#;t work, and you verify there is no overpressure or high-temperature condition, you simply replace the valve.

Repair Before Replacing

A little preventative maintenance goes a long way toward extending an electric water heater&#;s lifespan and repairs can yield higher margins for plumbers doing the work. Additionally, massive quantities of non-recyclable waste in scrapyards and landfills can be avoided. To learn more about how a water heater functions and how to troubleshoot and repair it, go to SkillMill&#;s Electric Water Heater Operation course.

Frank Garro

Interplay Learning Plumbing & HVAC Expert

Frank is Interplay&#;s resident plumbing and HVAC expert. While he began his career in the plumbing trade, his passion for HVAC led him to become an Associate Professor and HVAC Program Chair, at Ivy Tech Community College. Frank is a Licensed Mechanical, Plumbing and Electrical Contractor, and owns his own mechanical services company. As an educator, Frank is a firm believer in teaching the fundamentals and has a gift for simplifying complex concepts.

­­­Introduction

The heating system uses two hot water heaters, sometimes also referred to as boilers (although technically, boilers involve phase changes to steam), to supply hot water to the air-handling units and heat exchangers. These hot water heaters are of the water-tube variety, as depicted below, employing the combustion o­­f natural gas as a heat source. Each hot water heater is capable of producing 9.0 MBtu/hr (2.64 MW) of heat with a thermal efficiency of 80%. These two water heaters are designed to supply the heating system with water at roughly 180 °F (82.2 °C) during the winter months [1] .

Background

Water Tube Boilers:

The purpose of a boiler is to transfer heat from hot combustion gases to the working fluid (water). In a water tube boiler, water-filled pipes are passed through the combustion chamber, or furnace. At the bottom of the furnace, there are several natural gas burners that produce hot combustion gas. This gas is passed over the water tubes, consequently heating the flow. Figure 2 illustrates the manner in which water tube boilers operate.

Figure 1. Water Tube Boiler at 41 Cooper Square.

Figure 2. Water Tube Boiler Schematic. [2]

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The schematic in Figure 2 shows the combustion gases passing over the array of water tubes only once before being exhausted. This configuration is not very effective, as the hot gases are not given a chance to transfer all of their heat to the fluid. In Cooper Union&#;s 41 Cooper Square building, the boilers are of the multiple pass variety, in which the combustion gases are circulated over the water tubes many times. This allows more of the heat from the combustion to be transferred to the fluid, making the boiler more efficient and consequently allowing it to consume less fuel than with a single pass. (Note: condensing boilers are water heaters that can achieve even higher efficiency (typically greater than 90%) by extracting additional heat by condensing water vapor in the exhaust gas to liquid water.)

Forced Draft Combustion:

The combustion process inside the furnace is also controlled in order to boost efficiency and reduce fuel consumption. Each boiler is outfitted with blowers that force air into the combustion chamber. The rate of air supplied to the combustion chamber by these blowers is balanced in relation to the rate of natural gas being fed to the burners. In this manner, an optimal air-to-fuel ratio is constantly maintained, producing combustion that yields high heat with minimal fuel consumption.

Control

During the winter months, the BMS controls the boilers so that they are operating about their nominal set point, providing the heating system with water at 180 °F [3]. In order to achieve this constant set point temperature, the BMS actuates various valves, pumps, and burners that regulate the temperature and flow of the primary water flow. The figures below are screen shots of the BMS Hot Water System and BMS Boiler Control Panel that regulate the operation of the boilers.  

Figure 3. BMS Hot Water Boilers Panel with Outside Air Temperature of 62° F and 61% Relative Humidity.

Figure 4. BMS Boiler Control Panel Screenshot with Outside Air Temperature of 62° F and 61% Relative Humidity

Boiler Sequence of Operation*

The flow of water to each boiler is controlled by a single valve that is either fully opened or fully closed. The BMS takes frequent measurements (about every 15 minutes) of the supply temperature and return temperature. Based on these readings, various control decisions are made.

Once the BMS gives a heating command to the boilers, the lead boiler is turned on. In order to protect the boiler&#;s water tubes, the system verifies that proper flow is established before firing the burners. If the burners are activated while the tubes are dry, the tubes will overheat, expand, and crack because they are not being cooled internally by a constant flow of water. Thus, after taking a differential pressure reading across the water tubes that verifies proper flow rates, the burners are ignited.

The burners are regulated in order to maintain a nominal set point temperature of 180 °F.  The Boiler Control Panel shows that each boiler has two burners, or &#;Flames&#;, that can be turned on together or individually to control primary water temperature. If the nominal set point temperature of 180 °F is reached or exceeded, the burners are turned off and the pumps switched off 5 minutes later. This sequence allows the water in the system to cool back down to the desired temperature range.

The BMS also monitors the water temperature returning from the heat exchanger and air-handling units back to the boiler. If this return water flow temperature is below 160 °F (71 °C), this indicates that the primary flow is not meeting heating demands, and the BMS turns on a second feedwater pump (HHWP-2), shown in Figure 3. Turning on this pump increases the flow rate through the primary water loop, and consequently increases the rate of heat transfer occurring in the heat exchanger and air-handling units. This increase in heat transfer rate allows the primary return water flow&#;s temperature to rise, lowering the boiler&#;s heating load. During times of large heating loads, the return water flow temperature can dip to 150 °F (65.6 °C). When the return flow reaches this temperature, the BMS activates the building&#;s second boiler. With the second boiler turned on, the gas flow to both sets of burners is regulated to maintain a supply temperature of 180 °F while using only a single feedwater pump. These control sequences, amongst others, allow the temperature of 41 Cooper Square&#;s primary hot water flow to be regulated efficiently [3].

[1] GMP Set - The New Academic Building of Cooper Union &#; Water Tube Boilers. Syska Hennessy Group, New York, NY, , p. -5.

[2] Brain, Marshall. "HowStuffWorks: How Steam Engines Work" HowStuffWorks: Science. N.p., n.d. Web. 23 June . <http://science.howstuffworks.com/transport/engines-equipment/steam2.htm>.

[3] Gruzen Sampton, Sequence of Operation &#; The New Academic Building of Cooper Union, Morphosis Architects, Los Angeles, CA, Rep. , June . p. 15.

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