Smart Water Heater Design: Three Important Technologies You Shouldn’t Forget to Use

November 21, 2022

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This article provides designers with information on three critical technologies: protection, control, and sensing, for developing an electric water heater robust to external overloads, transients, and ESD. The components representing these technologies will help create a safe, efficient, long-life water heater that safely controls water temperature. By using the recommended components, design engineers will achieve the goal of developing high-quality, reliable, smart water heaters.

New technologies, including the Internet of Things (IoT), enable every product, including appliances, lighting, and HVAC systems, to be computer-controlled smart devices. In essence, any product that performs some level of control can become a smart device. That concept is also true for the ubiquitous water heater. As a result, the market for electric water heaters is growing at an attractive rate with a CAGR of 7.4%. Governments are spurring growth by incentivizing consumers to convert their gas-based water heating systems to electric systems to achieve the Net-Zero Emissions Initiative. In addition, new building codes in some regions will not permit a natural gas hook-up, further accelerating the growth of all-electric products such as water heaters.

The addition of intelligence to a water heater adds sensitive electronic circuitry. It is incumbent upon the designer to ensure the reliability of the water heater by protecting the electronics from current overloads, voltage transients, electrostatic discharge (ESD), and overheating. Protecting the electronics also ensures that individuals who contact the water heater are safe from thermal and electric hazards. The designer also needs to ensure the use of long-life components.

Overview of a Smart Electric Water Heater

Like almost every electrical and electronic product available today, electric water heaters use a microcontroller to process programming information and deliver water at the required temperature. The water heater uses AC line power to control the heating element and power the microcontroller and the other DC circuitry, such as the touch-screen display and the wireless interface. Temperature probes provide the input and output water temperatures, enabling the microcontroller to control the heating element. Figure 1 shows a typical water heater, its primary circuit blocks, and recommended components for protection, efficient control, and sensing.

Figure 1. Example tankless electric water heater with recommended components that provide protection, efficient control, and sensing
 

Figure 2 shows the block diagram for an electric water heater in greater detail with the recommended protection, control, and sensing components listed in the adjacent table. This article presents how to protect the circuit blocks and offers appropriate component solutions that apply control and sensing.

Figure 2. Block diagram of a smart water heater and the table showing the recommended protection, control, and sensing components

Protecting the Water Heater from Overloads, Transients, and ESD

AC Input Protection circuit

The AC Input Protection block protects the water heater circuitry from the current overloads and high voltage transients inherent on the AC line. Use a fuse to protect against current overloads and a metal oxide varistor (MOV) to absorb voltage transients induced on the AC line.

Consider using a cartridge fuse to allow easy replacement in the event a current overload causes the fuse to trip. Ensure the fuse holder that supports the fuse is UL/CSA/IEC compliant.

Use a metal oxide varistor (MOV) to absorb AC line-induced voltage transients from machinery, appliances, and even lightning. Look for a MOV with:

• The ability to absorb as much as 10 kA of transient current
• The capacity to absorb at least 400 J of transient energy
• Compliance to IEC 61051-1 and 61051-2, Varistors for Use in Electronic Equipment.

AC/DC Auxiliary Power Supply

The AC/DC Auxiliary Power Supply powers the MCU, the Touch-Screen Display, and the Wireless Interface. Consider secondary fusing and transient protection for the sensitive semiconductors. Use a fast-acting DC fuse in this circuit. Also, consider a transient voltage suppressor (TVS) diode which can absorb up to 400 W of pulse power. TVS diodes can clamp a transient in less than 0.1 ps to avoid damaging sensitive semiconductors. To save PC board space, look for a surface mount TVS diode.

Touch-Screen Display

Users have direct contact with the display to set controls for the water heater and obtain the water heater's status. As a result, the display is susceptible to ESD from human contact. Use a TVS diode to protect the touch-screen display circuitry. Models of TVS diodes comply with IEC 61000-4-2, Electrostatic Discharge Immunity Test, and provide ESD protection up to 30 kV for either a through-the-air strike or through direct contact.

Wireless Interface

The Wireless interface transmits data and receives control information from PCs, PLCs, tablet computers, and smartphones. Protecting the data lines from ESD avoids corruption of data transmission and reception. Consider bi-directional TVS diode arrays shown in Figure 3. Versions of this TVS diode can safely absorb ±12 kV of contact ESD discharge and 40 A from a fast transient. These diodes minimize signal distortion with less than 0.1 pf capacitance added to the circuit.

Alternatively, consider polymer ESDs for protecting data lines. These components have low capacitance, typically under 0.25 pF. The leakage current is under 10 nA to minimize power consumption in the Wireless Interface circuit

Figure 3. Bi-directional TVS diode array showing two anode-connected diodes

Maximizing Efficiency and the Life of the Water Heater

Electric water heaters will typically use a TRIAC to control the heating element. A TRIAC only needs a low-power drive circuit for control, and it eliminates contact bounce when switching power to the heater and has no audible noise. TRIACs offer high reliability, with versions offering surge capacity up to 250 A and a voltage capacity up to 100 V. The solid-state switching provided by a TRIAC provides reliable long-life operation.

Tankless water heaters typically have one heating element, but larger systems can have two heating elements. One element is at the top of the tank, and the second heating element is at the bottom of the tank. For fast heating, tankless systems typically use a 40 A or a 90 A TRIAC which is a much smaller component than an electromechanical relay with a similar contact current rating. Tank systems typically use lower power, 16 A or 25 A TRIACs.

Controlling Water Temperature and Protecting the Water Heater from Overheating

Use thermistor temperature probes to monitor the water temperature at the Water Inlet and Water Outlet. A thermistor provides more accurate temperature monitoring than other temperature sensor types. The fast thermal response of a thermistor probe can contribute to the water heater's efficiency. Also, take advantage of a thermistor's small form factor.

The Benefits of Including Components for Circuit Protection, Efficient Control, and Sensing

It does not take many components to protect a water heater and enhance its reliability. Save development and compliance testing time by including the three technologies, circuit protection, efficient control, and sensing, into the project plan. Not emphasizing these aspects of the design can lead to re-design, introduction delays, and cost overruns.

Fortunately, the component manufacturer's application engineers can help save development time with:

• Cost-effective component selection
• Guidance on applicable standards
• Pre-compliance testing to help catch problems before the water heater is submitted to a standards body for compliance testing.

Using the recommended components for protection, control, and sensing will ensure a high-reliability water heater with low in-warranty repair costs and high customer satisfaction.

Littelfuse References

• Smart Building and Home Automation Application Guide
• Circuit Protection Products Selection Guide
• Sensing Products Selection Guide
• Power Semiconductor 2022-2023 Selection Guide

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Ryan Sheahen is the Global Strategic Marketing Manager for the Electronics Business Unit. Ryan joined Littelfuse in 2011 as an inside sales specialist. He was previously the Global Product Manager for the magnetic sensing product portfolio. His current responsibilities include developing marketing collateral, managing marketing activities for new product launches, and performing marketing studies and feasibility analysis for new product ideas. Ryan earned his BS in Mechanical Engineering Technology from Purdue University.