Instant Water Heater: Design Rules and Process Limits

An instantaneous water heater switches several kilowatts of resistive load under the control of a small electronic board, and it does so in a wet environment with a person standing next to it. Almost every design decision follows from that sentence.

What the Board Controls

The board measures the inlet temperature and the flow, calculates the power required to reach the set point, and switches the heating element to deliver it. It also manages the display, the user interface and the safety chain.

The measurement and the switching are separated by the isolation barrier, and the layout has to respect that boundary on every layer rather than only on the surface.

The Heating Load

A resistive element of several kilowatts draws tens of amps from a single phase supply. The switching device, the terminals and the tracks that carry that current are sized for the continuous rating plus a margin for the tolerance of the element.

The element itself is a large thermal mass. Its resistance changes with temperature, so the power delivered for a given duty cycle falls as it heats, which the control loop has to accommodate rather than assume away.

Switching Options

A relay is cheap and has a low conduction loss, but it switches slowly and its contacts wear. A triac switches quickly and silently, at the cost of a forward drop and a need for thermal management of the device itself.

Where fine control is required, the triac is operated with phase control so that the power is set by the conduction angle. Where the load can tolerate it, a burst mode that switches whole cycles produces less harmonic disturbance.

Zero Crossing and Phase Control

The controller needs to know when the mains crosses zero, both to synchronise the phase control and to switch the element on and off at the least disruptive moment. The detection is done through a high value resistor and an optocoupler.

The detection circuit is on the mains side and its output crosses the isolation barrier. The resistor chain, the creepage across the optocoupler and the layout of the tracks carrying mains voltage are all part of the safety design.

Flow and Temperature Sensing

The flow is measured with a turbine, a paddle or a differential pressure sensor, and the temperature with a thermistor or a thermocouple. Both signals have to be accurate enough for the control loop and fast enough to detect a loss of flow.

The outlet temperature is the variable that matters for safety. It should be measured close to the element rather than at the outlet spout, so that the protection acts before the water leaving the product becomes dangerous.

The Control Loop

The loop adjusts the power to hold the outlet temperature at the set point. Its response has to be fast enough to follow a change of flow without overshooting the temperature limit, and slow enough not to react to the ripple of the measurement.

The loop also has to handle the case where the flow starts and stops. When the tap is closed, the element has to be switched off before the water in the heater boils, and the detection of that condition is a safety function rather than part of the control.

The Safety Chain

Protection covers over temperature, loss of flow, leakage current and the behaviour of the electronics itself. A thermal cutout that operates independently of the controller is required, because the controller is the component that might fail.

A residual current device tripping at thirty milliamps within thirty milliseconds is part of the product specification. The board provides the sensing, and the device that interrupts the supply is separate from the switching device used for control.

Isolation and Creepage

The mains section and the control section are separated by a distance that depends on the working voltage and on the pollution degree of the environment, which for a bathroom product is severe.

Slots in the board are used to increase the creepage without increasing the area, and they must be present on all layers. A plane that approaches the barrier on an inner layer cancels the separation achieved on the surface, which is a point the EMC and layout review should confirm.

Supply for the Control Section

The control electronics need a low voltage supply that is isolated from the mains. A small flyback converter with a wide input range provides it, and it has to work when the mains is at its lowest specified value.

The supply also has to survive the surge that the product is tested with. A varistor at the input and a series impedance before the converter are the usual measures, and their placement matters more than their value.

Thermal Design

The switching device, the bridge rectifier and the supply converter all dissipate heat inside a small enclosure. The board is usually mounted in the water path, which helps, but the copper under each device decides the junction temperature.

Thick copper on the power layers reduces both the conduction loss and the thermal resistance, which is why a product of this kind is often built on a heavier laminate or thicker copper than a consumer board.

Test and Certification

Testing covers the electrical safety tests, the leakage current, the temperature rise at the rated flow, the behaviour with a blocked outlet and the electromagnetic compatibility. Each of them is a requirement of the standard rather than an optional check.

The temperature and flow tests are performed at the extremes of the supply voltage and of the inlet water temperature. A product that passes at the nominal conditions may fail at the corner, and the corner is where the certification body will look.

Additional Considerations for This Build

Practical attention to instant water heater pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating instant water heater explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

Deliberate attention to flow sensor pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating flow sensor explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

Process Control and Verification

On a design of this kind, safety is the item that decides how the rest of the board is arranged. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.

Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

FAQ

Why is a thermal cutout needed if the controller measures temperature? Because the controller is a component that can fail. The cutout operates independently and removes the risk of a single point failure.

Should the element be switched by a relay or a triac? A relay is cheaper and has a lower loss, a triac is faster and silent. The choice follows from the control resolution the product needs.

What has to be verified for the flow signal? Its accuracy at the rated flow and its response to a sudden loss of flow, since that condition has to switch the element off before the water boils.