Sep 30, 2026
If you're specifying a capillary thermostat for a water heater, the SPST vs. SPDT decision shows up early. Both switch types use the same liquid-expansion sensing principle. Both mount in the same location. But the circuit topology inside the switch determines how many heating elements the thermostat can control, how the tank manages power demand, and what happens when the upper element reaches setpoint. Get it wrong and you either waste energy heating both elements simultaneously or fail to deliver adequate hot water recovery.
Most residential electric water heaters use two heating elements-upper and lower-but only one runs at a time. This "non-simultaneous" operation keeps peak electrical demand within the limits of a standard 30-amp circuit. The thermostat that makes this sequencing possible is the SPDT capillary switch. Specifying an SPST where an SPDT is needed means both elements could energize together, tripping breakers or requiring a larger circuit. Specifying SPDT where SPST is sufficient adds cost and wiring complexity with no functional benefit.
An SPST (Single-Pole Single-Throw) capillary thermostat is a simple on/off switch with two terminals. When the sensed water temperature drops below setpoint, the contacts close and power flows to one heating element. When temperature rises above setpoint, the contacts open and power stops. One thermostat, one circuit, one element.
An SPDT (Single-Pole Double-Throw) capillary thermostat has three terminals: common, normally closed, and normally open. As temperature changes, the switch transfers the common terminal from one contact to the other. In a water heater, this diverts power between two separate circuits-typically the upper element and the lower element circuit. The upper SPDT thermostat senses water at the top of the tank. When the upper portion reaches setpoint, the switch transfers power away from the upper element and down to the lower thermostat, which then controls the lower element.
| Design Consideration | SPST Capillary Thermostat | SPDT Capillary Thermostat |
|---|---|---|
| Terminals | 2 (input, output) | 3 (common, NO, NC) |
| Elements controlled | 1 | 2 (with companion SPST) |
| Typical position | Lower element control | Upper element sequencing |
| Circuit function | Simple on/off | Diverts power between circuits |
| Wiring complexity | Two-wire connection | Three-wire connection |
| Best for | Single-element tanks, lower element control | Dual-element non-simultaneous designs |
| When to avoid | Dual-element tanks needing sequencing | Single-element tanks (unnecessary cost) |
Single-element water heaters-common in small point-of-use units and some commercial tanks-have only one heating element. There is nothing to sequence. A single SPST capillary thermostat controls the element directly. Specifying SPDT here adds cost and wiring complexity with no benefit.
Dual-element residential water heaters require the upper thermostat to sequence power between the two elements. The upper SPDT thermostat also typically includes an integrated high-limit cut-off, calibrated to trip at 170°F (77°C) with manual reset. This is the industry-standard architecture. The lower thermostat is usually SPST, controlling only the lower element once the upper tank is satisfied.
Some commercial or high-recovery water heaters allow both elements to operate at once. These designs do not use an SPDT thermostat for sequencing. They typically use two separate SPST thermostats with independent control, or a controller that manages both circuits. An SPDT capillary switch is not the right component for this architecture.
Temperature range. Standard water heater ranges are +30°C to +110°C, with custom options available.
Electrical rating. Confirm the thermostat's rating matches your element wattage and voltage-common ratings are 16(3)A 250VAC and 30A at 240V.
Capillary length. Match the tube length to your tank geometry; standard lengths run from 300mm to 3000mm.
High-limit function. Verify whether the high-limit cut-off is integrated into the SPDT thermostat or provided as a separate limit control.
If you're designing a water heater and need help specifying the right capillary thermostat configuration, send us your tank layout, element wattage, voltage, and target temperature range. Our engineering team at Jaye Heater will recommend the correct switch type-SPST or SPDT-with the capillary length, bulb diameter, and electrical rating that matches your design, provide samples for testing, and quote production pricing. We've been making capillary thermostats for water heater OEMs for over a decade.
Click here to request a quote or email us . We'll respond within one business day.
The three terminals are labeled common, normally open, and normally closed. If lead wires are supplied, color coding is common: COM, NO (blue), NC (black). Confirm labeling with your supplier before finalizing assembly instructions.
Behavior depends on design. Some capillary thermostats include fail-safe action that opens the contacts if the expansion fluid leaks or if sensed temperature drops below a threshold-often around -20°C. Without fail-safe, a loss of fluid could leave the switch stuck closed, allowing continuous heating. Specify fail-safe action if your risk assessment requires it.
Most water heater thermostats operate with a differential of about 5% of the action temperature. At a 120°F setpoint, that's a 6°F differential. A wider differential means fewer cycles and longer contact life but greater temperature swing. A narrower differential means tighter control but more frequent cycling. Match the differential to your product specification.
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