Sep 11, 2026
A millivolt gas valve can control a gas appliance without a conventional external electrical power supply. But how does the system know when to turn the gas burner on or off?
A capillary thermostat can provide the temperature control by sensing temperature through a bulb and mechanically opening or closing the low-voltage control circuit.
This combination is commonly used in gas-fired equipment where simple, self-powered temperature control is required.
A millivolt gas valve is designed for gas appliances that use a small electrical signal generated by a thermopile or thermoelectric system.
For example, Resideo's millivolt gas valve systems use a thermopile to generate the electrical energy required to operate the gas valve, while a millivolt thermostat completes the control circuit.
The basic control sequence is:
Pilot Flame → Thermopile → Millivolt Signal → Thermostat → Gas Valve → Burner
Unlike a conventional 24 VAC control system, the control circuit works with a very small electrical voltage.
A capillary thermostat consists mainly of:
The sensing bulb is installed where temperature needs to be monitored.
As the temperature changes, the fluid inside the sensing system expands or contracts. This movement operates the thermostat contacts.
The thermostat can therefore open or close the millivolt control circuit according to the set temperature.
Temperature Changes
↓
Capillary Bulb Senses Temperature
↓
Thermostat Contacts Open / Close
↓
Millivolt Signal Controls Gas Valve
↓
Gas Burner Turns On / Off
This allows the gas appliance to maintain a relatively stable operating temperature without requiring a conventional powered electronic controller.
The millivolt system can obtain its electrical energy from the thermopile heated by the pilot flame.
This makes it useful for gas appliances where a conventional electrical supply is unavailable or undesirable. Resideo describes its millivolt gas controls as self-powered systems using a thermopile generator.
The sensing bulb can be positioned directly in the area where temperature needs to be measured.
For example, commercial gas cooking equipment can use a capillary bulb to sense cooking temperature and control the gas supply.
A capillary thermostat does not require a digital controller, software, or complicated electronics.
This makes the design relatively simple and suitable for equipment that requires straightforward temperature control.
The sensing bulb can be connected to the thermostat through a capillary tube, allowing the temperature-sensing point to be separated from the thermostat body.
This is useful when the control knob needs to be installed on a panel while the sensing bulb must be located inside the heated area.
This type of temperature-control system can be found in various gas-fired appliances and equipment, including:
For example, millivolt thermostats with capillary bulbs are commonly specified for commercial gas fryer temperature control.
The exact thermostat design depends on the equipment temperature range, sensing location, gas valve, and required switching characteristics.
For a millivolt gas valve application, the thermostat should not be selected only by temperature range.
Important parameters include:
| Parameter | Why It Matters |
|---|---|
| Temperature range | Must cover the required operating temperature |
| Bulb size | Must fit the sensing location |
| Capillary length | Determines installation flexibility |
| Switching action | Must match the required control logic |
| Electrical rating | Must be suitable for the millivolt control circuit |
| Contact resistance | Low resistance is important in low-voltage circuits |
| Mounting | Must match the equipment panel |
| Terminal configuration | Must match the gas valve/control circuit |
Because millivolt systems operate with very low electrical voltage, unnecessary resistance in thermostat wires, terminals, or connections can affect system performance.
Service documentation for millivolt gas equipment specifically emphasizes checking thermostat wiring and connections for excessive resistance.
A properly selected capillary thermostat can provide:
The key is matching the thermostat to the millivolt gas valve, temperature range, sensing position, and electrical characteristics of the complete system.
A capillary thermostat provides a simple way to control temperature in a millivolt gas valve system.
The basic relationship is:
Thermopile → Millivolt Power → Capillary Thermostat → Gas Valve → Burner
For equipment manufacturers, the most important factors are not only the thermostat's temperature range, but also its bulb dimensions, capillary length, switching action, electrical characteristics, and compatibility with the millivolt gas valve.
For custom gas appliance applications, these parameters should be evaluated together to ensure reliable temperature control.
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