Introduction To Thermostat Control

The thermostat responds to water temperature to turn the burner off or on so that a predetermined temperature is maintained. Thermostats are essentially electrical switches operated by changes in temperature. Most thermostats have two major components, a switch and a sensor. Temperature changes affect the sensor which, in turn, operates the switch. In most cases a rise in temperature causes a physical action of some sort, which results in activating a switch, which breaks or opens an electrical circuit. This break in the electrical circuit cuts power to the burner system and burner firing stops. When the thermostat’s sensor cools, the circuit is completed again and the burner resumes firing.

Thermostats In Brief, What They Are And How They Work

Thermostats are electrical switches operated by changes in temperature. Generally a thermostat has two major components, a switch and a sensor. Temperature changes affect the sensor, which, in turn, operates the switch. In most cases a rise in temperature causes a physical action of some sort, which results in activating a switch, which breaks an electrical circuit.

Breaking The Circuit To The Fuel Solenoid

In most oil-fired hot water cleaner designs the switch breaks the electrical circuit to the fuel solenoid. This break in the electrical circuitry cuts power to the fuel solenoid valve, the valve closes, and fuel is diverted into bypass and burner firing stops since no fuel is reaching the combustion area.

In a gas-fired system the thermostat will cut off gas flow at the gas valve. When the thermostat’s sensor cools, the circuit is completed and the burner resumes operation.

The most common types of thermostats, bi-metallic and capillary tube; rely on expansion of different materials over a certain distance as a result of a temperature increase. Scientific knowledge of the properties of materials when exposed to heat allows precise prediction of how far a material will expand or contract as the temperature varies.

Bi-Metallic Thermostats
The bi-metallic thermostat, as might be guessed by its name, relies on the difference in rates of expansion and contraction of various types of metals when they are exposed to temperature changes. The thermostat sensor is made up of two strips of dissimilar metal bonded to each other. When temperature changes, the two metal strips expand at different rates. The sensor bends in the direction of the strip, which expands more slowly. Moving a contact either closer to farther from the bi-metallic strip sets the thermostat. The further the metal strip has to travel to reach the contact, the greater the temperature change before the switch is activated.

The most common type of bi-metallic thermometer is the compact, single-knob Fenwall. A bolted-on protective cover shields the Fenwall’s electrical terminals. Different types of metals are used in bi-metallic thermostats and the important factors are the combination of metals and their different expansion qualities. In most cases, changing the distance between contacts sets these thermostats. With the contacts closer together, less change in temperature is needed to activate the device. With a larger gap, a greater temperature change is needed. These types of thermostats are usually the least expensive and are found on the least expensive cleaners.

Capillary Tube Thermostats
A capillary tube thermostat also relies on expansion to open or close a switch. However, in the case of these devices, a viscous liquid rather than metal is used to respond to temperature changes.

Essentially a capillary tube thermostat consists of a sensing bulb and tube with an extremely small internal diameter partially filled with a special compound. The compound expands when heated and activates a switch. The capillary tube thermostat may be mounted some distance from the sensing bulb. The length of the tube determines the distance of maximum separation.

Thermistors
A third type of thermostat uses a thermistor or temperature-sensitive resistor to detect temperature changes. The thermistor is seldom used directly as a control. Another device, usually a computer-type memory chip on the circuit board, responds to changes in the sensor’s resistance to control burner firing.

External Mounting
In most hot water high pressure cleaners, the thermostat’s sensor is mounted outside the heat exchanger’s pipe or tubing. Immersible sensors are available but generally cannot stand up to the pressure present on the output side of a high pressure cleaner.

The Strap-On Thermostat

One of the most popular thermostats is the strap-on variety, which is secured to the outside of some portion of the output side of the fluid heating system plumbing. The strap-on type thermostat can have a capillary tube sensor on its reverse side, which is pressed against the exterior of the boiler discharge pipe. Others may use a bi-metallic sensor, which responds to heat conducted through the mounting base. This mounting base is strapped or secured directly to a discharge fitting or pipe. Any strap-on thermostat must have firm, metal-to-metal contact with the discharge plumbing to get a good temperature reading.

The Thermo Well

Another method for installing a thermostat sensor is to use a thermo well. Essentially a thermo well is a threaded plug, which can be mounted in a fitting or tap block on the discharge side of the boiler. This plug has a cavity drilled in it from the outside, which is used for mounting the thermostat’s sensor. This configuration safely separates the fragile capillary tube sensor from the high pressure on the other side of the thermo well. Heat is conducted through this protective sheath. Some European cleaners have immersed sensors. This is generally a resistive-type sensor designed to hold up under high-pressure conditions.

Thermostat Testing And Service
The thermostat will generally (but not always) have three terminals (common, normally open and normally closed). It is generally wired for normally closed operation. Make sure you are testing the correct two terminals – usually the common and the normally closed. The manufacturer usually marks these terminals.

To test the thermostat, the system including the boiler, must be operating. While checking for continuity, turn down the thermostat until boiler firing stops. There should be continuity until firing stops. Adjust the thermostat upward until boiler firing begins again. There should be no continuity until boiler firing begins again. If a thermostat is found to be defective, it generally should be replaced.