Thermoelectric sensors - thermocouples - are used exclusively in gas boilers and water heaters equipped with non-volatile safety automatics. The element's task is to monitor the presence of a burner flame by continuously supplying voltage to the solenoid valve of the SIT 630 control unit (or the like). Our goal is to tell you what a thermocouple is, how it works and changes in the event of a malfunction.
Thermoelectric flame sensor device
A thermocouple is a safety feature of a gas boiler that generates voltage during heating and keeps the fuel supply valve open while the ignitor is on. The sensor shown in the photo acts autonomously, without connecting an external power source. The scope of thermocouples is gas-powered non-volatile installations: stoves, kitchen stoves and water heaters.
Let us explain the principle of operation of the thermocouple for the boiler, based on the Seebeck effect. If you solder or weld the ends of 2 conductors of different metals, then when this point is heated, an electromotive force (EMF) is generated in the circuit. The potential difference depends on the junction temperature and the material of the conductors, usually lies in the range of 20 ... 50 millivolts (for household appliances).
The sensor consists of the following parts (the device is shown below in the diagram):
- a hot junction thermoelectrode made of two dissimilar alloys, screwed with a nut to the mounting plate next to the pilot burner of the boiler;
- extension cord - a conductor enclosed inside a copper tube, simultaneously playing the role of a negative contact;
- positive terminal with a dielectric washer, inserted into the socket of the automatic gas valve and fixed by a nut;
- There are varieties of thermocouples connected to the automation using conventional screw terminals.
Note. A copper tube is needed to protect the positive conductor from external interference created by the 220 V house network and other electrical appliances. Remember: the minimum thermocouple voltage is only 20 mV.
For the manufacture of electrodes that produce EMF, special metal alloys are used. The most common thermal pairs:
- chromel - alumel (type K according to the European classification, designation - TXA);
- chromel - copel (type L, abbreviation - THC);
- chromel - constantan (type E, designated THKn).
Reference. Alumel is an alloy of nickel with aluminum, manganese and silicon. The composition of chromel is 90% nickel, 10% chromium. Copel also includes nickel combined with copper and silicon.
Principle of operation as part of the boiler
The connection diagram of the thermoelectric sensor in various gas-using devices is approximately the same. The measuring electrode is located in the area of the wick or the main burner, the conductor is connected to an electromagnet that opens the gas supply.
Reference Information. In turbocharged and atmospheric heat generators connected to the home electrical network, a photoelectric sensor can be used instead of a thermocouple. It detects the presence of fire without direct heating.
How does a thermocouple work on floor-standing boilers like AOGV and similar devices:
- The user presses the button with one hand and forcibly opens the gas supply solenoid valve.
- With the second hand, the homeowner turns on the piezo ignition while holding the first key. The igniter flashes.
- According to the instruction manual, the button must be held for 5-30 seconds (depending on the model of the unit), during which the wick warms up the measuring electrode.
- A direct current arises from the thermoelectrodes in the circuit of the electromagnet. The user releases the key, but the fuel supply does not stop - the valve now holds the thermocouple voltage.
If, for various reasons, the fire goes out, the heating of the thermocouple ends, the EMF disappears. The electromagnet will shut off, the spring will shut the valve and block the fuel path.
Reference. Electricity-free gas water heaters are equipped with automation from various manufacturers - EuroSIT, Zhukovsky Plant, Arbat, Orion, and so on. A thermocouple everywhere works on the same principle - while the electrode is heated by a flame, the gas supply will be open.
Differences from the temperature sensor
In addition to a thermocouple, a thermal bulb is connected to the automatic fuel valve of the boiler, which is responsible for turning off the main burner when the set temperature of the coolant is reached. Externally, the flasks of the elements and the copper connecting tubes are a bit similar. An ignorant homeowner can easily confuse these sensors.
We list the main differences between a temperature meter and a thermocouple:
- sensor design - a cylindrical bellows made in the form of a flask of copper with a sealed end;
- the thermoball is connected to gas automatics with a thinner capillary tube than the electricity generating sensor;
- the thermosensitive flask itself is installed inside the immersion sleeve or is hidden under the casing near the water jacket, and is not mounted near the igniter;
- the temperature meter does not disconnect from the automation at all or differs in the size of the fixing nut.
Note. The thermal bulb acts on a different principle: when heated, a special liquid expands inside the flask. The capillary pressure is transmitted to an automation valve that turns off the main burner. Igniter flame does not die out.
How to check and replace a thermocouple
The main symptom of a flame sensor malfunction is that the wick goes out simultaneously when the button is released. Sometimes the problem manifests itself differently - the light on the igniter remains, but after ignition of the main burner, the fuel supply is again shut off and the boiler goes out completely. The causes of such problems:
- the thermal electrode is covered with soot and does not warm up well, which means that the voltage in the circuit drops below a minimum;
- burnout of the meter body;
- violation of contact at the hot junction;
- the fastening nut was unscrewed, the working rod was skewed and did not warm well with the igniter;
- the traction sensor has become unusable or an open circuit has occurred.
Clarification. A malfunction of the traction sensor causes similar symptoms, since this “trailer” is connected in series with the thermocouple (in the open circuit). To exclude the influence of the sensor, temporarily short its wires.
For diagnostics, you will need a multimeter or another device that can measure low voltage (up to 100 mV). How is the check performed:
- Close the gas supply to the heat generator or the instantaneous water heater with a tap located on the inlet pipe. Remove the cover or front panel of the unit.
- Using open-end wrenches, unscrew the nuts securing the thermoelectrode and connecting tube. Remove the flame sensor.
- Carefully inspect the working electrode, clean it from soot with a brush and rags. If burnout is detected during visual inspection, the thermocouple must be replaced unconditionally.
- Once in the kitchen, light the gas stove burner. Connect the clamps of the multimeter to the center pin and the copper tube. Set the lowest measurement limit - 0.1 or 1 V.
- Heat the thermocouple with a gas stove burner, observing the voltmeter readings.
Main condition: a working thermocouple for the boiler should give a voltage of at least 0.02 volts. If the device shows zeros, the voltage jumps or does not exceed 20 mV, the element must be changed. Modern sensors cannot be repaired by soldering.
Tip.When buying a new thermocouple, always be guided by the brand and specific model of the boiler, so as not to get confused in the markings and designations.
If you do not want to remove the element ahead of time, diagnostics can be done directly on the boiler. Having unscrewed the nut, disconnect the thermocouple tube from the automation and connect the multimeter, as described above. While holding the key, light the igniter and take readings. The disadvantage of this method is the impossibility of visual inspection and cleaning of the electrode from soot.
When installing a new thermocouple in a gas boiler, adjust the position of the heated rod. Ideally, the electrode stands horizontally, does not deviate up or down and is well washed by the wick flame.