Many homeowners are concerned about installing a natural gas-powered heater in their home. For the most part, these fears are unfounded; as long as they are serviced once a year to ensure that all of their components are working properly, gas furnaces pose minimal health risks and should operate efficiently and safely for many years.
A device known as a thermocouple is one of the ways that gas furnace manufacturers make their products safe. To keep you secure, we’ll explain what it is and how it works.
The thermocouple
A thermocouple is a device that is made up of two metal strips of different materials. It’s positioned such that its ends face the furnace’s pilot light. The thermocouple senses heat from the pilot light, and if the pilot light is accidently turned off, the thermocouple will shut off the gas valve. This keeps unburned gas out of the combustion chamber, where a small spark could result in an explosion.
The thermocouple works by using a thermoelectric action known as the “Seebeck effect,” which was discovered in 1821 by Thomas Johann Seebeck, a German-Estonian researcher. Seebeck discovered that two different types of metal in close proximity will generate electrical voltage proportional to the heat they detect. The voltage rises as the temperature around the two metal pieces rises.
The electric voltage of a thermocouple in a gas furnace is meant to keep the gas valve open. When the pilot light goes out, the voltage between the two pieces of metal drops, which closes the valve and reduces the chance of combustion.
It’s a great achievement for such a basic and compact equipment. A thermocouple, on the other hand, can break, rust, or fall out of place, posing a threat if the pilot light fails. This is one of the reasons you should have your gas furnace serviced on a regular basis: technicians will detect faulty thermocouples and replace them with the suitable unit.
(Instead of a thermocouple, some modern gas furnaces utilize a mercury sensor.) Your technician will also know how to deal with any issues that may arise with mercury sensors.)
What is a gas thermocouple and how does it work?
When the temperature of the thermocouple varies, an electrical current is generated, which forces the gas valve that supplies the pilot light to open when the temperature is high or close when there is no direct heat source.
Is it necessary to place the thermocouple in the flame?
To access the burner assembly and pilot, remove the furnace cover panel. You should be able to see the pilot light’s flame clearly.
The color of the flame should be checked first. A natural gas flame should be bright blue with a hint of yellow at the very tip. A propane flame will be bluish-green in color with a yellow tinge at the tip. The flame should be powerful enough to cover the end of the thermocouple tip by about 1/2 inch.
The flame will be blue and may produce a hissing sound when it crosses the thermocouple if it is too strong and not regulated correctly. The intensity of the flame should be reduced by adjusting the pilot.
Incomplete combustion and a lack of oxygen generate a yellow flame. The pilot light will not get hot enough to heat the thermocouple to the temperature required to open the gas valve if it is a feeble yellow flame. A dirty pilot tube tip is frequently to blame.
A draft is the most common cause of a flickering or wavering flame. Examine the space for sources of drafts and take efforts to mitigate their impact on the furnace.
Is it possible to avoid using a thermocouple?
You’ve effectively disabled the thermocouple, but you can also reach in and unclip it from its holding bracket near the pilot light. It will be fastened on by a gravity clip in most cases, but others will be held on with a screw that will need to be removed with a small screwdriver.
When a thermocouple fails, what causes it to do so?
Thermocouples, like anything else, don’t last forever. A good thermocouple will be utilized, which can result in metal fatigue. Temperature changes can cause metal to expand and contract on a regular basis, causing thermocouples to deteriorate over time. Metal fatigue can cause a thermocouple to shatter over time.
Metal fatigue is more frequent in thermocouples that are exposed to significant heat stress or extreme circumstances on a regular basis. It’s possible that metal fatigue is causing strange readings from thermocouples.
Is it possible to clean a thermocouple?
A thermocouple is a sensor that consists of two metal wires joined at the sensing end. The temperature being measured is reflected in the voltage measured on the other end. Although their precision is slightly lower than that of an RTD, they have the widest temperature range, ranging from -200 to 1750 degrees Celsius, and are often more cost-effective.
To put it another way, a temperature sensor perceives the temperature of any substance that has to be measured, whether it’s solids, liquids, or gases. When these Thermocouples and RTDs are linked directly to a PLC, the measurement precision can be less than ideal. Electromagnetic interference is a common cause of precision loss (EMI). EMI mistakes can be efficiently reduced by converting thermocouple and RTD signals to industry standard 4-20 mA current.
Thermocouples produce a voltage signal of less than 50 millivolts, and they have almost no ability to generate current flow. As a result, any device that monitors a thermocouple needs an extremely large input resistance (often 1 million or more).
The thermocouples act like an antenna when they have a low mV signal and no current flow. A thermocouple can “receive” electrical noise from 50/60 Hz power mains, lightning bursts, static electricity, radio frequency interference from portable radios, commutator noise from electric motors, and many other sources and the longer the wires are, the more opportunities there are to “receive” electrical noise.
Yes, a thermocouple can be cleaned, but it must be done with caution. Remove the thermocouple from the process assembly and wipe the tip with steel wool or an emery cloth to remove any buildup.
Even when most of the details of an application are known, predicting useful thermocouple life is difficult. Unfortunately, such data is frequently difficult to come by. Installing the unit, using it, and evaluating the in-use performance of a design that is thought to succeed is the finest test for any application. When choosing an assembly style to put in a process, the recommendations and non-recommendations mentioned under the thermocouple type descriptions are a good place to start.
Because the thermoelectric emf produced by a given temperature difference is susceptible to changes in the wire’s chemical and metallurgical properties, the total emf produced by a used probe under the same conditions can differ from that of a new one. Over long periods of time, the changes are usually tiny (often negligibly small). However, under bad conditions, substantial drifts can occur at a quick rate.
The normal technique for achieving long and dependable thermocouple life is to keep the device at its maximum temperature and supply it with the cleanest possible working environment. Enclosures such as sheaths, protective tubes, and thermowells are commonly used to manage the conditions surrounding the thermoelements.
Corrosion or mechanical damage can cause protective tubes, sheaths, and even thermowells to fail. Processes can overheat, exposing thermoelements to temperatures that are higher than expected. If the output of a sensor controlling a process drifts low, the process, in reaction to its controller, may be forced to greater temperatures than intended. A few chemical compounds are capable of attacking base metal assemblages. They can be changed as well.
A portable temperature meter is a handy tool for troubleshooting thermocouple systems. A handful of these devices can work with two or more different thermocouple kinds, and some even have a “output” function that generates an electrical output to simulate a thermocouple at any temperature.
The instrument is generally connected to the wires of the circuit being tested at a convenient access point, such as a connecting head, when in use. Maintaining the correct polarity should be a priority. We utilize the ANSI color coding in Australia, where the negative is always RED. An working sensor’s output can be monitored and assessed there. Alternatively, a simulated thermocouple signal can be delivered back to the circuit’s permanent indicator or controller using an instrument’s ‘output’ function to ensure that the rest of the circuit is working properly. When driving a signal back towards an instrument, one side of the circuit must be broken to avoid ‘loading’ the portable tester with the thermocouple’s low resistance.
A portable tester can also be used to verify for appropriate connections in sections of extension wiring in thermocouple circuits. One end of an extension wire pair should be shorted together, and the segment being tested should be electrically isolated from the rest of the loop. If a tester is connected to the shorted pair’s opposite end, the tester should report the shorted end’s approximate temperature. If both ends of the extension pair are at the same temperature, it may be required to warm the shorted end a little and double-check that the tester’sees’ the temperature difference properly. In this test, the potential of an erroneous, reversed connection is investigated.
Which of the thermocouple wires is the positive one?
The negative lead of insulated thermocouple wire is usually red, according to industry standards. The positive lead is the same color as the thermocouple and the insulated extension grade wire in general. The thermocouple wire’s outer jacket is usually brown.
A thermocouple should read a certain number of ohms.
The resistance of a thermocouple should be very low. The negative lead should be connected to the red wire, while the positive lead should be connected to the yellow wire. The reading will be around three ohms in this scenario.
When a thermocouple is heated, what happens?
A voltage (potential difference) emerges when one end of the thermocouple is placed on something hot (the hot junction) and the other end is placed on something cold (the cold junction).
When I release the pilot light knob, why does my pilot light go out?
Remove the access cover at the bottom of the water heater to relight the pilot. The water temperature knob and the control knob should both be set to “Pilot.” Light the pilot light using a long match or wand lighter while depressing the control knob. Hold the knob down for a full minute after the pilot ignites to bleed air out of the line. (The relighting procedure is nearly same for a water heater with an automatic igniter.) You’ll push the striker knob repeatedly while pushing the control knob until the pilot ignites, rather than using a lighter or match.)
Relighting the pilot light may or may not be enough to get your water heater working again. Follow one of the following steps, depending on what happens after the pilot ignites:
- You’re fine to go if the flame stays lit! To return to a relaxing hot shower, simply replace the cover plate, turn the control knob to “On,” and pick the desired temperature on the water temperature knob.
- Clean the pilot orifice if the pilot light flickers and goes out immediately after relighting. Gas flow is hampered by a dirty pilot light aperture, but the solution is easy. Turn off the gas to the water heater first (look for a valve on the gas line that supplies the unit). By twisting the pilot orifice fitting to the left, you can remove it from behind the access cover. The orifice should then be unscrewed from the fitting. Clean all surfaces with a cotton swab soaked with rubbing alcohol after the fitting has been disassembled. Relight the pilot light as instructed above after reassembling and reattaching the fitting.
- The thermocouple should be replaced if you can ignite the pilot light but it goes out when you release the control knob. The thermocouple is a safety device that shuts off gas flow if it detects that the pilot light is out, but it loses its ability to regulate when it is broken. Although this repair is a little more difficult than the first two, a replacement is relatively inexpensiveoften less than $20.
The control panel is connected to the burner assembly, which is positioned behind the access panel, by this item, which resembles a copper tube. Turn off the gas to the water heater before attempting to disassemble anything. Next, detach the thermocouple tube, pilot light tube, and gas supply tube from the control panel using an adjustable wrench; the burner assembly should slide easily out. (Hint: Because thermocouples come in a variety of sizes and types, the best approach to ensure a precise match is to bring the damaged thermocouple with you when shopping for a replacement.) Slide the burner assembly back into place, rejoin the tubes, and relight the pilot light as indicated above after replacing the damaged thermocouple with a new one.