How the Gas Pilot Light Functions and 10 Reasons Why the Pilot Light Goes Out or Won’t Stay Lit!
Pilot lights can be found on furnaces, water heaters, fireplaces, ranges, ovens and other appliances. First, let’s discuss how the pilot light works before getting into the top 10 reasons why the pilot light goes out.
A pilot flame is lit with a manual push button spark ignitor or a handheld lighter while pushing down on the pilot plunger button found on the top of the gas valve. To light a gas pilot, the button on the combination gas valve must be pushed down which allows gas to enter the pilot tube and exit through the pilot termination head. Next to the termination head is the thermocouple rod. When the gas exiting the pilot termination is lit, the flame envelopes the thermocouple rod.
Pushing down on the pilot button also causes the plunger and rod to push down on the internal solenoid. The solenoid is connected to the thermocouple rod via an electrical conductor. When the thermocouple rod is enveloped by the flame, it heats up and sends millivolts through the conductor to the solenoid. This millivoltage powers the solenoid to keep the plunger down and therefore keeps the flame lit by allowing the gas to flow to the pilot tube.
When there is no heat at the thermocouple rod, no millivolts are sent to the solenoid. The result is the plunger pops upward and shuts off the gas to the main part of the gas valve and to the pilot tube.
A thermocouple is usually rated for up to 30 millivolts but only as little as 2 millivolts may be needed to power the solenoid in order to hold the plunger in place to allow gas through the pathway inside the gas valve.
Check out our "How a Combination Gas Valve and Thermocouple Work" video below if you are looking for additional info!
A thermocouple tester gives you a contact for the inner wire of the thermocouple. Measuring the millivoltage cannot be done when the thermocouple is connected directly into the solenoid. Therefore, a tester must be added. The thermocouple tester must be placed in line between the solenoid and the thermocouple. A millivolt reading of 10 mv or higher should be able to hold the solenoid plunger in the open position on most combination gas valves. Below are 10 possible problems for why the pilot flame is not staying lit!
Problem #1: A Bad Thermocouple Rod
· To test the thermocouple rod detach it from the combination gas valve. Use a multimeter set on DC millivolts with one alligator clip attached to the outer wire and the other alligator clip attached to the end of the wire (the inner wire)
· Light the pilot to get a reading on the multimeter.
· No reading or a low reading indicates the thermocouple rod is bad.
· Readings can also be taken in a closed circuit while the pilot light is lit by using a thermocouple tester. A 10 mv reading or higher would signal that the thermocouple rod is working properly. However a reading as low as 2 mv may be enough to keep the gas valve solenoid held in position. Ideally, you are looking for a higher reading closer to 10 mv.
Problem #2: A Dirty Thermocouple Rod
· A carbon deposit may build up on the end of the thermocouple rod. This build up acts like an insulator.
· Use non-soaped steel wool to clean the carbon off the rod.
· Do not use sandpaper or any other material that will use a residue on the rod.
· To prevent a clog inside the pilot tube orifice at the pilot termination head, make sure none of the steel wool gets inside it. You can either pull out the rod from the unit to clean it or simply blow out the pilot termination to get any steel wool out of it once complete.
Problem #3: The Thermocouple Rod is in the Incorrect Position
· The bracket holding the rod may be warped or the thermocouple rod may have slid down which will cause the rod to be in an incorrect position.
· If the rod is not in the correct position, the pilot flame will not make good contact with the end of the rod.
· If this is the case, the rod will not heat up and send the DC millivolt signal to the gas valve.
Problem #4: A Loose Connection
· The connection where the thermocouple connects to the gas valve may be loose.
· A loose connection will not allow the millivolts from the thermocouple into the solenoid.
Problem #5: The Solenoid is Bad
· To test the solenoid, connect your test meter between the thermocouple and where it connects into the solenoid using a thermocouple tester.
· Light the pilot and read the DC millivolts on the meter.
· If the reading on the meter indicates that there are enough millivolts (10mv or higher) to hold the solenoid open, but the pilot light goes out when the pilot button is released, then the solenoid is bad.
· You can double-check this by holding down the pilot button for a longer time just in case the solenoid needs a higher millivolt signal to remain open.
Problem #6: Low Gas Pressure or Low Pilot Flame Pressure
· Light the pilot.
· If only a small flame appears that barely touches the thermocouple and the correct inlet gas pressure is coming in, then an adjustment needs to be made to the pilot adjuster screw.
Problem #7: A Kinked or Clogged Pilot Tube
· If after you’ve followed the procedure mention in #6 above and the flame is still low, there may be a kink in the pilot tube or the tube may be clogged.
· Remove the tube from the pilot termination.
· Blow compressed air through the tube to remove any obstruction.
· There could be a clog in the orifice of the pilot termination.
· Do not use a needle to try to remove the clog as the needle may inadvertently enlarge the hole.
· If you are unable to remove the clog, the entire assembly needs to be replaced.
Problem #8: Gas is Off
· The manual gas valve outside the appliance may be in the off position (the handle is perpendicular to the direction of the gas flow).
· The gas meter outside the building may be turned off or the propane tank may be empty.
Problem #9: A Bent or Warped Pilot Termination Head
· Over time the pilot head may become bent or warped. Rather than bending it back into place, it may be best to replace it.
Problem #10: The Pilot Being Blown out by Wind
· Depending on the location of the appliance and the exhaust, a wind source may be blowing out the pilot flame.
· Check for any source of wind drafts and correct exhaust location and diameter.
Are you looking for more about understanding why a pilot light may go out? Be sure to check out our "TOP 10 Reasons Why the Gas Pilot Light Goes Out & Won't Stay Lit" video below or over at https://www.youtube.com/acservicetechchannel!
Published: 12/26/2019 Author: Craig Migliaccio
About the Author: Craig is the owner of AC Service Tech LLC and the Author of the book “Refrigerant Charging and Service Procedures for Air Conditioning”. Craig is a licensed Teacher of HVACR, Sheet Metal, and Building Maintenance in the State of New Jersey of the USA. He is also an HVACR Contracting Business owner of 15 years and holds an NJ HVACR Master License. Craig creates educational HVACR articles and videos which are posted at https://www.acservicetech.com & https://www.youtube.com/acservicetechchannel