Pilot Burner that Feeds the Flame to the Thermocouple (not pictured)
So you have found the pilot light will not stay lit when you try to light the pilot light. You know you have gas and everything seems to be fine so what is the problem? You ask a friend and the friend tells you the thermocouple is bad. So how do you test the thermocouple to see if it is bad or not? You will need a multi-meter that will read millivolts (most meters read small voltage like millivolts which is 1/1000th of a volt) and a source of fire such as a lighter or small torch.
Remove the thermocouple.
Turn on the meter and set it to Ohms. Touch the two leads together from the meter and you should get a reading close to zero. With the leads apart you will read infinity.
Start the flame and put the tip of the thermocouple in the fire.
Ensuring the tip is nice and hot from the flame take one lead from the meter and put it on the shaft of the thermocouple and the other lead and put on the end of the thermocouple where it makes contact with the gas valve.
If the reading is less than 25 millivolts replace the thermocouple as most standing pilot gas valves need 25 or more millivolts to keep the pilot flame lit.
Thermocouple fits securely in the pilot burner (pictured above)
If you test the thermocouple with the multimeter and you get a reading above 25 then you have another issue. Always make sure the tip of the thermocouple is directly in the pilot flame. If the tip is in the flame, you have held the nob on the gas valve down for longer than a minute, the pilot flame is strong and steady, and the pilot still does not stay lit then you likely have problem with the gas valve. Gas valves can not be repaired as per the manufacturer and need to be replaced. Good luck!
This is a Trane gas furnace. It is also called a Trane 90 plus gas furnace because it offers efficiencies up to 90 percent plus AFUE. Gas or oil furnaces generally cannot achieve efficiency at 100% because of impurities in the fuel including moisture. This is the reason why no fossil fuel furnace sold has a rating of up to 100%. This biggest impurity in many fuels is moisture which is difficult to remove because it is in the distribution and storage places where the fuel is distributed and stored. If these impurities were removed from the fuels before they were introduced to the furnace the cost of the fuel would be much higher and the cost to benefit ratio would not be effective.
Additional Efficiency Factors
Fuel purity cannot be overcome without costly and excessive filtering that makes the savings factor useless. Whether it is a Trane gas furnace or a Goodman gas furnace this factor cannot be overcome. There are other factors you can do to make sure you get the most out of the efficiency of the HVAC system.
You can have a 90 plus gas furnace and get efficiency but if your duct work is bad or your house has is drafty windows and doors with bad insulation in the walls you will not gain so much as far as efficiency is concerned. Of course it should be better than the older furnace but to increase efficiency for the whole system take care of other factors in addition to getting the new high efficient furnace. This is a good reason before you have a high efficiency gas furnace installed that you look at the big picture and have an effective plan in place to fix drafty windows and doors and add insulation to your home or business. It is also a good reason to do a complete evaluation of the entire HVAC system including the duct work. Leaking duct work will cause you to lose efficiency of the whole system because the conditioned or heated air from the high efficiency furnace is not being delivered to the space where it is needed. Instead it is wasted in the space where the duct work is located. The conditioned air should be delivered into the space and not in the attic or under the house. Give serious consideration to these factors and you will not regret it especially when you the gas or electric bill.
Everyone who has owned a gas heater including furnaces or water heaters will know that from to time you have to light the pilot light. Sometimes when you try to light the pilot it does not want to stay lit and you know you have a problem. Often times the problem turns out to be a bad thermocouple that needs to be replaced. But how do you know if the thermocouple is bad and the problem isn’t something else?
Pilot Flame should be directly on the tip
Some may be asking what a thermocouple is? A thermocouple is a safety device used in gas heating for many years. A thermocouple is made up of two dissimilar metals. At the end of the thermocouple is a welded end which is at the tip of the thermocouple. The welded end is referred to as a hot junction. The other part of the thermocouple are not welded and this is called the cold junction. When heat is applied to the hot junction a small amount of voltage is produced at the cold junction. This small amount of voltage is measured in millivolts. One millivolt is equal to 1/1000th of a volt and the thermocouple for a gas heating appliance produces enough millivolts to keep a safety solenoid inside the gas valve open. If the flame goes out that helps the thermocouple produce millivolts that voltage goes away and the safety valve closes. With the safety valve closed the main valve or primary valve inside the gas valve will not open. This prevents the main valve from opening and spewing un-ignited raw gas into the atmosphere where it can be potentially explosive.
Multi-Meter and Milivolts
To make certain the thermocouple is bad you have to test it. For testing a thermocouple you first have to remove it from the furnace, water heater, or gas appliance where it is installed. Once it is removed you will need a multimeter that needs to have the ability to read millivolts and a source of fire. Usually a lighter will suffice for the test. Attached alligator clips to your multimeter and attach one to the very end of the thermocouple where the connection is made to the gas valve. Attach the other end to the stem of the thermocouple. The stem is not the tip or any part of the tip but the area between where the connection is made to the gas valve and thermocouple tip itself. Light the fire and hold the tip of the thermocouple inside the fire. As the thermocouple tip heats up you should see a response on the multimeter. Make sure the meter is set for millivolts. You read 25 millivolts all the way up to a maximum of 100 millivolts although a reading that high is unusual. Anything less than 25 millivolts and the thermocouple needs to be replaced.
If it is above the 25 millivolt threshold then you are good to go. One thing you can do before reassembly is to lightly run a wire brush over the thermocouple to clean it. Only lightly though as it can easily be damaged. You reassemble the thermocouple and then replace it back in the pilot burner and restore everything else back to normal as it was before. Light the pilot light and everything should be good again. Good luck.
Two-stage gas furnaces make up over 30% of all gas furnaces installed in residences in the USA. These furnaces are rated from 80% AFUE all the way up to 95% AFUE and are more efficient than single-stage gas furnaces. When purchasing furnaces always check the ARI website for and efficiency comparisons. The 2-stage gas furnace runs at two different levels. A low fire and a high fire rate where the low fire runs at low demand and the high fire rate runs when there is a bigger demand. Of course high fire or the second stage is 100% and usually only kicks in when the temperatures are really cold outside. When the temperatures are milder outside the furnace runs at low fire and therefore uses less gas and in theory less electricity.
It depends on the type of blower motor installed in the 2-stage furnace as to the savings of electricity. A PSC blower motor will consume more energy than an ECM blower motor so it is best, if you are purchasing a two-stage gas furnace, to purchase the furnace with the ECM motor rather than the PSC blower motor. The payoff with an ECM motor is in electrical use but down the road if the blower fails it will cost more for the repair as ECM motors are more expensive than PSC motors. The PSC motor is a standard motor that has two wingdings inside of it. One for low speed and one for high speed. The low speed is used when the furnace is in low fire and high speed is used when the furnace is in high fire. It also uses a run capacitor whereas the ECM blower motor is variable speed and is electronically controlled to work at one speed or another depending on the firing rate the same as the PSC motor.
The 2-stage furnace had a little controversy to it when they first entered the market as there was no official test for the furnace except the single stage furnace test. The NIS or National Institute for Standards made hay out of it and ASHRAE made official tests for the 2-stage furnace that are now used by ARI. ARI (formerly AHRI) is the official testing agency that certifies efficiency ratings for HVAC equipment. All 2-stage furnaces on the market now are certified using the new test so efficiency ratings are accurate.
Buying a two-stage gas furnace is a big decision because usually the cost for this new furnace is bigger than a single gas furnace. When considering the purchase of any new furnace you have to take into account the future cost of fuel which never goes down. So you have to consider your budget for buying a new furnace versus the future cost of operating the gas furnace you are purchasing. Another consideration to have when purchasing higher efficiency furnaces is the resale value of the home. The more efficient the house the more attractive the house is for a potential buyer. Always remember the big variables when replacing any HVAC equipment. Good luck.
Gas furnace manufacturers have for many years been working on modulating furnaces to make furnaces more efficient. Before modulating furnaces the highest rated (in efficiency) were two-stage gas furnaces. Then came the modulating furnaces and efficiency ranges started increasing to 98% and fractionally more than 98% for some modulating gas furnaces. Always check the ARI website for certified efficiency of any HVAC product you purchase What makes this furnace different than the other types of gas furnaces you can purchase?
A modulating furnace will operate in a range from 10 to 20% all the way up to 100% and it depends on demand. For the modulating furnace to operate properly the controls need to be set up properly and in many cases the controls including the thermostat are proprietary so you will need the manufacturer of the furnace to provide the thermostat. Other gas furnaces such as the two stage gas furnace only offers two stages from 30 to 50% and 100% depending on the manufacturer and furnace set up. The two-stage gas furnace is more efficient than the single stage gas furnace which only operates at 100%. The cost also varies from furnace to furnace with the modulating furnace costing more than the other two. In many cases a lot more so it is important to compare the cost to the operating expense of the furnace and figure out the payback for the furnace. If you are going to stay in the house for a long period of time then it is better in most cases to purchase the most efficient. However if you are going to sell the home a new top of the line HVAC system will add value to the resale value of the home. It will definitely be a plus for the buyer and possibly help you sell faster.
When you purchase expensive equipment like a modulating gas furnace it is important that a certified HVAC contractor install the equipment as the higher end HVAC equipment usually comes with bells and whistles and the set up and final start up is important so that you get the proper rated efficiency out of the equipment. Additionally I always recommend that you purchase the extended warranty that includes labor because the parts are expensive and the troubleshooting can often take longer to repair because these systems are often complex.
You can see the rust holes up near the sheet metal plates in this heat exchanger that made the occupants sick
Richard, a technician just departed my house after looking at my gas furnace. My mom thought she smelled something so we called the furnace company we usually call when we need furnace maintenance to check it out. He told us our heat exchanger in our furnace failed. I asked him how much it would cost to fix and he told me his service manager would call me with a price. I just got off the phone with the service manager and he said the furnace is still under warranty but we would have to pay for labor. The new heat exchanger is covered under the warranty. He said the labor cost would be $850.00 to replace the heat exchanger. The furnace is over 7 years old and the heat exchanger has failed? Isn’t this a major component of the furnace and if it failed in 7 years won’t the new replacement heat exchanger have this issue in another 7 years? What is supposed to be the life span of a furnace? If so isn’t it a better decision to just replace the furnace altogether? And why would I want to buy this brand again if it failed so soon? We are cold right now but going to get some portable electric heaters. We need to make a decision soon. Thank you and I look forward to your answers. Jenna
Jenna, I am happy to help you. You’ve given me a lot of questions so let get on my soap box and answer them for and add a few things. Carbon monoxide is known as the silent killer. You can’t see it or smell it and it kills people every year. It is always my recommendation that anyone with a fireplace, gas or oil furnace, or any type of appliance that burns fossil fuels that you must have a carbon monoxide detector installed in your home. Your mother is not a good carbon monoxide detector although she did good in this circumstance.
A gas furnace heat exchanger should last for the life of the gas furnace. Many manufacturers offer a lifetime warranty on heat exchangers or even 20 years on the heat exchanger warranty. The average life any given furnace is 15 to 20 years and then it needs to be replaced. There are a few reasons to replace a gas furnace before this time. First of all HVAC furnace manufacturers are constantly improving efficiency’s of gas furnaces not only because of government regulations but also because gas gets more expensive over time and people can’t afford the old “gas hogs” of the past. It’s a judgement call on your part for replacing the furnace or just replacing the heat exchanger. If you are planning on staying in the house for a long time I would replace the furnace. If you are only going to be there for a few years then I would replace the heat exchanger. That would be how I would arrive at a decision and of course it would depend on the budget also as a new furnace will cost much more than just having the heat exchanger replaced. As far as the brand is concerned check out our HVAC reviews website and pick out a good brand from our consumer furnace reviews.
When the heat exchanger is installed or you get the new furnace make sure the HVAC technician properly checks all the gas furnace components and ensures the pressures for the manifold are correct. I have seen some instances of where the manifold pressure was way off and led to a cracked or failed heat exchanger. It is important these technical issues are checked to prevent future problems with the gas furnace. I’ve seen some installation technicians install a new system and turn it on and that’s it as long as it provides heating and cooling they did their job. It is important to tweak settings to make sure everything is within range. All these little things make a big difference in operating costs and longevity of the HVAC equipment.
Richard, Can you please explain to me in basic terms how a gas furnace works? I have a standing pilot gas furnace if that helps you better.
This question is closely related to my article on the Gas Furnace Sequence of Operation but each different type of gas furnace works differently. Therefore the sequence of operation or how the gas furnace works will work differently from gas furnace type to gas furnace type. Since you have a standing pilot gas furnace it, of course will work different than other types of gas furnaces.
How a Standing Pilot Gas Furnace Works
The first thing that happens with standing pilot gas furnace, or for and gas furnace for that matter is a call for heat from the thermostat. Providing all the safeties are good then the main burner will fire. The safeties for a standing pilot furnace include limits switches and the gas valve safeties. A standing pilot gas valve includes the thermocouple. The thermocouple needs a flame applied to it so it can produce the millivolts necessary to keep the pilot solenoid open inside the gas valve. If the thermocouple does not has a flame on it producing heat then the pilot solenoid closes and prevents the main solenoid inside the gas valve from opening. This is a safety for standing pilot gas valves that prevents the main valve from opening unless a pilot light is present to light the main burners. That keeps raw un-ignited gas from being spewed out into the atmosphere and becoming an explosive hazard. Aside from that the other safeties in the system include a high limit safety that would only open if the heat exchanger temperature reaches a preset temperature as defined by the limit and the design engineer. Other safeties including limit switches can be on the blower in case the blower overheats and a spill switch (also a limit switch) near the draft diverter in case the flue or chimney fails in some way. All these limits are either in series with the main gas valve or the pilot safety on the gas valve itself and prevent a safety hazard if it exists.
If all the safeties are good and the main burner ignites then the furnace begins heating up. As it heats up a fan limit control senses the heat and engages the blower once the furnace heats to a preset temperature according to what the fan limit control is set for in temperature. The fan kicks on and distributes the heat through the duct system until the thermostat satisfies. When the thermostat satisfies the main burners shut down but the blower continues running. The blower continues running to cool the furnace down and to use the usable heat still inside the furnace. If the thermostat is properly set up the heating anticipator will end the call for heat before the thermostat satisfies taking into account the blower continuing to run to cool down the furnace. The anticipator prevents overheating or overshooting the thermostat set point as the blower expels the additional heat inside the furnace after gas main burners shut down. When the additional heat is expelled the fan limit control shuts down the blower and the system is in stand by until the thermostat again makes a call for heat.
Electronic Control Board for an Electronic Ignition Gas Furnace
Electronic ignition for gas furnaces is the modern way for furnaces to light the main burners on modern gas furnaces. Electronic ignition gas furnace technology has evolved from the standing pilot gas furnace ignition systems that continue to be used today however there are less and less standing pilot gas furnaces used today. If your furnace is not working and you want to troubleshoot an electronic ignition gas furnace system it is important to make sure you know which type of ignition system you have for your furnace. Additionally there are different types of electronic ignition gas furnace systems and it is important to differentiate between the types of electronic ignition systems before troubleshooting a gas furnace electronic ignition. We will break it all down for you in this series article: “Troubleshooting an Electronic Ignition Gas Furnace“.
Two-stage gas furnaces are ever increasing in popularity among HVAC manufactures of gas furnaces. The product lines are ever increasing from single stage gas furnaces to two stage gas furnaces. The reasoning is that the two-stage gas furnaces are more efficient than the single-stage gas furnaces. This is according to tests performed using the Department of Energy’s test guidelines in accordance with testing AFUE ratings for gas furnaces. Is this true and is the test of gas furnaces correct in making the claim that the two-stage is more efficient than the single stage gas furnace? Before we make any claims to about the validity of the test we must first observe where the origin of the DOE guidelines for testing gas furnaces and boilers come from. These test guidelines come from ASHRAE or the American Society of Heating, Refrigerating, and Air Conditioning Engineers. The current standards used to determine AFUE or Annual Fuel Utilization Efficiency was written and adopted by DOE in 1993 from multiple revisions dating back to 1982 where the origin of AFUE was made. The tests are applicable to gas furnaces, oil furnaces, gas boilers, and oil boilers for determining AFUE.
You can see the rust holes up near the sheet metal plates in this heat exchanger that made the occupants sick
Everyone wants heat when old man winter arrives in the fall. Many times we take it for granted that when the temperature falls all we have to do is go to the thermostat and turn the heat on. Usually, this is not a problem. We turn the thermostat on and the heat comes on. Whether we have boilers or a furnaces at the beginning of the season when we do first turn the furnace on we have a slight burning odor which comes from the system. Usually, many of us ignore this odor because it does eventually, usually, go away. This is normal and is usually dust particles that have accumulated on the furnace’s heat exchanger burning off. How many people actually think it is carbon monoxide and called a professional HVAC technician to make sure it is not a problem? There are some people who do call but a majority of people just ignore and enjoy the heat being produced by their furnace. After all, carbon monoxide is a colorless and odorless byproduct of burning fossil fuels so even if an unsafe condition existed with their furnace they would probably never know it. This is why it is important, before old man winter arrives, to call a professional, qualified furnace technician to inspect the furnace. First, the furnace technician can make sure that the furnace will start and second of all the furnace technician can make sure you will have a safely operating furnace. You want to make sure the furnace does not have a cracked heat exchanger and a good qualified HVAC technician will look for a furnace cracked heat exchanger. Heat safety is paramount so have your furnaces inspected immediately for a cracked heat exchanger. Have a safe heating season.