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When the thermastat turns on the furnace doesn't ignite the burners and the pilot light is lit. Everything appears to be normal, fan runs, thermastat appears to work properly.
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The electronic ignition system in a gas furnace is a modern development that provides more reliable performance than a standing pilot and offers energy savings through better furnace efficiency. There are two basic types of electronic ignition systems:
An intermittent pilot system uses an electronically controlled high-voltage electric spark to ignite a pilot light and subsequently the main burners when the thermostat calls for heat.
A hot surface ignition system uses an electronically controlled resistance heating element, not unlike a light bulb filament, to ignite the gas burner.
Understanding some of the basic components of a modern furnace will help you identify the type of furnace you have and narrow down the list of potential problems.
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Types of Electronic Ignition Furnaces
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Most furnaces with electronic ignition are either conventional induced draft furnaces or high-efficiency condensing furnaces.
Induced draft furnaces use a small fan to draw the combustion gasses from the combustion chamber and out of the house through the flue (chimney). This system is more reliable and safer than old conventional furnaces, which relied on a natural draft coming in from an opening at the front of the furnace. Induced draft furnaces use either an intermittent pilot (IP) or hot surface ignition (HSI) instead of a standing pilot light. The combination of electronic ignition, electronic controls, and artificially created draft improves the efficiency of induced draft furnaces over older conventional models. Condensing furnaces use two heat exchangers, where conventional furnaces use only one. After the gas is burned to heat the primary heat exchanger, the secondary heat exchanger draws heat out of the hot exhaust gasses, cooling them to the point that the water vapor in the exhaust condenses into water. The resulting flue gases are so cool they can be vented outside via a plastic (PVC) pipe, while the condensed water is run to a floor drain. Condensing furnaces use hot surface ignition (HSI).
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Troubleshooting an Intermittent Pilot (IP)
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Typically found in induced draft furnaces, an intermittent pilot ignites the burner gas with a high-voltage spark only when the thermostat calls for heat. Once the pilot is lit, and the main burner senses the pilot flame (using a flame sensing rod), the main burner ignites. The intermittent pilot flame goes out after the heating cycle and remains off until the next time the thermostat calls for heat.
The furnace gas valve for this type of furnace is identifiable with its solenoid designations: MV, PV, and PV/MV - where MV = main valve, PV = pilot valve, and PV/MV = common.
Common problems with IP ignition include:
Ignition doesn't occur, and the furnace won't go on
A spark is present, but the pilot won't light
Pilot lights, but the main burner doesn't come on
Burners ignite but turn off after a few seconds
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Troubleshooting a Hot Surface Igniter (HSI)
The hot surface igniter (HSI) is the most commonly used electronic ignition system. It works like a light bulb filament, heating up when electricity is passed through it. Most are made from silicon nitride or silicon carbide. The igniter is located in the flow of gas entering the burners.
When the thermostat calls for heat, 24 volts are sent to the electronic ignition control module, which energizes the furnace blower and other systems and sends 120 volts to the HSI. The HSI heats up to around 1,800°F to 2,500°F and glows red-hot. Then the gas valve opens, and the burner gas is ignited by the HSI. Once a proper flame is sensed by the flame sensor, the HSI turns off and stays off until the next heating cycle. In some furnaces, the HSI also serves as a flame sensor (called local sensing) instead of having a separate sensing rod in the burner flame (remote sensing).
Under normal conditions, an HSI should last for three to five years, and they eventually crack and need to be replaced. This can happen sooner if the oils from your skin get on the element through improper handling. That means: Don't touch the element with your bare hands!
HSIs usually cost between $25 and $50, and it's a good idea to keep a spare on hand, in case one goes out at night or on the weekend, when you can't easily get a new part.
Here are some of the common causes of HSI failure:
Premature failure due to improper handling (oil from skin got onto element)
Improper igniter
End of normal usage life cycle (keep a replacement on hand)
Voltage in the furnace electrical circuit is too high (over 125 VAC)
Severely dirty air filter, causing the high-temperature limit switch to cycle the furnace ignition on and off frequently
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Reading Control Board Codes
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Most modern furnaces with electronic ignition systems have an integrated circuit board that controls many sophisticated processes. The boards can perform self-diagnostics and provide failure codes, similar to the diagnostic computer in your car.
If a failure occurs, LED lights on the control board light up in a blinking pattern to indicate a specific failure code. The codes are listed in a chart in the owner's manual and usually on a door or access panel on the furnace. Control boards are usually located inside the blower compartment of the furnace. If the board has blinking lights, refer to the chart to decipher the code and help identify the problem area with the furnace.
Oct 26, 2013 - i have a gas furnace with a lit pilot, but the burners won't light. ... I hv gas furnace we turn thermosate on upstairs but still wont turn on coud turn ...
? 3:15
https://www.youtube.com/watch?v=Ecd5CzJjss4
Jan 4, 2012 - Uploaded by Home Improvement Videos
If you woke up to find that your gas furnace is not starting or it's ... If this sensor gets dirty (and they almost always do over time) then your furnace will try to light, ... working, you can hear it starting, but the burners on not coming on. ... The furnace nowcomes on every time and the house is nice and cozy.
? 10:11
https://www.youtube.com/watch?v=yJxDc90hs4g
Sep 12, 2012 - Uploaded by grayfurnaceman
Our property manager is great about sending a repairman, but the house is a mess. ... While troubleshooting my downstairs furnacethat wouldn't turn on, I turned off the ... Not sure what to do now about flame rollout switch..
Bad igniter? When you 1st try it the gas valve will give enough gas that you would get the burner lit if the igniter was glowing red. If you're desperate you can light the igniter area w/ an aim n flame, but you definitely want to do that w/ caution and crank up your t-stat so the furnace stays on awhile Cuz you'll have to do it again for heat or buy an igniter($40 or so)
? 2:44
https://www.youtube.com/watch?v=yXIxgzwMkPs
Nov 17, 2012 - Uploaded by grayfurnaceman
This furnace should light its pilot with a spark then the pilot proves its on ... Troubleshoot an ignition problem on a Lennox gasfurnace .... Pilot and no inducer but is the same model as that great video! ... You probably have dirty burners. ... How do you know spark module is not sending 24 volts to mv and pv ...
The first thing I would do is make sure the flame sensor is clean, this is a metal rod immersed in the burner flame on the opposite side of the igniter. If too much oxidation builds up on it, it will not allow the burner to stay lit. To clean, just turn off furnace and remove flame sensor and clean with steel wool and then reinstall. If this does not solve the problem there may be a problem with the gas valve.
I would first check the gas pressure to your furnace. Then make sure that burners are clean and watch flames when burner first ignites. I have seen burners that were dirty cause delayed ignition that would create a suction when they came on and extingush pilot and burners.
The next thing I would do is change gas valve. Thank you
I would check to make sure control on gas valve is in on position and pilot is lit if you have a standing pilot. If you have a spark or hot surface ignitor make sure they are trying to lit pilot. If you have standing pilot model and pilot lit is not on. Turn control on gas valve to off,wait five mins. then turn to pilot. Depress botton and try to light pilot. If it lights hold down for a couple of mins. then release.. If pilot stays lit turn control to on. If thermostat is calling for heat this should bring main burner on. If pilot does not stay lit then replace thermocouple. Hope this helps. Thanks
If the burners do not fully ignite, the sensor will shut the furnace down. The lack of sufficient burner flame is a gas valve or gas pressure problem within the valve. Replacing the gas valve with the original one may get you going. I surely suggest a professional service company for safety reasons and a thorough inspection. A 20 year old furnace is past it's normal life span.
you should have 24 volts between yellow and white wires at the stat when the stat is set to heat and set as hot as it will go. burners should light first then 3 to 5 minutes later the fan motor should fire up. if burner doesn't light check the wires going into the top of the gas valve see if you have 24 volts arriving there from the stat. if you do then the gasvalve is probably bad. you can bypass the action of the stat by jumping red to white on 90 percent of the stats made and in a few seconds the gas valve should click open gas flow to main burner where pilot ignites it. another easy check you can do is flip stat fan switch to on. fan should come on at once. if not then you have either no high voltage to unit or no low voltage ( as in a bad transformer). if the fan comes on you know you have high volts and low volts...by cover i assume you mean the decorative cover but there second cover with the mercury bulbs and all is still there. if you have only the bare subbase then new stat time. sounds like an old furnace if it has a standing pilot, it should be checked for cracked heat exchanger by a pro with a n electronic carbon monoxide detector. NOT a diy item due to the real danger of co poisoning.
There are 4 -steps for full run time to kick in a furnace.. step-1, inducer fan runs & prove thru the air tube which closes a switch and tell the circuit board to go to step-2, the igniter lights the pilot & prove its lit by heating the thermocouple, which tells the circuit board to go to step-3, opens the gas valve to dump gas in, which will light from the pilot light, and prove by a flame sensor metal stick on the last burner , which heats up and tell the circuit board Saying, ok , I've got flame.. step-4. when the box heats up, it closes a switch and turn on the main fan...! (Some gas-valves has a brain -or circuit board on top of them and are real sensetive to moisture..)
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