Furnace pilot light turns on furnace lights then shuts off
The pilot light works just fine and the furnace starts up but then shuts off immediately. It keeps trying to start but the same thing keeps happening. Then out of the blue the furnace will work just fine!?!?!
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Re: Furnace pilot light turns on furnace lights then...
If you have a hot surface igniter (glow coil) it has a secondary purpose after ignition and that is to sense that there is actually flame present. If the HSI is located on one end of the burners look to the other and see if there is a small rod that protrudes in to the flame as well. If so it is part of this flame proving cycle. Take some sand cloth and clean it. This may buy some time but the rod should soon bee replaced.
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If its a high or mid efficiency furnace most of the time the sequence of operation goes as follows: tstat calls for heat (R&W at tstat ) ventor motor starts, pressure switch closes, spark or hsi lights pilot or main burners. main blower motor comes on. From your description of a humming noise it sounds like maybe the ventor motor is seized.
If your furnace uses an electrical glow element to light the pilot light, the Glow Element will need to be replaced. Glow Elements are general used on 90+ High Efficiency Gas Furnaces.
If it uses a standing pilot light rather than the Electrical Glow Element, you need to manually relight the pilot. Since I can't see your furnace, you will need to locate and read your Owners Manual on how to do this or look in the side or inside the pilot light compartment for instructions on how to light it. Be sure you have stick match or a butane grill lighter with you before you try to light it.
If it's a manual pilot ... if after lighting it, it doesn't hold when turned to the ON position, more thank likely you'll need to have the Thermocouple replaced.
And above all, BE CAREFUL. If you don't feel comfortable lighting a gas furnace, call a professional.
Newer, high efficiency furnaces have solid state controls and NO pilot light. To light the gas the new furnaces use a ceramic heating element rather than a pilot light. If the furnace isn't turning on you'll need to get a service tech to look at it for you. The circuit board has built-in diagnostics. Your furnace may have a window that lets you see the diagnostic light inside the blower box. Turn off the light in your furnace room to make it easier to see the flashing LED. See what pattern the light is flashing and look it up in your owners manual. It's usually something simple, like no power (the light doesn't flash at all, obviously), or too much back-pressure on the exhaust line, or a faulty temperature sensor or faulty ceramic element. I can't tell how you're trying to turn on the fan, but it could be as simple as a blown fuse or tripped circuit breaker. There are just too many possibilities to make guesses.
The pilot is iside the burner compartment of the furnace. Remove the cover and follow the 1/8" tubing to an orifice. This is where you light the pilot. the gas valve must also be turned off for a couple of minutes before trying to light the pilot. After a few minutes, turn the valve to pilot and light the pilot. Hold it for 30 seconds and then turn the valve to "ON"
Make sure your cleaning company didn't shut off the furnace at its disconnect located on or adjacent to and forgot to turn it back on. Other than that nothing they did should have effected the furnace unless they cleaned the furnace too and maybe left the blower door loose so the door switch can't connect.
There is a small tube that comes off the inducer for cracks or water. If there are cracks then the pressure switch that it is attached to may not be closing all of the time. Also sometimes you can get water in that tubing which will also cause the pressure switch to not close. If thats the case shorten the tube so there is no low droop in it where water can collect. Another thing that I would do is disconnect the tube at the indicer, take a straightened out paper clip or stiff wire and poke it in the nipple on the inducer to make sure it is clear too. Let me know if any of these work. There may be other things we can do.
The furnace is not sensing the flame and is shutting off the gas. What kind of pilot do you have? If you have a Hot Surface Igniter(glow coil) It will try to igniter three times and go into safety default for awhile to clear any gas that may be in the area. Check to see if you have a small rod that protrudes into the flames. It's usually located on the burner on the opposite end of the pilot but may be closer. Clean it with sand paper. if that works then it bought you time to get a new one. They are inexpensive. If you have a spark igniter then clean the rod(s) on the igniter. This may work but not as often as the HSI. Good luck.
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.
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..)