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My furnace will keep trying to start, but will not fire for more then one second.

Many People Call a furnace a boiler and a boiler a furnace. So the answer you are looking for may be on the boiler page.

From a plumbers perspective this is the rule of thumb, If it blows warm air through ducts, It is a Furnace, If it has baseboard or radiators it is a Boiler. If you need help with your boiler or furnace please use The 24/7 Help Line.

  Click On Any Image to see a Larger Image.
This is what the Janitrol furnace looks like with the cover closed. in order for it to work with the air supply and return, this unit was installed upside down, this unit is designed to be  installed up right or upside down. 
  Once the door lid removed you can see all of the parts that make up the furnace, the control board, the blower motor door switch, gas valve and other parts, including the flame sensor, the cause of this furnace's problem. 
  Here are your combustion fan your burner tubes, gas control valve, hot surface igniter and most important to this page the flame sensor.
  The orange wire in this picture leads to the flame sensor, there is a screw or a nut holding it in place, remove the screw or nut to remove the flame sensor. 
  This is the flame sensor, with the orange wire connected, i inspected the wire and found that it was in good shape, it had no brakes in the insulation, and did not need to be replaced.
  This is a close up of the flame sensor, you may think that it is not that dirty and could not be the cause of the problem, sadly some inexperienced plumbing and heating technicians, will look at a flame sensor like this and assume it is not the problem.
But they are very wrong, that little misunderstanding has costs many customers a small fortune in replacing other parts that did not need to be replaced. 
  A short explanation, on what this does, Very simple it is a flame sensor, so it feels the flame. But it only has 1 to 2 seconds to feel the heat from the flame, and send the signal to the control board. any delay and the board shuts off the gas, and the furnace tries again. So that little discoloring on the flame sensor acts as insulation preventing the flame sensor from feeling the heat, in the very short time allowed. In short, it needs to be cleaned.
  I use a fine file to clean the flame sensor, I found that grit cloth does not get all of the carbon build up off of the flame sensor. Note:
 how it shines. this is good for another year. So simple if you know what you are doing.
I hope you found this page useful and that you return to this website the next time you have a problem with your warm air furnace. 

Is this happening to you? There are a number of reasons why a furnace will short cycle.
But I am only covering this one for now. This is a Janitrol Furnace. And it is installed upside down. So it may look like yours but it is upside down. On this unit, sometimes it would start and run for hours even days then it would recycle over and over again, then it stopped all together.
I could see the igniter was working because I could see it glow. so I knew it was not that. The next step for me is the flame censor.
This is the way it works thermostat calls for heat, combustion motor comes on draft is good, power is sent to igniter, igniter gets hot, power is sent to gas valve to open, gas valve opens for a few seconds waiting for a signal from flame sensor. if it gets a signal the gas valve stays on till limit is reached, if there is no signal from flame censer, the gas valve is turned off. and the cycle starts again, and will do this a few times, after it fails a number of times the circuit board locks out and the red light blinks out a warning.

The problem is the flame sensor (on this unit), (you can have the exact symptoms and it could be another problem) And the solution is simple. take it out, clean it with a fine file, get all the carbon off of it and put it back. the only time you have to replace then is if the porcelain is cracked. the fourth picture from left shows where it is mounted (remember this furnace is upside down) next how dirty it is, then how to clean it. and how it should look. Good luck, for other instructions, Click here

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