Ignition issue

mburg

I am working on a Lennox G40UH-36A-070-15. It’s a natural gas 80% direct ignition furnace. It intermittently does not light off. The gas valve is energized, the burner manifold has 3.5” Wc, the offices and burners are clean. This issue may happen a few times in a row and than may not happen again all day. It’s very sporadic. Hopefully someone has a thought on what could be causing this. 

SuperTech

First thing you have to do is check the status of the diagnostic LEDs when the problem occurs.  This will point you in the direction of where to look for the problem.  I know its tough to do when the problem is intermittent. 

JohnNY

So you’ve got a pilot, the gas valve opens, and no ignition? Check your draft through the flue and heat exchanger. Play with your combustion air. You need 5%-15% natural gas for combustion and I suspect your air/gas mixture is being diluted. 

Ironman

When you say “the burners are clean”, what does that mean? 

Have they been removed, washed in hot water, and blown out with compressed air? If not, then they may not be truly clean even though they appear to be. Particular attention must be paid to the crossover passages that connect the burners.