When you say the heater hums is the fan turning? Providing the fan is turning remove the top cover on the heater. Turn the heater on and watch the back of the burner area. Do you see any fuel mist coming from the nozzle? Do you see the ignitor sparking or glowing. Providing you see the ignitor working check the air pressure on the back on the unit. Remove the plug on the back of the unit that does not have a hole drilled in it. Insert a gauge into the hole (1/8" pipe thread) The pressure should be 5 to 6 psi. Let me know what you find and I can tell you how to proceed from there.
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When you say the heater hums is the fan turning? Providing the fan is turning remove the top cover on the heater. Turn the heater on and watch the back of the burner area. Do you see any fuel mist coming from the nozzle? Do you see the ignitor sparking or glowing. Providing you see the ignitor working check the air pressure on the back on the unit. Remove the plug on the back of the unit that does not have a hole drilled in it. Insert a gauge into the hole (1/8" pipe thread) The pressure should be 5 to 6 psi. Let me know what you find and I can tell you how to proceed from there.