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Questions about furnace start up sequence

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videobruce:
I had a problem with what I thought was a sticking gas valve. It turned out it was a working igniter that wasn't hot enough to light the flame. I understand the basic chain of events, but wanted to clarify the various stages of protection when/if something goes wrong.

During the time flue exhaust blower builds up pressure and the burner actually ignites, on this furnace (Rheem 90plus model from 1994 no flame or igniter sensors that I could find), doesn't the system confirm the igniter is actually glowing before it energizes the solenoid for the gas valve? If there is no sensor for the igniter, how does the system know it is working? Why energize the solenoid if the igniter isn't hot enough?

What was happening was the igniter would glow, I would hear the solenoid energize, then de-energize within 5 seconds with no igniting of the gas. I assumed the gas valve was sticking since by taping on the valve (not with a hammer) after a couple of tries, the flame would ignite. The furnace would be fine for a day or so, then fail again. I had a service guy out and we found it was the igniter even though it was working. By removing the existing igniter and comparing that with the 'glow' from my original igniter (that I saved when it was replaced), the replacement had less of a 'glow'.

What I don't understand why taping on the valve caused the gas to fire if it was the igniter.

Hope that made sense.

jumptrout51:
The igniter sends a resistance signal(ohms) to the gas valve to open.
If the resistance is too low due to a bad ignitor the board times out before ignition is established.
A flame sensor keeps the valve open after the flame is established.
A dirty or defective flame sensor will cause premature flame out.
Low gas pressure could also be a issue.
A valve pressure test should be performed.

videobruce:

--- Quote ---If the resistance is too low due to a bad igniter the board times out before ignition is established.
--- End quote ---
If the resistance was too low (assuming that is the problem) why does the solenoid even energize?
--- Quote ---A dirty or defective flame sensor will cause premature flame out.
--- End quote ---
Once it flames it holds, doesn't 'flame out'.
--- Quote ---Low gas pressure could also be a issue.
--- End quote ---
By swapping the igniter's, doesn't that rule out low gas pressure?

This is a 'double' (two flats) the other furnace is identical.

tgoods:
I would have thought you had low pressure through the gas valve or plugged orifices.  But, since it's working fine with a new ignitor, I would say that was your problem.  If you don't have a separate flame sensor, the ignitor is acting as the flame sensor.

videobruce:
Then can I assume the control board senses resistance of the coil or current draw?

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