Rheem Gas Furnace ignition issues

[bearshead]

Obviously I should have known that the moment I chose to lend my multimeter to a friend (now out of town!) would be the moment that the furnace chose to act up. The glow ignitor glows merrily, and if I massage the Robertshaw HS780-34NL-30GA it will click in a promising solenoidal way, the gas strip in front of the main burners will light, and then just as quickly shut down.

Does this sound like a faulty HSI? I know that we can't be 100% sure without the tester, but does this sound like it might be the culprit?

Also I see that several of the rods that should be running parallel, above and either side of the main burners seem to have fallen out and are now lying beneath the burner (although still attached at the far end). What are they for, and how bad is it that they are not all in place?

cheers!

[RegUS_PatOff]

How is the Flame Sensor ?

It may need cleaning.

Furnace model number ?

[applianceman18007260692]

The module probably ain't getting confirmation of flame ignition due to a loose corroded burner ground or contaminated flame sensor.

[bearshead]

Thanks to both of you for the input. I will check the ground. There is no sensor on this furnace. I had a technician have a look and he confirmed my suspicions about the HSI, although it has been working since he looked at it. I have a spare on order though for when it finally kicks the bucket, but if I can eke some extra life by checking the ground I will.

Cheers!

[Bobice]

[align=center]Troubleshooting HSI

[/align]WARNING!

Troubleshooting the system consists of :

Turn power off before servicing.

Line voltage ( 120 VAC) could be present on the surface of the ignitor, if the system is

not correctly wired. Such voltage can cause serious injury or death.

The following 4 steps must be performed first before any troubleshooting begins.

1)  Disconnect electric power to system at main fuse or circuit breaker.

2) Remove draft shield (if necessary to gain access to the ignitor.

3) Visually inspect equipment for apparent damage. Check wiring for loose

    connections.

4) Check for Reversed Polarity. (HSI models with diagnostic light are polarity

sensitive and may lock out if 120 volt service wiring is reversed.)

A. Disconnect the ignitor socket from the wiring harness. Connect an AC

voltmeter across the terminal connected to the white wire and chassis ground.

B. Re-connect electric power to system. If no voltage exists, polarity is correct;

proceed to step 4D.

C. If voltage exists between the terminal connected to the white wire and the

chassis ground, the main power supply lines are improperly connected to the

furnace (Reversed Polarity). Again disconnect electric power to the system;

then reverse incoming line voltage leads and repeat step 4B.

D. Re-connect the ignitor socket to the wiring harness. Re-check the system

for proper operation.

 

[dkpd1581]

Your Hot Surface Igniter serves as your flame proving sensor.

Hot Surface Igniters are generally a silicon carbide construction and their resistance values increase with age and use.  When that resistance value increases above a certain threshold the igniter may still glow and ignite the burners; however, the flame rectified DC voltage - flame proof to the board - may not be sufficiently high enough due to the increased resistance (the old E=IR thing).  If so, you will experience intermittent problems similar to what you have described.