Furnace Lockout

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  #1  
Old 10-10-00, 05:38 PM
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I have a 5 year old Fraser Johnston furnace with a surface ignition system. I've been having lockout problems. When the problems began, during startup, everything worked fine up until the flame had been going for approx. 7 seconds. The flame went out and another ignition sequence was automatically attempted for several cycles. The furnace then went into lock out with the combustion air motor still going, then the main blower starting a few seconds later until I unplugged the furnace. Turning off the t-stat had no affect. I would get the trouble light code "no ignition". I tried the troubleshooting guide and found no problems. I changed the circuit board anyway, which was still under warranty at the time, no change. Then later, I changed the flame sensor.

That helped a bit. Now the flame stays lit during start up, but it eventually goes out during the heating cycle and immediately starts another ignition sequence. It does this several times. Eventually, it goes into lockout and I must unplug the furnace again to stop the blowers. Now, I get a "loss a flame" light. If the t-stat is still calling for heat, it will again start up as usual and go through this sequence of events until lockout or when the t-stat is satisfied, whichever occurs first.

I thought that a limit switch might be opening, but if that were the case, it would take, at the very least, a few seconds to cool off before the circuit board could attempt another start. The igniter coil starts heating up immediately after a "flame out".

I've been able to fool the system by manually applying power to the gas valve, I know, DANGEROUS! It also does OK if the t-stat is satisfied before the furnace has a chance to go through its flame out cycles, and eventually, lockout.

If anyone can help, I can provide more details if necessary.

Thanks!

Larry
 
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Old 10-10-00, 06:36 PM
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Hi Larry:

First, please stop fooling the gas valve. I would like ti talk to you again someday. I think that eithe ryou're still not getting good flame sensing, the pressure switch is erratic or the limit switch is erratic. You can place a voltmeter across each of the switches one at a time and monitor them to see if they are opening during operation. if they are ok, see if your furnace has a flame rod or does it only use the ignitor to sense the main burner flame? If ignitor sense only, sometimes the ignitor just gets too hot and loses its' ability to sense the flame. The fix is to add a flame rod the the circuitry. Make sure that your polarity is right. In other words, the hot wires goes to the hot wire terminal on the board and nuetral goes to nuetral. if it's reversed you'll gets hutdowns. Also, since the sensing sysytem uses ground to work, Make all ground connections in the furnace are clean and tight and the ground all the back to the panel is good. It's also possible that you have a bad module but they are usually go or no go. Double check the grounds and the connections on the flame sensing circuitry. The flame sensing current is microamps, so a little corrosion will stop it. Thanks. John.
 
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Old 10-10-00, 06:57 PM
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Hey John. Thanks for replying!

I don't think it's a limit switch or the pressure switch, the board "should" tell me via the trouble code light if it's one of those. A person on another forum suggested measuring the flame sensor current. I couldn't do that last year cause my multimeter was broke. Since he reminded me, I'm going to try that first.

I also thought of installing another sensor, as you suggested. I wasn't sure if that was a common fix. I assume that I would install it at another burner and connect the wires in parallel, IOW, both sensor wires spliced together and connected to the same board terminal.

Thanks again, I'll let you know what happens.

Larry
 
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Old 10-11-00, 04:47 AM
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Hi Larry:

An flame rod will only have one wire and it splices into the nuetral that feeds the ignitor glowbar. You splice in BEFORE the connector plug and NOT into the wire going to the ignitor glowbar AFTER the plug. If you want to go this route, you need to contact Fraser-Johnston to get their retrofit kit. Before pursuing this avenue, make darn sure the polarity and grounds and all connections are up to snuff or the flame rod won't help. Thanks. John.
 
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