No heat and cant seem to find the problem....please help

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Old 12-14-12, 03:03 PM
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No heat and cant seem to find the problem....please help

Hi,
I have a Carrier 58gp furnace system and need some help. I have been using this forum as a helpful tool as well as testing all I can to find the problem. My furnace stopped working earlier in the week unfortunately at the only time of the year that we received snow and freezing temps. My pilot was on and the fan kept blowing but only cool air. The main burners would not kick in. I have replaced the thermocouple, heat sensor, limit switch and the transformer but still no luck. I tired troubleshooting and diagnosing the problem before replacing these but could not trace the problem and didnt spend too much to do this anyway. With the age of the unit I figured why not to save myself from future problems. Even after all this I still have no heat and cannot seem to figure out this problem. Any help would be greatly appreciated. Thanks. I do have a voltage meter and can do all testing and have done my best so far but just seem to be overlooking something. Please help!!!
 
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Old 12-14-12, 05:39 PM
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Pretty sure this is a standing pilot unit....pilot is always lit. Am I correct ?

If the thermocouple was bad the pilot wouldn't stay lit.

You said the pilot was on and the fan was blowing ?? On a standing pilot furnace the fan only runs when the heat exchanger is heated (burner on for at least a minute or so) OR the thermostat was in the FAN ON mode and not the AUTO mode.

Looking at your model number I think your missing the last few characters. (Like 5 SI)
 
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Old 12-15-12, 04:51 AM
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PJ, another thought. The "fan" to which the OP refers could be the exhaust evacuation fan that comes on prior to ignition. He just isn't getting the ignition. Just thinking out loud.
 
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Old 12-15-12, 05:40 AM
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pictures and or a fulol model number will help us help you. Do you have a volt meter and know how to use it?
 
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Old 12-15-12, 08:26 AM
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yes Pilot is always lit, as far as the thermostat it is heat and auto fan mode but yes the main fan is constantly blowing unless i depress/take off the switch. I do have a voltmeter and do know how to use it but just happen to be a little stuck with this model and at this time...attached is a picture of my unit. As far as full model number i believe but am unsure because i cannot see it on the unit is 58gp-100. thank you for the help!
 
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Old 12-15-12, 08:38 AM
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check the siwtch on the shelf above the gas valve. Follow the wires back from the gas valve. Do they go through any switches? If so (with power off) remove the wires and check for continuity through the switch or component. You blower is coming on due to a timer in the control board.
 
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Old 12-15-12, 12:09 PM
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There is no inducer motor on this gravity vented furnace.

The fact that the fan is running suggests that the low voltage (24 VAC) power is not operating properly. Usually on these furnaces, you hear a "chunk" when the 120 VAC power is turned on. That's the fan switch on the circuit board being energized.

If the fan switch IS energized as described, the furnace fan is shut off. The fan is turned on when the fan switch is DE ENERGIZED.

So if the 24 VAC power supply fails, the fan runs continuously when the power is on and the door switch closed.

This cxould be caused by a blown fuse on the circuit board or another switch early in that circuit.

I'd start by using a multimeter to see if you have 24 VAC to the R terminal on the circuit board.
 
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Old 12-15-12, 01:42 PM
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SeattlePioneer,

At the R terminal on the circuit board i have a reading of .27vac. As far as any other switches....the only switches I have are the limit switch(brand new) and the main switch that the door holds shut. I don't see any regular fuses on my circuit board that could be replaced but I have attached a picture of the circuit board so maybe someone might see something that I dont know.

thanks for all the help this far guys. really appreciate it.
 
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Old 12-15-12, 01:50 PM
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Did you check the fusible link that's plugged into the high limit switch?
 
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Old 12-15-12, 04:37 PM
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Measure tghe voltage at the LIM1 terminal on the gas valve. You probably have 24 VAC there.

Check the voltage at LIM2. You probably have no voltage there.

If that's the case, check the voltage at the limit switch, the fusible link attacked to the limit switch or the vent safety switch. One of those is open and blocking the voltage. All these parts are in series and you should be able to follow the wires to locate the parts to check.

If it's the fusible link, you may have a heat exchanger plugged with soot and dirty burners.

If the vent safety switch is open you may have a plugged chimney.

Check back here for further directions. Don't operate a furnace that has potential serious safety issues.
 
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Old 12-16-12, 07:16 AM
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Guys,
Thanks for all the help. I have found the problem. It turned out to be the fusible link at the heat sensor. I feel a bit silly now but was not familiar with fusible links and just never even thought that could have been the problem. You learn something everyday. Again thank you for all the help and input.
 
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Old 12-16-12, 07:48 AM
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If the link was open there is a furnace problem call a professional.
 
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