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diyguy777 10-15-2012 01:16 PM

Carrier Furnace Burners Cycling On/Off
Furnace: Carrier 58GP150-LA Upflow (about twenty years old with match lit pilot)

Last week I turned on the furnace and experienced the following problem. Thermostat calls for heat, burners fire up, blower turns on approximately 90 seconds later.

After a few minutes of operation, the burners turn off, fan continues to run for several minutes and turns off. Pilot light stays on and seems to be fine.

Immediately after the blower turns off or concurrent to turning off, I hear a click (from HFR relay?), burners fire up and cycle repeats. This continues until the thermostat setting is reached.

Checked resistance across fusible link connected to limit switch terminal - 0 Ohms.
Put the ohmeter across limit switch and noticed that it goes high when burner turns off.

Connected a jumper across limit switch terminals and furnace resumed normal operation until Tstat setting reached. Concluded the LS was likely the problem as a couple of years ago the blower ran continuously with out burner and tstat off. Tapped on the LS to check it and it started working again fine with no problem until now.

Ordered a new limit switch. Tried to solder on the old fusible link to the new LS terminat and unfortunately it burned out. Nevertheless, since it was good before, went ahead and installed the new LS this morning without a fusible link (Iíll buy it later since Iím confident there shouldnít be a flame rollout problem). Fired up the furnace and the exact same problem occurs again.

So what could it be if not the LS? Schematic in the ownerís manual shows Blocked Vent Shutoff Switch (BVSS) which must be reset if it triggers according to manual, so canít be a blocked vent?

So now Iím stumped. What else could be causing these symptoms? Something causing the gas valve to de-energize but what? Pilot light seems fine. Could the furnace be overheating and if so, what could be causing that.

Any suggestions on how to proceed next will be appreciated. Thanks.

Technow 10-15-2012 02:03 PM

Low airflow

Marty S. 10-15-2012 04:21 PM

I enjoy helping the DIY crowd on here but in this case it's time to stop and call a pro. Leaving the roll out off on a 20 year old furnace clearly shows this job to be beyond your ability to work on a furnace safely. Sorry to be a stick in the mud but it needed to be said.

ben's plumbing 10-15-2012 06:23 PM

agree with technow maybe the limit switch is doing its job.....opening up on tempeture rise....check air flow ...filters,,blower..coil if is has a/c...go from there..ben sr

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