Carrier furnace (59SC5A series) stops heating about 30 minutes after start up. The blinking LED shows a pattern of 3 short flashes and 3 long flashes, i.e. status code 33.
From the Troubleshooting Guide,
“33 LIMIT CIRCUIT FAULT – Indicates the limit, flame rollout is open. ……. Flame rollout switch requires manual reset. Check for:
- Loose blower wheel.
- Dirty air filter or restricted duct system.
- Improper gas input adjustment.
- Defective switch or connections.
- Improper limit switch or no gasket unless noted on switch.
If open is longer than 3 min, code changes to 13 LIMIT CIRCUIT LOCKOUT. ”
So, what does it mean?
It means (A) overheating in the heat exchange, (B) defective switch or connections, (C) improper limit switch, or (D) fire outside the heat exchange.
Troubleshooting steps:
(A) Overheating:
Higher than expected temperature in the heat exchange.
- 1. Loose blower wheel (i.e. not providing sufficient air circulating through the heat exchange)
– fix or replace the blower wheel. - 2. Dirty filter or resisted duct system (i.e. not providing sufficient air circulating through the heat exchange)
– replace a clogged furnace filter, unblock all cold-air return vents and floor/ceiling registers, and/or increase blower speed. - 3. Improper gas input adjustment (i.e. improper gas input to furnace burners)
– adjust gas input by a gas technician or furnace contractor.
(B) Defective switch or connections:
- 4. Defective switch or connections (i.e. limit switch and flame rollout switches)
– check wire connections to limit switch and flame rollout switches.
– check the limit switch and flame rollout switches are not open in a normal condition, i.e. resistance value of the limit switch and flame rollout switches should not be very high.
– check the temperature inside the heat exchange when the limit switch is open by a gas technician or furnace contractor.
(C) Improper limit switch:
- 5. Improper limit switch (i.e. the limit switch opens at the temperature below the heat exchange operating temperature )
– check both the limit switch cutoff temperature (on the label) and the furnace specification. For example, if the furnace specification requires a limit switch of cutoff temperature at 170 degree F and the existing limit switch cutoff temperature is 140 degree F, the furnace will shut off at 140 degree F instead of the specified cutoff temperature of 170 degree F.
(D) Fire outside the heat exchange:
- Fire is detected outside the heat exchange by the flame rollout switch (i.e. burners are designed to fire in the heat exchange cells only)
– contact a gas technician or furnace contractor for further investigations.
– possible causes: negative pressure in the furnace room, burner damage, … etc.
Reference: Installation, Start-up, Operating and Service and Maintenance Instructions, 59SC5A Single Stage 4-Way Multipoise Condensing Gas Furnace Series 1 and 2.