No heat or a problem
Error codes, symptoms and components
Start with the code on the control if there is one. If not, start from what you can see and hear. Either way the page ends with safe things to look at, things not to do, and how to reach BC Wide.
Search by code
Smell gas?
- Do not operate light switches, appliances or phones inside.
- Leave the building and leave the door open behind you.
- From outside, call the FortisBC emergency line at 1-800-663-9911 or 911.
- Only after that, call BC Wide about the furnace.
A furnace that will not run is a breakdown, not a gas emergency. Gas smell, a CO alarm, soot or scorching are.
Documented codes
- Carrier code 23Stuck closed Main pressure switch · Stuck closed secondary pressure switchCarrier 59SC6A Comfort 96 furnaces with the on-board 3-digit display control
- Carrier code 24Fuse faultCarrier 59SC6A Comfort 96 furnaces with the on-board 3-digit display control
- Carrier code 25No program info in micro, no heating operation. · Corrupted program file, reprogram control using service app or super plug (supplied separately), no heating operation. · Twinned units are not identical, program #'s do not match. Ensure furnaces are identical (same program #) when twinning. Main unit will not operate in heating. · Main program invalid, using backup program to operate. Reprogram control to correct issue. Control still operates normally. · Installer settings corrupted. Try reselecting installer settings, or reprogram control. Control will operate using default installer settings. · Wrong program for control used - reprogram control so program matches control used. No heating operation. · Non-Condensing program detected in a Condensing model.Carrier 59SC6A Comfort 96 furnaces with the on-board 3-digit display control
- Carrier code 27Program in super plug is missing, not recognized, or corrupted. Remove super plug then retry. · If still have 27 code, try a different super plug. · If still fails, replace control.Carrier 59SC6A Comfort 96 furnaces with the on-board 3-digit display control
- Carrier code 31Open secondary pressure switch (HPS) · Secondary PS Lockout - failed to close secondary PS after three trialsCarrier 59SC6A Comfort 96 furnaces with the on-board 3-digit display control
- Carrier code 32Open Main pressure switch (LPS)Carrier 59SC6A Comfort 96 furnaces with the on-board 3-digit display control
- Carrier code 33A switch has opened in the Main Limit circuitCarrier 59SC6A Comfort 96 furnaces with the on-board 3-digit display control
- Carrier code 34Ignition fault - during four consecutive ignition trials. · After successful ignition (flame proven) flame loss before heating blower on delay. · After successful ignition (flame proven) flame loss after heating blower on delay.Carrier 59SC6A Comfort 96 furnaces with the on-board 3-digit display control
- Carrier code 45Control failure - flame circuit fault, memory mismatch or sequence error · Control failure - gas valve relay will not close · Control failure - EEPROM memory issueCarrier 59SC6A Comfort 96 furnaces with the on-board 3-digit display control
- Carrier code 46Momentary loss of powerCarrier 59SC6A Comfort 96 furnaces with the on-board 3-digit display control
- Carrier code 10Rapid flash LED L1 polarity faultCarrier 59SC6A Comfort 96 furnaces with the on-board 3-digit display control
- Carrier code 12W on at power upCarrier 59SC6A Comfort 96 furnaces with the on-board 3-digit display control
- Carrier code 13Limit Lockout - switch open longer than 3 minutes in Main Limit circuit · Ignition Lockout after 4 consecutive ignition triesCarrier 59SC6A Comfort 96 furnaces with the on-board 3-digit display control
- Carrier code 14Flame lost 3 times after 70s of heating · Lockout - 7 loss of flame events during a heat requestCarrier 59SC6A Comfort 96 furnaces with the on-board 3-digit display control
- Carrier code 2124VAC sensed on gas valve when shouldn't beCarrier 59SC6A Comfort 96 furnaces with the on-board 3-digit display control
- Carrier code 22False flameCarrier 59SC6A Comfort 96 furnaces with the on-board 3-digit display control
- Carrier code 31Open secondary pressure switch (HPS) · High Heat Only for next cycle – HPS failed to close in 75s when transitioning from low to high heatCarrier 59TP6C Performance 96 furnaces with the on-board 3-digit display control
- Carrier code 10Rapid flash - L1 polarity faultCarrier 59TP6C Performance 96 furnaces with the on-board 3-digit display control
- Carrier code 12W on at power upCarrier 59TP6C Performance 96 furnaces with the on-board 3-digit display control
- Carrier code 13Limit Lockout - switch open longer than 3 minutes in Main Limit circuit · Ignition Lockout after 4 consecutive ignition triesCarrier 59TP6C Performance 96 furnaces with the on-board 3-digit display control
- Carrier code 14Flame lost 3 times after 70s of heating · Lockout - 7 loss of flame events during a heat request · Lockout - 7 LPS open events during a heat requestCarrier 59TP6C Performance 96 furnaces with the on-board 3-digit display control
- Carrier code 15Lockout - no Blower rpms detectedCarrier 59TP6C Performance 96 furnaces with the on-board 3-digit display control
- Carrier code 19InteliSense™ communications lossCarrier 59TP6C Performance 96 furnaces with the on-board 3-digit display control
- Carrier code 2124VAC sensed on gas valve when shouldn't beCarrier 59TP6C Performance 96 furnaces with the on-board 3-digit display control
- Carrier code 22False flameCarrier 59TP6C Performance 96 furnaces with the on-board 3-digit display control
- Carrier code 23LPS stuck closed · HPS stuck closedCarrier 59TP6C Performance 96 furnaces with the on-board 3-digit display control
- Carrier code 24Fuse faultCarrier 59TP6C Performance 96 furnaces with the on-board 3-digit display control
- Carrier code 25No program info in micro, no heating operation. · Corrupted program file, reprogram control using service app or super plug (supplied separately), no heating operation. · Twinned units are not identical, program #'s do not match. Ensure furnaces are identical (same program #) when twinning. Main unit will not operate in heating. · Main program invalid, using backup program to operate. Reprogram control to correct issue. Control still operates normally. · Installer settings corrupted. Try reselecting installer settings, or reprogram control. Control will operate using default installer settings. · Wrong program for control used - reprogram control so program matches control used. No heating operation. · If program in furnace control is missing, not recognized, or corrupted. Reprogram control with correct program # as listed on the rating plate. Replace control if issue remains.Carrier 59TP6C Performance 96 furnaces with the on-board 3-digit display control
- Carrier code 27Program in super plug is missing, not recognized, or corrupted. Remove super plug then retry. · If still have 27 code, try a different super plug. · If still fails, replace control.Carrier 59TP6C Performance 96 furnaces with the on-board 3-digit display control
- Carrier code 32Open Main pressure switch (LPS)Carrier 59TP6C Performance 96 furnaces with the on-board 3-digit display control
- Carrier code 33A switch has opened in the Main Limit circuit · Lockout - High Heat Only for current cycle · Ignition fault - during four consecutive ignition trialsCarrier 59TP6C Performance 96 furnaces with the on-board 3-digit display control
- Carrier code 34Low heat flame lost before on-delay · Low heat flame lost after on-delay · High heat flame lost before on-delay · High heat flame lost after on-delayCarrier 59TP6C Performance 96 furnaces with the on-board 3-digit display control
- Carrier code 41No blower RPM at start up · No RPM when blower motor is runningCarrier 59TP6C Performance 96 furnaces with the on-board 3-digit display control
- Carrier code 43HPS closed before LPSCarrier 59TP6C Performance 96 furnaces with the on-board 3-digit display control
- Carrier code 45Internal control error · GVR relay not closing · Micro EEPROM error · Micro loss of CommCarrier 59TP6C Performance 96 furnaces with the on-board 3-digit display control
- Carrier code 46Momentary loss of powerCarrier 59TP6C Performance 96 furnaces with the on-board 3-digit display control
- Carrier code 52SAT - Open · SAT - ShortCarrier 59TP6C Performance 96 furnaces with the on-board 3-digit display control
- Carrier code 53RAT - Open · RAT - ShortCarrier 59TP6C Performance 96 furnaces with the on-board 3-digit display control
- Carrier code 31Open secondary pressure switch (HPS) · High Heat Only for next cycle – HPS failed to close in 75s when transitioning from low to high heatCarrier 59TP7A Performance 97 furnaces with the on-board 3-digit display control
- Carrier code 10Rapid flash - L1 polarity faultCarrier 59TP7A Performance 97 furnaces with the on-board 3-digit display control
- Carrier code 12W on at power upCarrier 59TP7A Performance 97 furnaces with the on-board 3-digit display control
- Carrier code 13Limit Lockout - switch open longer than 3 minutes in Main Limit circuit · Ignition Lockout after 4 consecutive ignition triesCarrier 59TP7A Performance 97 furnaces with the on-board 3-digit display control
- Carrier code 14Flame lost 3 times after 70s of heating · Lockout - 7 loss of flame events during a heat request · Lockout - 7 LPS open events during a heat requestCarrier 59TP7A Performance 97 furnaces with the on-board 3-digit display control
- Carrier code 15Lockout - no Blower rpms detectedCarrier 59TP7A Performance 97 furnaces with the on-board 3-digit display control
- Carrier code 19InteliSense™ communications lossCarrier 59TP7A Performance 97 furnaces with the on-board 3-digit display control
- Carrier code 2124VAC sensed on gas valve when shouldn't beCarrier 59TP7A Performance 97 furnaces with the on-board 3-digit display control
- Carrier code 22False flameCarrier 59TP7A Performance 97 furnaces with the on-board 3-digit display control
- Carrier code 23LPS stuck closed · HPS stuck closedCarrier 59TP7A Performance 97 furnaces with the on-board 3-digit display control
- Carrier code 24Fuse faultCarrier 59TP7A Performance 97 furnaces with the on-board 3-digit display control
- Carrier code 25No program info in micro, no heating operation. · Corrupted program file, reprogram control using service app or super plug (supplied separately), no heating operation. · Twinned units are not identical, program #'s do not match. Ensure furnaces are identical (same program #) when twinning. Main unit will not operate in heating. · Main program invalid, using backup program to operate. Reprogram control to correct issue. Control still operates normally. · Installer settings corrupted. Try reselecting installer settings, or reprogram control. Control will operate using default installer settings. · Wrong program for control used - reprogram control so program matches control used. No heating operation. · If program in furnace control is missing, not recognized, or corrupted. Reprogram control with correct program # as listed on the rating plate. Replace control if issue remains.Carrier 59TP7A Performance 97 furnaces with the on-board 3-digit display control
- Carrier code 27Program in super plug is missing, not recognized, or corrupted. Remove super plug then retry. · If still have 27 code, try a different super plug. · If still fails, replace control.Carrier 59TP7A Performance 97 furnaces with the on-board 3-digit display control
- Carrier code 32Open Main pressure switch (LPS)Carrier 59TP7A Performance 97 furnaces with the on-board 3-digit display control
- Carrier code 33A switch has opened in the Main Limit circuit · Lockout - High Heat Only for current cycle · Ignition fault - during four consecutive ignition trialsCarrier 59TP7A Performance 97 furnaces with the on-board 3-digit display control
- Carrier code 34Low heat flame lost before on-delay · Low heat flame lost after on-delay · High heat flame lost before on-delay · High heat flame lost after on-delayCarrier 59TP7A Performance 97 furnaces with the on-board 3-digit display control
- Carrier code 41No blower RPM at start up · No RPM when blower motor is runningCarrier 59TP7A Performance 97 furnaces with the on-board 3-digit display control
- Carrier code 43HPS closed before LPSCarrier 59TP7A Performance 97 furnaces with the on-board 3-digit display control
- Carrier code 45Internal control error · GVR relay not closing · Micro EEPROM error · Micro loss of CommCarrier 59TP7A Performance 97 furnaces with the on-board 3-digit display control
- Carrier code 46Momentary loss of powerCarrier 59TP7A Performance 97 furnaces with the on-board 3-digit display control
- Carrier code 52SAT - Open · SAT - ShortCarrier 59TP7A Performance 97 furnaces with the on-board 3-digit display control
- Carrier code 53RAT - Open · RAT - ShortCarrier 59TP7A Performance 97 furnaces with the on-board 3-digit display control
- Carrier code 23Stuck closed Main pressure switch · Stuck closed secondary pressure switchCarrier 59SP6B Performance 96 single-stage furnaces with the on-board 3-digit display control
- Carrier code 24Fuse faultCarrier 59SP6B Performance 96 single-stage furnaces with the on-board 3-digit display control
- Carrier code 25No program info in micro, no heating operation. · Corrupted program file, reprogram control using service app or super plug (supplied separately), no heating operation. · Twinned units are not identical, program #'s do not match. Ensure furnaces are identical (same program #) when twinning. Main unit will not operate in heating. · Main program invalid, using backup program to operate. Reprogram control to correct issue. Control still operates normally. · Installer settings corrupted. Try reselecting installer settings, or reprogram control. Control will operate using default installer settings. · Wrong program for control used - reprogram control so program matches control used. No heating operation. · Non-Condensing program detected in a Condensing model.Carrier 59SP6B Performance 96 single-stage furnaces with the on-board 3-digit display control
- Carrier code 27Program in super plug is missing, not recognized, or corrupted. Remove super plug then retry. · If still have 27 code, try a different super plug. · If still fails, replace control.Carrier 59SP6B Performance 96 single-stage furnaces with the on-board 3-digit display control
- Carrier code 31Open secondary pressure switch (HPS) · Secondary PS Lockout - failed to close secondary PS after three trialsCarrier 59SP6B Performance 96 single-stage furnaces with the on-board 3-digit display control
- Carrier code 32Open Main pressure switch (LPS)Carrier 59SP6B Performance 96 single-stage furnaces with the on-board 3-digit display control
- Carrier code 33A switch has opened in the Main Limit circuitCarrier 59SP6B Performance 96 single-stage furnaces with the on-board 3-digit display control
- Carrier code 34Ignition fault - during four consecutive ignition trials. · After successful ignition (flame proven) flame loss before heating blower on delay. · After successful ignition (flame proven) flame loss after heating blower on delay.Carrier 59SP6B Performance 96 single-stage furnaces with the on-board 3-digit display control
- Carrier code 45Control failure - flame circuit fault, memory mismatch or sequence error · Control failure - gas valve relay will not close · Control failure - EEPROM memory issueCarrier 59SP6B Performance 96 single-stage furnaces with the on-board 3-digit display control
- Carrier code 46Momentary loss of powerCarrier 59SP6B Performance 96 single-stage furnaces with the on-board 3-digit display control
- Carrier code 10Rapid flash LED L1 polarity faultCarrier 59SP6B Performance 96 single-stage furnaces with the on-board 3-digit display control
- Carrier code 12W on at power upCarrier 59SP6B Performance 96 single-stage furnaces with the on-board 3-digit display control
- Carrier code 13Limit Lockout - switch open longer than 3 minutes in Main Limit circuit · Ignition Lockout after 4 consecutive ignition triesCarrier 59SP6B Performance 96 single-stage furnaces with the on-board 3-digit display control
- Carrier code 14Flame lost 3 times after 70s of heating · Lockout - 7 loss of flame events during a heat requestCarrier 59SP6B Performance 96 single-stage furnaces with the on-board 3-digit display control
- Carrier code 2124VAC sensed on gas valve when shouldn't beCarrier 59SP6B Performance 96 single-stage furnaces with the on-board 3-digit display control
- Carrier code 22False flameCarrier 59SP6B Performance 96 single-stage furnaces with the on-board 3-digit display control
- Carrier code 15Lockout - no Blower rpms detectedCarrier 59SP6B Performance 96 single-stage furnaces with the on-board 3-digit display control
- Carrier code 19InteliSense™ communications lossCarrier 59SP6B Performance 96 single-stage furnaces with the on-board 3-digit display control
- Carrier code 41No blower RPM at start up · No RPM when blower motor is runningCarrier 59SP6B Performance 96 single-stage furnaces with the on-board 3-digit display control
- Carrier code 52SAT - Open · SAT - ShortCarrier 59SP6B Performance 96 single-stage furnaces with the on-board 3-digit display control
- Carrier code 53RAT - Open · RAT - ShortCarrier 59SP6B Performance 96 single-stage furnaces with the on-board 3-digit display control
- Carrier code 31Open secondary pressure switch (HPS) · High Heat Only for next cycle – HPS failed to close in 75s when transitioning from low to high heatCarrier 59TN7A Infinity 97 furnaces with the on-board 3-digit display control
- Carrier code 10Rapid flash - L1 polarity faultCarrier 59TN7A Infinity 97 furnaces with the on-board 3-digit display control
- Carrier code 12W on at power upCarrier 59TN7A Infinity 97 furnaces with the on-board 3-digit display control
- Carrier code 13Limit Lockout - switch open longer than 3 minutes in Main Limit circuit · Ignition Lockout after 4 consecutive ignition triesCarrier 59TN7A Infinity 97 furnaces with the on-board 3-digit display control
- Carrier code 14Flame lost 3 times after 70s of heating · Lockout - 7 loss of flame events during a heat request · Lockout - 7 pressure switch open events during a heat requestCarrier 59TN7A Infinity 97 furnaces with the on-board 3-digit display control
- Carrier code 2124VAC sensed on gas valve when shouldn't be, power reset onlyCarrier 59TN7A Infinity 97 furnaces with the on-board 3-digit display control
- Carrier code 22False flameCarrier 59TN7A Infinity 97 furnaces with the on-board 3-digit display control
- Carrier code 23LPS stuck closed · HPS stuck closedCarrier 59TN7A Infinity 97 furnaces with the on-board 3-digit display control
- Carrier code 24Fuse faultCarrier 59TN7A Infinity 97 furnaces with the on-board 3-digit display control
- Carrier code 25No program. · Corrupted program file, reprogram control. · Main program invalid, using backup program to operate. · Installer settings corrupted. · Wrong program, reprogram control. · If program in furnace control is missing, not recognized, or corrupted. Reprogram control with correct program # as listed on the rating plate. Replace control if issue remains.Carrier 59TN7A Infinity 97 furnaces with the on-board 3-digit display control
- Carrier code 27Program in super plug is missing, not recognized, or corrupted. Remove super plug then retry. · If still have 27 code, try a different super plug. · If still fails, replace control.Carrier 59TN7A Infinity 97 furnaces with the on-board 3-digit display control
- Carrier code 32Open Main pressure switch (LPS)Carrier 59TN7A Infinity 97 furnaces with the on-board 3-digit display control
- Carrier code 43HPS closed before LPSCarrier 59TN7A Infinity 97 furnaces with the on-board 3-digit display control
- Carrier code 45Internal control error · GVR relay not closing · Micro EEPROM error · Micro loss of CommCarrier 59TN7A Infinity 97 furnaces with the on-board 3-digit display control
- Carrier code 52SAT - Open · SAT - ShortCarrier 59TN7A Infinity 97 furnaces with the on-board 3-digit display control
- Carrier code 53RAT - Open · RAT - ShortCarrier 59TN7A Infinity 97 furnaces with the on-board 3-digit display control
- Carrier code 15Lockout - no Blower rpms detected · Lockout - Failed to reach 500rpm during start upCarrier 59TN7A Infinity 97 furnaces with the on-board 3-digit display control
- Carrier code 41No blower RPM at start up · No RPM when blower motor is running · Failed to reach 500rpm at startupCarrier 59TN7A Infinity 97 furnaces with the on-board 3-digit display control
- Carrier code 51OAT - Open · OAT - ShortCarrier 59TN7A Infinity 97 furnaces with the on-board 3-digit display control
- Carrier code 33A switch has opened in the Main Limit circuitCarrier 59TN7A Infinity 97 furnaces with the on-board 3-digit display control
- Carrier code 34Ignition fault - four consecutive ignition trials. · Low heat flame lost before blower on-delay · Low heat flame lost after blower on-delay · High heat flame lost before blower on-delay · High heat flame lost after blower on-delayCarrier 59TN7A Infinity 97 furnaces with the on-board 3-digit display control
Symptoms
- Furnace not heatingA gas furnace that produces no heat is usually stopped by one of its own safety checks: no power, a locked-out ignition sequence, a pressure-switch or limit fault, or a thermostat that is not calling. What to look at safely, what the code on the control tells you, and when to call.
- Furnace short cyclingShort cycling is a furnace that starts, runs briefly and shuts off, over and over. Common causes are airflow restriction tripping the limit, a flame sensor that cannot prove flame, a pressure switch that opens under load, an oversized furnace, or a thermostat location problem.
- Furnace blowing cold airA furnace whose blower runs but delivers cold air has a burner that is not lighting or not staying lit: an ignition, gas-supply or flame-proving problem, or a thermostat demand mismatch. The fan is often running on its own (continuous fan) while the heat side has failed.
- Furnace repeated lockoutA furnace that locks out repeatedly is failing one of its own safety checks over and over: ignition, flame proving, draft proving or limits. Each lockout has a code; the code plus the fault history tells which check keeps failing.
- Furnace making abnormal noiseFurnace noises are diagnostic: rumbling or popping at start-up suggests delayed ignition, squealing points at bearings or airflow, rattling can be loose panels or ductwork, and a roar can be normal combustion or a blocked vent. Each sound family points to a different check.
- Weak airflow from registersWeak register airflow comes from a restriction somewhere in the path: filters, returns, ducts, dampers, or the blower itself. The diagnosis is measurement: static pressure and register CFM identify where the air is being lost.
- Uneven heating between roomsRooms that never reach temperature usually have an airflow imbalance, a heat-loss difference, or both. The fix starts by measuring where the air goes and where the heat is lost, then balancing ducts, registers or zoning rather than turning the thermostat up.
- Furnace overheating and limit tripsA furnace that trips its limit switch is overheating because airflow is not removing heat fast enough: a loaded filter, blocked registers, a failing blower, or ducts too small for the furnace. The limit stops the burner; the cause is an airflow problem, and it needs measurement, not a new switch.
- Water around a condensing furnaceWater around a condensing furnace almost always means the condensate path is blocked, frozen, disconnected or mis-routed. The furnace may keep running for a while, then stop with a pressure-switch or drain code, or the water may simply appear and keep coming.
- Smell of gas around the furnaceA gas smell is the one furnace symptom that is never investigated by the homeowner: leave the building first, call FortisBC's emergency line or 911 from outside, and let the gas utility make the building safe before anyone touches the furnace.
Components
- Furnace pressure switchThe pressure switch is a small diaphragm switch that proves the inducer is moving combustion air before the control allows gas to flow. Two-stage furnaces have two: a low-heat switch and a high-heat switch. A pressure-switch code usually points at venting, condensate or the inducer rather than at the switch itself.
- Inducer motorThe inducer is the small blower that starts before ignition to pull combustion air through the heat exchanger and push flue gas out the vent. When it fails or slows, the pressure switch cannot close and the furnace will not light. Two-stage furnaces use a two-speed inducer.
- Condensate trap, drain and pumpA condensing furnace makes water: several litres a day in cold weather. The trap, drain and, where needed, a condensate pump carry it away. When they block or freeze, water backs into the furnace, the pressure switch opens and the furnace stops, or water appears on the floor.
- Blower motorThe blower motor moves air through the heat exchanger and ducts in heating, cooling and continuous-fan modes. Furnaces today use ECM motors (constant-torque or constant-airflow) whose speed is controlled electronically; failures show as no airflow, weak airflow, noise, or RPM codes on the control.
- Hot-surface ignitorThe hot-surface ignitor is the ceramic element that glows to ignite the gas on modern furnaces. It heats during every cycle and is a wear item: failures show as no ignition, ignition lockouts, or a furnace that only lights sometimes.
- Flame sensorThe flame sensor proves the burner is actually burning before the gas valve is allowed to stay open. It is a small rod in the flame that conducts a tiny current; when it fouls or fails, the furnace lights and then shuts off seconds later, over and over.
- Limit switchThe limit switch stops the burner when the heat exchanger area gets too hot, protecting the furnace from airflow failures. It is a temperature switch in the furnace's airflow path; a limit that keeps opening means the furnace is overheating, not that the switch is bad.
- Control boardThe control board runs the furnace: it sequences the inducer, ignitor, gas valve and blower, reads the safeties, stores fault history and shows status codes. Board failures are real but less common than the parts it monitors; diagnosis rules out the field parts first.
- Gas valveThe gas valve admits gas to the burners only when the control commands it after ignition conditions are met. Gas-valve faults are serious by nature: diagnosis checks that the valve opens when commanded, closes when not, and delivers the right pressure, and any work on it is licensed-gas-fitter work.
- Heat exchangerThe heat exchanger transfers combustion heat to the air while keeping flue gas separate from the house air. Condensing furnaces have a primary and a stainless secondary exchanger. Cracks and corrosion are the serious failures because they can let combustion gas into the airstream.
- Furnace filterThe filter protects the furnace and the ducts from dust, and every airflow problem starts here: a loaded filter raises resistance, cuts airflow, raises temperature rise and can trip limits. Filter choice is a balance of MERV and pressure drop for the specific furnace.
- Duct restrictionsDuct restrictions are the physical limits on airflow: undersized trunks, crushed flex duct, undersized returns, closed dampers and wrong transitions. They are found by measuring static pressure and register airflow, not by feeling vents.
- ThermostatThe thermostat is the user's half of the furnace: it calls for heat, cooling or fan, and on communicating systems it is the control interface. A surprising share of no-heat calls start at the thermostat: dead batteries, wrong mode, or a set point below room temperature.
- Rollout switchThe rollout switch is a manual-reset safety in the burner area that trips when flame rolls out of the burner chamber instead of burning inside it. A tripped rollout switch means a combustion problem that must be found before the furnace is reset.