Why a Rheem Furnace Won’t Turn on and How to Fix It

The Rheem furnace won’t turn on is a common issue for homeowners that can stem from electrical, control, or mechanical causes. This article explains likely reasons, step-by-step troubleshooting, safety considerations, expected repair costs, and preventive maintenance to restore reliable heating. Clear diagnostic steps help decide when to DIY and when to call a technician.

Symptom	Quick Check	Possible Fix
No power at furnace	Check breaker and switch	Reset breaker or restore power
Furnace cycles but won’t ignite	Listen for gas valve click	Clean or replace ignitor; check gas supply
Fan runs but no heat	Check thermostat settings	Replace limit switch, thermostat, or control board

Common Reasons A Rheem Furnace Won’t Turn On

Several issues commonly cause a Rheem furnace won’t turn on. Power problems such as a tripped breaker or switched-off furnace switch are frequent culprits.

Thermostat settings or wiring faults often prevent the furnace from receiving a call for heat. A malfunctioning thermostat or incorrect programming can give the impression that the furnace is dead.

Combustion-related components including the ignitor, gas valve, or flame sensor can fail and stop ignition even when the blower runs. Safety interlocks like the pressure switch or limit switch may also keep the unit off if they detect unsafe conditions.

Airflow restrictions from a clogged filter or blocked vents can trigger safety cutouts. Control board failures are less common but possible, especially in older systems or after electrical surges.

Initial Safety And Preparation Steps

Before troubleshooting a Rheem furnace won’t turn on, ensure personal safety and avoid creating hazards. Turn off power at the service switch or breaker before inspecting internal components.

Shut off the gas supply at the appliance or the meter if a gas smell is present. If gas odor persists, leave the property and call the gas company or emergency services.

Wear appropriate protective gear such as gloves and safety glasses when working inside the furnace cabinet. If the homeowner is uncomfortable with electrical or gas systems, contact a licensed HVAC technician immediately.

Diagnostic Checklist: What To Inspect First

Use a logical sequence to diagnose why a Rheem furnace won’t turn on. Start with the easiest checks and move to more technical items.

• Power Supply: Verify the breaker hasn’t tripped and the furnace switch is on.
• Thermostat: Confirm it’s set to heat, with a proper temperature above room temperature and using correct system type (gas/heat pump).
• Air Filter: Replace a dirty filter that can cause overheating and shutdowns.
• Indicator Lights: Note any error codes on the control board LED; refer to the Rheem manual for code meanings.
• Vents and Exhaust: Ensure intake and exhaust pipes are unobstructed, especially on newer sealed-combustion units.

Step-By-Step Troubleshooting For Homeowners

1. Check Power And Switches

Confirm the furnace breaker is in the ON position and the furnace switch on the cabinet is flipped on. Reset the breaker if it has tripped.

If the breaker trips again immediately, do not keep resetting; this indicates an electrical fault requiring professional diagnosis.

2. Verify Thermostat Function

Replace thermostat batteries if present and ensure settings are on Heat with a temperature above current room temp. If available, set the thermostat to a higher temperature to force a call for heat.

Test by temporarily jumping the thermostat wires at the furnace to simulate a call for heat when safe and if the homeowner is confident with low-voltage wiring.

3. Inspect The Air Filter And Vents

Replace disposable filters if visibly dirty. A clogged filter restricts airflow and can trip the high-limit switch, preventing the furnace from operating.

Check supply and return vents for closures, furniture obstruction, or blocked registers that could cause airflow issues.

4. Observe Furnace Behavior When Powered

Turn power back on and set the thermostat to call for heat while listening for the sequence of operations: inducer motor, gas valve click, ignitor glow, ignition, and blower operation.

Document what is heard or not heard. For example, if the inducer runs but the ignitor does not glow, that points to ignitor or control issues.

5. Check The Ignitor And Flame Sensor

For furnaces that crank but don’t light, a cracked or weak ignitor and a dirty flame sensor are common causes. Visually inspect the ignitor for cracks and the flame sensor for soot buildup.

Clean the flame sensor gently with fine emery cloth and replace the ignitor if it shows visible damage or if the control board indicates failure to ignite.

6. Examine Safety Switches And Pressure Switch

Blocked venting or a malfunctioning inducer motor can cause a pressure switch to remain open, preventing ignition. Inspect the vent pipes for ice, nests, or debris.

Verify the pressure tubing is intact and not collapsed or disconnected. Replace the pressure switch only after confirming venting and inducer operation.

7. Review Control Board Codes

Most Rheem furnaces have a status LED that flashes error codes. Compare the flash pattern to the furnace manual to identify faults like limit switch open, ignition lockout, or flame failure.

Record the code and consult Rheem documentation or an HVAC technician for interpretation if unsure.

When To Call A Professional Technician

Contact a licensed HVAC technician if the homeowner encounters persistent electrical issues, gas valve concerns, cracked heat exchanger suspicion, or repeated ignition lockouts.

Complex diagnostics such as gas pressure checks, control board replacement, heat exchanger inspection, and sealed combustion adjustments require professional tools and certification.

If the furnace shows signs of carbon monoxide or rusted/breached heat exchanger surfaces, immediately stop using the unit and schedule an emergency service call.

Common Parts That Fail And Typical Repair Costs

Understanding which parts commonly cause a Rheem furnace won’t turn on helps anticipate repair costs.

Part	Typical Issue	Estimated Cost (Parts+Labor)
Ignitor	Cracked or weak, no ignition	$150–$350
Flame Sensor	Dirty or failing, flame not detected	$80–$200
Control Board	Electrical faults or communication errors	$300–$800
Pressure/Limit Switch	Safety cutouts prevent operation	$100–$300
Gas Valve	Failing to open or partial operation	$200–$600

Prices vary by region, model, and whether the furnace is under warranty. Always request a written estimate from the service provider.

Preventive Maintenance To Avoid Future Problems

Regular maintenance reduces the chances a Rheem furnace won’t turn on. Replace the air filter every 1–3 months depending on occupancy and pet presence.

Schedule annual heating tune-ups that include cleaning burners, testing safety switches, checking gas pressure, and verifying proper airflow and venting.

Keep the area around the furnace clear, and ensure exterior intake and exhaust terminations are unobstructed seasonally.

Safety Tips And Carbon Monoxide Awareness

Install and maintain carbon monoxide (CO) detectors near sleeping areas and on each level of the home. A furnace that doesn’t ignite properly can produce CO even without obvious signs.

Never block combustion air openings or vent terminations. If the home has a single-barrel chimney or shares flues, consult a professional about proper venting and draft safety.

Turn off the gas and call for help if the gas smell is strong. Do not test ignition repeatedly in the presence of a gas odor.

Frequently Asked Questions

Why Does The Thermostat Show Heat But The Rheem Furnace Won’t Turn On?

This can result from a faulty thermostat, wiring issues, or a control board not receiving the 24VAC signal. Verify thermostat function or bypass the thermostat briefly to test direct furnace response.

Can A Dirty Filter Prevent The Furnace From Turning On?

A severely clogged filter may cause the furnace to overheat and trip the high-limit switch, causing shutdowns. Replace filters regularly to avoid this problem.

How Long Does It Take To Fix An Ignitor Or Flame Sensor?

Replacing an ignitor typically takes 30–60 minutes for a trained technician. Cleaning or replacing a flame sensor is usually a 20–40 minute job.

Is It Safe To Try DIY Repairs On A Gas Furnace?

Simple tasks like replacing filters and checking breakers are generally safe. Anything involving gas piping, ignition components, or internal electrical parts should be left to licensed professionals.

Resources And Further Reading

Reference the Rheem furnace owner’s manual and service guides for model-specific LED codes and wiring diagrams. Manufacturer documents provide reliable troubleshooting flows and part numbers.

For safety and code compliance questions, consult a licensed HVAC contractor or local building department. Gas utilities often offer safety guidance and emergency response instructions.

When the Rheem furnace won’t turn on, following a systematic approach reduces unnecessary service calls and helps prioritize repairs. Document symptoms and any troubleshooting steps before calling a technician to speed diagnosis and reduce service time.