> By: The Cooling Company > Published: 2025-12-24 > Last updated: 2025-12-24
If your central AC runs but the heat does not, check thermostat mode, breakers, and the furnace power switch first. Replace a dirty filter and test batteries. For heat pumps, watch reversing-valve and defrost cycles. If basic checks fail, note codes and call a licensed HVAC technician for safe repair. (Source: ASHRAE Technical Resources)
Key Takeaways
- Set thermostat to HEAT and raise the setpoint.
- Replace the filter and clear return grills.
- Check breakers and the furnace power switch.
- Note any error codes, smells, or unusual noises.
Why is my heat off when AC runs?
Many systems use different parts for heating and cooling. Cooling often runs through the outdoor compressor and indoor coil. Heating may use a gas burner, a reversing valve, or electric strips that only engage in heat mode.
Homeowners often find simple causes quickly. A wrong thermostat mode, a tripped breaker, or a clogged filter can stop heating. If those checks fail, the fault could be electrical, gas, or refrigerant related and need a licensed tech to fix.
Is the thermostat set to heat?
Check the thermostat display and mode first. Many units show HEAT, COOL, or AUTO. Make sure the setpoint is above the room temperature and the screen is awake.
Also check for active holds or schedules that lower heat. For smart thermostats, open the app and verify no remote hold is active. If settings look right but no call goes out, move to power checks.
Are thermostat batteries dead?
Some thermostats keep settings on batteries even when wired. Low batteries stop controls from sending a heat signal. Replace batteries and test the command again.
If the display stays blank or the thermostat does not respond, document wiring colors and terminal letters so a technician can diagnose transformer or control wiring issues quickly.
Are the breakers or switches tripped?
Locate your main electrical panel and inspect HVAC breakers. Many systems use separate breakers for the outdoor unit and the furnace or air handler. A single tripped breaker can stop heating while cooling previously ran.
Also check the small on/off switch near the furnace or air handler. That switch is often turned off for service and left off. Turn it on and wait a minute for the system to respond before testing again.
Could a tripped fuse or switch be the cause?
Some furnaces use fuses on the control board instead of breakers. A blown fuse prevents ignition and control operation. Inspect fuses visually and replace only with the same type and rating.
Outdoor disconnects by the condenser often contain fuses too. Confirm they are intact. Never leave covers off or touch fuses with power applied. If unsure, call a pro for safe inspection.
How do I check thermostat, breakers, and switches?
Start with simple visual checks you can do safely. Confirm thermostat mode, raise the setpoint, and listen for clicks at the furnace. Then inspect breakers and switches for obvious trips or off positions.
If those steps do not restore heat, note what you observed. For example, whether the outdoor unit runs on a heat call, or if the furnace clicks but does not ignite. Those notes speed a technician’s diagnosis. (Source: ASHRAE Technical Resources)
Have you checked the circuit breaker?
Breakers protect motors and control circuits and are a common failure point. Reset any breaker that looks tripped and watch if it trips again. Repeated trips point to a motor or short.
If you see immediate re-trips, do not keep resetting. Record which breaker trips and when. That information helps the technician find whether a compressor, blower, or capacitor is failing.
Is there a blown fuse in the furnace?
Locate the control board inside the furnace or air handler. Small glass or blade fuses sit on the board and protect 24V controls. A blown fuse stops the thermostat from talking to the system.
Replace fuses only with the same type and rating. If a new fuse blows quickly, shut off power and call a technician to avoid damaging the control board or creating a safety hazard.
Could thermostat wiring be loose or damaged?
Loose wires, corroded terminals, or rodent damage can interrupt control signals. Look at the thermostat wiring at the stat and at the furnace. Take a clear photo before you move any wires.
Do not pull terminals hard. If wires are frayed or insulation is burned, note that for the technician. Proper testing with a multimeter is best left to pros to avoid shorting the transformer.
Could thermostat programming confuse heat mode?
Programmable and smart thermostats can use schedules, geofencing, or remote holds. These features sometimes keep heat off until a scheduled time. Check both the local display and any linked app for active holds.
If you use zoning, confirm you are adjusting the correct thermostat for the affected zone. Zoning dampers and controls can be miswired or stuck, causing one zone to lose heat while others stay warm.
Is the heat pump failing to switch to heat?
Heat pumps move refrigerant both ways to cool and heat. A reversing valve flips flow direction for heat. If that valve or its control fails, the system may remain in cooling mode or produce no heat.
Heat pump problems are often electrical or sealed‑system related. Refrigerant work and reversing valve repairs need certified technicians. Use the checks below to gather helpful details before you call.
What is a stuck reversing valve?
A reversing valve is a solenoid valve that shifts refrigerant flow. If it sticks, the system cannot change modes. You might hear a loud clunk when it tries to shift or notice the unit running but not heating.
Technicians test the valve coil, apply a shift command, and check pressures. Valve replacement is labor intensive and can range in cost widely. Note if you hear clicking or if auxiliary heat engages during a heat call.
Is the outdoor contactor or capacitor faulty?
The outdoor contactor sends power to the compressor and fan. If it fails, the compressor may not start on a heat call. Capacitors that weaken often let fans run but prevent compressors from starting.
If the fan runs but the compressor stays quiet during a heat call, record this symptom. That observation narrows the fault to the contactor, start capacitor, or the compressor itself.
Could low refrigerant stop heat mode?
Heat pumps need a correct refrigerant charge to move heat effectively. Low refrigerant can reduce heating performance while leaving cooling somewhat functional. Leaks often worsen over time.
Finding and fixing leaks requires licensed refrigerant handling. Running the system long on auxiliary electric heat will raise bills. If you suspect a leak, note oil marks on lines and avoid long operation.
Are defrost cycles preventing warm air?
Heat pumps defrost in cold weather to remove frost from the outdoor coil. During defrost the system briefly runs reversed and may use auxiliary heat. That can feel like lost heat for a short period.
Frequent or long defrost cycles suggest a defrost sensor or control problem. Note if the outdoor coil shows heavy ice or if the unit changes modes often. Those observations help a tech pinpoint the control issue. [Point 1] (Source: ASHRAE Technical Resources)
How does auxiliary electric heat work?
Auxiliary heat uses electric resistance strips when the heat pump cannot meet demand. It kicks in during very cold weather or when defrost is active. Expect much higher power use while it runs.
Avoid using EMERGENCY HEAT unless necessary. Running auxiliary heat always can triple your energy bills. Use it briefly to restore warmth, then call a technician to fix the primary heat source.
Could electrical issues be blocking heat?
Electrical faults often stop the heating sequence while leaving cooling intact. Control boards, relays, transformers, and fuses can fail. These failures need safe testing and sometimes part replacement.
Only do visual and safe external checks. Turn off power before opening panels. For internal diagnostics, hire a licensed technician to avoid shock and to meet local electrical codes.
Could a failed transformer be the culprit?
The 24V transformer powers the thermostat and control relays. If it fails, the thermostat may show power loss or the furnace may not respond. Symptoms include a blank stat and no clicking from the furnace.
A pro will measure transformer output with a meter and replace it if needed. Transformer swaps are common and relatively low cost, but they require correct matching to your control system.
Are wiring connections loose or corroded?
Wiring connections that are loose or corroded cause intermittent faults. Terminals can loosen and wires can fray from vibration. Corrosion at outdoor terminals is common in humid areas.
Take photos before you touch wires. Simple fixes like tightening screws may help, but voltage checks are needed for a reliable repair. Let a tech test for hidden shorts and fix bad splices.
Can a bad relay stop the heat cycle?
Relays route power to the compressor, blower, and ignition systems. A stuck or failed relay can prevent the heating sequence. You may see the control board indicate a heat call, yet nothing runs.
A technician will test relays for coil resistance and switch continuity. Relay replacement is often moderate in cost. Record audible clicks or lack of clicks when you call for heat to help diagnostics.
Is there a control board error code?
Modern furnaces and air handlers use LED blink codes to show faults. These codes identify ignition failures, pressure switch faults, and sensor problems. Look on the control board for a legend or a sticker with codes.
If you photograph a blinking pattern, include that in your service call. Many diagnostics begin with those codes, letting the tech bring the likely replacement parts faster.
Should you reset power before testing?
A power reset can clear minor electronic faults on control boards. Shut off the furnace switch and breaker, wait thirty seconds, and restore power. Then re-test the heat call.
If resetting brings back temporary heat and it fails again, schedule service. Intermittent failures point to marginal components or wiring that need professional repair.
What safety switches can disable heat?
Limit switches, pressure switches, and float switches protect the system from unsafe conditions. A tripped switch can stop ignition or the compressor to avoid overheating or water damage.
When a switch trips, fix the root cause. For example, remove blockages that caused a condensate float trip. Technicians test switches and replace or recalibrate them when needed.
Might gas supply or furnace parts be faulty?
When heating uses gas, proper fuel delivery and ignition are essential. Faulty gas valves, ignitors, flame sensors, or heat exchangers will stop heating while leaving cooling unaffected.
Combustion work requires a licensed technician. Gas and combustion issues risk leaks, fire, or carbon monoxide. Use the checks here only to gather safe information for the service call. [Point 2] (Source: ASHRAE Technical Resources)
Is the gas valve open and supplying fuel?
Confirm the manual gas shutoff near the furnace is open. Utility work or a contractor may close the valve. If you do not feel comfortable checking, call your utility for guidance.
If you smell gas at any time, evacuate and call emergency services. Do not light pilot lights or operate switches until a qualified technician inspects the system.
Pilot light out on older furnaces?
Older furnaces may use a standing pilot that must remain lit. If it is out, follow the posted lighting instructions on the furnace. If it will not stay lit, the thermocouple may be weak.
Avoid extensive DIY work on pilot assemblies. A trained technician can replace thermocouples and verify safe operation of older combustion systems.
Could ignition or hot surface igniter fail?
Modern furnaces use hot surface igniters or electronic spark modules. Igniters can crack or wear out. Symptoms include repeated clicking without flame or a glowing igniter that does not light the gas.
Ignitor replacement is common and usually moderate in cost. If ignition seems weak or flame sensors fail, a technician will check gas pressure and combustion safety before finishing repairs.
Are filters, vents, and coils causing the issue?
Airflow problems cause many heating complaints. Dirty filters, blocked returns, and clogged coils reduce heat transfer and can trip safety switches. These issues are often fixable by homeowners.
Duct leaks or closed dampers create uneven heat and extra strain on the blower. Before calling, replace filters, clear returns, and check vent temperatures to give the technician a clear picture.
When were furnace filters last changed?
A clogged filter limits airflow and can trigger a high‑limit switch to shut the burner off. Inspect filters monthly and change them every one to three months depending on pets and dust.
After replacing the filter, run a heating cycle and listen for steady blower operation. Weak airflow after a fresh filter can mean a failing blower or duct restriction.
Are supply vents blocked or closed?
Check all supply and return vents for furniture, curtains, or rugs that block airflow. Closed vents in unused rooms can unbalance the system and raise pressure, causing uneven heating.
If only one room is cold, check zone dampers and thermostat settings. Note any rooms that stay cold to help the technician test dampers and airflow balance.
Could dirty evaporator coils affect heating?
Dirty indoor coils reduce heat transfer and can cause the heat pump to struggle or a furnace to overheat. Coil contamination lowers efficiency and may trigger defrost or shutdowns.
Cleaning coils usually requires opening the air handler and using appropriate cleaners. Avoid wetting electronics; let a professional handle coil cleaning if you are unsure.
Is airflow restricted by duct issues?
Duct leaks, crushed flex ducts, and disconnected joints reduce airflow and can deliver uneven temperatures. Visible ductwork should be inspected for loose seams and sagging runs.
Sealing large leaks and insulating ducts in unconditioned spaces often improves performance greatly. Technicians use pressure tests and thermal checks to find hidden leaks.
How do clogged condensate drains impact heat?
High-efficiency furnaces and some coils drain condensate. A blocked drain can trip a float switch that disables the blower or burner to prevent overflow and water damage.
Clear visible drain traps and flush lines if accessible. If a pan remains full or the float stays tripped, a technician will snake lines and repair the drain slope. [Point 3] (Source: ASHRAE Technical Resources)
Can uneven room temps indicate duct leaks?
When some rooms heat and others stay cold, duct leaks or closed dampers are likely. Inspect attic and crawlspace ducts for disconnections and holes. Feel vent temperatures with a thermometer.
Technicians can seal ducts and rebalance dampers. Fixing large leaks near the air handler gives the best comfort improvements for the cost.
Should you clear vents and registers first?
Yes. Clearing vents and replacing filters are the fastest home fixes. They restore airflow and let the system operate normally in many cases. Vacuum returns and move furniture away from grills.
If clearing vents does not restore heat, note which rooms are affected and how the system behaves during a heat call. Those observations help a technician diagnose faster.
Will routine maintenance prevent future failures?
Regular HVAC tune-ups catch wear and dirt before they cause failures. Annual checks include filter inspection, coil cleaning, electrical tests, and combustion analysis for gas furnaces. Maintenance extends equipment life.
Schedule service before cold seasons to avoid emergency calls. A technician can flag parts that show wear and offer cost-effective replacement options before a breakdown.
How often to schedule professional HVAC service?
Most homes need annual service for gas furnaces and heat pumps. Homes in dusty or pet-filled environments may need more frequent checks. Heat pumps in cold climates can benefit from two visits yearly.
Ask for a maintenance plan with reminders. Preventive care often costs less than emergency repairs during peak weather.
What parts commonly fail in heating mode?
Common failures include ignitors, flame sensors, pressure switches, reversing valves, capacitors, contactors, and control boards. Blower motors and belts also wear and reduce airflow, risking safety cutouts.
Smaller parts like sensors or capacitors cost less, while sealed‑system parts and heat exchangers are costly. Knowing likely parts helps you budget for repairs.
How much do typical repairs cost?
Minor fixes like filters, thermostat batteries, or simple wiring often cost under $150. Mid-range jobs like ignitor or blower motor replacements run $300–$900. Sealed‑system or heat exchanger work can cost $800–$3,500 or more.
Prices vary by region, model, and access difficulty. Always get a written estimate and ask about parts warranties before work begins.
When is replacement more cost effective?
Consider replacement when repair costs approach half the price of a new system. Also replace when equipment is older than 15 years or efficiency is poor. New systems save energy and lower bills.
A technician can perform a Manual J load calculation to size a new unit correctly. Proper sizing and matched indoor and outdoor units preserve warranty coverage and system life.
Are emergency heating options safe?
Portable electric heaters can warm a room safely when used with care. Keep them away from flammable items and never leave them on unattended. Avoid unvented combustion heaters indoors.
If gas issues cause heat loss, do not attempt gas line work. For extended outages, consider staying elsewhere when temperatures drop and vulnerable people are at risk.
Call to action
If you need fast, expert heating service in Las Vegas, call The Cooling Company at 17029308411. Our technicians serve Las Vegas, Henderson, and North Las Vegas. Tell us your system make, model, serial number. Any error codes to speed diagnostics and get the right parts on the first visit.
Outside our area? Look for NATE-certified technicians at natex.org and use the same checklist you used here. If you are local, call The Cooling Company for licensed repairs, clear pricing. Quick emergency response tailored to Nevada homes.
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About The Cooling Company
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References
- U.S. Department of Energy (Energy.gov) (accessed 2025-12-24)
- U.S. Environmental Protection Agency (EPA) (accessed 2025-12-24)
- ASHRAE (Standards and guidance) (accessed 2025-12-24)
- ENERGY STAR (Heating & cooling) (accessed 2025-12-24)