HVAC repair goes faster and costs less when you know which part of the system failed: the heating side, the cooling side, the airflow path, or the controls that tell everything when to run. A surprising number of "broken" systems are a tripped breaker, a dead thermostat, a clogged filter or a full condensate pan. Sort the symptom first, then go to the guide for that part of the system.
The Four Parts of an HVAC System
- Heating. A gas furnace, an electric furnace or heat strips, a heat pump in heating mode, or a boiler.
- Cooling. The outdoor condenser (compressor, condenser fan, capacitor, contactor), the indoor evaporator coil, and the refrigerant lines between them. A heat pump uses the same circuit for heating too.
- Airflow. The blower motor, the filter, the coil it blows across, and the supply and return ducts.
- Controls. The thermostat, the low-voltage wiring, the control board, breakers and disconnects, and safety switches like the condensate float switch.
The blower and controls are shared by heating and cooling. When both fail at once, look there first.
HVAC Symptoms by System Area
| What you notice | System area | Likely causes | Where to go next |
|---|---|---|---|
| Nothing runs in any mode, blank thermostat | Controls | Thermostat batteries or power, tripped breaker, blown board fuse, broken C wire | Thermostat wiring and troubleshooting |
| AC stopped, drain pan full of water | Controls | Float switch tripped by a clogged condensate drain | AC leaking water |
| Indoor fan runs, outdoor unit silent | Cooling | Failed capacitor, contactor, outdoor breaker or disconnect | AC repair guide |
| Outdoor unit runs, air not cold | Cooling | Low refrigerant from a leak, dirty condenser coil, compressor fault | AC not cooling |
| Ice on the indoor coil or refrigerant line | Airflow or cooling | Clogged filter, weak blower, low refrigerant | AC not cooling |
| Weak air at every vent, both modes | Airflow | Dirty filter, failing blower motor or capacitor, dirty blower wheel | Furnace blower motor |
| Some rooms hot or cold, others fine | Airflow | Leaky or disconnected ducts, closed dampers, undersized returns | Ductwork installation |
| Furnace lights, then shuts off | Heating | Dirty flame sensor, limit trips from poor airflow | Gas furnace repair guide |
| Heat pump blows lukewarm air, aux heat always on | Heating | Outdoor unit off or iced, defrost fault, refrigerant problem | Heat pump repair |
| System short-cycles in either mode | Controls or airflow | Thermostat placement, oversized equipment, limit or pressure trips | Depends on mode |
| Burning smell, buzzing at the outdoor unit | Controls | Overheating wiring, failing contactor or capacitor | Shut it off at the breaker, then a tech |
Start With the Controls
Controls problems are common and often cheap.
- Thermostat. Fresh batteries, correct mode, and a setpoint that should trigger a call. A smart thermostat with a weak or missing C wire can reboot, drop out or short-cycle the system.
- Breakers and disconnects. The furnace or air handler and the outdoor unit each have their own breaker, and the outdoor unit has a disconnect box beside it. Reset a tripped breaker once. If it trips again, leave it off.
- Float switch. Many air handlers have a safety switch in the drain pan or drain line that kills cooling when water backs up. The system looks dead, but the real problem is a clogged drain.
- Furnace door switch. A loose blower door shuts everything down.
If the thermostat is new or was just rewired, the thermostat wiring and troubleshooting guide shows how to check each terminal.
Then Check Airflow
The blower, filter and ducts serve heating and cooling, and restricted airflow causes problems in both. In summer it freezes the evaporator coil. In winter it trips the furnace limit switch. A clogged filter is the most common cause, so replace it before anything else. Then make sure supply registers are open and return grilles are clear. Weak airflow from a clean filter points to the blower motor, its capacitor or a dirty blower wheel.
Heating Side or Cooling Side
Once controls and airflow check out, the fault is in the equipment for the mode that's failing:
- Cooling only: the outdoor unit and refrigerant circuit. Capacitors and contactors are frequent failures. Low refrigerant always means a leak. See the AC repair guide.
- Heating only, gas furnace: the ignition sequence of inducer, pressure switch, ignitor, gas valve and flame sensor. See the gas furnace repair guide.
- Heat pump: heating and cooling share one refrigerant circuit, plus defrost controls and backup heat. See heat pump repairs.
What You Can Safely Check Before Calling Anyone
- Thermostat batteries, mode and setpoint.
- Breakers for the indoor and outdoor units, and the outdoor disconnect.
- The filter. Replace it if it's dirty.
- The condensate drain and float switch. If the pan is full, shut cooling off.
- Registers open, returns clear.
- Leaves, grass clippings and debris cleared from around the outdoor unit, with the power off.
- If the indoor coil is iced, turn cooling off and run the fan to thaw it before calling anyone.
What Not to Touch
Never open the refrigerant circuit or add refrigerant yourself. EPA Section 608 certification is required to handle it. Capacitors hold a dangerous charge after the power is off. Never bypass a limit, rollout, pressure or float switch. If you smell gas, leave the house and call the gas utility from outside. If a CO alarm sounds, get everyone out and call 911.
What an HVAC Repair Visit Involves
A good technician measures before replacing anything. Expect checks like:
- Voltage and amp draw on motors and the compressor, against their nameplate ratings
- Capacitor readings against the printed rating
- Temperature difference across the indoor coil or furnace
- Refrigerant superheat and subcooling, on the cooling side
- Total external static pressure, to measure duct restriction
- Flash codes and a combustion check on a gas furnace
Ask which reading condemned the part. The answer separates a real diagnosis from a parts-swapping guess.
What Drives HVAC Repair Cost
- The part. Capacitors, contactors, fuses and drain clearing sit at the low end. Ignitors, pressure switches and thermostats are lower middle. Blower and condenser fan motors and control boards land in the middle to high range. A compressor, an evaporator coil or a furnace heat exchanger is at the top and often triggers a repair-or-replace decision.
- Refrigerant type and leak repair. Finding and fixing a leak costs more than the refrigerant itself. R-22, which ended US production on January 1, 2020, is scarce.
- Warranty. Registered equipment often carries parts coverage for years. Labor usually isn't included.
- Timing and access. Peak-season emergencies, nights and weekends, and attic installs all raise the bill.
The AC repair cost breakdown covers the cooling side in more detail.
Repair or Replace When the Furnace and AC Are Both Aging
Air conditioners and heat pumps commonly last 15 to 20 years, and furnaces 15 to 30. The 50 percent rule is a reasonable start: if a repair costs more than half the price of new equipment and the system is past the middle of its life, replace it.
When both halves are old, think of them as one system:
- Matched equipment. An outdoor unit's efficiency rating applies only when it's paired with a compatible indoor coil and blower. Putting a new condenser on an old coil can cut efficiency and cause comfort problems.
- One installation. Replacing both at once shares labor, permits and the line set work.
- R-22 systems. A major cooling repair on an R-22 system is usually money better put toward replacement.
- Heat pump option. When the AC fails, a heat pump paired with the existing furnace, or a full heat pump system, is worth pricing.
If the furnace is much younger than the AC, replacing only the cooling side with a matched coil can still make sense. Use the HVAC installation cost guide to frame the replacement side.
How to Vet an HVAC Repair Contractor
Questions to ask:
- What did you measure, and which reading shows this part failed?
- If refrigerant is low, where is the leak and will you repair it?
- Is the part under manufacturer warranty, and what labor warranty do you offer?
- Are your technicians EPA 608 certified, and do you hold the license my state requires?
- Does the diagnostic fee apply toward the repair?
Red flags: topping off refrigerant every year without finding the leak, recommending full replacement before diagnosing, quoting a mismatched outdoor unit on an old coil, scare tactics about carbon monoxide without readings, and pressure to sign the same day. The HVAC companies guide covers comparing contractors in more depth.
Frequently Asked Questions
What is the most common HVAC repair?
Failed run capacitors on air conditioners and heat pumps, clogged condensate drains that trip the float switch, dirty flame sensors on furnaces, and thermostat or wiring faults are among the most frequent calls. Many of them trace back to a skipped filter change or missed annual maintenance.
How do I know if the problem is my furnace or my AC?
Check which mode fails. If heating and cooling both fail and the blower doesn't run, look at shared parts: the thermostat, breakers, the blower and the control board. If only cooling fails and the indoor fan runs, the outdoor unit or refrigerant side is the likely culprit. If only heat fails, start with the furnace or heat pump heating sequence.
Is it worth repairing a 15-year-old HVAC system?
A small repair like a capacitor, contactor or flame sensor usually is. For a compressor, coil, heat exchanger or control board on a system that age, get a replacement quote alongside the repair, especially if it uses R-22 refrigerant or has needed several repairs recently.
Should I replace my furnace and AC at the same time?
Often, if both are near the end of their lives. The indoor coil and outdoor unit need to be a matched pair to deliver their rated efficiency, and one installation costs less than two. If the furnace is much younger than the AC, replacing only the cooling side with a correctly matched coil can still make sense.
Can I repair my HVAC system myself?
You can check the thermostat, breakers, filter, float switch and condensate drain, and clear debris from the outdoor unit. Refrigerant work requires EPA Section 608 certification, and capacitors, wiring, gas valves and burners belong to a technician.
What certifications should an HVAC technician have?
Anyone handling refrigerant must hold EPA Section 608 certification. Check that the contractor holds the license your state or city requires and carries liability insurance. NATE certification is a voluntary credential that shows tested knowledge.