How to Repair Your HVAC

Basic Fixes for Your HVAC Unit

HVAC Thermostat

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Project Overview
  • Total Time: 30 mins - 1 hr
  • Skill Level: Intermediate
  • Estimated Cost: $10 to $75

When you turn on the thermostat, you expect a rush of cool or warm air from the vents. If this isn't happening, your first thought may be to start scanning your phone for HVAC repair companies. Not so fast: You can perform minor repairs to your HVAC system by yourself, and you'll save considerable time and money over having the pros make a house call.

What You'll Need

Equipment / Tools

  • Screwdriver
  • Cordless drill
  • Kitchen lighter
  • Shop vacuum
  • Flashlight


  • Replacement air filter
  • 18/5 thermostat wire


If A/C Is Blowing Warm or Hot Air

  1. Possible Causes

    When set at the correct temperature, the air conditioner should blow cold air, not warm or hot air. Your system may have a dirty air filter. The filter is always the easiest and first thing to check. Otherwise, the condensate drain line might not be properly moving water, or the refrigerant might be low or leaking.

  2. Possible Solutions

    • Dirty Air Filter: Changing out air filters on your HVAC system should be part of your regular maintenance schedule. If you are off-schedule, the filter may become clogged. Or activities that create dust, such as burning a wood-burning fireplace or building inside, can clog up filters faster than usual. Remove and dispose of the current filter. Put a new filter in place. Be sure to point the filter in the correct direction, as indicated by arrows on the filter.
    • Condensate Drain Line: Remove the service panel to access the line and condensate drip pan. Remove the threaded plug and vent. With a shop vacuum on wet mode, clean out the drain. Outdoors, locate the end-point of the drain line and suck out any obstructions with the shop vacuum.
    • Refrigerant: If you suspect that the refrigerant is low or leaking, call in an HVAC technician. The technician will perform a leak test. If leaks are discovered, they can fix the leak. If the refrigerant needs to be recharged, the technician can do this, as well. Do not try to recharge the refrigerant by yourself.

If the Furnace Is Blowing Cool or Cold Air

  1. Possible Causes

    It's always a cause for concern if the furnace component of your HVAC system isn't producing warm air. The filter may be clogged, which slows the rate of airflow. By the time the warm air has reached the vents, it will have cooled down already. If it's not the filter, the pilot light may have extinguished. Or the furnace may heat up, but the blower isn't working, so it can't push hot air to the vents.

  2. Possible Solutions

    • Filter: Change the furnace filter. Make sure it is turned in the correct direction for maximum airflow.
    • Relight Pilot Light: Turn off the furnace at the thermostat. Locate the pilot light at the bottom of the furnace. Turn it to the OFF position and wait about three minutes. Turn the knob to PILOT and press the knob for the automatic piezoelectric lighter. If it does not have a piezoelectric lighter, light the pilot with a kitchen lighter.
    • Blower: Gas furnaces have electrically powered blowers. Even if the gas does come on, the blower may not be working. Check the electric service panel to see if a circuit breaker has flipped off. If so, turn the breaker back on by flipping it backward (toward the outer edge of the box), then forward again until it clicks into place.

If the Thermostat Isn't Working Right

  1. Possible Causes

    If the thermostat isn't working right, nothing else with the HVAC system will work right. The thermostat is the brain of the entire system. All thermostats need power. Thermostats can be powered by batteries, by a low voltage wire, or by both. If power is compromised, the thermostat will not work. Thermostats aren't always well-placed. Yours may have been installed where sunlight hits it or next to a cool area with a draft.

  2. Possible Solutions

    • Battery Power: If the thermostat has a battery, the first step is always to replace the battery with a fresh one.
    • Low Voltage Power: To see if your thermostat has a low voltage wire, remove the front plate and turn out the screws holding the thermostat from the wall. Gently pull the thermostat away from the wall to see if all of the wires are attached. Often, there will be some unnecessary wires and they may still have plastic coating on their ends. Replacing the 18/5 thermostat wire is inexpensive, but threading it from the HVAC system to your thermostat can be challenging.
    • Placement Issues: Short of relocating the thermostat, you can change the conditions that are confusing the thermostat, such as by putting up curtains to block sunlight.

If the Heat Pump or Furnace Never Shuts Off

  1. Possible Causes

    The house may be so poorly insulated that the heat pump or furnace is forced to constantly run to keep pace with the demands. Or the outdoor unit might be iced over or have a snowdrift banked against it, blocking the vents. Another cause might be that the return air is so cold, the unit is overtaxed. Interior air returned to the unit should be warm when it reaches the unit. If the air is too cool, the unit has to work extra hard to keep up.

  2. Possible Solutions

    • Insulation: Sometimes, the house is so poorly insulated in the walls and ceiling that interior temperatures drop precipitously in the winter. The solution is to insulate the walls, either by pulling off drywall from the interior and adding roll or batt fiberglass insulation or by adding cellulose insulation to closed walls. Attics can be insulated by laying insulation batts between the joists or by blowing cellulose insulation into the attic.
    • Ice or Snow on Unit: Scrape away ice on top of the exterior unit. Brush off snow and shovels away any snowdrifts.
    • Return Air: A likely cause is a disconnected or leaking return air vent that is pulling in cold air from the attic. Inspect the ducts while the unit is running. Sometimes, the duct can be accessed in the attic. Locate the duct and inspect it with a flashlight for audible or visual clues of leakage.
  3. Signs That a Duct Is Leaking

    • Loose duct connection
    • Holes in ducts
    • Crushed flexible ducts
    • Duct tape missing or not holding
    • Whistling sound when system is running

If Airflow Is Weak

  1. Possible Causes

    HVAC systems create a loop by sending air out from the system, through ducts, and out into your house via vents. That same air is sucked back through separate return vent and ducts, back to the unit for re-heating or re-cooling.

    Any of the vents in this loop might be closed or blocked. Dust, pet hair, and other debris might be blocking the grating or the first few feet of the duct. Another little-known cause of low airflow might be the dampers: metal plates that open and close to control the amount of air that flows through the ducts.

  2. Possible Solutions

    • Vents: With the cordless drill, turn out the screws holding the vents (including the return vent) in place. Clean them with a shop vacuum. While whole-house duct cleaning can only be done with special equipment, you can clean the 3 or 4 feet by inserting the hose into the duct at each vent point.
    • Damper: Dampers are located near your unit. Find them near the branches of the duct system. The dampers are sometimes adjusted with a screwdriver or with a lever that you manually turn. If it's a lever, usually you'll need to rotate the lever until its handle is parallel with the ducts. This opens up the damper to its fullest position.