HVAC Emergency Guide: What to Do Right Now

Last updated: March 2026

By the HVAC Pricing Guide Team

This guide covers every common HVAC emergency with step-by-step instructions. Find your situation below and follow the steps in order. The most critical and time-sensitive emergencies are listed first.

Gas Smell Near Your Furnace or Gas Lines

Immediate Steps

1. Do not touch any electrical switches. Do not turn lights on or off, do not plug or unplug anything, and do not use your phone while inside the house. Any electrical spark can ignite natural gas.
2. Do not start your car in the garage. If your furnace is near the garage, gas may have accumulated there. A car's ignition can create a spark.
3. Get everyone out of the house immediately. Gather all family members and pets. Leave the door open behind you as you exit to allow the gas to dissipate.
4. Move at least 100 feet away from the home. Do not stand on the porch or in the driveway. Get to a safe distance.
5. Call 911 from outside. Tell the dispatcher you suspect a gas leak. The fire department will respond.
6. Call your gas company's emergency line. This number is on your gas bill. The gas company will send a crew to locate and shut off the leak at no charge.
7. Do not re-enter the home until the fire department or gas company confirms it is safe.

Understanding the Gas Smell

Natural gas is odorless in its raw state. Utility companies add a chemical called mercaptan that produces a distinctive rotten-egg or sulfur smell. This additive exists specifically so you can detect leaks. If you smell it, take it seriously every time, even if you are not sure. A false alarm costs you nothing. A missed warning can be fatal.

Gas leaks in HVAC systems commonly originate from a cracked heat exchanger, a faulty gas valve, a loose gas line fitting near the furnace, or a corroded gas pipe. After the immediate danger is resolved, you will need an HVAC technician to inspect the furnace and repair the source. For information on what this repair costs, see our emergency HVAC cost guide.

Faint Gas Smell vs. Strong Gas Smell

Some homeowners notice a faint gas odor only when the furnace first lights and it dissipates within a few seconds. This can happen with older furnaces and may not indicate a major leak. However, it still warrants a service call during normal business hours to inspect the burner assembly and gas connections.

A strong, persistent gas smell that is noticeable throughout a room or the house is a different situation entirely. This indicates a significant leak that requires immediate evacuation and emergency response. Do not try to determine whether the smell is "strong enough" to worry about. If it is noticeable, leave the house.

Carbon Monoxide Alarm Sounding

Immediate Steps

1. Get everyone out of the house immediately. This includes all people and pets. Open windows as you leave if you can do so quickly, but do not delay your exit.
2. Call 911 from outside the house. Tell the dispatcher your carbon monoxide detector is alarming. If anyone has symptoms (headache, nausea, dizziness, confusion), mention this. The fire department will respond with CO detection equipment.
3. Do not re-enter the home until the fire department has tested the air and confirmed CO levels are safe.
4. Seek medical attention if anyone has been experiencing headaches, dizziness, nausea, or confusion. These are symptoms of CO poisoning and can be serious even after leaving the contaminated area. The emergency room can test blood CO levels and provide treatment.
5. After the fire department clears the home, call an HVAC technician to inspect the heating system. Do not operate the furnace, water heater, or any gas appliance until the source of the CO has been identified and repaired.

Common Sources of Carbon Monoxide

In homes with gas-burning appliances, the most common sources of CO are a cracked heat exchanger in the furnace, a blocked or disconnected flue or chimney, a malfunctioning gas water heater, and a furnace or water heater with inadequate combustion air supply. Running a car, generator, or charcoal grill in an attached garage can also produce lethal CO levels inside the home.

A cracked heat exchanger is the most serious and expensive HVAC-related cause. The heat exchanger separates the combustion gases from the air that circulates through your home. When it cracks, those gases (including CO) can enter your living space. Repairing a cracked heat exchanger costs $1,500 to $3,500. On a furnace over 15 years old, replacement of the entire furnace is usually more practical.

Preventing Carbon Monoxide Emergencies

Install CO detectors on every level of your home and near sleeping areas. Test them monthly by pressing the test button. Replace batteries annually (or use models with sealed 10-year batteries). Replace the detector itself every 5 to 7 years, as the sensor degrades over time.

Schedule annual furnace maintenance that includes a CO test and heat exchanger inspection. A qualified technician uses a combustion analyzer to measure CO levels in the flue gases. Elevated levels indicate a problem before it becomes dangerous. This is one of the most important reasons to maintain a regular HVAC maintenance schedule.

No Heat in Winter: Step-by-Step Troubleshooting

Losing heat during cold weather is stressful, but in most cases, the cause is something you can check yourself before calling a technician. Work through these steps in order. Each one takes only a minute or two and rules out the most common causes.

Step 1: Check the Thermostat

This sounds obvious, but thermostat issues cause a surprising number of "emergency" calls. Verify the thermostat is set to "heat" mode (not "cool" or "off"). Make sure the set temperature is higher than the current room temperature. Check that the fan is set to "auto" (not "on," which runs the fan without heating). If the thermostat display is blank, replace the batteries. If it uses power from the HVAC system rather than batteries, a tripped breaker will kill the thermostat display.

If you have a programmable or smart thermostat, check the schedule. A programming error or a recent time change can cause the thermostat to be in a setback or "away" mode when you expect it to be actively heating. Reset it to manual mode temporarily and set the desired temperature.

Step 2: Check the Circuit Breaker

Go to your electrical panel and find the breaker labeled "furnace," "heating," or "HVAC." If it is in the middle position (tripped), switch it fully to "off" and then back to "on." A tripped breaker can cut power to the furnace, including the blower motor, ignition system, and control board. If the breaker trips again immediately, there is an electrical issue that requires professional diagnosis. Do not keep resetting it.

Note that some furnaces also have a dedicated power switch that looks like a standard light switch, usually mounted on or near the furnace. Make sure this is in the "on" position. It is not uncommon for someone to accidentally flip this switch while working near the furnace.

Step 3: Check the Air Filter

A severely clogged air filter restricts airflow through the furnace, causing it to overheat. When the furnace overheats, the high-limit safety switch shuts it down to prevent damage. The furnace may attempt to restart and shut down repeatedly (short cycling) until the filter is replaced. Pull out the filter and hold it up to a light. If you cannot see light through it, replace it immediately.

Some furnaces will not restart automatically after a high-limit shutdown. After replacing the filter, try turning the thermostat off, waiting 5 minutes, and turning it back on. This resets the system and allows it to attempt a fresh startup.

Step 4: Check the Pilot Light or Ignition

Older furnaces (pre-2000) use a standing pilot light that can be blown out by a draft. If your furnace has a pilot light, check whether it is lit by looking through the small viewing window on the front of the furnace. If the pilot is out, follow the relighting instructions printed on the label inside the furnace access panel. Turn the gas valve to "pilot," hold the reset button, and use a long lighter to ignite the pilot. Hold the button for 30 seconds before releasing.

Newer furnaces use electronic ignition (hot surface ignitor or spark ignition) and do not have a pilot light. If the ignitor has failed, you will not be able to fix it yourself. The furnace may flash an error code through a small LED light visible through the viewing window. Note the flash pattern (for example, three flashes) and report it to the HVAC technician when you call. This helps them diagnose the problem and bring the right parts.

Step 5: Check the Gas Supply

If you have a gas furnace, verify that the gas valve near the furnace is in the open position (the handle should be parallel to the pipe, not perpendicular). Also check that your other gas appliances are working. If your stove and water heater are also out, the issue may be with the gas supply to your home rather than the furnace itself. Call your gas company to check for service disruptions.

Step 6: If Nothing Works, Call a Technician

If you have worked through all five steps above and the furnace is still not producing heat, the problem requires professional diagnosis. Common culprits include a failed ignitor ($150 to $350 to replace), a faulty flame sensor ($100 to $250), a bad gas valve ($300 to $800), or a failed control board ($500 to $1,200). See our furnace repair cost guide for a complete breakdown of repair costs.

Staying Safe and Warm While You Wait

While waiting for the technician, focus on two priorities: keeping people warm and preventing pipes from freezing.

For warmth, close off rooms you are not using and gather in one or two rooms. Use electric space heaters, keeping them at least three feet from any flammable material and never leaving them unattended. Layer clothing and blankets. Use your fireplace if you have one. Do not use your gas oven or stovetop for heat, and never bring outdoor heating equipment (propane heaters, charcoal grills, generators) indoors. These produce carbon monoxide.

To protect pipes, open cabinet doors under sinks on exterior walls. Let faucets drip slightly. Keep the indoor temperature above 55 degrees Fahrenheit if possible. In extremely cold conditions (below 20 degrees), pipes in poorly insulated exterior walls can freeze within 3 to 6 hours after the house drops below 55 degrees. A burst pipe can cost $1,000 to $5,000 or more in water damage, far more than the furnace repair.

No AC in Summer: Step-by-Step Troubleshooting

A failed air conditioner during summer heat is uncomfortable and can be dangerous for vulnerable household members. Work through these troubleshooting steps before calling for service.

Step 1: Check the Thermostat

Verify the thermostat is set to "cool" mode with the desired temperature set below the current room temperature. Check that the fan is set to "auto." Replace the batteries if the display is blank. If you have a smart thermostat, check for firmware updates or connectivity issues that might be interfering with normal operation.

Step 2: Check the Breaker Panel

Your AC system typically has two breakers: one for the indoor air handler or furnace (which contains the blower) and one for the outdoor condensing unit. Check both. If either has tripped, reset it by flipping fully to "off" and then back to "on." If the outdoor unit breaker trips repeatedly, there is likely a compressor or electrical issue that needs professional attention.

Step 3: Check the Outdoor Unit

Go outside and look at the condensing unit. Is the fan spinning? If the fan is not running but you can hear a humming sound, the compressor may be trying to start but failing, which often points to a bad capacitor or contactor. If the unit is completely silent, check the outdoor disconnect switch (a small box mounted on the wall near the unit). Open the box and make sure the disconnect is engaged. Some disconnects are a pull-out handle; others are a breaker-style switch.

Also check for debris around the unit. Leaves, grass clippings, and other material can block airflow through the condenser coils, causing the system to overheat and shut down. Clear any debris and ensure at least 2 feet of clearance on all sides.

Step 4: Check the Air Filter

Just as with heating, a severely clogged filter can cause the AC to shut down. A clogged filter restricts airflow across the evaporator coil, which can cause the coil to freeze. If you see ice on the refrigerant lines or on the indoor coil, a dirty filter is a likely cause. Replace the filter, turn the system to "fan only" mode for 2 to 3 hours to let the ice melt, and then switch back to cooling mode.

Step 5: Check for a Frozen Evaporator Coil

If the system is running but not cooling, check the refrigerant lines at the indoor unit. If the larger (suction) line is covered in ice, the evaporator coil is frozen. This can be caused by a dirty filter (most common), low refrigerant from a leak, a malfunctioning blower motor, or a dirty evaporator coil. Turn the system off and let it thaw for 2 to 4 hours before restarting. If it freezes again, call a technician.

Step 6: Check the Condensate Drain

Many AC systems have a safety float switch that shuts down the system if the condensate drain line is clogged and the drain pan fills up. If your system stopped working and you notice water near the indoor unit, a clogged drain is the likely cause. You can try clearing it by using a wet/dry vacuum on the outdoor end of the drain line or by pouring a cup of distilled white vinegar into the drain access point near the indoor unit.

Step 7: If Nothing Works, Call a Technician

If troubleshooting has not resolved the problem, the issue likely involves a failed capacitor ($150 to $400), a faulty contactor ($150 to $400), a refrigerant leak ($200 to $1,500), a compressor failure ($1,000 to $3,000), or a fan motor failure ($200 to $700). See our AC repair cost guide for detailed pricing.

Staying Cool While You Wait

Close all blinds and curtains, especially on south-facing and west-facing windows. Use portable fans and ceiling fans to circulate air. Avoid using the oven, stove, or other heat-generating appliances. Drink plenty of water. Take cool showers. Move to the lowest level of the home, as hot air rises and basements are naturally cooler.

If you have elderly residents, infants under 1 year, pregnant women, or anyone with heart disease, respiratory conditions, or other heat-sensitive conditions, consider relocating them to an air-conditioned location. Local cooling centers, libraries, shopping centers, and the homes of friends or family are all options. Heat-related illness can develop quickly and can be life-threatening for vulnerable individuals.

Strange Noises from Your HVAC System

Unusual noises from your heating or cooling system are alarming but rarely life-threatening. In most cases, you have time to identify the type of noise and schedule a repair during normal business hours rather than paying for emergency service.

Banging or Booming from the Furnace

A loud bang when the furnace starts up often indicates delayed ignition. Gas accumulates in the burner chamber before the ignitor lights it, creating a small explosion. This is typically caused by dirty burners, a weak ignitor, or a faulty gas valve. While a single occurrence is not dangerous, repeated delayed ignition can crack the heat exchanger over time. Turn off the furnace and schedule a service call within a day or two.

Banging from the ductwork when the furnace turns on or off is usually caused by thermal expansion and contraction of the metal ducts. This is annoying but not harmful. The ductwork flexes as air pressure changes, producing a popping or banging sound. A duct contractor can sometimes reduce this by adding reinforcement to the affected sections.

Squealing or Screeching

A high-pitched squeal from the furnace typically indicates a worn blower motor bearing or a slipping belt (on older belt-driven models). The system is still functioning, but the component needs attention before it fails completely. Schedule a regular service call. If the squealing suddenly changes to a grinding noise, the bearing has seized, and you should turn off the system to prevent further damage to the motor.

Squealing from the outdoor AC or heat pump unit usually points to a fan motor bearing issue. The compressor can also produce a screeching sound when internal pressure builds up abnormally. If the outdoor unit produces a loud screech and then shuts off, turn it off at the thermostat and call a technician. Running it in this condition can cause compressor failure.

Rattling

Rattling from the outdoor unit is commonly caused by loose screws, bolts, or panel covers. Check for loose fasteners and tighten them. Debris inside the unit (sticks, leaves, or small stones) can also cause rattling against the fan blades. If you can safely remove the debris, do so. If the rattling is coming from inside the compressor, the unit needs professional service.

Rattling from the ductwork may indicate loose connections, detached duct sections, or objects that have fallen into the ducts through registers. Inspect accessible duct sections for obvious problems.

Clicking

Rapid clicking from the outdoor unit when it tries to start usually indicates a failing contactor or capacitor. The system is trying to engage the compressor but cannot deliver enough power. This is not dangerous, but the system will not cool until the part is replaced. Schedule a service call.

Clicking from the furnace control board area can indicate a relay issue. If the furnace clicks repeatedly without igniting, the control board may be failing, or the ignitor is not reaching the temperature needed to light the gas. Turn off the system and call for service.

Hissing or Bubbling

A hissing sound from the refrigerant lines of your AC or heat pump can indicate a refrigerant leak. Refrigerant under high pressure escaping through a small hole produces a distinct hiss. A bubbling or gurgling sound in the lines can also indicate low refrigerant charge, as air enters the system where refrigerant has escaped. Both warrant a service call but are not emergencies in the immediate-danger sense.

Burning Smell from Your HVAC System

A burning smell from your HVAC system can range from completely normal to potentially dangerous. The type of smell and when it occurs determines the appropriate response.

Dusty Burning Smell at Startup (Normal)

When you turn on your furnace for the first time in the fall or winter season, a dusty, burning smell is completely normal. Dust has settled on the heat exchanger, burners, and inside the ductwork during the months the system was idle. When the furnace heats up, this dust burns off. The smell typically fades within 30 to 60 minutes of the first startup. Open some windows to ventilate if it bothers you.

If the dusty smell returns every time the furnace runs (not just the first time of the season), the system may have a clogged filter that is allowing excess dust to reach the heat exchanger, or there may be a buildup on the burner assembly that needs cleaning. Schedule a regular service call.

Burning Plastic or Rubber Smell (Take Action)

Common causes of an electrical burning smell include an overheating blower motor (the motor windings are burning out), a failing capacitor that is causing the motor to overheat, wiring insulation melting due to a loose connection, or a foreign object caught in the blower fan that is creating friction. These problems can lead to electrical fires if the system continues to run.

Rotten Egg or Sulfur Smell

As discussed in the gas smell section above, a rotten egg or sulfur smell near your furnace indicates a potential gas leak. This is not a burning smell per se, but homeowners sometimes describe it as a "chemical" or "burning chemical" smell. If you detect this odor, follow the gas leak evacuation procedure described at the top of this guide.

Musty or Moldy Smell

A musty smell from the vents when the AC starts running is not a burning smell, but it is common enough to mention here. It usually indicates mold or mildew growth on the evaporator coil, in the drain pan, or inside the ductwork. While not an emergency, it can affect indoor air quality and should be addressed through professional coil cleaning and, if necessary, duct cleaning. Changing the filter and ensuring the condensate drain is clear can also help.

Water Leak from Your HVAC System

Water around your indoor HVAC equipment can range from a minor nuisance to a serious problem depending on the volume and location. Here is how to assess and respond.

Small Amount of Condensation (Usually Normal)

Air conditioners and heat pumps produce condensation as they cool the air. This water is supposed to drain through a condensate line to the outside or to a floor drain. A small amount of moisture around the base of the indoor unit, especially during high-humidity days, is normal. Keep an eye on it, but it does not require immediate action.

Significant Pooling of Water (Take Action)

If you find a noticeable pool of water around the indoor unit, on the floor, or dripping from the ceiling near the unit, turn off the HVAC system at the thermostat to stop the flow. Place towels and buckets to contain the water. Check whether the water is near any electrical outlets or the electrical panel. If so, turn off the breaker for that area.

The most common cause is a clogged condensate drain line. Algae, mold, and debris can block the drain over time, causing water to back up into the drain pan and overflow. If you are comfortable with basic maintenance, you can try clearing the line by applying suction with a wet/dry vacuum to the outdoor end of the drain line, or by pouring a cup of diluted bleach or vinegar into the drain access point.

Other causes include a cracked or rusted drain pan, a disconnected drain line, a frozen and then thawing evaporator coil, or a condensate pump failure (if your system uses one). These typically require a technician to repair. Schedule a service call, but this is usually not an after-hours emergency unless the water is causing active damage to finished spaces.

Preventing Water Damage

Document the leak with photos before cleaning up, in case you need to file an insurance claim. Move any electronics, documents, or valuable items away from the water. If the leak is above a finished ceiling, the drywall may be saturated and could collapse under the weight of the water. Stay clear of any sagging or discolored ceiling areas.

When to Call 911 vs. an HVAC Technician

Knowing which situations require emergency responders versus a professional HVAC technician can save lives and ensure you get the right help at the right time.

Call 911 Immediately

The fire department responds to gas leaks and carbon monoxide emergencies because these involve immediate threats to life. They have specialized detection equipment, respiratory protection, and the authority to shut off utilities. An HVAC technician does not have these capabilities and is not trained for hazardous material response.

Call Your Gas Company

After calling 911 for a gas leak, also call your gas company's 24-hour emergency line. The gas company dispatches a crew specifically trained to locate gas leaks, shut off the gas supply if necessary, and determine when it is safe to re-enter the home. This service is free. The gas company's emergency number is printed on your gas bill and is available on their website.

Call an HVAC Technician

For all other HVAC emergencies, an HVAC technician is the right call. This includes no heat, no AC, strange noises, non-gas-related burning smells (after you have shut off the system), water leaks, and systems that are not running properly. For guidance on what emergency HVAC service costs and how to find a qualified technician quickly, see our emergency cost guide.

Before calling, gather this information to help the dispatcher and technician:

• The brand and approximate age of your system
• The symptoms (what is or is not working)
• Any error codes flashing on the furnace or thermostat
• What troubleshooting steps you have already tried
• The current indoor and outdoor temperatures
• Whether anyone in the home has health conditions affected by temperature

This information helps the dispatcher prioritize your call and helps the technician arrive prepared with the right parts and tools. It can also reduce the time (and therefore cost) of the repair.

Preparing for HVAC Emergencies Before They Happen

The best time to prepare for an HVAC emergency is before you have one. A few simple steps now can make a future emergency less stressful, less expensive, and less dangerous.

Build Your Emergency Contact List

Identify two or three HVAC companies in your area that offer 24/7 emergency service. Save their phone numbers in your phone. Ask about their emergency service fees so you know what to expect. Your regular maintenance provider should be the first name on the list, as many companies prioritize existing customers. See our guide to finding an HVAC contractor for tips on choosing the right company.

Install and Maintain Safety Equipment

Install carbon monoxide detectors on every level of your home and near sleeping areas. Install a smoke detector near your furnace and water heater. Test all detectors monthly and replace batteries annually. Keep a fire extinguisher rated for Class B (flammable liquids) and Class C (electrical fires) near your furnace. Make sure all household members know where the fire extinguisher is and how to use it.

Know Your System

Learn the location of your furnace power switch, the HVAC breakers in your electrical panel, the outdoor AC disconnect switch, the gas shut-off valve, and the condensate drain line. Label these items if they are not already labeled. Show other household members where they are. This knowledge lets you take immediate action and communicate effectively with technicians.

Keep Spare Filters on Hand

A clogged air filter is the most common preventable cause of HVAC shutdowns. Keep two or three replacement filters stored near the furnace so you always have one available. Write the filter size on a piece of tape on the side of the furnace so anyone in the household can purchase the correct replacement.

Schedule Regular Maintenance

Professional maintenance twice per year (fall for heating, spring for cooling) catches most problems before they become emergencies. A technician who inspects your system regularly knows its history and can spot developing issues. This is the most effective way to avoid the situations described in this guide. See our maintenance cost guide for what to expect.

Frequently Asked Questions