Category: Heating & Cooling

Short answer: When the AC does nothing at all, the cause is usually power or control, not refrigerant. Carrier’s checklist: confirm the thermostat is on Cool and calling for cooling, look for a tripped breaker or blown fuse, check the disconnect box by the outdoor unit, and replace a clogged filter. Many systems also have a condensate float switch that cuts power when the drain backs up. If a breaker trips again right after a reset, stop and call a pro.

Quick diagnosis

What you notice	Most likely cause	What to check	Call a pro?
Nothing happens, thermostat blank	No power to the thermostat or system	Batteries, breaker, furnace switch	If still blank
Indoor fan runs, outdoor unit silent	Outdoor breaker, disconnect, capacitor or contactor	Check breaker and disconnect	Yes, for parts
Outdoor unit hums, fan does not spin	Failed capacitor or fan motor	Turn it off	Yes
System shut off, water near indoor unit	Condensate float switch tripped	Clear the drain line	If you cannot
Breaker trips right after reset	Electrical fault or failing compressor	Leave it off	Yes
Runs but blows no air at vents	Blower motor, frozen coil or very dirty filter	Check filter; look for ice	Often

Check in this order

1. Thermostat: set to Cool (or Auto), setpoint below room temperature, fan on Auto. Replace batteries; check a schedule is not overriding you.
2. Breakers: the AC usually has its own breaker, sometimes two (indoor and outdoor). Reset a tripped one once.
3. Outdoor disconnect: the gray box on the wall by the condenser must be on (some have a pull-out block or fuses).
4. Furnace or air handler switch: the indoor unit has a light-switch-style power switch; make sure it is on.
5. Filter: a badly clogged filter can freeze the coil and shut airflow down.
6. Condensate: look for water in the drain pan or around the indoor unit. A tripped float switch cuts power until the drain is cleared.

Leave these to a technician

Carrier warns that capacitors, motors and compressors are high-voltage parts, and that refrigerant work requires certification. If you see ice, turn the system off to protect the compressor.

When to call a professional

• The breaker trips again after one reset.
• The outdoor unit hums, clicks or does not start while the thermostat calls for cooling.
• The blower does not run with a clean filter and power on.
• You see ice or suspect a refrigerant leak.

What it usually costs

HomeGuide (2026) puts most AC repairs at about $150 to $650. Typical prices: service call $75 to $200, capacitor $100 to $250, condenser fan motor $200 to $700, thermostat replacement $140 to $350, and clearing a condensate drain $75 to $250.

Frequently asked questions

Why is my AC not turning on but the thermostat is?

Check the AC breaker, the outdoor disconnect, the indoor unit switch and the condensate float switch. If all are on, a capacitor, contactor or control board may have failed.

Why does my AC turn on but not blow air?

A frozen coil, a very dirty filter or a failed blower motor are common causes.

Related problems

• AC not cooling
• AC leaking water
• Thermostat not working
• Breaker keeps tripping

Sources

• Carrier: Troubleshoot an air conditioner not working
• HomeGuide: AC repair cost (2026)

This guide is general information, not a substitute for an inspection by a licensed professional. See our disclaimer.

Short answer: Start with batteries: Lennox calls dead batteries the most common cause of no power to a battery-powered thermostat. Next check the HVAC breaker and the furnace or air handler power switch. Water around the indoor unit can trip a condensate float switch that cuts power to the thermostat. If the screen still stays blank, a blown HVAC fuse, loose wiring or a failed transformer is likely, and those need a technician.

Quick diagnosis

What you notice	Most likely cause	What to check	Call a pro?
Screen blank	Dead batteries	Replace batteries; clean terminals	No
Screen blank, wired model	Tripped breaker or furnace switch off	Check panel and unit switch	If it trips again
Blank, water near the indoor unit	Condensate float switch tripped	Clear the drain line	If you cannot
Blank after a power surge	Blown fuse on the HVAC control board	Look for a fuse on the board (power off)	Usually
Screen on but system does not run	Wrong mode or schedule, or a problem in the AC or furnace	Check settings; then see AC or furnace guides	Often
Reads the wrong temperature	Sunlight, a nearby vent or loose wiring	Check placement	If wiring

Quick checks

1. Replace the batteries (even on many wired models that also have batteries) and wipe corroded terminals.
2. Check the HVAC breaker. Lennox says to call a professional if it trips again right away.
3. Make sure the furnace or air handler power switch is on and the front panel is closed.
4. Replace a clogged filter; a clogged filter can make the system shut down.
5. Look for water around the indoor unit or in the drain pan. Clear the condensate line to reset the float switch.

Settings that look like failures

• Mode set to Off, or Heat in summer and Cool in winter.
• A program or schedule overriding your temperature, or a hold that expired.
• Fan set to On, which blows air even when the system is not heating or cooling.
• Smart thermostats: check the app or the manufacturer’s help code; some report “no power” when the C-wire is missing.

Placement

Trane notes that direct sun or a spot near the kitchen or a supply vent can make a thermostat read the wrong temperature and run the system at the wrong times.

When to call a professional

• The screen stays blank after new batteries and a breaker check.
• The breaker trips again, or a fuse keeps blowing.
• Wiring is loose, corroded or you are installing a smart thermostat that needs a C-wire.
• The thermostat works but the furnace or AC still does not respond.

What it usually costs

HomeGuide (2026) lists thermostat replacement at about $100 to $200 for a basic model, $160 to $340 for programmable and $200 to $450 for smart, installed. Adding a C-wire runs about $90 to $135.

Frequently asked questions

Why is my thermostat blank?

Most often dead batteries or no power from the HVAC system (tripped breaker, furnace switch off, tripped float switch or blown fuse).

Can a clogged AC drain turn off the thermostat?

Yes. Many systems have a float switch that cuts power to the thermostat when the condensate drain backs up.

Do I need a C-wire?

Many smart thermostats need one for steady power. HomeGuide notes most modern thermostats require it.

Related problems

• AC not working
• Furnace not turning on
• AC leaking water
• Breaker keeps tripping

Sources

• Lennox: No power to thermostat?
• Trane: Thermostat not working, causes and solutions
• HomeGuide: Thermostat installation and replacement cost (2026)

This guide is general information, not a substitute for an inspection by a licensed professional. See our disclaimer.

Short answer: Some cool air is normal. Carrier explains that heat pumps periodically run a defrost cycle of about 5 to 10 minutes that sends cooler air indoors, and that heat pump air is typically around 85 to 100°F compared with 120 to 140°F from a gas furnace. It is a problem if the air stays cold, the outdoor unit is covered in thick ice, or the system cools fine in summer but will not heat.

Quick diagnosis

What you notice	Most likely cause	What to check	Call a pro?
Cool air for 5 to 10 minutes, steam from outdoor unit	Normal defrost cycle	Wait	No
Air warm but not hot	Normal heat pump supply temperature	Feel at a vent near the unit	No
Air cold all the time	Thermostat on Cool or fan ON	Set Heat and fan AUTO	If it continues
Thick ice (more than 1/4 inch) stays on outdoor unit	Airflow, refrigerant or defrost problem	Clear snow and debris around unit	Yes
Cools fine, will not heat	Stuck reversing valve	Note when it started	Yes
Outdoor fan not running	Power, capacitor or motor	Check outdoor disconnect and breakers	Yes

Checks before you call

1. Thermostat set to Heat, setpoint above room temperature.
2. Check the breakers for both the indoor and outdoor units; call a pro if they keep tripping.
3. Make sure the outdoor disconnect and the indoor air handler switch are on.
4. Replace or clean the filter.
5. Clear snow, leaves and debris; Carrier recommends keeping at least 24 inches of clear space around the outdoor unit.
6. Reset: thermostat off, both breakers off for one minute, back on, wait a few minutes, then set Heat.

About emergency heat

Emergency (auxiliary) heat uses electric resistance strips or a backup furnace. It is useful if the outdoor unit fails, but Carrier warns it can raise your electric bill significantly, so use it only until the system is repaired.

Ice on the outdoor unit

A light frost under about 1/4 inch is normal and should clear during defrost. Heavy ice that remains, or very frequent defrosting, points to restricted airflow, low refrigerant or a defrost control problem.

When to call a professional

• The air stays cold for more than a defrost cycle.
• Thick ice stays on the outdoor unit.
• The system cools but does not heat.
• The outdoor fan does not run or the breakers keep tripping.

What it usually costs

HomeGuide (2026) puts general HVAC repairs at about $150 to $450. Refrigerant work on AC and heat pump systems commonly runs $100 to $600 for a recharge, plus leak repair if needed.

Frequently asked questions

Why does my heat pump blow cold air for a few minutes?

It is probably in defrost mode, which lasts about 5 to 10 minutes and is normal.

Should I use emergency heat?

Only if the heat pump is not working. It costs more to run.

Related problems

• Furnace blowing cold air
• AC not cooling
• Breaker keeps tripping

Sources

• Carrier: Heat pump not heating? Troubleshooting cold air
• HomeGuide: HVAC repair cost (2026)
• HomeGuide: AC repair cost (2026)

This guide is general information, not a substitute for an inspection by a licensed professional. See our disclaimer.

Short answer: Start with the simple things Trane lists: thermostat set to Heat and above room temperature (with fresh batteries), the furnace power switch on, the breaker not tripped, and a clean filter. On newer furnaces with electronic ignition, a failed igniter or a control board fault needs a technician. If you smell rotten eggs, leave the house and call your gas company.

Quick diagnosis

What you notice	Most likely cause	What to check	Call a pro?
Thermostat blank or not calling for heat	Dead batteries or wrong setting	Set Heat, raise setpoint, replace batteries	If it still does nothing
No sound at all from the furnace	Power switch off or breaker tripped	Check the switch near the furnace and the panel	If the breaker trips again
Furnace starts, then shuts off	Dirty filter (overheating) or flame sensor	Replace the filter	Yes, if it continues
Blinking light on the furnace	Error code from the control board	Read the code chart inside the panel door	Usually
Gas smell	Possible gas leak	Leave the house now	Call the gas company

Safety first

If you smell rotten eggs, Trane advises leaving the house and calling your gas company. Do not operate light switches or try repairs until the danger has passed.

Homeowner checks

1. Thermostat: set to Heat, a few degrees above room temperature. Replace batteries; check for an error message.
2. Power switch: look for a light-switch-style switch on or near the furnace. Make sure it is on.
3. Breaker: reset a tripped furnace breaker once. If it trips again, there is an electrical problem; call an HVAC company.
4. Filter: replace a dirty filter, and make sure vents and returns are not blocked.
5. Panel door: many furnaces will not run if the front panel is not fully closed.

Ignition problems

On newer furnaces with electronic ignition, Trane recommends calling a technician, who will check the control board for error codes. On older furnaces with a standing pilot, you can relight the pilot by following the instructions on the unit; if it will not stay lit, call a professional.

When to call a professional

• The furnace still will not start after these checks.
• The breaker trips again after one reset.
• The pilot will not stay lit, or the igniter does not glow.
• There is any gas smell or a blinking error code you cannot clear.

What it usually costs

HomeGuide (2026) puts most furnace repairs at about $125 to $480. Typical prices: flame sensor $80 to $250, igniter $100 to $250, control board $300 to $650, blower motor $300 to $900, draft inducer motor $400 to $1,100. Technicians charge about $75 to $150 per hour, more for emergency service.

Frequently asked questions

Why won’t my furnace kick on when the thermostat calls for heat?

Common reasons are a power switch turned off, a tripped breaker, an overheating shutdown from a dirty filter, or an ignition problem.

Is there a reset button on a furnace?

Some furnaces have one, but repeated resets can hide a real problem. Turning power off and on at the switch is the usual reset; if it keeps failing, call a technician.

My heater is not working at all. Where do I start?

Work through the same order: thermostat (Heat mode, setpoint, batteries), the furnace power switch, the breaker, the filter and the front panel door. Most “heater not working” calls end at one of those.

Related problems

• Furnace blowing cold air
• Heat pump blowing cold air
• Breaker keeps tripping
• No hot water
• Thermostat not working

Sources

• Trane: Furnace not turning on? Troubleshooting tips
• HomeGuide: Furnace repair cost (2026)

This guide is general information, not a substitute for an inspection by a licensed professional. See our disclaimer.

Short answer: Check the thermostat first: Trane notes that when the fan is set to ON instead of AUTO, the blower keeps running between heating cycles and blows unheated air. Next, replace the filter, because a clogged filter can make the furnace overheat and shut off the burners. If those are fine, the flame sensor, igniter, gas supply or (on high-efficiency furnaces) a blocked condensate line may be keeping the burners from running.

Quick diagnosis

What you notice	Most likely cause	What to check	Call a pro?
Cold air between warm cycles	Thermostat fan set to ON	Set fan to AUTO	No
Warm, then cold after a few minutes	Overheating from a dirty filter (limit switch)	Replace the filter	If it continues
Burners light, then shut off; blower keeps going	Dirty flame sensor	Call for cleaning	Yes
No flame at all	Igniter, pilot or gas supply	Check gas valve open, propane/oil level	Yes
High-efficiency furnace stops heating	Clogged condensate line	Look for water around the furnace	Yes
Some rooms cold, others fine	Duct leaks or closed vents	Check vents are open	If it persists

Thermostat and fan

Set the thermostat to Heat, the fan to AUTO and the temperature above room temperature. Replace the batteries if needed.

Filter and overheating

A clogged filter restricts airflow. The furnace overheats, the high-limit switch shuts the burners off, and the blower keeps pushing air that is no longer heated. Trane suggests changing filters every 30 to 90 days. Turn the furnace off before you change it.

Fuel supply and ignition

• Natural gas: make sure the gas valve to the furnace is open.
• Propane or oil: check the tank level.
• Older furnaces: a pilot that went out can be relit following the label on the unit.
• Newer furnaces: a failed igniter or a dirty flame sensor needs a technician.

When to call a professional

• The burners do not light, or light and shut off within seconds.
• The furnace keeps overheating after a new filter.
• There is water around a high-efficiency furnace.
• You smell gas: leave the house and call the gas company.

What it usually costs

HomeGuide (2026) lists flame sensor replacement at about $80 to $250 and igniter replacement at about $100 to $250. Most furnace repairs fall between $125 and $480.

Frequently asked questions

Why is my furnace blowing cold air but the pilot is on?

The burners may not be lighting from the pilot, the flame sensor may be shutting them off, or the limit switch may be cutting heat due to overheating.

Is it normal for a furnace to blow cold air at first?

A short delay is normal: many furnaces wait until the heat exchanger warms before the blower starts. Steady cold air is not.

Related problems

• Furnace not turning on
• Heat pump blowing cold air
• AC not cooling

Sources

• Trane: Furnace not blowing hot air
• HomeGuide: Furnace repair cost (2026)

This guide is general information, not a substitute for an inspection by a licensed professional. See our disclaimer.

Short answer: Your AC pulls water out of the air and sends it down a condensate drain line. When water leaks inside, Carrier lists a clogged drain line as a main cause, followed by a cracked or rusted drain pan and a frozen evaporator coil (often from a dirty filter or low refrigerant). Turn the system off, dry the area, check the filter, the pan and the drain line.

Quick diagnosis

What you notice	Most likely cause	What to check	Call a pro?
Water around the indoor unit or furnace	Clogged condensate drain line	Look at the drain outlet; is water coming out?	If you cannot clear it
Drip from a rusty or cracked pan	Damaged drain pan	Inspect the pan with a flashlight	Often
Ice on the coil or lines, water when it thaws	Frozen coil (dirty filter or low refrigerant)	Turn off, replace filter	If it refreezes
Water near a small pump with a tube	Failed condensate pump	Check the pump runs and is plugged in	Often
Water dripping from a ceiling vent	Condensation on ducts or a clogged attic unit drain	Check attic unit and pan	Yes

Stop the damage first

1. Turn the system off at the thermostat.
2. Soak up the water and move anything that can be damaged.
3. If water reached a ceiling, see our guide to a water stain on the ceiling: wet drywall can sag.

Clear the drain line

Find where the drain line exits (often outside near the condenser, or into a floor drain). Carrier suggests turning the AC off and using a wet-dry vacuum on the outdoor end of the line to pull out the clog. Do not use a wet-dry vacuum if you are not sure which line is the condensate drain.

Check that the line slopes downward all the way and that water flows freely once it is clear.

Check the filter and the pan

• Replace a dirty filter; Carrier suggests checking it monthly and replacing every one to three months.
• Look at the drain pan for cracks, rust or debris.
• If the coil is frozen, let it thaw with the system off before running it again.

When to call a professional

• You cannot find or clear the clog.
• The drain pan is cracked or rusted through.
• The coil freezes again after you replaced the filter.
• Water is coming through a ceiling or the leak may involve refrigerant.

What it usually costs

HomeGuide (2026) lists clearing a clogged condensate drain line at about $75 to $250 and a condensate pump at about $100 to $165. A service call is about $75 to $200.

Frequently asked questions

Is it normal for an AC to drip water?

Water should drip outside or into a drain. Water inside the house is not normal.

Can I keep running the AC while it leaks?

Carrier advises turning the system off. Pooling water can damage ceilings and floors and lead to mold.

Related problems

• AC not cooling
• Water stain on ceiling
• Mold on bathroom ceiling
• Thermostat not working
• Water leaking from ceiling

Sources

• Carrier: Why is my AC leaking water?
• HomeGuide: AC repair cost (2026)
• U.S. EPA: A brief guide to mold, moisture and your home

This guide is general information, not a substitute for an inspection by a licensed professional. See our disclaimer.

Short answer: When the AC runs but the house stays warm, Carrier lists the most common causes as a thermostat set wrong, a dirty air filter and a blocked or dirty outdoor unit, all things you can check yourself. A frozen indoor coil, a refrigerant leak or a damaged unit need a technician. If you see ice on the copper lines, turn the system off: running it frozen can damage the compressor.

Quick diagnosis

What you notice	Most likely cause	What to check	Call a pro?
Air from vents is room temperature	Thermostat on fan “on” or heat, not cool	Set to Cool and fan to Auto	No
Weak airflow, house slowly warms	Dirty air filter	Replace the filter	No
Outdoor unit covered in grass, leaves or dirt	Blocked condenser coil	Clear debris; gently rinse fins	If it does not help
Ice on the copper lines or indoor unit	Frozen evaporator coil	Turn the system off to thaw	Yes
Hissing, oily spots, cooling gets worse each week	Refrigerant leak	Turn it off	Yes
Outdoor fan not spinning, unit hums	Failed capacitor or fan motor	Turn it off at the breaker	Yes

Check these first

1. Thermostat: set to Cool, fan to Auto, and the temperature a few degrees below the room. Replace thermostat batteries if the display is dim.
2. Air filter: turn the system off and look at the filter. If it is gray and clogged, replace it. Make sure supply vents and return grilles are open.
3. Outdoor unit: clear grass, leaves and anything stacked against it, then gently rinse the fins with a garden hose (no pressure washer).
4. Reset: Carrier suggests setting the thermostat to off, switching off the AC breaker, waiting at least 5 minutes, then turning it back on and setting it to Cool.

Frozen coil

Frost on the copper tubing, weak cooling, or more water than usual at the drain are signs of a frozen evaporator coil. Turn the system off (you can leave the fan on to speed thawing) and replace the filter. If it freezes again, the cause is usually low refrigerant or an airflow problem that a technician must find.

Refrigerant problems

AC systems do not use up refrigerant. If it is low, there is a leak. Carrier warns not to try to fix a refrigerant leak yourself; a licensed technician has to find the leak, repair it and recharge the system.

When to call a professional

• The system still does not cool after the thermostat, filter, outdoor unit and reset checks.
• You see ice that comes back after thawing.
• The outdoor fan does not spin, or the unit hums, clicks or trips the breaker.
• You suspect a refrigerant leak.

What it usually costs

HomeGuide (2026) puts most AC repairs at about $150 to $650. Typical prices: service call $75 to $200, capacitor $100 to $250, condenser fan motor $200 to $700, refrigerant recharge $100 to $600, refrigerant leak repair $200 to $1,500, and compressor replacement $600 to $2,500 or more.

Frequently asked questions

Why is my AC running but not cooling?

The most common reasons are a thermostat setting, a dirty filter or a dirty outdoor unit. If those are fine, a frozen coil, low refrigerant or a failed part is likely.

How long should it take an AC to cool a house?

It depends on the house and the weather. If the temperature does not drop at all after an hour, or keeps rising, something is wrong.

Can a dirty filter stop the AC from cooling?

Yes. A clogged filter restricts airflow and can even freeze the coil.

Related problems

• AC leaking water
• Heat pump blowing cold air
• Breaker keeps tripping
• Furnace not turning on
• AC not working
• Thermostat not working

Sources

• Carrier: Why is my AC not blowing cold air?
• HomeGuide: AC repair cost (2026)

This guide is general information, not a substitute for an inspection by a licensed professional. See our disclaimer.