No Heat? Emergency Guide

It’s 11 p.m., the thermostat says 68°F, and the house is getting colder by the hour. Whether your furnace is blowing cold air, your heat pump is stuck in defrost, or the boiler is silent, a no-heat situation in winter is a genuine emergency that needs a fast, safe, and methodical response.

This guide walks you through the checks you can safely do yourself, the failures that require a licensed technician, and what repairs realistically cost in US dollars in 2026 — so you can make a calm decision instead of a panicked one.

Rule Out the Obvious Before You Panic

Roughly one in five “no heat” service calls ends with a technician flipping a switch or replacing two AA batteries. Before you assume the worst, verify these basics:

• Thermostat mode and setpoint. Confirm it’s set to HEAT, not COOL or FAN. Raise the setpoint 3–5°F above room temperature to trigger a call for heat.
• Thermostat batteries and power. Battery-powered thermostats go blank or behave erratically at about 20% charge. Check for a blank or flickering display.
• The furnace service switch. Many furnaces have a light-switch-style shutoff on or near the unit (often on the side of the cabinet or at the top of the basement stairs). If someone flipped it while painting or doing electrical work, the unit is dead.
• The breaker. Look for a tripped breaker in the main panel and any subpanel. A furnace typically draws 15–20 amps on its own circuit; a heat pump’s air handler has its own breaker plus a separate one for the outdoor condenser.
• Fuel supply. Check that the gas valve at the furnace is parallel to the pipe (open). If you’re on propane, check the tank gauge — a “no heat” call in rural areas is frequently an empty tank. Oil customers should check the tank and the reset button on the burner (press once only).
• The air filter. A filter loaded with dust and pet hair restricts airflow and can trip the high-limit switch, shutting the burner down repeatedly. A 1-inch filter should be changed every 30–90 days; a 4–5 inch media filter every 6–12 months.

If none of that restores heat, move on to diagnosis by system type. For a deeper walk-through of general fixes, our HVAC troubleshooting library covers symptom-by-symptom checks for every major system.

Safety First: When No Heat Is a True Emergency

Before you touch anything, know the signals that mean stop and get out:

• You smell natural gas (rotten eggs). Do not flip switches, do not use your phone inside, and do not attempt to relight anything. Leave the building, then call your gas utility’s emergency line from outside.
• A carbon monoxide alarm is sounding. Get everyone — including pets — outside and call 911. CO is odorless, and a cracked heat exchanger or blocked flue can push it into your living space.
• You see soot, scorch marks, or a yellow, lazy flame instead of a crisp blue one. Shut the unit off at the service switch and call a professional. This is a combustion problem, not a DIY repair.
• You hear banging, rumbling, or a boom on startup. That’s delayed ignition, a genuine safety hazard.

Every year, US emergency rooms treat thousands of CO exposures linked to heating equipment. A functioning CO alarm on every level of the home and outside sleeping areas is non-negotiable, and it costs less than $30. Combustion appliances should be installed and vented to NFPA 54 / ANSI Z223.1 standards, and any gas work must be performed by a licensed technician.

Diagnose the Problem by System Type

Furnace Blowing Cold Air or Not Running

A gas furnace follows a fixed ignition sequence: thermostat calls for heat → draft inducer motor starts → pressure switch closes → ignitor glows → gas valve opens → flame sensor proves flame → blower motor starts (usually after a 30–90 second delay). Any break in that chain produces the same result: no heat.

Most common failures, in order of frequency:

1. Dirty flame sensor. The sensor is a thin metal rod that must be cleaned with steel wool annually. A dirty sensor lets the burner light for 5–10 seconds, then shuts it down. This is the single most common “no heat” repair in the US.
2. Failed hot-surface ignitor. The silicon nitride ignitor is fragile and typically lasts 4–8 years. It fails open, so the burner never lights.
3. Pressure switch or blocked flue. High-efficiency (90%+ AFUE) furnaces vent through PVC pipe, which can be blocked by snow, ice, bird nests, or a frozen condensate trap. In January, a frozen condensate line is one of the top reasons a condensing furnace locks out.
4. Tripped high-limit switch from restricted airflow — almost always the filter or a failing blower motor.
5. Failed control board or inducer motor.

Our dedicated furnace troubleshooting guide lists the exact blink codes for Carrier, Trane, Lennox, Rheem, Goodman, and Bryant units, which is the fastest way to narrow it down.

Heat Pump Blowing Cold (or Room-Temperature) Air

Heat pumps feel cool at the register — supply air is typically only 90–105°F versus 120–140°F from a gas furnace — so “it’s blowing cold” is sometimes just normal operation. What is not normal:

• Aux/Emergency heat locked on. If the display shows EM HEAT or AUX, the heat pump itself has failed and the backup electric resistance strips (typically 5–15 kW, or 17,000–51,000 BTU/h) are carrying the load. That works — at roughly two to three times the cost per BTU of the heat pump alone.
• Reversing valve stuck. The valve that flips the system between heating and cooling fails, so a heating call delivers air conditioning. A telltale sign is hot air from the outdoor unit in heat mode.
• Defrost cycle stuck. Heat pumps periodically run in cooling mode to melt frost off the outdoor coil. Normal defrost lasts 3–10 minutes; a system that stays in defrost for 30+ minutes has a failed defrost board or sensor.
• Low refrigerant charge from a leak, or a failed capacitor/contactor on the outdoor unit.
• Balance point issues. Below the system’s balance point (often 25–35°F in a standard air-source heat pump), capacity drops and the aux strips must take over. If your system was sized for cooling load rather than heating load, this becomes obvious on the first hard freeze.

See the heat pump troubleshooting guide for defrost, reversing valve, and refrigerant symptom charts.

Boiler or Radiator Not Heating

If you have a hydronic system, the boiler may be firing without delivering heat to the rooms. Look for:

• A failed circulator pump (no flow, cold radiators, hot boiler).
• Zone valve stuck closed — check whether one zone is cold while others are warm.
• Air in the lines — bleed radiators at the bleed valve until water runs steadily.
• Low system pressure — most residential boilers run at 12–18 psi cold. Below 10 psi, the low-water cutout may prevent firing.
• A frozen or failed expansion tank, causing pressure spikes and relief valve discharge.
• Baseboard or radiator fins blocked by furniture, curtains, or paint.

Typical Repair Costs (US, 2026)

Prices below reflect installed cost — parts plus labor — in a mid-cost US metro. Expect 15–30% more in high-cost areas like Boston, Seattle, San Francisco, or New York.

Two things drive the wide ranges: equipment tier and whether the repair is a “drop-in” swap. A control board for a 20-year-old builder-grade furnace may be backordered or unavailable, which pushes the decision toward replacement.

A Safe DIY Troubleshooting Sequence

Work through this in order, and stop the moment you’re uncomfortable with anything involving gas, wiring, or disassembly beyond the filter door.

1. Confirm the thermostat is calling for heat. Raise the setpoint 5°F. Listen for a click at the thermostat and the furnace.
2. Check the breaker and the service switch at the unit.
3. Replace the air filter. Note the size and airflow arrow printed on the old one.
4. Check the flue and intake terminations outside for snow, ice, nests, or leaf debris. Clear them with a broom — never with your hands.
5. Check the condensate trap and drain line on a 90%+ furnace. If it’s frozen or clogged, the pressure switch won’t close.
6. Reset the unit once using the power switch (off for 30 seconds, then on) and watch the ignition sequence through the sight glass if your unit has one.
7. Record the LED blink code from the control board — that code is the fastest path to a diagnosis and will save you money on the service call.

Do not attempt to clean a flame sensor, replace an ignitor, jump a pressure switch, or open the burner compartment unless you’re trained. Beyond the risk of gas leaks and electrical shock, unauthorized gas work can void your homeowner’s insurance and violate local code.

When to Call a Pro — and How to Choose One

Call a licensed technician immediately if:

• You smell gas or your CO alarm sounds.
• The unit has power but produces no ignition attempt at all.
• You see a cracked heat exchanger (often visible as a distorted or rusted cell through the sight glass).
• The system has tripped its breaker more than once.
• Your home will be below 55°F within a few hours and you have infants, elderly residents, or anyone with medical needs.

When you call, ask three questions: Are you NATE-certified? What’s your after-hours diagnostic fee, and does it apply toward the repair? Do you stock common parts on the truck? In most states, HVAC contractors must hold a state or local license, and any work involving refrigerant (R-410A, or the newer R-454B and R-32 blends being phased in under the AIM Act) requires EPA Section 608 certification.

How to Stay Warm Safely While You Wait

• Use UL-listed space heaters only, plugged directly into a wall outlet — never an extension cord or power strip. Keep them 3 feet from anything flammable and never run one unattended while sleeping.
• Never use a gas range, oven, or charcoal grill for heat. This is a leading cause of fatal CO poisoning in winter storms.
• Keep indoor temperatures above 55°F. Below that, pipes in exterior walls become vulnerable. Open cabinet doors under sinks on exterior walls and let faucets drip.
• Close off unused rooms and hang blankets over drafty single-pane windows. A door draft stopper and plastic window film cost under $30 combined and make a measurable difference.
• Consolidate into one room with a single space heater and dress in layers. A 1,500-watt space heater produces about 5,100 BTU/h — enough to take the edge off one room, nowhere near enough to heat a house.

Prevent the Next No-Heat Emergency

Most winter breakdowns are predictable. Schedule a professional tune-up every fall (typically $89–$200), and ask the technician to check the flame sensor, clean the burners, test the pressure switch, verify the temperature rise (a gas furnace should run 35–70°F above return-air temperature, per the data plate), and inspect the heat exchanger.

Other high-value habits:

• Change filters on schedule. A dirty filter is the number one cause of preventable furnace failure and can add 5–15% to heating bills.
• Replace the thermostat batteries every fall, even if the display looks fine.
• Keep two feet of clearance around the outdoor heat pump unit and clear snow away from it — but never chip ice off the coil with a tool.
• Consider an ENERGY STAR certified unit when replacing. ENERGY STAR gas furnaces meet a minimum of 90% AFUE, well above the 80% federal minimum for standard models. Current federal minimums for split-system heat pumps are 14.3 SEER2 / 7.5 HSPF2 in the South and 13.4 SEER2 / 7.5 HSPF2 in the North.
• Right-size the equipment. Contractors should run a Manual J load calculation rather than guessing. A typical 2,000 sq ft home in a cold climate needs roughly 60,000–90,000 BTU/h of furnace output (about 30–45 BTU per sq ft), while a heat pump delivers about 12,000 BTU/h per nominal ton.

Frequently Asked Questions

Why is my thermostat on but there’s no heat coming out?

The most common culprits are a dead thermostat battery, a tripped breaker, a clogged air filter, a dirty flame sensor, or a failed ignitor. Check the thermostat mode and batteries first, then the breaker and the furnace service switch before calling a technician.

How much does an emergency HVAC call cost for no heat?

Expect $150 to $350 for an after-hours emergency diagnostic visit, versus $75 to $150 during normal business hours. Common repairs like a flame sensor cleaning run $100 to $200, while an ignitor, blower motor, or control board can push the total to $300 to $1,200 installed.

How long can a house go without heat before pipes freeze?

With outdoor temperatures below 20°F, exposed pipes in exterior walls can begin to freeze in as little as 6 to 12 hours. Keep indoor temperatures above 55°F, open cabinet doors on exterior walls, and drip faucets until heat is restored.

Conclusion

A no-heat emergency is stressful, but it’s rarely mysterious. In the vast majority of cases, the problem traces back to one of a handful of causes: a thermostat setting or battery, a tripped breaker or service switch, a clogged filter, a dirty flame sensor, a failed ignitor, a blocked flue, or a dead circulator pump. Work through the safe checks in order, write down any blink code you see, and stop immediately at the first sign of gas odor, soot, or a CO alarm.

If you can’t restore heat within a couple of hours — or if outdoor temperatures are heading below 20°F — call a licensed, NATE-certified technician rather than waiting until morning. Budget $150–$350 for the emergency visit, keep the house above 55°F in the meantime, and use only UL-listed space heaters. Then, once the system is running again, book a fall tune-up and stay ahead of the next breakdown: the $89–$200 you spend on annual maintenance is the cheapest heating insurance you can buy.