Emergency HVAC · Heating
A flame-sensor clean is often one labor hour, and after-hours HVAC labor runs $275 to $600 (2026 rate aggregator data, read 2026-08-16). A furnace blowing cold air has usually lost the flame rather than the fan: incorrect thermostat settings, a dirty air filter, a dirty sensor or ignition failure.
Cold air at a vent is either the system behaving as designed at the start of a heating cycle, or it has failed to light and hold a flame. Those two situations are told apart by timing a single cycle, which takes about five minutes and costs nothing.
Five-minute diagnosis
Stand at a vent as the thermostat calls for heat. Cold for a minute then warm is normal.
Fan on runs the blower continuously between cycles, so room-temperature air comes out of every vent. Fan auto runs it only during heat.
The burner is lighting and dropping out, usually because the flame sensor is dirty. The pattern repeats identically every time, which is the giveaway.
The burner never lit: either the board locked out, or a limit switch cut the burner while leaving the blower running.
“If the flames light and then it only lasts for about three seconds, that is almost guaranteed to be a flame sensor issue.”
Thermostats have a fan control with two positions, and most people never touch it.
Set to auto, the blower runs only while the furnace is producing heat. Set to on, it runs continuously, circulating room-temperature air instead of heat for most of the hour.
The giveaway is a heater blowing cold from every register while the unit is not running, alternating with warm air when it is.
Cold air for a full cycle, with the blower running throughout, means the burner never lit.
A modern furnace tries to light three or four times, looks for a flame with a sensor each time, and stops trying if it never finds one. That state is called lockout, and it is deliberate: it prevents unburnt gas accumulating in the heat exchanger.
Our page on a furnace that will not turn on covers the door switch and the flash codes.
This has a signature you can time: warm air arrives, lasts thirty to ninety seconds, then goes cold while the blower keeps running.
The cause is almost always the flame sensor, a rod in the burner flame that confirms combustion by conducting a tiny current. Oxide builds up, the current drops, and the board shuts the gas valve.
The working technician behind Word of Advice TV calls the dirty flame sensor the champion of all furnace problems: burners that light and die after three or four seconds almost always point to flame sense.
Longer than that points at the limit-switch pattern further down this page.
It is one of the cheapest repairs in the trade, often inside a labor hour, and usually a cleaning rather than a part: the rod is scrubbed and refitted.
The third pattern is the one that matters most, and most guides name the part without explaining the cycle.
If airflow across the exchanger is restricted by a dirty air filter or closed vents, it runs hotter than design. The high-limit switch cuts the burner and leaves the blower running, which is why cold air comes out.
Once things cool the burner relights and it repeats, which is the furnace that heats a bit then blows cold all evening.
The loop is self-terminating. Word of Advice TV puts the threshold at three to five limit trips inside a cycle, after which the board drops into a multi-hour lockout.
An evening that starts as intermittent cold air and ends with a dead furnace is one restriction escalating, not two faults.
His most common cause is the dullest: a dirty air filter. The giveaway is a furnace that never quite keeps up, set to 72 with the house at 70, before it trips out.
A locked-out unit is already producing no heat, one cycling on the limit switch is fatiguing its exchanger, and a flame-sense dropout is the least harmful.
Plenty of people ask about electric furnaces specifically, yet almost nothing online is written for them.
An electric furnace has no burner or flame sensor. Heat comes from a resistance heating element brought in by sequencers, so cold air means a failed sequencer, a tripped high-limit, or elements failed open.
A gas furnace can vent combustion products; an electric one cannot, so cold air there is no carbon monoxide risk. Everything about airflow and limit switches still applies.
Know the line
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Tell us the timing pattern, warm then cold or cold throughout. Local HVAC contractors covering your area will contact you directly. No obligation to hire.
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Indicative US ranges. Always get a written quote.
| Job | Typical range | Notes |
|---|---|---|
| After-hours diagnostic fee, weeknight | $150–$250 | Before any repair work |
| After-hours diagnostic fee, holiday | $225–$500 | Top of the band on major holidays |
| Emergency labor multiplier | 1.5×–3× | Applied to the standard hourly rate |
| Furnace igniter, standard hours | $150–$350 | $275–$600 after hours |
| Flame sensor clean or replace | usually inside one labor hour | Often the cheapest fix on this page |
| Blower motor | $400–$900 | Standard hours |
Source: 2026 emergency HVAC rate aggregator data; HomeGuide 2026 HVAC repair cost guide (retrieved August 2026). Figures vary by region and timing.
Figures last checked
The flame sensor is the good-news case, often one labor hour and a cleaning. Ignition failure at the igniter is the common one, and the limit-switch loop is where diagnosis matters more than the part.
Check vents and the filter first: that is the whole of the DIY here. The trade sits in the the heating and cooling section, summer faults on our cooling page, and Emergency Services 24H routes the call.
Full rate structure sits in our emergency HVAC rates, including whether the diagnostic fee is credited.
Emergency Services 24H is a referral service. We do not employ HVAC technicians and we do not perform gas work.
Your request routes to local contractors nearby. Describe the timing pattern rather than just saying cold air, because it covers three faults at three prices.
Straight answers
Short, plain answers to the questions people ask us most about this page.
Browse all guidesIntermittent heat is the limit-switch pattern: the burner lights, the unit overheats behind a restriction, and the switch cuts the burner. Furnace heating that comes and goes in winter is an airflow fault.
A furnace blowing cold air but not heating has lost the flame rather than the fan: ignition failure, a dirty flame sensor, or a pilot light that has gone out. An oil furnace follows the same logic at a different burner.
No. An RV furnace and a car heater share the symptom and none of the parts. This page covers a house furnace: gas, propane or oil, with a blower and a heating element or burner behind it.
Either it is early in a cycle, when the blower starts before the exchanger is warm, or it failed to establish a flame. Check the thermostat settings first: fan on runs the blower continuously.
Yes if it locks out repeatedly, lights and drops out every cycle, or trips the limit switch. No if the cold air lasts thirty to sixty seconds and then turns warm.
Switch the furnace off at its own power switch, usually a light-switch-looking control on or near the unit, wait a full minute, then switch it back on. That clears a control-board lockout. If it locks out again within a cycle or two, stop and call a technician rather than resetting repeatedly.
The three real causes are a control-board lockout after failed ignition attempts, a flame that lights and then drops out because the flame sensor is dirty, and overheating that trips the limit switch and cuts the burner while leaving the blower running. All three produce cold air at the vent.
Rising run times with falling output, repeated lockouts, a burner flame that is yellow rather than blue, rust or soot around the cabinet, escalating repair frequency, and age beyond about fifteen to twenty years. None of those alone is decisive, and a combination of three usually is.
Yes, at the thermostat or at the furnace power switch. Both are safe. If you switch it off at the breaker mid-cycle the blower stops immediately, which leaves residual heat in the exchanger with no airflow, so the thermostat or the unit switch is the better choice.
Independent, publicly published sources, quoted with attribution and linked in full. None is affiliated with Emergency Services 24H, reviewed this page, or works in the referral network.
Reviewed on . If a figure here no longer matches its source, that is a bug, so tell us and we will correct it.
24/7 dispatch network
Tell us what has failed and how soon you need someone. We route your request to local contractors covering your area, and they contact you directly.
Fire, a gas smell, or an injury? Leave the building and call 911 first, then your gas utility or power company. Use this form once you are safe.