Emergency HVAC → Carbon Monoxide
This is the one household hazard your senses give you nothing on. No colour, no smell, and early symptoms that look exactly like flu. The response is not investigation.
Carbon monoxide furnace leaks happen when combustion products that should leave through the flue enter the house instead. CPSC records more than 200 US deaths a year from carbon monoxide produced by non-automotive consumer products, with an average of 63 a year attributable to heating systems in its review period.
Every other page on this site starts with a diagnosis. A carbon monoxide furnace problem starts with an evacuation instead, because that is the correct order for this hazard.
If an alarm is sounding right now
Move every person and pet out to fresh air immediately. Not to another room, not to the porch with the door open. Outside.
Fire crews carry calibrated CO meters and will tell you whether the building is safe to re-enter. That is not a judgement to make yourself.
CPSC lists this explicitly, because carbon monoxide causes confusion and people have been left behind while others assumed they had followed.
Searching for the source is the instruction CPSC gives specifically to avoid, and it is the one people ignore. You cannot see or smell what you would be walking into.
Does carbon monoxide have a smell?
No. It is colourless, odourless and non-irritating. Natural gas smells because a sulphur compound is deliberately added to it so leaks are obvious. Nothing is added to carbon monoxide, because it is a combustion product rather than a supplied fuel.
This single fact drives everything else. If you could smell it, the standard advice to look for a yellow burner flame would be reasonable. Since you cannot, walking toward the appliance to inspect it means walking toward the source of an invisible gas.
It is also why the two hazards get confused. A gas smell near a furnace is unburnt fuel and a different problem, though the first instruction is the same: leave, then call from outside.
CPSC describes the initial symptoms of CO poisoning as similar to flu: headache, fatigue, shortness of breath, nausea and dizziness. Taken individually, every one of those is also just a bad week.
What distinguishes carbon monoxide is the pattern rather than the symptom.
This is not a self-diagnosis page
If anyone is confused, drowsy, unsteady or unconscious, that is a 911 call from outside the building and nothing else. Carbon monoxide poisoning is treated with oxygen and it needs medical assessment. Nothing on this page is a substitute for that, and no symptom checklist should be used to talk yourself into staying.
A correctly working gas furnace produces carbon monoxide as a normal combustion by-product and sends all of it up the flue. Problems arise when the gas ends up somewhere else, or when combustion goes wrong.
Does ventilation fix a carbon monoxide leak?
No. Opening windows dilutes what is already in the air and does nothing about a source that is still producing. Open them on your way out if it costs you no time, but the action that protects you is leaving the building.
This question appears in the People Also Ask block for this search and is answered on none of the ranking pages, which is a notable gap on a safety topic.
The reasoning is straightforward. Ventilation helps once the source has stopped, which is why fire crews ventilate after they have shut the appliance down. Before that, all it does is change the concentration you are standing in.
A furnace that is not burning fuel cannot produce carbon monoxide. That is the honest short answer, and there are two caveats worth knowing.
Older furnaces with a standing pilot light are burning something continuously, at a very small scale. And in most homes the furnace shares its flue with a gas water heater, so a blocked chimney can spill the water heater's combustion products into the house on a summer day when the furnace has not run for months.
If you have symptoms out of the heating season, a carbon monoxide furnace fault is unlikely and the shared flue is the first thing to have looked at, along with any gas range or fireplace.
The stakes, in real numbers
Every figure is sourced
Source: ESFI
Source: EPA
Source: CPSC
Source: NFIP
Placement is where most households get this wrong, and it is stated on none of the pages currently ranking for this search.
A smoke alarm is not a CO alarm unless it is explicitly a combination unit. That confusion is common and it costs nothing to check the label tonight.
CPSC recommends that a qualified technician inspect the home heating system, including the furnace or boiler and its controls and safety switches, once a year. This is what that inspection actually consists of.
The technician inspects the heat exchanger, sometimes with a borescope camera, because cracks are frequently invisible from outside. They then test draft at the flue and check for spillage at the draft hood.
That spillage test is the direct check of whether combustion products are actually leaving the building.
Then they run a combustion analysis, sampling flue gases to measure CO and oxygen levels and confirm the burner is running correctly. A furnace can pass a visual inspection and fail a combustion analysis, which is why the instrument matters more than the look of the flame.
If the furnace also short cycles, mention it, because incomplete cycles worsen combustion problems.
Know the line
24/7 dispatch network
A CO alarm that has sounded, or a furnace you want combustion-tested. Send the details and your ZIP code and licensed, insured HVAC contractors covering your area will contact you directly. If an alarm is sounding now, call 911 first.
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.
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 |
| Combustion safety inspection | usually 1 hour + diagnostic | Draft test, spillage test, combustion analysis |
| Annual heating system inspection | standard-hours service call | CPSC recommends this once a year |
Source: 2026 emergency HVAC rate aggregator data; HomeGuide 2026 HVAC repair cost guide (retrieved August 2026). Figures vary by region, severity and after-hours timing.
Figures last checked
An annual inspection at a standard-hours rate is one of the few pieces of home maintenance with a mortality statistic attached to skipping it. It is also the cheapest version of this visit you will ever buy. Full rate detail sits in our breakdown of what emergency HVAC repair costs.
If a cracked heat exchanger is found, expect a replace-or-repair conversation. On an older furnace the economics usually favour replacement, and any contractor recommending you continue running a furnace with a confirmed crack is giving you advice worth declining.
Emergency Services 24H is a referral service. We do not employ HVAC technicians, we do not test appliances, and we cannot tell you whether your house is safe to re-enter.
Your request routes by ZIP code to licensed, insured contractors covering your area, and they contact you directly. For an active alarm, the fire department is the correct first call and the form can wait until you are outside.
Straight answers
The questions homeowners actually search for on this problem, answered in plain language.
Browse all guidesBy alarm, not by sense. Carbon monoxide is colourless and odourless, so a working CO alarm is the only reliable detector. Supporting signs include soot or staining around the furnace, a yellow rather than blue burner flame, and excessive condensation on windows, but none of those is a substitute for an alarm.
The most reliable is a CO alarm sounding. The second is a pattern of flu-like symptoms, headache, fatigue, nausea and dizziness, affecting several people or pets at once and improving when everyone is outside. CPSC notes the initial symptoms are similar to flu, which is exactly why the pattern matters more than any single symptom.
Because combustion products that should leave through the flue are entering the house instead. The usual causes are a cracked heat exchanger, a blocked, corroded or disconnected flue, poor combustion air supply, or a burner that is out of adjustment. A powerful extractor fan can also pull flue gases back into the room.
It depends entirely on the concentration. High levels can cause confusion and unconsciousness within minutes, while low-level exposure produces headaches and fatigue over hours or days that are easily mistaken for illness. That variability is the reason the response to an alarm is to leave immediately rather than to assess how you feel.
No. Ventilation dilutes the concentration but does nothing about the source, which is still producing. Open windows on your way out if it is convenient, but leaving the building is the action that matters. CPSC guidance when an alarm sounds is to move everyone outside, call 911, and do a head count.
A furnace that is not burning anything cannot produce carbon monoxide. Two caveats matter: a standing pilot light is still combustion, and other appliances such as a water heater often share the same flue, so a blockage can push their combustion products into the house while the furnace sits idle.
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24/7 dispatch network
Tell us what has failed and where you are. We route your request to licensed, insured contractors covering your ZIP code, 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.