Placement, sensor age and the difference between a code-minimum alarm and a low-level monitor, laid out room by room.
Get the detector placement right →Generator carbon monoxide kills more than 80 people in the United States in an average year, according to CPSC, and it does it mostly to careful people who thought they had allowed enough distance. Carbon monoxide has no smell, no colour and no taste, it is roughly the same density as air so it does not conveniently sink or rise, and the early symptoms are indistinguishable from a bad headache. Where you put a generator is not a preference. It is the single decision most likely to hurt someone in your house during an outage.
Safety: Never run a portable generator indoors, in a garage, in a carport, in a shed, in a basement, in a crawlspace, or on a porch — including with the door or windows open. CPSC’s guidance is blunt: a generator used in an enclosed or partly enclosed space can kill you in minutes. There is no ventilation arrangement that makes an attached garage safe.
The Twenty-Foot Rule
CPSC’s placement guidance is that a portable generator runs outside only, at least 20 feet from the house, with the exhaust directed away from the home and away from any other building. It should be kept well clear of windows, doors and vents.
Twenty feet is a floor, not a target. It assumes the exhaust has somewhere to go. If your generator sits 20 feet away but tucked into an alcove between the house and the garage, or against a fence that traps the plume, or upwind of an open kitchen window, the number has done nothing for you. Two things have to be true at once: enough distance, and a clear path for the exhaust to disperse away from every opening in the building.
Why the Open Garage Still Kills People
This is the specific mistake that fills the incident reports, so it deserves the mechanism rather than just a prohibition. A portable generator produces carbon monoxide at a rate that swamps the natural air exchange of a garage. Opening the overhead door increases the exchange rate; it does not come close to matching the production rate. Concentrations climb into dangerous territory in minutes, and CPSC’s own testing has documented lethal levels in attached garages with the door open.
Then the house draws it in. In cold weather your home is at slightly negative pressure relative to outside, because the furnace, the dryer and the bathroom fans are all pushing air out and the stack effect is pulling warm air up and out of the top of the house. That negative pressure is made up by air coming in through every gap it can find, and the door between the garage and the mudroom is one of the biggest. The generator does not have to be in the house. The house will fetch the exhaust.
The Placements That Show Up in Incident Reports
- Attached garage, door open — the most common fatal scenario by a wide margin.
- Under a covered porch or a deck, where the roof holds the plume against the wall.
- In a shed or an outbuilding, “just to keep it dry.”
- In a breezeway or an alcove between two wings of the house, where there is no crosswind.
- Beside a window air conditioner, a fresh-air intake, a dryer vent or a bathroom exhaust — all of which either pull air in or sit right next to something that does.
- Beneath an open second-floor window on a still night, when nothing carries the exhaust away.
- Inside a tent or a tarp enclosure rigged for rain, with no top clearance for exhaust.
Choosing the Spot, Concretely
| Consideration | What to do |
|---|---|
| Distance from the house | At least 20 feet, further if there is no wind |
| Exhaust direction | Pointed away from your house and your neighbour’s |
| Openings nearby | No windows, doors, vents, intakes or window AC units in the plume path |
| Overhead | Open sky. No deck, porch roof, low eave or tarp above it |
| Weather protection | An open-sided, listed generator cover or canopy only — never an enclosure |
| Ground | Level, dry, not where meltwater will pool under it |
| Neighbours | Check their windows too. CO does not respect property lines |
The Second Layer: Alarms That Actually Work
Placement is the first defence and detection is the second, because your judgement about wind direction at 2 a.m. is worse than you think. UL 2034 sets what a residential CO alarm has to do:
| CO concentration | Must alarm within | Must NOT alarm before |
|---|---|---|
| 30 ppm | — | 8 hours |
| 70 ppm | 240 minutes | 60 minutes |
| 150 ppm | 50 minutes | 10 minutes |
| 400 ppm | 15 minutes | 4 minutes |
Read the right-hand column carefully, because it is the part nobody mentions. A code-compliant alarm is designed not to sound at 70 ppm for a full hour, deliberately, to avoid nuisance trips. That is a defensible engineering choice for a house with a slightly dirty water heater. It is a poor fit for a house with a generator running outside, where a low-level monitor that reads and displays concentrations from single digits upward gives you an hour of warning the code-minimum device is designed not to give.
On placement, NFPA 72 wants CO detection outside each separate sleeping area, within 21 feet of any bedroom door, and on every habitable level including the basement. Alarms in the same room as a permanently installed fuel-burning appliance go on the ceiling. Otherwise, mounting height is flexible, because CO mixes readily with room air rather than collecting at floor or ceiling level. Sensors age: most units carry an end-of-life date five to ten years out, and a chirping alarm that predates your phone is not protecting anyone. The full breakdown is in CO detector placement and failures.
CO Shutoff Generators Help, and They Are Not a Pass
Newer portable generators built to ANSI/PGMA G300 carry a sensor that shuts the engine down when carbon monoxide builds up around it, and the 2023 revision of that standard — effective for units manufactured on or after 1 January 2025 — lowered the shutoff thresholds. UL 2201 covers similar territory. If you are buying a generator now, this feature is worth prioritizing, and it has demonstrably prevented deaths.
It is still an interlock of last resort. The sensor measures CO at the generator, not in your child’s bedroom, and it does nothing about a plume drifting toward a window 20 feet away where the air around the machine itself stays clean. Treat it the way you treat a seatbelt: a very good thing to have, and not a reason to drive differently.
Recognizing It, and What to Do
Early carbon monoxide poisoning looks like flu without a fever: headache, dizziness, weakness, nausea, shortness of breath. It progresses to confusion, blurred vision, vomiting and loss of consciousness. The signal that should make you act is more than one person, or a pet, feeling unwell at the same time, or symptoms that ease when you go outside.
Safety: If a CO alarm sounds or you suspect exposure, get everyone outside into fresh air immediately, then call 911 from outside. Do not stop to open windows, do not hunt for the source, and do not go back in until the fire department says the building is clear. Anyone who is confused, unconscious, pregnant, or having chest pain needs emergency medical assessment even if they feel better outdoors.
The Rule I Would Give Someone
Pick the generator’s spot on a calm afternoon in good weather, mark it, and never improvise it again. Twenty feet minimum, open sky above, exhaust pointed at the field or the driveway rather than at anyone’s wall, and nothing that opens or breathes within the plume. Buy a model with CO shutoff if you are buying new, put a low-level monitor in the hallway outside the bedrooms, and check the manufacture dates on every alarm in the house this weekend. The decision you make once, calmly, in daylight is the one that protects you at three in the morning during an ice storm when you are tired and the wind has shifted. If you would rather move the whole problem outdoors permanently, that is one of the honest arguments for a permanently installed standby unit — though it has its own exhaust to think about.
Frequently Asked Questions
How far should a generator be from the house?
At least 20 feet, per CPSC, with the exhaust pointed away from the house and clear of windows, doors and vents. Add distance on a still night, and never place it where a roof, deck or fence traps the exhaust.
Can I run a generator in the garage with the door open?
No. CO production outpaces the air exchange even with the door fully open, and negative pressure inside the house then draws the exhaust indoors. This is the most common fatal generator scenario there is.
How do I run a generator in the rain?
Use a listed open-sided generator canopy or a purpose-built cover that leaves the exhaust and airflow unobstructed. Never a tent, a tarp draped over the frame, or a shed. Keep the machine on a dry, level surface and keep your hands dry when operating it.
Where should CO detectors go if I use a generator?
Outside each sleeping area within 21 feet of the bedroom doors, and on every habitable level including the basement. Add one near whichever side of the house the generator sits on, and consider a low-level monitor that displays ppm.
Do CO shutoff generators make placement rules unnecessary?
No. The sensor protects against CO building up around the generator itself. It cannot detect a plume drifting into a window across the yard, so the 20-foot rule and exhaust direction still apply exactly as before.