Is it safe to warm up a car with the garage door open in an attached garage beneath living spaces?

Short answer

Why people ask this

People with attached garages located under bedrooms or living areas hope cracking the garage door makes idling safer. They want to avoid frosty starts without risking the spaces directly above. The worry is carbon monoxide and other exhaust drifting through the garage ceiling, stairwells, and utility penetrations into the home. Many also assume a strong opening to outdoors will dilute emissions enough to be harmless.

When it might be safe

There are no commonly accepted situations where this is considered safe.

When it is not safe

• Door-open dilution is unreliable in a garage directly under living spaces; exhaust can rise into the floor cavities and rooms above
• Stack effect in cold weather can draw fumes upward through cracks, drywall gaps, and recessed lights in the garage ceiling
• Shared chases for plumbing, wiring, or ductwork between the garage and upstairs can channel CO into bedrooms
• Running a car near a door to the mudroom or basement stairwell increases the path for gases to enter the home
• Furnaces or dryers creating negative pressure can pull garage air into the living spaces above

Possible risks

• Carbon monoxide poisoning in rooms over the garage, especially bedrooms on cold, still mornings
• Delayed buildup: CO accumulating in joist bays and seeping into the home after the car is turned off
• Headache, nausea, dizziness, or impaired judgment that may be mistaken for illness
• Heightened risk for children, older adults, and anyone sleeping in rooms above the garage
• Possible soot and benzene exposure from incomplete combustion, not just CO

Safer alternatives

• Start and warm the car outdoors, well clear of the house, then drive gently to complete warm-up
• Use a block heater or battery/engine preheater so the engine starts cleanly without idling in the garage
• Precondition an EV or plug-in hybrid with the vehicle outside; avoid running any ICE in the under-house garage
• Scrape windows, use a windshield cover, and set cabin defrost after pulling the car outside
• Improve air sealing between garage and living spaces (self-closing gasketed door, sealed ceiling penetrations) for general safety, but still avoid idling beneath the home

Bottom line

In an attached garage beneath living spaces, idling a car is not safe even with the door open. Exhaust can be drawn upward into rooms above via gaps, chases, and pressure differences. Move the car outside to start and warm it, or use non-idling warm-up methods.

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