Carbon Monoxide & Gas Appliance Safety on an Inspection

Most safety and environmental findings in a report describe a slow-building risk — radon accumulating over years, asbestos disturbed during a future renovation. Carbon monoxide (CO) findings are different in character. They describe conditions that can turn dangerous in hours, which is why inspectors treat them with a directness you don't see elsewhere in the report. This guide covers what generates a CO finding, what inspectors actually write, and why these tend to be some of the cheapest fixes in the entire inspection paired with some of the most urgent language.

Where carbon monoxide actually comes from in a house

CO is a byproduct of incomplete combustion: anything that burns fuel (natural gas, propane, oil, wood) produces some CO, and normal venting is designed to carry it safely outside. Problems arise when that venting fails. The three sources inspectors focus on:

• Furnaces and boilers, particularly an aging or cracked heat exchanger, which can let combustion gases mix into the air actually being heated and blown through the ductwork. This is serious enough to warrant its own dedicated guide.
• Water heaters, especially older atmospherically vented gas units where a blocked or improperly sloped flue can send exhaust back into the room instead of up and out.
• Any fuel-burning appliance affected by backdrafting, a broader phenomenon (below) that can involve furnaces, water heaters, fireplaces, and even a gas range in the wrong conditions.

What backdrafting is and why inspectors specifically test for it

Backdrafting happens when a house's air pressure goes negative relative to outside, often because exhaust fans, a clothes dryer, or a tightly sealed modern house are competing for the same limited air supply. That negative pressure pulls combustion exhaust back down the flue and into the living space instead of letting it rise and vent normally. Many inspectors perform a basic backdraft test as part of the furnace and water heater evaluation: running exhaust fans and the dryer simultaneously while checking whether flue gases hold their upward draft or get pulled backward.

The finding might read: "Backdrafting observed at water heater flue when bathroom exhaust fan and clothes dryer operated simultaneously. Recommend evaluation by licensed HVAC or plumbing contractor to correct venting and/or provide combustion air." This is a real, present-tense finding. It describes something the inspector watched happen during the test, and it belongs at the top of your action list regardless of what else is in the report.

Missing or non-functional CO detectors

A large share of findings in this category are simply the absence of what's now standard, inexpensive safety equipment. Most states and many local codes require CO detectors near sleeping areas in homes with fuel-burning appliances or an attached garage, and a general inspection routinely checks for their presence (and sometimes tests whether existing units respond and aren't past their own expiration date; CO detectors have a service life, typically 5–10 years, printed on the unit). This is the cheapest, most clear-cut finding in the whole safety section: buy the detectors, install them, done.

Gas leaks and appliance connections

Separately from combustion venting, inspectors look for the smell of gas, visibly damaged or improperly connected gas lines (especially flexible connectors behind stoves and dryers), and missing shutoff valves. Any suspected active gas leak is an emergency that gets escalated immediately. A general inspector will typically stop and recommend the gas utility or a licensed plumber be called right away rather than including it as a routine report line item.

Attached garages: a CO source that isn't an appliance at all

Not every CO finding traces back to a furnace or water heater. An attached garage is itself a recognized CO risk, because a car idling inside it (even briefly, even with the garage door open) can push exhaust into the house through the shared wall, especially through the door between the garage and living space if its weatherstripping and seal are poor. This is part of why building codes require a fire- and gas-rated separation between an attached garage and the house, the same "firewall" requirement that shows up as its own finding on a garage inspection. It's also why a functioning, self-closing door between the garage and the house matters as much for air quality as for fire safety. Homeowners who plug in a portable generator during a power outage face a related and more acute version of this risk. Generators are only ever safe to run well outside and away from windows and doors, never in a garage even with the door open. That's worth knowing regardless of what the report says, since it's a use pattern rather than a fixed condition an inspector can flag in advance.

What corrections typically cost

How to handle a CO or gas safety finding

1. Treat an active backdrafting or suspected leak finding as immediate. These are the rare inspection findings where the right response is a phone call to a licensed contractor this week, not a line item for the closing table.
2. Missing detectors are a same-day DIY fix. Buy them before you even finish reading the rest of the report, and don't spend negotiating capital asking a seller to install $30 worth of hardware.
3. Ask what caused the backdrafting, since the fix ranges from a $150 flue adjustment to a more involved combustion-air correction. A contractor's diagnosis, not the report alone, tells you which.
4. If a heat exchanger crack is suspected, get an HVAC contractor's confirmation before assuming full furnace replacement. But don't run the furnace in the meantime until it's checked.

Act on these first

Carbon monoxide and gas safety findings are uncommon compared to the rest of a typical report, but they're written with more urgency for good reason: the failure mode is fast, and the fixes are usually inexpensive and quick once identified. Read these findings first if they're present, act on them promptly, and don't let their rarity in the report make you skim past them.

For the rest of this category, see the safety and environmental hazards hub.