The "High or Low" Debate Is a Myth: Where to Actually Place Your Carbon Monoxide Detector

Of all the safety devices in your home, the carbon monoxide (CO) detector guards against the most insidious threat. You can see and smell smoke. You can feel heat. But carbon monoxide is invisible, odorless, tasteless, and deadly. It’s rightfully called “the silent killer.”

Because the threat is so unique, the technology to detect it is completely different from a smoke alarm. And, critically, the rules for where to install it are widely misunderstood.

One of the most persistent—and dangerously incorrect—myths is the debate over whether a CO detector should be installed “high” on the wall (like a smoke alarm) or “low” near the floor.

Let’s settle this debate with science, and then create a clear, actionable plan for placing your alarms correctly.

The Myth That Won’t Die: “High vs. Low”

The myth goes something like this: “Carbon monoxide is heavier than air, so it sinks. You must install your detector near the floor.” Or, the exact opposite: “Carbon monoxide is lighter than air and rises with warm air, so install it on the ceiling.”

Both are wrong. Or, more accurately, they are both missing the point.

Here is the simple, scientific fact: Carbon monoxide’s density is almost identical to that of air.

• The molecular weight of carbon monoxide (CO) is 28.01 g/mol.
• The average molecular weight of air (a mix of nitrogen, oxygen, etc.) is about 28.97 g/mol.

They are, for all practical purposes, the same weight. CO does not sink like a rock, and it does not float like a helium balloon. It travels wherever the air in your home travels. It mixes perfectly, carried by natural convection currents and your home’s HVAC system.

So, if “high” or “low” doesn’t matter, what does? Location, location, location. The key isn’t the height on the wall; it’s the room you put it in.

Where Experts Say You MUST Install CO Detectors

The National Fire Protection Association (NFPA) provides the definitive guide (NFPA 720). The goal is to place alarms where they will wake you up if CO builds up while you’re asleep—the most vulnerable time.

Here is the expert-backed minimum requirement:

1. On Every Level of Your Home: This includes finished basements and attics. CO can travel far from its source.
2. Outside Each Sleeping Area: This is the most critical one. Place a detector in the hallway outside the bedrooms. If a furnace in the basement leaks CO, you want the alarm to sound before the gas reaches lethal levels inside the rooms where you are sleeping.
3. Inside Any Bedroom with a Fuel-Burning Appliance: If you have a gas fireplace in your master bedroom, for example, that room needs its own detector.

As for the height? Follow the manufacturer’s instructions. Most will tell you it’s fine to: * Plug it directly into a wall outlet (low). * Mount it on the wall, anywhere from outlet-height to chest-height (middle). * Install it on the ceiling (high).

The fact that all three (high, medium, and low) are acceptable proves that height is not the primary concern. The location is.

How Does a CO Detector Even Work?

It’s important to know why a CO alarm is different from a smoke alarm. Many people assume a combination alarm (like a First Alert SMCO100) just has one sensor for everything. This is not true. It has two completely different sensors.

• A smoke sensor looks for particles (using light or ions).
• A CO sensor is a miniature chemical laboratory that looks for a specific gas molecule.

The most common type is an electrochemical sensor. Here’s a simple analogy:

1. Inside the sensor is a small container of a chemical solution (an electrolyte) and some electrodes.
2. When CO gas from your home’s air enters the sensor, it reacts with the solution.
3. This chemical reaction produces a tiny, measurable electrical current.
4. The alarm’s microprocessor measures this current. The more CO, the stronger the current.
5. It doesn’t just trigger instantly. It “counts” over time. A low level of CO for many hours (e.g., 50 parts per million, or ppm) or a very high level for a few minutes (400 ppm) will both trigger the alarm, mimicking how your body would absorb it.

This is also why CO sensors have a limited lifespan (typically 7-10 years). The chemical solution inside them eventually dries out or degrades, and the sensor stops working. (This is a topic for another day!)

Beyond the Furnace: The Overlooked Sources of CO

Now you know where to put your detector. But do you know what you’re protecting yourself from? Most people say “my furnace.” That’s true, but in a modern home, the sources are more varied.

The Obvious Sources: * Gas furnaces * Gas water heaters * Gas stoves/ovens * Wood-burning or gas fireplaces * Gas clothes dryers

The Overlooked (And Deadly) Sources:

1. The Attached Garage: This is one of the biggest risks in modern homes. Simply starting your car and letting it “warm up” for 5-10 minutes—even with the garage door open—can push deadly levels of CO into your home. It seeps through the connecting door, through drywall, and into adjacent living spaces (like a kitchen or family room). You should always have a CO alarm in the living area closest to your garage entryway.
2. Blocked Vents & Flues: After a heavy snowstorm, the PVC exhaust pipes for your high-efficiency furnace can become blocked by snow. This forces the exhaust (full of CO) back into your home. The same goes for a bird’s nest in a chimney or lint buildup in a dryer vent.
3. Portable Generators: Never, ever, ever run a portable generator inside your home, garage, or even on a covered porch. They must be at least 20 feet away from the house, with the exhaust pointing away.
4. Gas Grills & Heaters: Using a charcoal grill, propane heater, or camp stove indoors can turn a home into a death trap in minutes. These are for outdoor use only.

Your Final CO Safety Checklist

You can’t see it or smell it, so you must detect it. * Check your locations: Do you have an alarm on every level, and critically, outside your bedrooms? * Check the date: Look on the back of your CO alarm. Is it more than 7-10 years old? If so, it’s time to replace the entire unit, not just the batteries. * Use combination alarms: Devices that detect both smoke and CO are an efficient and effective way to get 2-in-1 protection, ensuring you’re covered from both particles and gas.

Don’t guess. Don’t follow myths. Read the science, check your alarms, and protect your family from a threat you’ll never see coming.