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Carbon Monoxide Detector Buying Guide

Expert advice on buying indoor carbon monoxide detectors to protect your family from toxic gasses.

carbon monoxide detectorKidde

Low-cost battery-powered carbon monoxide detector has large display and shows highest measured levels of CO.

Carbon monoxide (CO) is an unavoidable gas produced by burning many types of carbon-based fuels such as heating oil, natural gas, gasoline, propane, kerosene, coal, and wood. In a properly built and maintained home, these toxic combustion gasses are vented outside so that they don’t pose a risk. But carbon monoxide becomes a serious threat—in fact it becomes deadly—when it accumulates as a result of ventilation equipment that is improperly installed, obstructed, disconnected from vents, or in need of repair. CO also can be formed by open flames, space heaters, blocked chimneys, using a barbecue indoors, or running a car inside a closed garage. As you might expect, most carbon monoxide poisonings occur during the fall and winter heating seasons.

As many as 500 Americans die each year from carbon monoxide inhalation, and 15,000 are hospitalized with symptoms associated with carbon monoxide exposure. Children and seniors are the most vulnerable to the hazardous byproduct of utilizing fossil fuels in the home.

Fortunately, special, inexpensive carbon monoxide detectors are available for warning you when CO levels become dangerous. These have sensors to detect dangerous levels of this colorless and odorless gas. With these, an alarm sounds, based on the level of gas detected. The alarm gives you time to ventilate the area or safely leave your home. CO detectors do not serve as smoke detectors and vice versa, although dual smoke/CO detectors are made to serve both purposes.

Types of Carbon Monoxide Detectors

Dual-power plug-in CO alarm has a battery backup.

A basic carbon monoxide detector typically costs from about $20 to $40. Look for models with digital displays, which, rather than simply beeping, show relatively precise CO levels in parts per million (ppm).

CO detectors may be battery-powered, designed to plug-in to an electrical outlet, or designed to be hard-wired to an electrical box that’s part of the home’s electrical system. Battery-operated smoke detectors are easier to install, but batteries must be changed twice a year. Plug-in smoke detectors, which typically have a battery backup, are popular because they’re very easy to install—as long as an empty standard 120-volt electrical outlet is available. Hard-wired smoke detectors are permanent fixtures, but these only make sense if an non-switched electrical box has been provided to receive them. Most have a battery backup in case the power goes out.

Before buying a CO detector, consult with your local building department to see if it requires a specific type of CO alarm, for example hardwired. Also make sure the detector meets Underwriters Laboratories Standard 2034. If you buy a battery detector, make sure to follow the manufacturer’s instructions for correct battery replacement, and adhere to its replacement schedule.

Nest Protect combination CO detector and smoke alarmNest Protect


Combination detectors that combine smoke and carbon monoxide detection are gaining in popularity. They are available as CO and ionization alarms and CO and photoelectric alarms. By purchasing these combo alarms, you have fewer alarms to deal with, however placement is limited because smoke alarms should be located only near the ceiling (see Smoke Detectors Buying Guide).

A hugely popular new innovation is the Nest Protect Smoke & Carbon Monoxide Alarm. When connected to Wi-Fi, this combination smoke alarm and carbon monoxide detector sends an alert to your mobile phone if the alarm is activated or the batteries run low. When the alert comes in, a voice tells you whether the problem is smoke or CO and where it is occurring. The connection to your phone is operated through a companion app on your phone. The unit’s carbon monoxide sensor has a 10-year life. Both battery-operated and hard-wired models are available.

Some CO alarms are designed to integrate with your home security system. The alarm can then be heard inside and outside the house, and the central monitoring station will also receive a signal and can the proper authorities or call your cell phone.

Test your CO alarm once a month, and vacuum out dust and other debris that may collect on your CO detector as needed.

CO alarm warranties usually last no more than five years because carbon monoxide sensors lose sensitivity over time.

Where to Put CO Alarms in Your Home

For a small, one-level home, one CO alarm is sufficient. For a larger home or multi-level home, consider using one per floor, including the basement and a walk-in attic.

Choose a central location just outside or within sleeping areas and other living spaces. Don’t put alarms inside garages where initially high CO from car exhaust can trigger false alarms. Also don’t place them near doors or windows where fresh air can cause a misleadingly low reading.

CO detectors can be placed near the ceiling or near the floor as CO is very close to the same density as air.

How Carbon Monoxide Detectors Work

CO alarms differ from smoke detector alarms in that they incorporate a concentration-time function. At lower concentrations (100 parts per million), a detector will not sound an alarm for up to an hour. At 400 ppm, the alarm will sound within a few minutes. They also warn you within four hours when they reach 70 ppm or within 15 minutes if the level reaches 400 ppm. This concentration-time function is intended to mimic the uptake of carbon monoxide in the body while also preventing false alarms due to common sources of carbon monoxide such as cigarette smoke. The newest models incorporate an end-of-life signal that will tell you when you need to replace the alarm.

Some basic models of detectors only display current ppm levels, while others display both current ppm levels and spikes over a period of time. The latter type of detector provides important information to emergency responders evaluating the level of past or ongoing exposure.

Three types of CO sensors are available. They vary in cost, accuracy, and speed of response. Any of the three are fundamentally adequate because CO levels normally increase very slowly, and there is a large range between safe and unsafe levels.

1. Biomimetic. A biomimetic sensor (chem-optical or gel cell) works with a form of synthetic hemoglobin that darkens in the presence of CO and lightens without it. You can either see the colors directly or have them connected to a light sensor and alarm.

2. Electrochemical. This type of fuel cell is designed to produce a current that is precisely related to the amount of carbon monoxide in the atmosphere. Its advantages over other technologies are that it is highly accurate in its readings of carbon monoxide, requires minimal power, and has a five-year life.

3. Semiconductor. This sensor has thin wires of the semiconductor tin dioxide on an insulating ceramic base, which is monitored by an integrated circuit. The carbon monoxide reduces resistance and by doing so allows a current high enough to trigger the alarm.

Preventing Carbon Monoxide Poisoning

The best way to avoid carbon monoxide hazards is not to produce it in the first place. Preventative measures against carbon monoxide poisoning include:

• Have your heating system inspected. Making sure your heating system is in proper working order. If your furnace has a cracked heat exchanger or a blocked flue, the unit can release CO into your home.

• Have your chimney and fireplace inspected annually, even if you rarely use the unit. A nest in the chimney can cause a smoke and/or CO backup into the living space. (If you use your fireplace frequently, the chimney may also have a build-up of soot that can become a fire hazard.) Do not close the flue of a smoldering fire in the fireplace until the ashes have fully cooled because embers can generate a large quantity of CO.

• Never operate combustion appliances (such as barbecues) or gas motors indoors. Generators can be serious CO hazards. If you have one in the case of a power failure, be sure to place it away from your house so that engine-exhaust gases, which emit CO, do not migrate back into the living space. Never operate a generator in a garage, basement, or other confined spaces where CO can leak back into the house.

• Don’t allow exhaust into the house. If you have an attached garage, warm up vehicles after you have pulled them out from the space.



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