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Oxygen Depletion Sensor vs Carbon Monoxide Detector

vent free fireplace

If you own a propane space heater, a vent-free heater or a vent-free fireplace, chances are it has an oxygen depletion sensor (ODS). These sensors are designed to shut the fuel off if the oxygen level in the air gets too low.

On the surface, this makes sense. A lack of oxygen sounds like a bad thing. But in order for oxygen levels to get too low, some other gas has to take its place. The “other gas” can be carbon dioxide - which is bad – or carbon monoxide – which can kill.

So when customers ask if an oxygen depletion sensor will protect them around gas space heaters, in virtually every case the answer is, “it isn’t the lack of oxygen that kills, it is the increase in carbon monoxide."

Where are oxygen depletion sensors used?

Oxygen depletion sensors (ODS) are used on virtually all modern vent-free heaters and fireplaces. The sensor is designed to shut off the fuel if the oxygen level in the room drops from 21% (normal air) to 18.5% oxygen by volume or below. The reduction in oxygen triggers a thermocouple which closes the gas valve. As a result, the heater is starved of gas and will shut off automatically.

Beginning in 1980, all ANSI certified vent-free units require an ODS. Gas and vent-free heater and fireplace manufacturers are proud of their record of no recorded deaths attributed to lack of oxygen when their heaters were used.

Do oxygen depletion sensors really protect you?

The simple answer is no. While the idea of an oxygen depletion sensor is good, in reality it will not protect you from the deadly gases carbon dioxide or carbon monoxide.

The problem with this is one of logic. For room oxygen levels to drop from 21% to 18.5% by volume, 2.5% of another gas has to take its place. Most of this will be carbon dioxide, which is bad for your health above 3%. 

Carbon monoxide is even more deadly. Breathing air with CO levels above 400ppm is deadly in hours and above 1,600ppm is fatal within minutes.

So while oxygen depletion sensors might make you feel safe when you read about them in your heater’s manual, they are not a complete answer. That’s why carbon monoxide detectors are more important.

Carbon monoxide detectors are more important

Carbon monoxide poisoning (CO) is the second most common cause of non-medicinal poisonings death in the US and the most common cause of poisoning worldwide. According to the CDC, over 10,000 are poisoned by CO each year and more than 438 people in the U.S. die annually from CO poisoning. That’s why the majority of states in the US now require CO detectors in homes.

Because CO detectors are relatively cheap and easy to install, most heaters and furnaces don’t have separate CO sensors. Instead, home owners are strongly encouraged – and in some states legally required - to place a CO detector in every bedroom and near the home’s furnace.

Even if you are using a heating device with an oxygen depletion sensor, you should always have a carbon monoxide detector nearby.

What about electric space heaters?

This discussion only relates to flame heaters that burn gas or propane. Since electric heaters do not burn fuel, they don't consume oxygen or give off carbon monoxide. Therefore, you don't need an ODS or a CO detector if you use an electric heater.

Why an oxygen depletion sensor won’t save you

No matter how dependable a heater with an ODS sensor or a CO detector is, at the end of the day it would be unwise to trust you and your family’s health to a part or device that only costs a few dollars. Even the heater manufacturer’s includes these warnings in their manuals:

  • Don’t use an open-flame heater indoors without opening a window
  • Don’t use an open-flame heater while sleeping

And every furnace manufacturer includes this warning:

  • Have your furnace services yearly to check for carbon monoxide leaks

All good advice, even if you have an oxygen depletion sensor or carbon monoxide detector in your gas heater.


Photo by Andrea Davis .


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