Un-vented gas heaters are NOT safe to operate indoors.  (Wrong!)

If you don’t read and follow directions, Myth #9 is actually true!

The very first two or three paragraphs of every owner’s manual of every gas-fired, non-vented space heater cautions the user to operate the heater in a well ventilated space.

In other words, open a window.

Opening a window might be counter productive if you’re trying desperately to heat a room in the middle of a cold winter, but facts are facts.

If you use your gas-fired, non-vented space heater in a confined area for an extended period of time, you may die!

Because of all the inconvenient law suits from families who lost people because they couldn’t or didn’t read directions, the manufacturers of gas-fired (non-vented) space heaters came up with a solution. They started making their heaters with “oxygen depletion sensors“.


Images of various oxygen depletion sensors. These controls will shut off a burner on a non-vented gas-fired heater if the space runs low of oxygen.

The O2 (oxygen) sensor (ODS) is supposed to shut off the heater if the oxygen level in the area dropped too low.

This would be a great solution except for a few things worth considering.

First, when a switch or device fails, it usually fails or sticks in its current position. In the case of an “oxygen depletion sensor”, if it sticks, it will probably stick closed, and the heater will continue to run.

Second, when it comes to manufacturing, businesses usually seek the lowest cost for their raw material. So, in essence, the people making the oxygen sensor are the lowest bidders for that product.

Third, read the warnings on the first few pages of the owner’s manual. (Every heater manufacturer includes them in one form or another.) The key phrases to remember are “use with adequate ventilation” and “do not use in a confined space” (or something similar.)

In my opinion, using a non-vented space heater with closed windows (with or without an oxygen sensor) while you sleep is like asking to meet the Angels. You might want to take up parachute jumping or high-wire walking, it’s safer.

If you have an unvented, gas-fired space heater … open a window.

Get the full story from “Carbon Monoxide Myths“.

Here is cautionary information straight from a vendor that sells unvented heaters, log kits and fireplaces.  They specifically caution about using the heaters with ventilation and only briefly mention that as room air is depleted of oxygen that the heater will continue to burn, converting CO2 into CO.


Improper use of a gas appliance usually involves one of two misuses:

  • not sufficiently venting the room or area by cracking a window or keeping a door open to a wider area
  • running the appliance continuously

If you misuse a vent-free gas appliance you can develop a situation that certain people who complain about vent-free heaters themselves cause: condensation.

A vent-free gas appliance must breathe. They need a fresh oxygen supply, and this is best accomplished by cracking a window, even in winter. The ventilation of the room and proper function of the heater or logs more than compensates for the letting in of outside air, even in the winter, and will not affect the ambient heat of the room.

Carbon monoxide in any measureable amount is not generated by a vent-free gas appliance. Carbon monoxide can occur, though, if the room or space where a vent-free appliance is running ISN’T being ventilated properly.

Carbon dioxide can accumulate over a long period of time in an unventilated space, and then the flame of the vent-free appliance begins to convert carbon dioxide to carbon monoxide.

This should NEVER occur if a vent-free appliance is installed and used properly.