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Should You Have a Window Open With a Gas Heater?

Many people wonder if they should have a window open with a gas heater. Unlike a furnace or boiler, you typically install a gas heater to heat a limited space. So, when you have combustion in the limited space of a room, it is practical to know if you should open a window. 

You should have a window open with a gas heater if the heater doesn’t have a flue or vent. Else, exhaust gasses may accumulate in your room and make everyone sick. Also, gas heaters require sufficient air for the fuel to burn, so a room should have adequate ventilation.

Thus, rely on a window or air vents when using a gas space heater. However, flued gas heaters within a sealed system and a combustion air intake duct or vent don’t require an open window. Keep reading to know when and why you should open a window with a gas heater. 

Why You Should Have a Window Open With a Gas Heater

Gas heaters burn an air-fuel mixture. The fuel is usually natural gas or propane. You may have piped natural gas or a propane tank fitted to the unit. However, neither is a problem if you have sufficient air flows into the venturi, burners, and combustion chamber.

Also, a flued gas heater with a fully sealed system has a fixed exhaust duct and combustion air intake vent. Thus, you need not worry about the heater burning any indoor air and blowing the exhaust gasses in the room. However, a ventless gas heater is a cause for concern. 

Here are two worrying facts about ventless gas heaters and those with an open flue:

  • A gas heater with no combustion air intake duct uses the oxygen in the room to burn the fuel.
  • A gas heater with an open flue or no exhaust vent will blow the combustion byproducts or harmful gasses into the room.

An open flue gas heater is not ventless. However, the flue and the gas heater aren’t sealed units. Some exhaust or flue gasses may blow into the room instead of flowing up and out through the duct.

Both open-flue and ventless gas heaters draw the necessary combustion air from the room or space where you have installed them. Depending on the type and model, these gas heaters may have air intake vents at the front, side, or from underneath. 

Thus, you may encounter one or more of the following effects on your indoor air quality. 

An Unflued Gas Heater Depletes Oxygen in an Unventilated Room

Every gas heater needs combustion air or oxygen for the burners to work, be it a radiant, convection, or radiant-convection model. 

All manufacturers have an oxygen depletion sensor in their ventless gas heaters to shut the unit when it senses a shortage of fresh air. Thus, in theory, the safety feature should suffice. But in reality, you will spend substantial time in an oxygen-deficient room before the gas heater stops.

The oxygen level in an unventilated room is never more than ~20%, the optimum proportion in ambient air. However, a gas heater will continue to deplete this limited oxygen. Thus, everyone in the room will have chronic exposure to an oxygen-deficient indoor environment. 

A lack of sufficient fresh air or oxygen will adversely affect your health. Simultaneously, the limited oxygen available in the room will lead to incomplete combustion in the gas heater. In effect, the gas heater will produce carbon monoxide, a toxic gas with grave health risks. 

A Ventless Gas Heater Can Generate Carbon Monoxide

If your heater uses natural gas, the exhaust includes the following:

  • Carbon dioxide
  • Nitrogen oxides
  • Carbon monoxide
  • Sulfur dioxide
  • Particulates.

The two predominant byproducts of burning propane in a gas heater are carbon dioxide and water vapor. The quantum of carbon monoxide is nominal when propane burns cleanly, which means the gas heater has adequate fresh air and the burners have consistently blue flames.

However, both natural gas and propane emit more carbon monoxide if a gas heater encounters incomplete combustion. Thus, you are at risk of carbon monoxide poisoning if you do not have air vents in the room or keep a window open when using a ventless or open-flued gas heater. 

Like oxygen, carbon monoxide does not have an odor. Hence, you are unlikely to realize if a gas heater with an open flue or without an exhaust vent is blowing carbon monoxide into a room due to incomplete combustion. 

Some ventless gas heaters have carbon monoxide detection sensors. But I wouldn’t rely on only a sensor if opening a window or having air vents in the room can prevent or mitigate the risks in the first place. Besides, a triggered sensor will shut the gas heater so that you won’t have any heat. 

Additionally, the heater’s incomplete combustion means you will simultaneously endure oxygen deficiency and carbon monoxide. You won’t have enough oxygen to breathe in. Also, the carbon monoxide molecules will replace the oxygen in your bloodstream when you inhale the toxic gas. 

Unflued or Open-Flued Gas Heaters Can Lead to Poor Indoor Air Quality

A lack of fresh air or oxygen is inevitable in an unvented room with a ventless gas heater. Also, the indoor air quality will get alarmingly worse in time. 

The combustion in propane or natural gas heaters without a flue or exhaust vent will increase your indoor air’s carbon dioxide and water vapor content. 

Carbon dioxide is not a toxic gas, but inhaling too much of it can cause the following:

  • Headache
  • Sleepiness
  • Increased heart rate
  • Lack of attention
  • Poor concentration
  • Nausea.

Additionally, a significant increase in carbon dioxide naturally leads to oxygen deficiency in an unventilated room or space. Also, excessive water vapor in the indoor air causes moisture or humidity-related problems. 

Unusually high carbon dioxide and humidity in indoor air are a serious concern for people with asthma, respiratory ailments, and many allergies. 

Furthermore, incomplete combustion in ventless gas heaters due to a lack of oxygen in an unventilated room increases the pollutants or particulates in the indoor air, including the toxic nitrogen dioxide. Inhaling nitrogen dioxide adversely affects the lungs and respiratory tract.  


You should have a window open with a ventless or open-flued gas heater if there are no other air vents. If you have a flued gas heater but the unit is not fully sealed, avoid using a blower or any fan that can draw the exhaust gasses into the room. Or, keep a window open.

The only scenario when you don’t need an open window or air vents is if the gas heater is fully-sealed with a flue and a dedicated duct to draw combustion air from outside. 


  • Steve Rajeckas

    Steve Rajeckas is an HVAC hobbyist with an avid interest in learning innovative ways to keep rooms, buildings, and everything else at the optimal temperature. When he's not working on new posts for Temperature Master, he can be found reading books or exploring the outdoors.

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