Can Ventless Heaters cause Carbon Monoxide Poisoning?

Can you get CO poisoning from a ventless heater?

Quick answer: Yes, you can theoretically get carbon monoxide poisoning from a ventless heater. However, it is very unlikely as long as you vent your room every few hours. Ventless space heaters are among the most common source of carbon monoxide poisoning in homes. Modern ventless heaters are designed not to produce CO when functioning properly, but many factors, including poor ventilation and dirt, can make the heater less than safe.

Because of no vent pipe or chimney, vent-free space heaters are easy and inexpensive to install, you can move them easily, and they are quite efficient. More importantly, ventless heaters heat up a room quickly and quietly. They do not require electricity, so they are suitable for emergency blackouts since gas is cheaply available in most areas.

But all these advantages are a far cry from the dangers this type of ventless heater poses.

How Ventless Gas Heaters Produce CO

Again, the best ventless gas heaters are designed to produce minute amounts of carbon monoxide when maintained and adjusted properly.

Carbon monoxide occurs due to incomplete combustion caused by insufficient oxygen. Modern heaters come with an oxygen depletion sensor (ODS) that shuts down the heater if oxygen levels go below 18%. For reference, normal oxygen levels remain around 21%.

If everything is well done, normal by-products of burning gas and fuel are carbon dioxide, water vapor, and some nitrogen dioxide, which are all released into the room instead of being led outdoors via a chimney pipe.

Unfortunately, the ODS does not respond to a lack of oxygen caused by the accumulation of dust inside the heater, improper gas pressure, rust in the burner, disruption of the burner by air currents, and improper placement of artificial logs in a fireplace.

This means it’s possible for a ventless heater to continue working even when it’s not getting enough oxygen for ideal combustion, hence producing carbon monoxide.

A lot of care must be taken to ensure the heater is working perfectly at all times, and it’s impossible to know when something is amiss sometimes. For this reason, approximately 48 people are killed by carbon monoxide poisoning produced by fuel-burning heaters.

Other Dangers of Ventless Gas Heaters

As it turns out, carbon monoxide poisoning is the least of your worries when dealing with a ventless gas heater. This is because the other by-products of burning gas aren’t that safe either. And since they are being released directly into the air you breathe, you may want to take a few precautions before turning on a ventless heater.

  • Carbon dioxide. This is a non-toxic gas that makes the biggest percentage of by-products resulting from burning gas. At such high concentrations, however, CO2 causes headaches, sleepiness, and a stuffy atmosphere in the house. This is the main reason why people who use fuel space heaters wake up with a headache every morning.
  • Water vapor. A 30,000 BTU/h ventless heater produces 1.3 liters of water per hour. Too much water vapor in your house elevates the humidity level, leading to condensation on your walls, window panes, concrete slabs, and ceilings. This dampness will encourage mold development, wood rot, and deterioration of other parts of the house, including furniture.
  • Nitrogen dioxide. Like CO, nitrogen dioxide is a toxic gas which, at low concentrations, causes upper respiratory issues such as sore throat, headaches, nausea and coughing. This gas also causes asthma. Any burning flame produces NO2, and the quantity is based on the size and design of the heater.

How to improve your ventless heater’s safety

First things first, an unvented gas heater is designed for supplemental use only. This means a few hours when the power goes out, or your generator is on break. It cannot be your regular heat source, and you MUST not operate it for more than 4 hours at a time.

Aside from that,

Use only approved gas heaters with an ODS pilot.

This will take care of the room oxygen issue and shut off the heater if oxygen levels are low.

Get professional installation

All gas heaters should be installed by a professional. They are aware of the hazards, and you need their input to choose the right-sized heater for your house. A 3800 BTU/hr ventless heater is the largest you can use in a tight 10 by 20 room.

Even if you are buying a portable gas heater, a professional will assess the house for the best position to place it based on ventilation access and adjust everything to perfection.


Fuel heaters consume a lot of oxygen during combustion. This means the oxygen in your room will only suffice with proper ventilation. With today’s tight houses, keep a window open in the room to allow oxygen in freely and eliminate the CO2, NO2, and water vapor released.

That said, do not use an unvented gas heater in confined places like small bedrooms, bathrooms, and kitchen.

Follow all operation and maintenance instructions

Depending on the design of the ventless heater, follow all the operation instructions to the letter. These include ventilation, placing the artificial logs properly in a fireplace, proper adjustment, and so on.

Maintenance-wise, ensure you clean the heater at least twice a year, or as instructed in the owner’s manual. It’s also important to have the heater checked and readjusted by a professional annually to rule out aging issues like burner rust and improper gas pressure.

Install a UL-listed carbon monoxide detector

Because it’s almost impossible to know when the heater is producing carbon monoxide, always have a digital CO detector listed as sensitive in the room. The detector will go off when CO levels go over 70ppm for an hour or so, or 400ppm for over 4 minutes.

However, this is a high concentration for pregnant women, sick people, and babies. Instead of waiting for the detector alarm to go off, keep a close eye on how you feel when a ventless heater is on. A slight headache, nausea, or breathlessness should necessitate shutting down the heater.

Final Thoughts

While the design of new ventless gas heaters makes carbon monoxide production very low, they are still quite dangerous. The chances of CO poisoning are higher than any other type of heater, not to mention all the other by-products are also released in the room.

To make things worse, the Gas Appliance Manufacturers Association says it’s impossible to create comfortable temperatures with a correctly sized ventless heater because of the required ventilation. A larger heater would pollute the air beyond allowable levels.

Knowing that it’s better to invest in a good vented space heater, electrical space heater, or an outdoor generator for heating.