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How Does an ODS Make A Vent Free Gas Log Fireplace Safe

Updated on October 18, 2011

Ventless Gas Birch Log Fireplace

Realistic Birch Logs in this vent-free gas log fireplace set are designed for safety indoors without a chimney or other ventilation.
Realistic Birch Logs in this vent-free gas log fireplace set are designed for safety indoors without a chimney or other ventilation. | Source

What are vent-free gas log fireplaces?

Vent-Free gas log fireplaces are designed to be safe indoors without a chimney or other form of ventilation. Unvented gas logs are often called "reduced vent" and can be used in wood burning fireplaces and in vented fireplaces that have been damaged but these are gas fireplaces that are designed to be safe indoors without the major construction otherwise necessary to install or repair a fireplace chimney flue. There are many safety features that distinguish a ventless gas fireplace set -- necessary aspects of the design to minimize carbon monoxide, soot, smells and air-particle emissions. We will look at the ODS or Oxygen Depletion System which functions by cutting off gas flow if oxygen levels are not safe while the gas fireplace is burning.

Gas Fireplace Pilot Thermocouple and ODS.

Ventless gas fireplace close up of pilot assembly showing the pilot flame, thermocouple and ignition electrode.
Ventless gas fireplace close up of pilot assembly showing the pilot flame, thermocouple and ignition electrode. | Source
The ignition electrode arcs against the sleeve where the pilot burns.  Once lit the pilot extends to heat the tip of the thermocouple and ignite the gas fireplace burners.
The ignition electrode arcs against the sleeve where the pilot burns. Once lit the pilot extends to heat the tip of the thermocouple and ignite the gas fireplace burners. | Source

But is a gas log fireplace safe?

All over America -- in the states that allows the use of vent-free gas fireplaces -- ventless gas log fireplaces have limits to the amount of gas that passes through the control valve. The limit for a unvented gas fireplace valve is forty thousand BTU's regardless of the size of the fireplace burner. The lower gas flow ensures air particle emissions can be reduced with other gas log safety features including log stacking designs.

Gas logs are designed to diminish direct contact between flames and logs. When gas flames touch a solid object carbon is produced (which is dangerous to humans). When propane or natural gas flames touch a solid steel reinforced ceramic log harmful carbon monoxide fumes can be produced in the home. However, logs must appear to be burning in the natural flames so gas log fabricators and installers are very creative when we stack logs for a realistic that is also safe.

Thermocouple and thermopile gas fireplace controls.

This is the same ventless gas log fireplace as the top image but using oak logs instead of birch logs.
This is the same ventless gas log fireplace as the top image but using oak logs instead of birch logs. | Source
This shows a thermocouple in front of the pilot and a thermopile above the pilot for hot surface ignitions.  The thermocouple acts as ODS and the thermopile - many thermocouples in one - works with the remote ignition.
This shows a thermocouple in front of the pilot and a thermopile above the pilot for hot surface ignitions. The thermocouple acts as ODS and the thermopile - many thermocouples in one - works with the remote ignition. | Source

ODS: Oxygen Depletion Sensor.

When ventless gas log fireplaces are installed minimal square footage requirements ensure the gas flow limit and log stack design keep the gas fireplace safe with minimal or no ventilation. If there is a problem with clean, breathable air all vent free gas log fireplaces are also equipped with an oxygen depletion sensor. The ODS immediately stops gas flow to the fireplace burner if oxygen levels are not above safe levels.

Gas fireplaces have a pilot flame just like gas water heaters, range-top cookers and ovens. When using the gas fireplace if the control valve is used manually like an oven's control knob, if accessed with a remote control, a wall switch or a hot surface ignition the first flame has to be the pilot flame. The pilot flame heats a thermocouple and the thermocouple opens the gas flow in the valve.

The technical stuff...

A thermocouple tip is made of different metals conducting heat against each other that generate a small electric (millivolt) charge. The other side of the thermocouple has a threaded piece of steel like a bolt and a wire within a copper sleeve runs between the tip and the bolt. The millivolt charge running through the copper to the steel causes the steel to become magnetized. There is a magnet inside the fireplace control valve so that the reverse pole of the thermocouple electro-magnet causes the internal magnet to be pushed away which opens the gas line. If the pilot goes out the thermocouple tip cools and the millivolt no longer magnetizes the bolt so the internal magnet snaps closed stopping gas flow.

In short, if the tip of the thermocouple is hot the gas will flow through the control valve. If the pilot flame moves away from the thermocouple the valve closes and stops any gas flow. In a fireplace and in a barbecue rotisserie this is a safety feature that ensures gas is never flowing when there is not a flame present to burn the gas.

ODS rising flame looking for oxygen.

In this image the gas fireplace pilot is touching the thermocouple keeping the gas flowing through the control valve.
In this image the gas fireplace pilot is touching the thermocouple keeping the gas flowing through the control valve. | Source
This image shows the pilot flame rising as it searches for oxygen.  Once this happens the thermocouple will cool slightly and the gas stops flowing.
This image shows the pilot flame rising as it searches for oxygen. Once this happens the thermocouple will cool slightly and the gas stops flowing. | Source

Oxygen Depletion Sensor

The Oxygen Depletion Sensor sounds pretty technical but in reality is as simple as the thermocouple design. The pilot shoots straight forward from the gas line sleeve searching for a spark to ignite and oxygen to continue burning. If the carbon monoxide levels get higher inside the fireplace -- before ever reaching the room -- or if oxygen is lower for any reason the flame coming out of the pilot sleeve begins to rise up to reach more oxygen. If the pilot flame raises up the thermocouple cools within seconds and the valve de-magnetizes and gas stops flowing. The "sensor" is very non-technical which makes this a very difficult process to fail.

Maintaining your gas log fireplace pilot.

Along the pilot gas line to the right of the flame in this image there is a small hole in the brass sleeve.
Along the pilot gas line to the right of the flame in this image there is a small hole in the brass sleeve. | Source

Gas fireplace pilot assembly.

Most of the time a customer calls us to say their fireplace will not light or will not stay lit the pilot assembly is the problem. Actually most of the calls we get are because the customer needs to change the batteries in the controls or the customer does not remember how to light the pilot and then turn the fireplace "on" so the remote transmitter will function. The occasions we get a service call for a gas fireplace that is not either remembering instructions or replacing batteries it is a problem with the pilot. The pilot must always be clean. Dust, dander, pet hair, soot or anything else can cause the pilot to burn ineffectively and read oxygen poorly. In this case the pilot will burn a very tiny flame or will raise up. Either way the pilot will not heat the thermocouple and allow gas to continue to flow.

Ventless gas fireplace options.

Ventless gas fireplace fireballs
Ventless gas fireplace fireballs | Source
Ventless gas fireplace fire shapes.
Ventless gas fireplace fire shapes. | Source
Vent-free gas fireplace fire glass.
Vent-free gas fireplace fire glass. | Source

In closing: Gas Fireplaces are easy!

Gas log fireplaces are easy to use and safe to enjoy. Although like any other industry there are companies who buy low-quality products overseas, make up misleading names ("The Commercial Series") and mass market cheap junk as long as we all remember that if we are lucky we will get what we pay for then we will be o.k. If a product seems too cheap it is probably not worth the expense. There are hundreds of ventless gas fireplace manufacturers that make good quality gas fireplaces that will provide many years of beautiful real fire beauty. Pilot assemblies, thermocouples, control valves all have very few moving parts and are too simple to be damaged during normal usage. Vent free gas fireplaces are safe and reliable and a beautiful addition to every room.

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      azggeorge 

      2 years ago

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