The Problem With Oxygen Depletion Sensors…

The problem with oxygen depletion sensors is that they don’t sense oxygen.  Oxygen depletion sensors (ODS) are found on gas log appliances and are intended to shut the appliance off before the oxygen in a space falls to a dangerous level.  ODS sensors are thermocouples which produce a millivoltage when heated.  For this reason, the sensing end of an ODS sensor should be in contact with the pilot flame.  When operating properly, the flame heats the thermocouple which produces a millivoltage which in turn keeps the main gas valve open and allows gas to flow to the main burner.  When the flame cools as a result of low oxygen levels, the thermocouple or ODS fails to produce the necessary power to keep the main gas valve open and allows it to close, preventing gas flow and subsequent ignition.  The real problem however, is not low oxygen level, but the production and distribution of soot.  Anyone who has ever dealt with a soot damaged home knows how difficult the process can be to recover from the damage.  You see, appliances that burn with a yellow flame are already burning natural or propane gas incompletely.  That is, the carbon not consumed by the combustion process, will be visible and deposited as soot on clothing, furniture, draperies and appliances.   As a result, the ODS will NOT shut down the unit in time to prevent a soot production problem.  The lesson to be learned here is that even though your appliance has an ODS, don’t be fooled into thinking that you are protected.  Remember, if your appliance is designed to burn with a yellow flame, it is already producing soot.  Keep an eye on your fireplace insert, particularly if yours is unvented.  You will eventually see soot on the inside walls and if you see it there, it’s in the house!  The only way to prevent further damage is to quit using the gas logs set.

 

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Soot Damage From Gas Logs

Although it has been several years since we’ve seen sooting damage from a set of gas logs, it still occurs. The main problem is that soot is created when a fuel such as natural gas or propane is burned incompletely.  That is, there is a lack of air mixed with the gas and as a result, carbon is not completely burned.  The excess carbon then appears as soot on solid surfaces.  Appliances, like gas logs units, that advertise the appearance of a realistic wood fire tend to burn with a yellow flame.  The yellow flame is an indication that the fuel is not burned completely.  Some older readers might remember when gas appliances, including log sets, were made to burn with a blue flame.  A blue flame indicated that your appliance was operating as efficiently as possible.  When the flame turned yellow, this was owner’s cue to have the appliance checked.  This is no longer true and hasn’t been true for at least 30 years.  As a result, homeowners don’t have any warning as to when their appliances need attention.  Many of the log sets made today come equipped with what is known as an oxygen depletion sensor.  The device is supposed to shut the log set off if the oxygen in the space drops to a point below what is required to operate the set.  In reality, soot can be produced before the oxygen level drops to an unacceptable level.  This is because the sensor does not sense oxygen, it senses heat from the pilot.  As long as the pilot is producing a flame and the sensor is detecting the heat, the main gas valve will remain open.

Christmas and Safety

Christmas is fast approaching and so is winter. Although it is cold in many parts of the country not everyone has begun using their heating appliances on a regular basis. Instead, every year people resort to using small free-standing heaters to warm a room. From a fire investigation standpoint, I’ve already encountered two instances where small space heaters have been suspected as being the cause of a fire. Small space heaters are not necessarily defective and inherently dangerous if used properly. If the instructions say to keep the unit a certain distance away from combustible material, then, it is imperative that the minimum distance be maintained. The trouble is that consumers have a tendency to forget about that distance. Even more troubling is the fact that consumers will operate a small heater all night and unattended when that is clearly not the manufacturer’s intention. Remember that warm spot on the corner of the sofa that was created when the heater was turned on? After several hours, that warm spot can turn into a hot spot and then a point of ignition; all because the heater should not have been left on. That scenario prompts a question: can the heater be left on all night if the heater is sufficiently far away from all combustible material? The short answer is no. Anything that could knock the heater over might cause a fire. Most small heaters are equipped with a tip switch that turns the heater off in the event that the unit is tipped over. However, even though the power is off, that doesn’t mean that whatever the heater is resting on won’t ignite because of the residual heat emitted by the heater until it cools. The same rules apply to the use of kerosene heaters. In the case of kerosene heaters however, there is a steady production of carbon monoxide as the kerosene is burned. People with breathing difficulties can be severely affected. If a unit is allowed to burn all night, the risk that occurs is that there will come a point where the kerosene level will be low enough to produce soot instead of heat. So, in either case, whether using an electric or kerosene heater, it is best not leave them on and unattended during the night.

Hopefully, this reminder will help someone avoid a disastrous holiday season.

Merry Christmas and Happy New Year everyone!

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