Digital Zeus™ HVAC Tool & Instruments Journal

Tech Notes on the Monoxor II Carbon Monoxide Sensor

There are several things to keep in mind with regards to this type of sensor.

All CO sensors have a life span (Generally 2 - 5 years).  Ten+ years of experience with this sensor has generally been 5 years of useful service life can be reasonably expected, depending on factors such as:

	•Hours of use.
	•Age of sensor
	•Amount of exposure to carbon monoxide
	•Care and maintenance

This type electrochemical sensor has a chemical reaction when exposed to CO.  As the chemical is exposed to CO, an electrical current is generated which is picked up by the circuitry and translated into a parts per million (ppm) reading.

As such, the more CO it is exposed to, the shorter its life.

To extend the life of the sensor, try to minimize exposure to particularly high levels of CO.  For example, if you see the reading quickly climbing towards the over range point (2000 ppm), remove the probe - or better yet, disconnect the probe from the top of the instrument and allow it to purge.

Typical heating equipment is designed to minimize CO production in the flue gas samples (i.e.. AGA - 400 ppm air free).  When you start getting a couple of hundred ppm’s above the allowable level, that’s all you need to know - there’s a problem.

However, do test every piece of equipment you are involved with.  Frequent use and exposure to ‘normal’ flue gas CO levels will not shorten it’s live significantly.

When this sensor is near the end of its service life, you will notice the reading wandering up and down (in clean air) and it will be hard to keep it adjusted to zero.

 A rough field test to see if the sensor is working at all is to light a match and stick it into the probe allowing the suction of the pump to extinguish it.  The reading should bump up to 600 to 800 ppm’s.

Do allow the instrument to post purge after normal use or when the sensor has been exposed to high CO levels.

Once testing is complete, disconnect the hose assembly and let the instrument run for 5 - 10 minutes.  This will purge out any residual CO and flue gas moisture.

The acids in flue gas condensate can also damage instrument parts and the sensor.  Remember, these sensors are designed to be exposed to flue gas condensate, but not soaked in it.

Also remember to keep the yarn in the moisture trap dry and clean.  When the yarn gets damp, put dry material in and let the original dry out, to be used again.

Along the same line, try not to expose the instrument to freezing temperatures.  Moisture in the sensor can freeze, expand and crack the sensor case or when the instrument in brought into a warm, moist environment, condensation may occur on the circuit board.

Another thing to keep in mind is that this is a non-hydrogen compensated sensor.

Sometimes, particularly on larger commercial burners, hydrogen can be produced by a flame and exhausted along with the flue gases.  This sensor reads hydrogen as carbon monoxide.

Hydrogen in the ambient air (i.e., charging automobile batteries) will also be read as carbon monoxide.

Hydrogen Compensated sensors are used in high end combustion analyzers such as the PCA and ECA 450.  They actually have two sensors, one to measure CO and one to measure hydrogen, then make a calculation to deduct the hydrogen from the reading.

Under ‘normal’ circumstances you might see a 5 to 10 ppm reading difference in the flue gases on a residential unit - generally none.

Commercial equipment could be much higher.  We’ve seen the difference as much as 60 to 70 ppm’s between compensated and non compensated sensors, then again, frequently there is no difference.

Combustion Testing: ON a Viessman CT3-46 (Sealed Combustion)
Using a Testo® 330-1 combustion analyzer and Easy Heat™  software, observed the combustion over time in high and low fire. The stability of the combustion is being affected by changes in CAZ (Combustion Air Zone) pressure due to intermittent loss of draft through the appliance due to the negative fluctuating mechanical room pressures. This was observed during the draft readings, CO readings, stack temperatures and O2 readings. Due to the instability of the combustion process, both boilers will require re-commissioning after the ventilation air issues are resolved. Although no CO was produced during operation as the technician left the boiler, changes in building pressures could result in CO formation and pose grave danger to the building occupants. 

Using a Combustion Analyzer to Test Heat Exchanger Integrity: http://hvacprotech.forumwise.com/hvacprotech-thread4956.html

Incomplete Combustion

Commonly used fuels like natural gas and propane generally consist of carbon and hydrogen. When a fuel has a large ratio of hydrogen, more excess air must be provided. Water vapor is a by-product of burning hydrogen. To maintain it’s vaporous state, it robs heat from the flue gases, which would otherwise be available for more heat transfer. Natural gas contains more hydrogen and less carbon per BTU than fuel oils and as such produces more water vapor. Consequently, natural gas is generally slightly less efficient than fuel oil.

Too much, or too little fuel with the available combustion air may potentially result in unburned fuel and carbon monoxide generation. A very specific amount of O2 is needed for perfect combustion and additional (excess) air is required for good combustion. Too much additional air can contribute to CO generation, lower efficiencies and perhaps unsafe conditions with heating equipment not out living its full service life.

 

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- Using a Manometer to Identify Carbon Monoxide Problems - Carbon Monoxide

The U.S. Centre for Disease Control and Prevention estimates that Carbon monoxide (CO) poisoning kills 600 Americans each year, many of them in their own homes. In many of these cases death is directly caused by faulty heating systems or construction that traps the deadly gas in the house. As a contractor responsible for the maintenance of these heating systems you will have to agree that “Killing your clients is bad for business”.

So what is making them ill? Many experts are beginning to realise that there are thousands of cases where people are sickened, to some degree, with low-level CO poisoning. Low level CO poisoning is simply not recognised because it has symptoms that mimic colds and the flu. Elderly people are particularly susceptible to this. The 1995 study by the American Journal of Epidemiology reported that even a slight rise in CO levels sends many to the hospital and the older you are the more likely a low level CO is going to affect you. The report concludes that, for just seven cities studied, low-level CO resulted in $33,000,000 in increased medical costs during the 1995 year.

CO is odourless, tasteless and invisible. You will not detect it unless you have special equipment. It is produced wherever there is incomplete combustion and combustion for furnaces, and household appliances, is never 100% efficient. Water heaters, boilers, gas cookers, gas hobs, gas fires, coal burning stoves are all a threat and the more airtight and energy efficient the house is the greater the threat.

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