Analyzing Combustion Analyzers

February 29, 2012
March/April 2012
A version of this article appears in the March/April 2012 issue of Home Energy Magazine.
Click here to read more articles about Tools of the Trade

Just talking about CO can be confusing. Although the terms used in this article are defined in the sidebar (see “Terminology”), CO can be deadly, and so I want to emphasize the basic information. Test equipment can provide great information, but only if the operator understands what that information means.

First, it is important to understand the difference between ambient carbon monoxide (CO) and CO air free or COAF (also known as CO undiluted and undiluted CO), which is a value calculated either manually or by the combustion analyzer. Ambient CO is the CO that is in the air that occupants breathe. Ambient CO should be measured and monitored during the entire building analysis in order to protect the health and safety of the occupants and the analysts. COAF is the CO that is present during the incomplete burning of the fuel in a combustion appliance. COAF comes from the combustion process and is generally measured before the exhaust gases have a chance to combine with dilution air introduced into the vent by a draft diverter.

Paul Raymer
is chief investigator of Heyoka Solutions, a company he cofounded in 2006. He has been wandering through the mysteries of building science since 1977. He has multiple BPI certifications and is a HERS Rater.

For most home assessments (including BPI), the probe of the analyzer must be inserted into the heating appliance exhaust air, before or upstream of the draft diverter, so that the CO will not have combined with dilution air. This will provide what BPI defines as an undiluted sample. The COAF value on the combustion analyzer should be used. Be certain to insert the probe in the correct location in the combustion appliance. Most of the product manuals describe how to accomplish this.

Another serious issue that we should address is the effect of nitrogen oxides (primarily NO and NO2) on the COAF reading. The electrochemical sensors that are used in these devices are targeted for CO, but they can react with other gases, such as nitrogen oxides. This may generate an additive output, giving an elevated reading of CO. The Testo 327-1 described below has a nitrogen oxide filter built into it. Bacharach advises adding nitrogen oxide filter or NOx scrubber to the input, and one can be added to the UEI input hose as well. (See “Did This at Home” for more on the benefit of a NOx scrubber.)

A combustion analyzer represents a serious investment both in money and in the time it takes to learn how to use it.

Combustion analyzers were developed for boiler and furnace technicians, who use the information to adjust the appliance properly, and the analyzers provide a great deal of information—most of which is not required by BPI, RESNET, or most weatherization protocols. This article focuses primarily on the COAF measurement capabilities of these analyzers. If you’re not a trained boiler or furnace technician, you should not be adjusting the performance of the appliance or passing information along to the homeowner that you don’t fully understand. Get the necessary COAF value and move on, and if you have doubts, record the CO, COAF, and oxygen values.

A combustion analyzer represents a serious investment both in money and in the time it takes to learn how to use it. You can get a CO sample with a basic device like the Bacharach Monoxor III, which costs $515; and you can also get a CO sample with a combustion analyzer from Nova Analytical that costs $5,265 (a product not described here). Before you invest in any of these devices, you need to think about what information you need now, and what information you might need in the future.

One other format note: I have used all upper case letters for control identification like ON/OFF or HOLD to differentiate control names from the article text. I have used italics for screen term identification like rEu X.XX or Adjust.

(You can find a summary of information about each device I reviewed in Table 1.)

Monoxor III Bacharach Monoxor III (Paul Raymer)
NOx filter (Paul Raymer)
UEI C75UEI C75 (Paul Raymer)
Testo 327-1
Table 1. Summary of Device Features
Summary of Device Features

Bacharach Monoxor III

The Monoxor III is a simple device for measuring CO. It does not provide information on stack temperature, CO2, efficiency, or many of the other readings that are useful to a combustion appliance repair technician, but are not necessary for the majority of home assessment technicians. Unfortunately it does not provide an oxygen reading, so it cannot be used to calculate a COAF value.

The Monoxor III is simple to set up. Install four AA batteries, connect the flue gas hose, check the water trap and filter assembly for pollutants, and turn the analyzer on (in fresh air). If the analyzer is set to Manual Zero, it takes 10 seconds to warm up (60 seconds if it’s set to Auto Zero). When the unit is turned on, the display screen will first read the software revision level as rEu X.XX, which is then replaced by a countdown, which is finally replaced by the CO screen reading in ppm.

The ˆ (Up) and ˇ (Down) buttons are only used to calibrate the analyzer. The HOLD button freezes the display and stops the pump. The ENTER button restarts testing after the HOLD button has been pressed. The LIGHT button controls the backlight, and the I/O button turns the analyzer on and off. The Monoxor III will turn itself off after 20 minutes of keyboard inactivity unless the detected level of CO is above 50 ppm. The backlight automatically turns off after 10 minutes of keyboard inactivity.

When the Monoxor III is used for testing ambient CO, the hose and probe assembly can be disconnected. Note that if the HOLD button has been pressed, there is no screen indication of the hold condition, but the pump will be off, saving the battery and making the unit quieter. But it won’t be taking readings.

During a test, the analyzer should be held above the water trap and filter, keeping the trap in a vertical position to optimize its effectiveness. Flue gas condensate is acidic and very corrosive. It should remain in any of these analyzers for as short a time as possible.

It is advisable to calibrate the Monoxor III every six months to ensure its accuracy. Cylinders of CO and a calibration kit can be purchased from Bacharach for this purpose, or the unit can be sent back to Bacharach. Note that calibration, CO sensor replacement, and pump replacement are all described in detail in the manual that comes with the analyzer. A NOx scrubber could (and in my opinion, should) be added to the hose to reduce elevated readings.

This is a simple and basic unit that can provide an ambient CO value. Inserted in the proper place in the flue, it will provide a CO value but it will not be a COAF value. It is not ideal as a personal safety device because it has no alarms. But it may be a great starting place for a moderately priced way to take both required CO readings. An infrared printer is available from Bacharach, but the basic analyzer comes with a hard carrying case, hose, and manual.

Bacharach Fyrite Tech60

The Tech60 measures these exhaust gas components: O2, stack temperature, ambient temperature, and CO. Then it uses that information to calculate steady-state combustion efficiency, CO2, excess air, and CO air free. It is a true starter combustion analyzer; it’s not meant to be used as an ambient CO monitor for personal safety. As the manual points out, “this analyzer does not have an audible alarm, and is not intended to be used as a safety device.”

The input hose and the thermocouple connect to the bottom of the analyzer. On the front panel there are a two-line display and six control push buttons. These buttons turn the analyzer on and off, scroll up and down through the display screens, enter the run mode, toggle the backlight on and off, and hold or freeze the display. (The HOLD button will also turn off the pump.)

For performing COAF analysis, the probe should be in fresh air when the analyzer is turned on, despite the confusing line in the instructions that says the “probe must be located in the area containing the burner’s combustion air supply,” by which they mean “fresh air.” If you choose to set the fuel, F1 displayed on the screen equals natural gas, F2 equals #2 oil, F3 equals propane, and F4 equals kerosene.

After turning the analyzer on and allowing it to warm up in fresh air, insert the probe into the correct location on the combustion appliance to get the COAF reading. The accompanying manual provides graphics of a variety of appliances, showing where the probe should be inserted on each one. The display screen will display CO in ppm on the top line and percent O2 on the bottom line. Scrolling through the other display screens will provide stack temperature, efficiency, ambient temperature, and CO air free.

When the test is finished, if the CO level is above 50 ppm, the analyzer will not turn off. The pump continues to run and the screen displays PUr6 CO (which looks sort of like purge CO) until the level drops below 50 ppm. The purging process can be bypassed by pressing the ON/OFF switch a second time.

Like the Monoxor III, the Tech60 comes with complete instructions for calibrating and replacing the filter and sensors at the back of the manual. A NOx scrubber could be added to reduce elevated readings. Kits are available that include the infrared printer, hard case, and a protective rubber “boot” or sleeve.

Did This at Home

I tested the UEI C75 and the Testo 327-1 on my own oil-fired boiler. The majority of measurements from the two analyzers were very similar. The COAF readings differed, however. The Testo 327-1 measured 6 ppm and the UEI C75 measured 37 ppm CO. When I added a NOx scrubber to the UEI C75 hose, the CO reading dropped to 5 ppm, clearly indicating to me the need for the NOx scrubber. I have repeated these tests in other houses with the Bacharach combustion analyzer, and have had similar results.

In my case, neither of the original readings posed a problem, but in other circumstances these variations could cause the appliance to be out of tolerance. At 26 ppm, for example, the BPI Building Analyst Standard states, “Recommend that the CO problem be fixed.” This might mean calling a service technician to service a problem that doesn’t exist, resulting in an unnecessary cost to the homeowner. The low additional cost of adding a NOx scrubber makes it prudent to do so.


The UEI C75 combustion analyzer measures O2, CO, and flue temperature and uses that information to calculate CO2, steady-state combustion efficiency, CO air free, and the amount of excess air. The temperature sensor and the flue gas probe plug into sockets in the bottom of the analyzer. On the face of the unit, there are a two-line display, four buttons, and a selector dial. The buttons allow the operator to turn the unit on and off, toggle the backlight, use the optional printer, turn the internal pump on and off, and hold the readings.

The analyzer in its protective covering is relatively heavy, and the pump is also somewhat noisy, but the pump can be shut off with the push of a button, although it won’t be taking readings when the pump is off. With the pump off, the C75 beeps occasionally to let you know that it is not able to do its job. Although this analyzer could be used to take ambient CO readings without its hose, it is a bit much to lug around. It does have strong magnets on the back of the covering that allow it to be attached to the side of most combustion appliances. An optional infrared printer is available.

The selector dial allows the operator to read any of the parameters that the C75 can measure. In the case of basic energy audits, the dial can be set to O2/CO and the O2 and ambient CO or COAF can be clearly read. So charge up the batteries (or install four AAs), hook up the hose, check the particle filter and the water trap, turn the C75 on in ambient fresh air, select O2/CO on the dial, and take your readings. There are 20 memory slots in the software where the data can be stored. (Or you can just write it down!) Even though the information is not necessary for just measuring the CO level, it’s good practice to enter the type of fuel the appliance uses, so if the analyzer is being used for complete combustion analysis, it will be set up properly.

Although the temperature sensor plug is polarized, it can easily be plugged in backward, which provides strange readings. Like the Bacharach units, the C75 could be fitted with a NOx scrubber to reduce elevated CO readings.

As with any of these devices, it’s a good idea to become familiar with all the functions and controls and read the manuals before you get into a real testing situation. Standing in front of a roaring boiler and fumbling to turn the backlight on or off or storing the reading in one of the memory slots can be frustrating.

All of these devices work well and reliably; just make sure that you understand the information they provide.

Testo 327-1

The Testo 327 is a comprehensive combustion analyzer with a bundle of capabilities that must be understood to be appreciated. It will allow you to gather all the information necessary to perform diagnostic analysis on a combustion system. It’s an investment. It can measure draft pressure as well as CO. It can also provide stack temperature, percent CO2, percent efficiency, percent excess air, percent O2, ambient temperature, and delta temperature.

The 327 is a solid-feeling device with strong magnets on the back of the case that allow it to be mounted on the side of most combustion appliances. The probes attach to connectors on the bottom of the analyzer. Along with the connectors there is a temperature sensor permanently mounted in a “cage,” and there are sockets for the flue gas probe, gas outlet, pressure input, and AC power input. The ON/OFF switch is on the top of the housing, and eight push buttons are located on the face. Three orange buttons have variable functions, with their functions displayed on different screens of the display. The ▲ (Up) and ▼ (Down) buttons scroll the display. There is a button to toggle the backlight on and off, a MENU button, and an ESC button to cancel an operation.

With the analyzer fully charged, the probe needs to be connected before the 327 is switched on. The manual says, “Any connected probe must be in the open air during the zeroing phase! [emphasis theirs]” The zeroing or start-up process takes 30 seconds. Pushing the MENU button allows one to scroll through Measuring, Adjust, Setup, Mode, Fuel Select, and Diagnostic menus. Pushing the middle orange button under OK when Measuring is displayed moves to the next menu level, cycling through Flue Gas, Draft, CO-AirFree, Smoke/Oil, diff prs, Delta Temp, and Ambient CO. To measure the COAF, select Flue Gas and read the ppm CO-AF line. Here you can also choose the type of fuel; choices are Oil #2, Nat Gas, Bioheat5, Propane, Oil #5, Oil #6, Kerosene, or Wood. If you are only interested in COAF, you can use any of these fuels, because the CO level will be the same for any of them. The beauty of this is that if you are testing in a combustion appliance zone (CAZ) with a gas water heater and an oil-fired boiler, there is no need to change the settings.

You can set up the display screen on the 327 so that the displays are in the sequence you want. For example, I have mine reading ºF T stac (stack temperature) first, then ppm CO, Pa draft, ppm CO-AF, %EFF, and then a bunch of other stuff.


Performing a typical home energy assessment requires testing for CO in the ambient air and in the exhaust products of combustion.

CO ambient is the CO that is present in the air that is not contained in the combustion equipment. It is the CO that the occupants are likely to be breathing.

CO air free or CO-AF or COAF is a mathematical calculation of the amount of CO in a sample of air with the percent of oxygen removed. It is equal to

CO ppm x ([20.9]/[20.9 – O2])

The combustion analyzers make that calculation after the operator takes a “clean-air” sample (generally outside). The COAF is the value that most combustion safety programs (including BPI) are looking for (along with the CO ambient value).

An interesting feature of the 327 is the draft measurement process. A bar graph is displayed on the screen indicating the highest flue gas temperature corresponding to the highest draft rate. Moving the probe moves the line on the bar graph. The physical opening in the probe should be oriented across the flow to allow for maximum airflow across the sensor. All of this information can be printed out on the optional, infrared connected printer.

The basic 327 comes with the analyzer, flue gas probe, and charger. Options include a hard case, IR printer, smoke pump, digital smoke spot meter, adapter for pressure readings, air temperature probe, flexible flue gas probe, and 9-foot hose extension.

The Testo 327 is capable of providing a lot more information than just COAF. The instructions are based on German documents and don’t always clearly translate into English, but there is good information on the web, including instructional webinars from tool suppliers like TruTech Tools that specifically teach how to use the Testo 327.


Testing for CO may be one of the most complicated tasks the building analyst faces. The four analyzers described in this article have been developed to do more complex things, like combustion analysis and system diagnostics and repair. The Monoxor III and the Fyrite Tech60 could also be used for ambient CO monitoring, but they don’t have alarms. The UEI C75 and the Testo 327 can also measure draft, supplementing the need for a second manometer. All of these devices work well and reliably (once you add the NOx scrubber to the ones that don’t have them). Just make sure that you understand the information they provide and how to use the equipment before you’re faced with a homeowner and a complicated combustion situation. Even the smartest equipment can provide useless information.

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