As HVACR technicians, assessing whether the furnace we’re servicing is working efficiently is critical to understanding the overall health of the HVAC system — and to the success of our service call. The best way to confirm that a furnace is operating efficiently is to evaluate its combustion process. Is the gas pressure set properly? Is the flue draft correct for safe ventilation? Is the stack temperature within the expected range? The best way to answer these questions is to test the system with a combustion analyzer.

What is combustion?

Combustion appliances like furnaces and boilers combine fuel, oxygen and an ignition source to create heat. This process is called combustion. As a technician, you can adjust a furnace to control how much fuel, oxygen or heat is part of this process. If the adjustments allow too much fuel or too much oxygen into the reaction, an imbalance that leads to incomplete combustion occurs. When this happens, the furnace won’t work as efficiently as it should. Also, it could create a dangerous amount of carbon monoxide (CO), an odorless, colorless, tasteless and toxic gas. By optimizing combustion and fine-tuning the furnace, a technician lowers energy consumption and costs. This also helps prevent pollution and eliminates the risk of CO poisoning for the home’s occupants.

What is combustion analysis? 

To diagnose imbalanced or incomplete combustion, a technician utilizes a combustion analyzer. Using a Fieldpiece Combustion Analyzer, CAT85 or CAT45, a technician can view all the key parameters involved in combustion on a single large, five-inch bright touchscreen display. Display readings include a variety of measurements and calculations such as ambient CO, CO air-free, oxygen, carbon dioxide (CO2), ambient temperature, stack temperature, dew point and draft. By understanding each of these values and how they relate to each other, a technician gets a clear picture of how efficiently a furnace is burning oxygen and fuel to create heat. They also understand how much CO is being created by the combustion process.

How to use a combustion analyzer

To use the Fieldpiece Combustion Analyzer, CAT85 or CAT45, start by locating the certified combustion test access fitting. It should be on the exhaust flue about one to two feet above the furnace or boiler. There is sufficient hose for the probe to reach the fitting, and a strong magnet on the back of the CAT85 and CAT45 to hold them in place on any metal ductwork.

This access fitting is where to insert the probe from your combustion analyzer. By measuring inside the flue, it becomes apparent how efficiently the exhaust flue is moving flue gases from the combustion chamber as well as measure how efficiently the appliance is turning fuel and air into heat. The CAT85 and CAT45 do more than measure, they also perform the calculations needed to determine how efficiently and safely the appliance is performing. Also, since they’re part of the Fieldpiece Job Link® System, techs can store readings for future reference and share them in real-time using the Fieldpiece Job Link® App on any mobile device. The app offers numerous other benefits for technicians like job tracking and geolocation tagging that make it worth using. These tools also have their own internal memory that let HVACR pros store reports directly on the device. Plus, compatible printer options allow for printing reports on the spot.

Ensuring safety

The Fieldpiece Combustion Analyzers, CAT85 and CAT45, also check for ambient CO which could be toxic to occupants. To perform this test, insert the probe into the supply air duct or supply vent. This shows how much CO is present in the house. Levels ideally should be zero, but they could be impacted by a running car in an attached garage, a gas range or fireplace that’s being used in the home, measurement noise at very low levels, or a natural gas water heater. Any level above zero should be investigated. If the CO is coming from the furnace and not from an external source, it could be from a cracked heat exchanger or another fault with the furnace. These leaks should be inspected with a CO detector and repaired.

Performing a service call

Under ideal conditions, a precise amount of air is required to completely react with a specific amount of fuel. But those ideal conditions only exist in a lab. In the real world, combustion conditions are never ideal, and additional or excess air must be supplied to completely burn the fuel. The CAT85 and CAT45 help a tech determine the best fuel-to-air ratio to achieve maximum combustion efficiency.

When a tech is servicing a furnace, a combustion analyzer helps to determine the optimum amount of excess air and fine-tune the appliance. This minimizes heat lost up the stack and improves combustion efficiency. To see how efficiently the furnace is working, the combustion analyzer also measures the stack temperature and flue gas oxygen or CO2 concentrations.

Determining the correct amount of excess air

If there’s not enough air, the reaction creates unburned combustibles in the exhaust gasses like fuel, soot, smoke and CO. We recommend users perform a smoke test before inserting the probe for a combustion test on oil-based furnaces. The tech should then adjust the amount of excess air to decrease the presence of these contaminants. But don’t adjust too far. If there’s excess air, it results in heat loss because of the increased flue gas flow. This causes the furnace to operate at less-than-peak efficiency. 

Saving money and lives

When performing a service call, use the Fieldpiece Combustion Analyzers, CAT85 and CAT45, to make occupants safer while ensuring that the system burns as efficiently as possible. 


Don’t test a cold furnace. Before you start taking measurements, make sure to run the system for at least 10 minutes. Why? Because when combustion starts on a cold furnace, the reaction produces excess CO and other gasses, and this impacts your readings. It is also important to allow a proper draft to begin, which usually takes at least 5 minutes. So, always let the system warm up first before taking measurements.

Don’t drill – use the access fitting instead. When using a combustion analyzer, make sure to use the access fitting on the flue if possible. Don’t drill a hole into the vent and seal it with foil tape when finished. Not only is this unsightly, but the tape wears off, allowing the flue to vent into the living space. 

Works for all fuel types. The Combustion Analyzers, CAT85 and CAT45, work on boilers and furnaces that burn all fuel types – from natural gas and propane to coal gas and biofuels. There are even options for the tech to add custom fuels for analysis. 

SensorVault™ extends life. The CAT85 and CAT45 each come with the patent pending SensorVault™ technology. When the power button is pressed on the device, the sensor is unsealed and ready to take measurements. When the device is powered off after use, the sensor automatically reseals using SensorVault™ and is protected from oxygen in the air resulting in a longer sensor lifespan. Also, the sensors are field-replaceable, and come calibrated so they are ready to roll upon startup. This patent-pending technology helps prolong the life of the sensor and offers a lower cost of ownership. 

The HydroCycle™ Pump makes work easier. Another innovation on the CAT85 is the HydroCycle™ Pump. This innovation eliminates the need for a water trap by returning condensate back into the flue. The detector cools off the air automatically, removes the water and returns it to the duct where it evaporates. A tech won’t need to empty or clean out water traps. When removing the probe after testing, point it downwards to empty any remaining condensate before storing.