Combustion analysis

A combustion analyzer measures and gives out results to you based on what’s taken place after combustion has happened.

Understanding exactly what your combustion analyzer is telling you is arguably one of the most important pieces of information to understand as a technician nowadays.

An essential step is running a combustion analysis in commissioning a newly installed appliance, troubleshooting, or even just on routine maintenance for both efficiency and safety.

Since you will need to use a combustion analyzer often, let’s go over some of the readings and just what they mean.

Reading: ambient temperature

This is the ambient air temperature is used to calculate the net stack temperature. When taking air from outside for combustion, an additional temperature sensor is used.

The ambient temperature will not change any of the combustion readings.

Net stack temperature:

Flue gas temperature – combustion air temperature

Gross stack temperature:

Flue gas temperature + combustion air temperature

Reading: stack temperature

The stack temperature is the hottest flue gas temperature reading, so you may have to move your probe around a little at your testing area to find that sweet spot.

The stack temperature is checked to ensure you do not face the possibility of creating condensation inside the stack on a non-condensing appliance. Condensation on a non-condensing appliance can lead to chimney rot or venting to rust through.

Standard efficiency (70-75%) – 325°F – 500°F

Mid-efficiency (80%) – 325°F – 425°F

High-efficiency (90%+) 80°F – 120°F

Reading: dew point

The temperature below which water vapor in the flue gas turns to condensation. If the exhaust temperature in the venting stays above dew point temperature, the flue gas is vapor and you are not producing condensate. If the exhaust temperature in the venting falls below the dew point temperature, water vapor will condense.

Reading: draft

Measuring draft tells you how well the appliance is moving the flue gases from the appliance out of the venging.

Do not drill into the venting and seal it with foil tape. That’s messy, and the foil tape will wear off eventually. Instead, use a certified combustion test access fitting about 1-2′ away from the appliance.

Maintaining draft is required to ensure all flue gases are removed from the heat exchanger or out of the draft hood on all induced draft, atmospheric, and power burner appliances.

The draft will be best at times where the flue is warmest and outside temperature is coldest. The draft will quickly stabilize in an appliance that has been operational recently. If the appliance has not run for some time, the draft could take longer since the flue needs to warm up.

Codes may vary depending on the area, but generally, you have 5 minutes to prove draft.

When multiple appliances, such as a furnace and water heater, are common vented, it’s essential to test for spillage. Spillage is when the flue gases from one appliance spill out the draft hood for the other. Both appliances should be operated and tested for spillage.

It’s best to test for spillage under worst-case scenario by running any appliances that exhaust air such as dryers, kitchen range hoods, and exhaust fans while checking for spillage.

Reading: ambient carbon monoxide (CO)

Some combustion analyzers may not have the ability to check for ambient CO. When testing for ambient CO, your combustion analyzer wand is placed into the supply air duct or at a supply vent.

Ambient air is the air we’re breathing. Now we want 0 ppm of CO to come up every time we complete a CO test inside a house, but this isn’t always the case.

Health Canada recommends 0 ppm – 10 ppm as an acceptable short-term CO level within a home. Running a car in a garage, gas range, or smoking in the house are reasons for ambient CO readings above 0 ppm. Even with an allowable limit, any CO reading inside a house needs investigating.

CO levels measures in homes are usually caused by something aside from a cracked heat exchanger in a heating system. Homeowners heating their vehicles in attached garages and unmaintained gas ranges are the two most likely sources. Water heaters, fireplaces, and gas heaters are other sources to check.

Reading: carbon monoxide (CO) – COAF and CO

COAF stands for CO air-free. Because we add excess air for combustion, flue gas CO readings are inaccurate since the added excess air dilutes the readings we take in the exhaust. The COAF measurement compensates for the excess air and is used to express CO levels in the flue gas reading as if that excess air added is not even there.

The maximum CO an appliance is allowed to produce and exhaust out the venting is 400 ppm (coaf), although as a general rule of thumb we like to get them below 50 ppm to ensure the best combustion, efficiency, and safety for the customer.

As a side note, natural draft boilers are a bit different. At least in Ontario Canada. They are required to produce under 100 ppm CO (not air-free) and receive a yearly inspection which expires May 1st each year. The HVAC technician is required to complete an inspection anytime one of these is seen with an out of date tag. You can read more about that here.

The standard CO reading is diluted by excess air while CO air free calculates the CO reading with the excess air removed for a more accurate representation of what’s taking place during combustion.

Reading: oxygen (O2)

Air is 20.9% oxygen and is what we need for combustion. The nitrogen in the air plays no role in combustion. The nitrogen only absorbs heat energy and passes it out the exhaust which can be wasteful and is the reason we want to limit excess air to not reduce the efficiency of the appliance.

When an O2 reading appears on a combustion analysis reading, you have supplied a larger amount of air (excess air) than needed for complete combustion.

The target is to provide the lowest excess air possible while still maintaining quality combustion and a low CO / COAF level in the flue. The appliance manufacturer will have acceptable levels of excess air and other readings you will need when setting up the appliance.

Reading: carbon dioxide (CO2)

As combustion air is decreased, the carbon dioxide percentage will increase. The opposite is also true. As you increase combustion air, the carbon dioxide percentage will decrease.

A theoretical maximum percentage of carbon dioxide produced during complete combustion for natural gas is 11.8% and 13.8% for propane.

The CO2 reading is dependant upon having an O2 sensor installed on your combustion analyzer.

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