Steps in order for furnace commissioning:

1. Set gas pressure to manufacture default
2. Set input of the unit. Change orifices if needed
3. Set gas pressure
4. Get the temperature rise in the manufacturer suggested range
5. Combustion analysis test

An essential step in commissioning a newly installed gas appliance or even just on routine maintenance for both efficiency and safety includes clocking the meter, running a combustion analysis, checking temperature rise, verifying stack/venting draft, and setting manifold pressures.

A 94% gas furnace installed fresh out of the box could be running at quite a bit less when not set up correctly. You’re not doing your customer any favors when not finishing an install as you could add thousands of dollars in gas usage and repairs that could be avoided over the life of the appliance.

Clocking the meter and setting gas pressure

Clocking the meter is an important first step in setting up a new install and is done to ensure you’re not over or under-firing your appliance which can cause stress on the appliance and possibly premature failure.

When clocking the meter, you’re matching the actual input to what’s printed on the appliance label. Furnace’s ship with the orifices for the highest calorific value in the country being shipped to, so you could have the incorrect orifices in the appliance.

Step 1: Set the fuel pressure to what’s listed on the appliance nameplate. Do not go by what is printed on the gas valve. This valve can be used in multiple different pieces of equipment requiring different pressures.

Step 2: Make sure all other gas appliances are turned off, pilots and all.

Step 3: Have the unit running at the highest output (highest heat stage). If you have a 2 stage unit, get it going in 2nd stage.

Step 4: A stopwatch is needed for this step, so break out your cell phone. Using the smallest test dial on the gas meter, usually 1/2 ft³ (imperial meter) or .01 m³ (metric meter), track how many seconds it takes for one full revolution.

I like to cover half the test dial on an angle using a debit or business card. Be sure to not move your head and change your line of sight.

Take at least 3 readings, add them together and divide by the number of readings taken to get your average. The smallest test dial does not always move smoothly and can give false readings which is why you get an average.

Example:

Revolution 1 – 16.84 seconds

Revolution 2 – 23.23 seconds

Revolution 3 – 20.79 seconds

Revolution 4 – 23.18 seconds

Total = 84.04 seconds / 4 readings = 21 seconds average

Step 5: Complete the calculation to get actual input using either metric (.01 m³) or imperial (1/2 ft³) calculation depending on the type of meter used.

Metric meter:

We will be using the 21 seconds average from above for the seconds of a revolution.

The actual equation to solve for is:

((# seconds per hour x dial size) / time in seconds) x calorific value x 35.31

• seconds per hour is always 3600
• seconds per turn is 21 seconds from the above example
• dial size is .01 m³
• 35.31 is used to convert m³ to ft³
• The calorific value for my area is 1000 btu. You will need to contact your utility for their average. Generally, it’s between 1000 btu to 1075 btu

3600 x .01 / 21 x 1000 x 35.31

= 60,531 btu

Imperial meter:

We will be using the 21 seconds average from above for the seconds of a revolution.

The actual equation to solve for is:

((# seconds per hour x dial size) / time in seconds) x calorific value

• seconds per hour is always 3600
• seconds per turn is 21 seconds from the above example
• dial size is .5 m³
• The calorific value for my area is 1000 btu. You will need to contact your utility for their average. Generally, it’s between 1000 btu to 1075 btu

3600 x .5 / 21 x 1000

= 85,714 btu

Step 6: Now if you have to adjust high fire plus or minus 10% of the nameplate manifold pressure to get it to the appliance input, the orifices will need to be changed. If you need to go above 10%, the orifices are undersized. If you need to go below 10%, the orifices are oversized.

You will need to access the gas output on the gas valve to measure the gas going to the burners. These are the two most common connections in residential and light commercial gas valves.

The 3/32 hex screw only needs to be cracked and not fully removed. From there you can use an adapter to slide over for easy access. I like the fieldpiece RMA316.

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A proper setting for manifold pressure is necessary to ensure the correct flame characteristics and appliance operation.

If the gas pressure is too high on the outlet side of the gas valve, you may cause too much heat which can shut the furnace down. It can also cause flame sensor failure or make the flame lift-off the burners or for the flame to actually touch the first pass of the heat exchanger and cause a crack. Too high of pressure on the inlet of the gas valve can cause the gas valve to not even open or even fail.

Too low of gas pressure can cause a no heat or intermittent heat failures, sooting, flame sensor lockout, or delayed ignition.

Temperature Rise

The temperature rise is performed to ensure proper airflow across the appliance heat exchanger. Moving too little airflow across the heat exchanger will cause the high limit safety to stop the call for heat to protect the heat exchanger from permanent damage. In this case of a high limit, the flame would be disabled, and the blower would continue to run to cool off the heat exchanger. After a set amount of time, the high limit would close and the call for heat would start again.

When setting temperature rise, the lowest end of the temperature rise will give the appliance the greatest efficiency, but will not necessarily be as comfortable for the customer. Most appliances have a temperature rise range of around 30 – 60℉.

The heat produced is going to go either to the space being heated or out of the exhaust venting. A 50℉ change in the stack temperature is somewhere around a 1% savings. Setting the appliance temperature rise somewhere mid-range will provide more comfort for the customer as well as leave a little room for the filter to get dirty.

Combustion Test

The next step is to do a combustion test. Please see the combustion testing article here.

It’s important for the condensate trap to be primed with condensate or water prior to completing a combustion test. Either pour water into the trap or allow the furnace to run 10-15 minutes.

If the trap is not primed, air will be pulled backward from the condensate trap instead of through the heat exchanger. Air pulls from the least restive path. We need the water weight in the condensate trap to allow the appliance to pull air from the correct location.

Combustion air supply and ambient CO

Ensure ambient CO is 0 and the combustion air supply is correct. Please see the previous module on combustion air supply for additional information.

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.

Boyle’s Law is a law for an ideal gas. An ideal gas is a perfect gas that follows all the gas laws. Ideal gases are hypothetical.

Boyle’s Law states that for a fixed amount of an ideal gas at a fixed temperature, P (pressure) and V (volume) are inversely proportional, which means at a fixed temperature, when pressure increases, the volume will decrease. If the volume increases, pressure decreases.

Example: A jar filled with an amount of gas.

You have a large jar (volume UP) with an amount of gas. The jar is large, so it gives the molecules a lot of room to bounce around (pressure DOWN) inside the jar.

Now you take that same amount of gas and move it to a small jar (volume DOWN). There’s a lot less room for those molecules to move around now, so they’re bouncing all over (pressure UP) the walls of the jar like crazy.

You can see how volume and pressure are inversely proportional. When one increases, the other decreases. 

It’s important to remember Boyle’s Law is based on measurements at the same temperature.

What this means on a real service call

Boyle’s Law is not just a classroom idea. It explains why a sealed system, a tank, or a pocket of trapped air behaves the way it does when its space changes. If you squeeze the same amount of gas into a smaller space at the same temperature, its pressure goes up. If you give it more room, its pressure goes down. That simple trade-off shows up whenever you are charging, recovering, evacuating, or checking a system, and whenever a restriction changes the volume available to the gas on one side of it.

Think of a compressor, a receiver, or even a duct or a sealed vessel. The amount of gas does not have to change for the pressure reading to change. A change in volume alone can do it, as long as temperature stays about the same. That is why a pressure reading by itself never tells the whole story. You need to know what changed: the amount of gas, the temperature, the volume, or a combination of them.

Worked example you can check

Use the simple form P1 x V1 = P2 x V2, where P is pressure and V is volume, at a fixed temperature and with a fixed amount of gas. The pressures in this kind of textbook example are absolute pressures, not gauge pressures, because gauge pressure leaves out the atmosphere pushing on the system from outside.

Example: a sample of gas at 100 kPa absolute occupies 4 litres. If it is compressed to 2 litres at the same temperature, the new pressure is (100 x 4) / 2 = 200 kPa absolute. The volume was cut in half, so the pressure doubled. If instead the same sample expanded to 8 litres, the pressure would fall to (100 x 4) / 8 = 50 kPa absolute. The volume doubled, so the pressure was cut in half.

That is the pattern to remember: halve the volume, double the pressure; double the volume, halve the pressure, when temperature and the amount of gas do not change. In real work, temperature usually does change a little during compression, so treat this as a guide to direction and size, not as a laboratory-perfect prediction.

Numbers and measurements a technician actually sees

On a gauge set you usually read gauge pressure, in psi or kPa, not absolute pressure. To use Boyle’s Law properly, add atmospheric pressure: about 14.7 psi, or about 101 kPa, at sea level. A reading of 0 psig is therefore about 14.7 psia, not zero pressure. Forgetting that step is one of the easiest ways to get a Boyle’s Law calculation wrong, especially when the pressures are low.

Also watch temperature. Boyle’s Law only holds when temperature stays fixed. Compressing a gas quickly usually warms it, and letting it expand usually cools it. So if you compress a sample and check the pressure right away, it may read higher than Boyle’s Law alone predicts, because the temperature has risen as well. Wait for the temperature to settle, or account for the temperature change, before you draw a firm conclusion.

A common mistake

A common mistake is to assume a high pressure means there is too much gas in the system, or a low pressure means there is too little. That can be true, but it is not the only explanation. A restriction, a closed valve, a blocked passage, or a change in the space the gas can occupy can raise pressure on one side and drop it on the other, even with no leak and no added charge. Check for a restriction and for temperature change before you add or remove gas.

A practical checking step

When pressures do not make sense, work in this order: first, record the pressures and temperatures on both sides of the component you suspect. Second, look for a temperature drop or a restriction, a closed service valve, or a kinked line. Third, compare the readings over time rather than trusting a single snapshot, because a system still warming up or cooling down will mislead you. Do not open a pressurized system, and do not defeat a safety control to force a reading. Wear eye protection, work with the system de-energized where the check allows it, and follow the equipment maker’s service procedure for recovery and charging.

Related posts

For more on the gases themselves, see Characteristics of Gas. For how heat moves and changes temperature, see About heat and Types of Heat Transfer.

Combustion is a rapid chemical reaction when oxygen combines with the fuel to produce heat. By modifying any of the three, oxygen, heat, or fuel, you create an imbalance and can produce incomplete combustion.

Carbon monoxide (CO) and aldehydes are products of incomplete combustion.

Carbon monoxide is the most harmful product of incomplete combustion. It’s colorless, odorless, tasteless, and toxic. In large quantities, it can be combustible.

Aldehydes are irritating to the eyes, throat, and nose. They smell terrible. Aldehydes taste somewhat metallic and are likely accompanied by carbon monoxide.

Completing a combustion test is the only way to be sure that you’re providing a safe appliance for the homeowner with the best possible efficiency possible.

There are many causes for incomplete combustion, and we’ll cover several reasons here.

Low oxygen (O2) readings in the flue

Low oxygen (O2) reading is probably the most common issue to have and will usually have high carbon monoxide (CO) readings with it.

Low oxygen (O2) readings could be caused by too little combustion air, weak draft in the flue, or an overfired system.

• Check for blockages in combustion air supply, combustion air sizing, exhaust equipment removing too much air from the structure, dirty burners or obstruction at the heat exchanger inlet. Adjust burner air shutters as needed
• Check for blockages in the venting/chimney/heat exchanger. Check for any barometric damper setup incorrectly or not opening.
• Check the length of venting or too many elbows used. Check for proper sizing of venting and chimney
• Check gas pressure and adjust if needed

High O2 readings in the flue

High O2 readings will be caused by excessive combustion air, high draft in the stack, or an underfired system.

• Check gas pressure and adjust as needed
• Check for missing plugs on sides of the appliance, missing door, or burner adjustment shutters
• Check venting size or barometric damper

Low Stack Temperature

Low stack temperature can be caused by an underfired system or too much air across a heat exchanger.

Check gas pressure and adjust if needed

Check temperature rise and adjust if needed. When you’re moving too much air across the heat exchanger with too high of fan speed, you will not allow enough heat to flow through the venting system and could cause condensation to form on the heat exchanger or venting and force it to rust out.

High Stack Temperature

High stack temperature can be caused by an overfired system or not enough air across a heat exchanger.

Check gas pressure and adjust if needed

Check temperature rise and adjust if needed. When you’re moving too little air across the heat exchanger, you will not provide enough air to absorb heat from the heat exchanger so the heat will escape out the flue. A dirty filter, dirty blower wheel, and undersized or blocked return or supply duct will cause this

What this means on a real service call

Incomplete combustion is not a single fault. It is the result of an imbalance: not enough oxygen, the wrong gas pressure, a dirty burner, a blocked vent, or heat being pulled away from the flame too quickly. The danger is that the appliance can still produce heat while it produces carbon monoxide, so a customer may not notice anything wrong except a headache, a strange smell, soot, or a burner that does not look right. Aldehydes, which irritate the eyes, throat, and nose and smell terrible, often appear alongside carbon monoxide, but carbon monoxide itself has no smell, color, or taste.

That is why the original post stresses a combustion test. A test does not just confirm the appliance lights. It shows whether the fuel is burning cleanly after warm-up, whether oxygen and carbon monoxide are in a sensible range, and whether stack temperature fits the firing rate and airflow. Without that test, you are guessing about safety.

Numbers and measurements a technician actually sees

On a combustion test, watch the pattern, not just one number. Low oxygen in the flue usually goes with high carbon monoxide and points to too little combustion air, weak draft, or overfiring. High oxygen usually points to excess air, high draft, or underfiring. Low stack temperature can point to underfiring or too much air across the heat exchanger. High stack temperature can point to overfiring or too little air across the heat exchanger. Temperature rise across the furnace, gas pressure, and draft should all be checked together, because changing one without checking the others can move the problem rather than fix it.

Also use the limit already given on this site: on the About Combustion post, the maximum carbon monoxide allowed in the vent is 400 ppm on an air-free basis. A reading that stays high after the first few minutes, rather than settling after light-off, should be treated as a fault to find, not as normal operation.

A common mistake

A common mistake is to replace a flame sensor, igniter, or control board for a sooting or carbon monoxide problem, when the real cause is air, venting, or gas pressure. A new part may let the burner light more reliably, but it will not fix incomplete combustion. Another mistake is to test only at light-off, when carbon monoxide is normally higher for a short time, and then either panic over a normal spike or, worse, test only briefly and miss a reading that stays high once the unit is warm.

A practical checking step

If you suspect incomplete combustion, do not just clean a burner and leave. First, look for soot, a yellow or lazy flame, and any blocked combustion air opening or vent. Second, check gas pressure and temperature rise against the rating plate. Third, run a full combustion test after warm-up, if you are trained and equipped, and write down oxygen, carbon monoxide, and stack temperature. Fourth, check the room and the vent for carbon monoxide with a proper detector, because the appliance is not the only place carbon monoxide can show up.

Safety caution: if a carbon monoxide alarm sounds, occupants feel ill, or you find high carbon monoxide that does not settle, shut the appliance down if it is safe to do so, get people out, ventilate, and do not restart it until a qualified technician has found and fixed the cause. Never disable a safety control, and never use a gas appliance as a space heater for a room it was not designed to heat.

Related posts

For the full picture, see About Combustion and Combustion Air Supply, and for the fuels involved, see Characteristics of Gas.

Gas requires quite a bit of air for proper combustion since the air is made up of mostly nitrogen and only contains about 20% oxygen. For adequate combustion to happen, you require combustion air, excess air, and dilution air when using a draft device.

Combustion air is made up of primary and secondary air when speaking about perfect combustion. Since we strive for complete combustion in the real world, excess air is added to ensure combustion.

Combustion air requires 10ft³ per 1000 Btuh of the appliance. Excess air is supplied to ensure complete combustion can happen. Excess air is provided at 5ft³ per 1000 Btuh for a total combustion air supply of 15ft³ per 1000 Btuh of the appliance rating.

Dilution air is 15ft³ per 1000 Btuh of the appliance rating.

Combustion air is required for complete combustion of the gas. Combustion air is made up of primary, secondary and excess air.

Primary Air

Primary air is air mixed with the gas before combustion takes place. Burners are used to prepare the air:gas mixture for combustion using the venturi effect. Gas flows through a small opening in the orifice. As the gas travels through the small opening in the orifice, it draws air in around the opening with it. Gas passing through the orifice which has a reduction in size causing an increase in speed and draws primary air in with it is the venturi effect.

Secondary Air

Secondary air is supplied around the burners at the heat exchanger inlet and is required to complete the combustion process. This air is provided to the flames.

Excess Air

Excess air is over and above what is theoretically required for complete combustion. It’s supplied to ensure enough oxygen for complete combustion. The downside of excess air is since it absorbs heat, it carries that heat through the venting, reducing the efficiency of the appliance. Too much excess air can cause the flame to waver and possible flame impingement, or cool too much below the required flame temperature and in either case cause incomplete combustion.

Dilution Air

Dilution air is the air that combines with the flue gases. It assists with venting and dilutes the exhaust. The draft hood on a natural draft furnace was used to separate the flue venting from the furnace and allow air to draft into the heat exchanger naturally for combustion.

Dilution air is provided in appliances with a draft hood or some form of draft control. No dilution air is required otherwise.

Flue Gas Spillage

A small amount of flue gas spillage upon appliance startup is normal. Anything more needs addressing right away. The chimney will fill with a substantial amount of cool air during the appliance off cycle that the appliance hot flue gases will have to overcome. Venting should correct within 30 seconds or so of the appliance burners starting. Once the chimney heats up again, venting will happen normally.

Checking for spillage is fairly easy to check. When the appliance is up and running, hold a smoke pen in front of the dilution air draft hood. The smoke from the pen should be pulled in toward the draft hood indicating proper draft through the chimney. If the smoke is pushed the opposite direction, this indicated the exhaust is spilling.

What this means on a real service call

Combustion air is easy to overlook because it is invisible, but a burner cannot burn cleanly without it. Most of the air around us is nitrogen, not oxygen, so the appliance needs a surprisingly large volume of air for every cubic foot of gas. When that air is missing, blocked, or being pulled away by another appliance, the flame gets lazy, soot appears, and carbon monoxide risk rises, even though the burner still lights.

The problem often appears after something else changed: a room was finished and sealed, a closet door was added, a vent was covered, a larger exhaust fan was installed, or another appliance was added to the same space. The furnace did not suddenly break. Its air supply did.

Numbers and measurements a technician actually sees

Use the numbers already given in this post as your starting point. Combustion air is 10 ft³ per 1,000 Btuh of appliance rating, excess air adds 5 ft³ per 1,000 Btuh, for a total combustion air supply of 15 ft³ per 1,000 Btuh, and dilution air is another 15 ft³ per 1,000 Btuh where a draft device is used. For a 100,000 Btuh appliance, that works out to 1,500 ft³ of combustion air, and the same again for dilution air in a draft-hood setup. That is why a small, tightly closed closet is so often the problem.

On a combustion test, a lack of air often shows up as low oxygen and high carbon monoxide. Too much air or too much draft can show up as high oxygen and, on a furnace, a temperature rise that is too low. Always check the appliance rating plate and the installation instructions for that unit before deciding whether an opening is big enough, because the general numbers here do not replace the requirements for a specific appliance.

A common mistake

A common mistake is to adjust gas pressure or clean a burner to fix soot, without ever looking at the air supply. If the room is starved for air, the burner will soot again. Another mistake is to block or reduce a combustion air opening because it feels drafty. Those openings are not comfort drafts to be sealed. They are part of the appliance system.

A practical checking step

Start with the room, not the burner. Check that combustion air openings are open, unblocked, and not covered by storage, insulation, or a new wall. Look for a sealed room, a missing or closed louver, or a vent that ends in a dead space. Then check other appliances in the same space, plus exhaust fans, dryers, and range hoods, because they can pull air away from the appliance and cause spillage.

Next, use the spillage check from this post: with the appliance running and the vent warmed up, hold a smoke pen in front of the draft hood. Smoke should be drawn into the vent. If it is pushed back into the room after startup, treat that as a problem to correct, not a quirk. Finish with a combustion test if you are trained and equipped, and check for carbon monoxide in the room.

Safety caution: if flue gas keeps spilling, a carbon monoxide alarm sounds, or occupants feel ill, shut the appliance down if it is safe to do so, get people out, ventilate, and do not restart it until a qualified technician has corrected the air supply and venting. Never block a combustion air opening to stop a draft, and never enlarge or seal an opening by guesswork.

Related posts

For what happens when air is short, see Incomplete Combustion and About Combustion, and for the fuels being burned, see Characteristics of Gas.

Natural Gas

Natural gas has the chemical formula of CH4. Natural gas, also known as processed natural gas, is a dry gas which means it has all the heavier hydrocarbons removed and stored in underground storage tanks for use. Natural gas has no color, no taste, no odor (your distributor adds mercaptan for safety which gives a rotten egg smell), is non-toxic (although you may die from insufficient oxygen if there is too much natural gas in the air), and is transparent.

The specific gravity of natural gas is .55, although some go by .6. This means a weight of a volume of a vapor gas weighs less than an equal volume of air. This is why when you have a natural gas leak, the vapor will rise towards the ceiling since the gas weighs less than air.

The ignition temperature for natural gas is between 1200°F and 1300°F, and the limits of flammability are 4% to 15%. This means if you have below 4% natural gas in the air, combustion cannot happen. Likewise, if you have above 15% gas in the air combustion cannot occur either. Opening a window during a gas leak can be dangerous due to this.

The products of incomplete combustion are carbon monoxide and aldehydes.

Aldehydes are irritating to the eyes, throat, and nose. They smell terrible. Aldehydes taste somewhat metallic and are likely accompanied by carbon monoxide.

Carbon monoxide (CO) is the most harmful product of incomplete combustion. It’s colorless, odorless, tasteless, and toxic. In large quantities, it can be combustible.

The products of complete combustion for natural gas are heat, water vapor, and carbon dioxide.

The calorific value of natural gas is 1000 which may vary from location to location. This means 1 ft³ of natural gas gives off 1000 BTUh.

The flame speed for natural gas is 12’’/second.

Natural gas has a boiling point of -260°F and an expansion ratio of 1:600 when boiling from a liquid to vapor.

Propane

Propane has the chemical formula of C3H8 and is a wet gas containing the heavier hydrocarbons. Propane has no color, no taste, no odor (your distributor adds mercaptan for safety which gives a rotten egg smell), is non-toxic (although you may die from insufficient oxygen if there is too much natural gas in the air), and is transparent.

Propane as a vapor has a specific gravity of 1.52. The specific gravity of a gas is the weight of a volume of a gas compared to an equal weight of a volume of air. This means propane as a vapor is heavier than air and during a leak will build up closer to the ground.

Propane as a liquid has a specific gravity of .51, which is less dense than water. The specific gravity of a liquid is the weight of the liquid, propane, compared to the weight of an equal volume of water.

The ignition temperature of propane is lower than natural gas. It takes about 920°F to ignite.

The limits of flammability for propane are 2.4% to 9.5%. So less than 2.4% or over 9.5% of propane in the air will not be enough for combustion to take place.

The products of incomplete combustion are carbon monoxide and aldehydes.

Aldehydes are irritating to the eyes, throat, and nose. They smell terrible. Aldehydes taste somewhat metallic and are likely accompanied by carbon monoxide.

Carbon monoxide (CO) is the most harmful product of incomplete combustion. It’s colorless, odorless, tasteless, and toxic. In large quantities, it can be combustible.

The products of complete combustion are heat, water vapor, and carbon dioxide.

The calorific value of propane is 2500. This means 1 ft³ of propane gives off 2500 Btuh.

The flame speed for propane is 11’’/second.

Propane has a boiling point of -44°F and an expansion ratio of 1:270 when converting from a liquid to vapor.

What this means on a real service call

Specific gravity is the number that tells you where a leaking gas will go. Natural gas, with a specific gravity around 0.55 to 0.6, is lighter than air, so it rises toward a ceiling. Propane vapor, at 1.52, is heavier than air, so it sinks and can collect in low spots, basements, pits, and crawl spaces. That difference changes where you look for a leak, where you place a detector, and why a propane leak in a low area is especially dangerous.

The other numbers on this page matter for the same reason. Natural gas and propane are not interchangeable. They have different calorific values, about 1,000 Btu per cubic foot for natural gas and about 2,500 Btu per cubic foot for propane on this page, different flame speeds, and different ignition temperatures. An appliance set up for one fuel will not burn the other correctly without the proper conversion, orifices, and regulator settings for that appliance. Never assume a conversion is just a matter of changing the gas supply.

Numbers and measurements a technician actually sees

On a gas job, the readings you check most often are manifold pressure and supply pressure, measured with a manometer in inches of water column, and the flame appearance. A lazy, yellow, lifting, or floating flame is a clue, not a diagnosis on its own. Pair what you see with a combustion check where you are equipped to do one, and with the appliance rating plate. Also remember the expansion ratios given above: natural gas expands about 1 to 600 and propane about 1 to 270 when changing from liquid to vapor. A small amount of liquid therefore makes a very large volume of vapor, which is why liquid propane handling needs care and why a leak must never be treated as minor.

A common mistake

A common mistake is to rely on smell alone to rule out a leak. Natural gas in its natural state has no odor; the rotten-egg smell is added for safety, and it can fade, be masked, or go unnoticed, especially if you have been around the odor for a while. Propane also sinks, so a check at standing height can miss gas collecting near the floor. Use a proper gas detector and a leak-check solution or method approved for the job, and check high for natural gas and low for propane.

A practical checking step

If you suspect a gas leak, stop, do not operate electrical switches, phones, or ignition sources in the area, ventilate only if it can be done safely, and leave the area if the smell is strong or a detector alarms. From a safe place, follow your gas utility’s emergency instructions and the site’s emergency procedure. Do not relight a pilot or restart an appliance until the leak has been found and repaired by a qualified person and the system has been tested. For a routine check, confirm the fuel type on the rating plate, check supply and manifold pressures against that plate, look at flame pattern, and check for soot, which can point to incomplete combustion.

Related posts

For what happens when that fuel burns, see About Combustion, Incomplete Combustion, and Combustion Air Supply. For how pressure and volume relate, see Boyle’s Law.

Combustion is a rapid chemical reaction when oxygen combines with the fuel to produce heat. Lighting a match is combustion.

You require three things to produce combustion; heat, fuel, and oxygen. Since the air we breathe is only about 20% oxygen and 80% nitrogen, we end up having to supply much more air to get the oxygen we need.

There are three types of combustion:

• Perfect combustion
• Complete combustion
• Incomplete combustion

Perfect combustion is known as stoichiometric combustion and provides an exact fuel to air ratio and extracts all energy from the fuel at 100% efficiency. Perfect combustion does not exist in the real world as some of the hydrogen (H) and carbon (C) atoms will not join the oxygen (O2) and instead create incomplete combustion.

Complete combustion is what we aim for. 1 cu ft gas to 15 cu ft air for natural gas and 1 cu ft gas to 18 cu ft air is necessary for propane. Carbon and hydrogen combine with oxygen (O2) during complete combustion to produce carbon dioxide (CO2) and water (H2O).

Incomplete combustion is a result of an unbalanced combustion process. Not enough combustion air, incorrect gas pressure, flame impingement, wrong gas orifices, blocked venting, etc. Products of incomplete combustion are carbon monoxide, soot, and aldehydes.

Carbon monoxide (CO) is the most harmful product of incomplete combustion. It’s colorless, odorless, tasteless, and toxic. In large quantities, it can even be combustible.

Measuring CO in the exhaust is done on an air-free basis and is referred to as COAF. Since the added excess air for complete combustion dilutes CO readings, the COAF reading compensates for this dilution using the O2 sensor in the analyzer.

The maximum carbon monoxide an appliance is allowed to produce and exhaust out the venting is 400 ppm (coaf). As a general rule of thumb, an appliance well-tuned should not exceed 50 ppm CO. Check local codes for your allowable maximum.

During initial light off, a high level of CO is produced but should level off within a few minutes.

Dirty burners can cause insufficient air to gas ratio. If dirty burners are an issue, they should be removed and cleaned. A soft wire brush may be used to clear any rust or debris. Ensure the burners are in proper alignment to allow for the flame to spread evenly along with the burners flame spreader.

Gas pressures should be checked and set to manufacturer suggested settings. Follow the pressures listed on the appliance rating plate. You should not assume the gas valve comes with the correct regulator settings because multiple manufacturers could use the same gas valve since gas valves are not always explicitly manufactured for each appliance.

Aldehydes are irritating to the eyes, throat, and nose. They smell terrible. Aldehydes taste somewhat metallic and likely accompanied by carbon monoxide.

Soot is unburned carbon.

What this means on a real service call

On a furnace call, the combustion triangle is your quick diagnostic frame. If any leg is weak, combustion suffers. Too little fuel, too little air, or not enough heat for ignition, and the burner will not light, will not stay lit, or will burn poorly. Too much fuel for the air available, and you get incomplete combustion, soot, and carbon monoxide. Too much excess air, and you waste heat up the vent and can cool the flame.

Complete combustion is the goal: the right fuel-to-air balance, a stable flame, and clean products of combustion, mainly heat, water vapor, and carbon dioxide for natural gas. Perfect, or stoichiometric, combustion is the exact theoretical balance, but in real equipment a small amount of excess air is needed to make sure the fuel and oxygen actually meet and burn fully. That is why the air numbers look large: air is only about 20 percent oxygen, so a lot of air is needed to supply enough oxygen, and most of the rest is nitrogen that simply passes through.

Numbers and measurements a technician actually sees

The original post gives a useful starting point: about 1 cubic foot of natural gas to 15 cubic feet of air for complete combustion, and a carbon monoxide limit of 400 on an air-free basis, known as COAF. Do not judge combustion by flame color alone. Where you are trained and equipped to do so, use a combustion analyzer to check oxygen, carbon monoxide, and stack temperature, and compare the results with the appliance maker’s instructions. A CO reading that stays high after the first few minutes of operation, rather than leveling off after light-off, is a warning sign, not something to ignore.

Gas pressure is the other everyday number. Check supply and manifold pressures with a manometer and set them to the pressures on the rating plate and in the maker’s instructions, not to a number you remember from another unit. A burner that is overfired or underfired will not burn cleanly, even if everything else looks right.

A common mistake

A common mistake is to adjust gas pressure to fix a flame problem without checking air supply, burner cleanliness, and venting first. Dirty burners can starve the flame of air, a blocked vent can change draft, and a starved combustion air supply can do the same. Turning the gas up or down to mask those problems can make incomplete combustion worse and can create a safety hazard. Find the cause, clean or repair it, and then set pressures correctly.

A practical checking step

Work through combustion in order: first, confirm the fuel type and that the appliance is set up for it. Second, check combustion air and venting for blockage, and look for soot around the burner or vent, which points to incomplete combustion. Third, check and set gas pressures to the rating plate. Fourth, if you are trained and equipped, run a combustion test after the unit has warmed up and compare it with the maker’s guidance. Safety caution: if you smell gas, see heavy soot, or a carbon monoxide alarm sounds, do not keep running the appliance to finish a test. Shut it down if it is safe to do so, ventilate, leave the area if needed, and get qualified help. Carbon monoxide is colorless and odorless, so never rely on smell to judge safety.

Related posts

For the fuels themselves, see Characteristics of Gas. For what goes wrong, see Incomplete Combustion, and for air supply, see Combustion Air Supply.