What is a Low NOx Furnace?

What is a Low NOx Furnace?

Have you ever heard about low NOx furnaces? Do you even know what NOx is?

Many Trane dealers were anxiously awaiting the release of the new S8 furnace line.

The main reason they’re looking forward to it was that the 34-inch cabinet is going to be a lot easier to deal with when we’re replacing a furnace in a home’s closet or attic.
But when the rest of the country was getting the new S8 furnaces, Californians had to wait for the release of the low NOx models mandated there in California.
We all know what greenhouse gases are and how they negatively affect the world we live in. It’s bad for our health and limits the quality of life for our generation, as well as future generations.
NOx is just another gas that needs to be reduced to help our planet remain stable and healthy for us humans to exist.
NOx is an abbreviation for Nitrogen Oxides. They’re poisonous and highly reactive gases that are created naturally during lightning strikes and wildfires.

Both events include the combustion of oxygen and nitrogen at very high temperatures, so it makes sense that the combustion that takes place in the gas-fired furnaces that we work on, would also produce these nitrogen oxides.
The two most dangerous oxides are Nitric Oxide and Nitrogen Dioxide.

Nitric Oxide (NO) is formed naturally in the body to help relax and dilate blood vessels in the body. It’s used in a lot of pre-workout energy drinks to help athletes and weight lifters recover faster but it’s also a product of high-temperature situations like combustion in a furnace.

The other oxide, Nitro Dioxide (NO2) are produced by vehicles and cigarette smoke in their combustion processes but also when gas and oil furnaces fire up.

NO2 causes inflammation in the airways, coughing, increased asthma attacks, and just a greater risk of negative breathing problems associated with pollution.

Although the end-user of the heat being delivered through the ductwork isn’t going to be exposed to these gases that way, the accumulation of it in the immediate area of the combustion chamber and onto the flue pipe adds to the overall production of greenhouse gases generated by humans.

According to the EPA, NOx contributes to respiratory problems, acid rain, smog, elevated algae levels, and global warming.

They also say greenhouse gas emissions that come from homes in California represent about 25% of the state’s total emissions. NOx emissions from a furnace are primarily influenced by the temperature of the flame right at the burner assembly.
So, what are manufacturers doing to produce low NOx furnaces?
Most efforts to reduce NOx emissions ultimately want to lower the temperature of the flame, air/fuel mixtures are dialed in, flue gas circulation is enhanced through advanced inducer motors, and ultra-lean pre-mixed burner technologies are all ways to achieve more control over the flame temperature inside the furnace.

Performing a combustion analysis will ensure the correct operation of the low NOx furnace once it’s installed. Installers just need to make sure that the airflow to the system is properly sized so the static pressure is going through the duct system are right.
The Installer also needs to make sure the gas valve inlet and outlet pressures are adjusted to the proper settings.
These two items alone will help create the recommended temperature rise across the heat exchanger as well as flue gas temperatures exiting the unit.

As we were researching this technology, we were reminded that carbon monoxide, which is another regulated pollutant and nitrogen oxides are both majorly influenced by the air-fuel mixture directly at the burners.
As that ratio increases the temperature of the flame increases and NOx levels increase. While carbon monoxide levels decrease and vice-versa.

Having the proper mixture is what low NOx is all about.
In most low NOx furnaces, the air mixture is sort of delayed to stretch out the chemical reactions happening at the burners. Low NOx furnaces also compensate for seasonal changes in the ambient air, humidity levels, and minor differences in the gas coming from the utility at any given time.

To go a little bit further with the whole CO2 and NOx discussion the typical start-up conditions of a furnace on a cold morning means that the metallic chambers are cold.

There’s an excess amount of combustion air to mix at the burners, the air inside the chambers is colder the gas temperature is initially lower and the flue gases themselves are moving slowly from the heat exchanger and on to the flue pipe.
All these conditions create higher levels of Carbon Monoxide at the startup because the system is not burning as hot. As a result, NOx emissions tend to be lower.
On the opposite side of, that the hotter that furnace gets after being on for several minutes and several times that day, contributes to higher levels of NOx and lower levels of Carbon Monoxide.
When we’re talking about these gases and if we had our choice as to which gas to focus on reducing? It would be NOx.
Because of the contribution to acid rain and the breathing problems associated with it. CO has its own detriment as well and we can’t really tell which gas is more harmful at certain concentrations.
Now, we’re not scientists – we’re just HVAC technicians and our knowledge really only goes so far about this topic but there are plenty of discussions online about NOx and Carbon Monoxide.
So, remember low NOx doesn’t have anything to do with the heat that enters the house through the air ducts or the air that the homeowner is breathing, it has more to do with what’s leaving the flue pipe and entering our atmosphere.
Nitric Oxides and Dioxides, that are produced by flames are part of the poisonous NOx family. The more we can reduce them while still heating our homes effectively, is really what it’s all about.
And we can do this by controlling the temperature of the flame at the burner assembly.
Replacing older gas furnaces with low NOx furnaces will help our country’s emissions goal and if you want to go even further than staying with a gas furnace, you could switch over to a zero-emissions heating solution, by replacing that gas furnace with a heat pump system.
This eliminates flue gas, flue pipes, NOx, and still heat your home just the way you like it.
Well, we hope that this has helped you understand what the fuss is about what NOx is and why the industry is pushing low NOx furnaces.
And you’d like to compare a low NOx furnace with the “regular” furnaces on the market get a hold of us – we’re Red Deer’s heating and air conditioning experts.