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Furnace Combustion: Atmospheric vs. Sealed

Posted by John Sims on Mon, Jan, 26, 2015 @ 16:01 PM

Over the years, furnace equipment and how it is installed has gotten more complex. So knowing this information is very helpful when it comes to getting the best value when you decide to replace your furnace.

As homes are insulated and built "tighter" to achieve better energy efficiency and comfort, our technicians and sales staff need to be up on all of the changes and how that affects you the homeowner.

Frequently, we have to explain what the difference is between atmospheric and sealed combustion.

Here's the basic process for combustion in furnaces:

  1. Pull in air to mix with the propane or natural gas.
  2. Burn the mixture of gas and air.
  3. Exhaust the combustion gases to the outside through the flue.

Now keep in mind, for every cubic foot of air that enters the furnace, another cubic foot of air has to come into your home to replace the air that gets used by the furnace. I don't want to assume everyone knows the basics.

Atmospheric Combustion: 

First, let me define atmospheric combustion because many people don't know what it means. An atmospheric combustion furnace draws air from your basement(most furnaces in this area are installed in the basement area next to the water heater) which in turn pulls the air from the rest of the house.

An atmospheric combustion furnace pulls room air into the combustion chamber through the grill on the front. You could, if you wanted to, pull the cover off and stick your finger into the blue flame.

Many older homes with atmospherically vented furnaces are 60 - 80% efficient (also known as low-mid efficiency) gas furnaces use the chimney to vent their combined combustion gases out of your home.

The way they remove the by-products of combustion, is the heat of the flue gases creates a negative pressure, or suction(hot air rises), inside the chimney, pulling the flue gases out of your home.

Remember, a basic rule of building science and physics, is that for every cubic foot of air that leaves the house, another cubic foot of air comes in. When an atmospheric furnace runs inside the  conditioned space(the space you pay to heat and cool), you're increasing the air infiltration that the house experiences. Hey, you're not running to crack open a window every time your furnace kicks on, right? Your energy bills will be higher as a result, and you may find the house a bit drafty while the furnace is running.

A lot of houses have atmospheric combustion furnaces. What that means is that a lot of houses have higher energy bills as well as the potential for serious health and safety problems. By the way, combustion safety is a huge issue

Sealed Combustion: 

The simple definition is all the air used by the furnace burners comes from outdoors. Sealed combustion furnaces have an air-intake pipe as well as an exhaust pipe. It uses outside air for combustion, not air inside your home. It's got two pvc pipes that bring in combustion air from outdoors and then exhaust the gases back to the outdoors.

It doesn't use already heated air inside your home for combustion and second, by separating the combustion air from house air, the furnace is not affected by other home appliances in a tight home, and that’s a great safety feature.

According to the folks at EnergyVanguard, there are 3 advantages of using outside air for combustion:

  1. Prevents furnace damage caused by vapors from laundry products. The vapors can mix with indoor combustion air to corrode furnace parts.
  2. Avoids using indoor air that you already paid to heat.
  3. Reduces the danger of backdrafts (pulling exhaust gases down the chimney).

All high-efficiency (90+ % AFUE) combustion furnaces and boilers use sealed combustion. Obviously, they do not vent flue gases through a standard chimney. Since these gases are relatively cool—about 100°F—they are vented through a sealed plastic vent pipe. All 90% appliances have built-in combustion blowers that draw air from the outside into the combustion chamber and then push the flue gases to the outside.

It's pulling combustion air from outdoors and sending the exhaust gases back outdoors, so it's only adding heat to your indoor air, not exchanging any of it with outdoor air.

When you plan to replace or upgrade, you best investment will be a sealed combustion furnace. Also, check out the 6 'SUREFIRE' Steps to getting the best value!

Topics: Best Value, Furnace Replacement, Building Science, Combustion

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