Sealed combustion furnaces draw combustion air from outdoors through a dedicated pipe, while vented (atmospheric) furnaces burn indoor air. That single difference is why sealed units reach 90 percent or higher AFUE, vent exhaust safely, and cut carbon monoxide risk – a real advantage in west-central Minnesota, where a furnace runs hard for months of well-below-zero cold.
Which one fits your home depends on more than efficiency ratings. Insulation, existing venting, budget, and local building code all shape the decision, and a vented model can still make sense in an older, drafty farmhouse. Below, Northern Services Today breaks down how each type works, what it costs to install and maintain, and how the two compare on safety and long-term reliability.
How a Vented (Atmospheric) Furnace Works
A vented furnace – also called an atmospheric or natural-draft furnace – pulls the air it needs for combustion straight from the room around it, usually a basement or utility closet. The burners ignite the gas, and the hot exhaust rises on its own natural draft up a metal flue or masonry chimney. Because the process relies on room air and a passive chimney, the design is simple and has been the standard in Minnesota homes for decades.
That simplicity has a cost. Every cubic foot of heated indoor air the burners consume has to be replaced by cold outside air leaking in somewhere else, so the furnace is fighting itself on the coldest nights. Older atmospheric units land in the 56 to 70 percent AFUE range, meaning roughly a third of the fuel you pay for leaves through the flue as waste heat. A single heat exchanger is all that stands between the combustion gases and your ductwork.
The bigger concern is draft. If the chimney is blocked, oversized, or the house is depressurized by exhaust fans, combustion gases – including carbon monoxide – can spill back into the living space instead of rising up the flue. That is called backdrafting, and it is the main reason atmospheric furnaces demand a working carbon monoxide detector and an annual inspection.
How a Sealed Combustion Furnace Works
A sealed combustion furnace closes off the burner from your indoor air entirely. Two PVC pipes run through the wall or roof: one draws fresh combustion air directly from outdoors, and the other carries exhaust back outside. The burner never touches the air you breathe, which is why these units are also called direct-vent furnaces.
Most sealed combustion models are condensing furnaces, and that is where the efficiency comes from. A second heat exchanger squeezes additional heat out of the exhaust until water vapor in the gases condenses to liquid – hence the name “condensing.” That recovered heat pushes AFUE ratings to 90 percent and higher, with many models rated 95 to 98 percent. The tradeoff is a small amount of acidic condensate that drains away through a tube, and cooler exhaust that can safely travel through plastic pipe.
Because the burner draws outdoor air, a sealed unit does not depressurize the house or compete with bath fans, range hoods, and the clothes dryer for air. It cannot backdraft the way an atmospheric furnace can, and it keeps combustion byproducts fully separated from your living space.
Vented vs. Sealed Combustion: Key Differences
| Feature | Vented (Atmospheric) | Sealed Combustion |
|---|---|---|
| Combustion air source | Indoor room air | Outdoor air via dedicated pipe |
| Typical AFUE | 56-70% (older units) | 90-98% |
| Heat exchangers | Single | Dual (condensing) |
| Venting | Metal flue or chimney, natural draft | Two PVC pipes, fan-forced |
| Backdraft / CO risk | Higher | Much lower |
| Upfront cost | Lower | Higher |
| Best fit | Older, naturally ventilated homes | Tight, well-insulated homes; cold climates |
Efficiency and What It Means for Your Heating Bill
AFUE – Annual Fuel Utilization Efficiency – tells you how much of the fuel a furnace actually turns into usable heat. A 95 percent AFUE furnace delivers 95 cents of heat for every dollar of gas or propane and sends the remaining 5 percent out the vent. An older 65 percent unit wastes 35 cents on the dollar, every dollar, all winter long.
In west-central Minnesota that gap adds up fast. A furnace here runs for months, often through stretches of well-below-zero weather, so the difference between a 65 percent atmospheric unit and a 95 percent sealed combustion model shows up on every heating bill from November through March. Propane customers feel it even more, since propane costs more per unit of heat than natural gas. Over the life of the equipment, the fuel savings from a high-efficiency sealed unit often offset a large share of its higher purchase price.
Safety and Indoor Air Quality
Carbon monoxide is the safety issue that matters most with any gas furnace. It is colorless, odorless, and dangerous at high levels. Atmospheric furnaces carry more CO risk simply because they rely on room air and a passive chimney – if the draft reverses, those gases have a path back into the house.
Sealed combustion removes that path. With the burner drawing outdoor air and exhausting through a dedicated pipe, there is no shared chimney to reverse and no room air being pulled into the flame. That is a meaningful advantage in a tightly built modern home, where powerful kitchen and bath exhaust fans can depressurize the house and pull against a natural-draft appliance.
One detail a licensed contractor watches for during an upgrade: if you replace an old atmospheric furnace that shared a chimney with your water heater, the water heater can be left venting alone into an oversized flue. That can cause the water heater to backdraft. A proper installation addresses the orphaned water heater, not just the furnace.
Whichever system you have, keep working carbon monoxide detectors on every level of the home and outside sleeping areas, and schedule an annual inspection of the heat exchanger, burners, and venting.
Installation and Maintenance Costs
Vented atmospheric furnaces cost less upfront. The equipment is simpler, and if a usable chimney or flue is already in place, installation is straightforward. Sealed combustion units carry a higher purchase price and require running two PVC pipes to the outdoors, so the install is more involved – especially in a retrofit where new vent penetrations are needed.
Maintenance runs a little differently between the two. Atmospheric furnaces need their flue and chimney checked for blockages and corrosion, along with the usual burner cleaning and filter changes. Sealed condensing units add a condensate drain that has to stay clear, and in our climate that drain and the outdoor vent terminations need attention so they do not freeze or get buried by snow. Both types should get a professional tune-up once a year and a fresh filter every one to three months during heating season.
Which Furnace Is Right for Your Home?
For most homes in Olivia, Willmar, Hutchinson, and the surrounding area, a sealed combustion furnace is the stronger long-term choice. It burns less fuel through our long heating season, it will not backdraft, and it suits the tighter construction of newer and well-sealed homes. Local building codes and federal efficiency standards have also moved steadily toward high-efficiency equipment.
A vented atmospheric furnace can still be a reasonable fit in an older, naturally drafty farmhouse – especially where the budget is tight and a sound chimney already exists. If your home is loosely built and well ventilated on its own, the depressurization concerns that push people toward sealed combustion are less pressing.
Before you decide, weigh these factors:
- Your home’s age, insulation, and how tightly it is built
- Whether a usable chimney or flue already exists
- Natural gas versus propane and your winter fuel costs
- Long-term energy savings against the higher upfront price of a sealed unit
Frequently Asked Questions
Are sealed combustion furnaces safer than vented ones?
Yes. Because a sealed combustion furnace draws air from outside and exhausts through a dedicated pipe, it cannot backdraft the way an atmospheric furnace can, which lowers the risk of carbon monoxide entering your home.
Which type is more energy-efficient?
Sealed combustion furnaces are far more efficient. Most are condensing units rated 90 to 98 percent AFUE, compared with roughly 56 to 70 percent for older atmospheric furnaces.
Do vented furnaces cost less to install?
Usually, yes – the equipment is simpler and often uses an existing chimney. But their lower efficiency means higher fuel bills over the life of the furnace, which can erase that upfront savings.
Is a sealed combustion furnace a good choice for a cold Minnesota winter?
Yes. Since it does not depend on indoor air, it heats efficiently through long cold stretches without depressurizing the house. Just keep the outdoor intake and exhaust terminations clear of snow and ice.
Talk to Northern Services Today
Still not sure which furnace fits your home and budget? The team at Northern Services Today installs and services both vented and sealed combustion furnaces across west-central Minnesota, from Olivia and Willmar to Hutchinson, Litchfield, and Redwood Falls. Call us at (320) 523-5862 to schedule an assessment, and we will help you choose a system that keeps your family warm and safe through the coldest months.