What’s the difference between a 95% furnace and an 80% furnace? The wise-guy answer is “15%.”
But the useful answer is more complex and helpful than that.
In this post, we cover the debate from all angles – some that aren’t typically considered.
There is more to a gas furnace than its efficiency. In this debate, you should also consider:
- Initial Cost of the Furnace
- Installation Cost
- House size
- Furnace Performance
- Possible Safety Issues
95% Furnace vs 80% Furnace: We’ll keep score as we go.
Initial Cost of the Furnace: 80 vs 95 AFUE
The difference is $500 to $1,200 for just the furnace, depending on its performance. That’s the difference you’ll need to make up through lower heating bills to break even.
Single-stage heating, CFM Blower: The 95% furnace will cost $500-$750 more than the 80% furnace.
Two-stage heating, ECM Variable Speed Blower: The 95% furnace will cost $900-$1,200 more than the 80% furnace.
Solely based on furnace cost, here’s a point for an 80% furnace.
1. Initial Cost
Score: 80% 1, 95% 0
Installation Cost – Vent & Drain
Let’s talk about venting the combustion gases including deadly carbon monoxide.
- 80% furnace lose 20% of their heat through the chimney. That’s why their steel vents must go up and through the roof: The gas is 400 degrees Fahrenheit.
- 95% furnaces waste just 5% of heat. They can be vented through the side of the house because exhaust gases are 90F to 110F.
First Furnace: In new construction, installation costs are about the same. A high-efficiency furnace might cost a bit less to install.
Replacement Furnace: If you’re considering upgrading the home’s 80% furnace to a 95% furnace, then you’ll have two extra installation costs – the vent and drain.
- Running a PVC vent out the side of the home will cost $200-$500 based on length and job difficulty. The average is $400.
- The 95% furnace will produce condensate, as explained next, which will require a drain. If the furnace is installed in a basement and there is a floor drain handy, cost will be negligible. A plastic drain tube running to the drain will be enough. If the furnace is on a first floor or in the attic, a drain will have to be installed. The cost averages about $250.
Q: Why can’t a 95% furnace use the existing steel vent?
A: Because the exhaust from a high-efficiency furnace will corrode the galvanized steel flues used for 80% furnaces. And that will eventually allow carbon monoxide, or CO, to infiltrate your living space. Obviously a bad idea.
If you’re interested, here’s why this happens. Furnace exhaust includes a lot of moisture and highly corrosive carbonic acid, two byproducts of combustion.
The 400F exhaust of an 80% furnace will keep the carbonic acid-laced moisture in vapor form, and it is easily carried by the rising hot exhaust up and out through the roof.
When the exhaust gases are cooled to 100F, the acidic moisture condenses. It would corrode steel. Therefore, 95% furnaces must be vented using PVC piping.
The bottom line is that if you upgrade, you’ll have the extra cost of installing a new furnace vent.
In upgrades from an 80% to a 95% furnace, the less efficient furnace gets another point because installation costs are lower.
2. Installation Cost – Vent & Drain
Score: 80% 2, 95% 0
Installation Cost – Air for the Furnace
Furnaces are often installed in vented locations – an attic is most common, but there are a few vented crawlspaces.
An 80% furnace will do just fine there because it pulls in air/oxygen from the surrounding area for combustion.
A 95% furnace uses sealed combustion, and so a pipe must be installed to feed air directly into the furnace’s combustion chamber. This is a second added cost to a 95% furnace. In vented space, the extra installation cost means the more efficient furnace is not a cost-effective option.
3. Installation Cost – Air for the Furnace
Score: 80% 3, 95% 0
House Size & Furnace Efficiency
The bigger the house, the larger the furnace must be.
In a small, well-insulated house, your winter heating bill might be $300 if you use an 80% furnace. The upgrade to a 95% furnace will save about 15% or $45 per year. And remember, the 95% furnace will cost $500-$1,200 more depending on its performance. If it’s a replacement, and you have to install a new vent, that’s another $400 to the total.
$500+$400 = $900 / $45 = 20 years. In a small house, the cheapest upgrade to a 95% furnace will take 20 years to pay off. It isn’t worth it.
In a very large home, the bill might be $1,200 or more for winter heating, especially in a cold climate and if the home doesn’t have as much insulation as it should.
In the big house scenario, the upgrade to a 95% gas furnace saves you about $180 per year. The cheap upgrade to a single-stage 95% furnace gets paid off in just 5 years. If you choose a high-performance 95% furnace, it’s 10+ years.
Bottom Line: The larger your home (2,500 square feet or more), the more it makes sense to buy a high-efficiency gas furnace.
If you pay $600 or less to heat your home, then it is even. Don’t upgrade.
But if you have a large home, it’s a point for the 95%. Keep your heating costs in mind as you keep your own score.
4. House Size
Large Home Score: 80% 3, 95% 1
Small Home Score: 4-0 in favor of the 80% furnace
Climate & Furnace Efficiency
This can be brief. If you live in a mild or warm climate, you’ll have low heating bills either way. It doesn’t make sense to install a 95% model.
In a cold climate it does.
Warm Climate Score: 80% 5, 95% 1
Cold Climate Score: 80% 4, 95% 2
Now here’s a twist. Let’s say you’ve got a two-stage 80% furnace and you’re considering upgrading to a 95% single-stage furnace.
The single-stage furnace will run at 100% capacity whenever heating. Call it “full blast.” These furnaces waste a little bit of energy and cause temperature swings of a few degrees.
This is because when the thermostat point is reached and the burner shuts off, there is still a lot of heat in the furnace. The blower keeps running until that heat is dispersed.
A two-stage furnace runs at low capacity, or about 65% on most models, most of the time. It only uses full capacity when running on low isn’t keeping up with rapidly dropping temperatures or when you walk into a chilly house and “crank up the heat.”
In this scenario, an 80% furnace running on low most of the time will be about as efficient as a 95% furnace running on high all the time.
The price of the furnaces will be similar too, since the 80% unit has a two-stage gas valve probably a nicer blower motor while the 95%’s is a single-stage valve with a cheaper motor.
5. Furnace Performance
It’s a tie here. Stick with the 80% furnace.
Possible Safety Issues in Non-Vented Space
Carbon monoxide leaking from any gas appliance can cause fatalities, as was the case with this family of 4 in 2020.
It’s rare, but when it happens, it is dangerous.
Vented Space: If the furnace is in a vented attic or vented crawlspace, the carbon monoxide might be carried out the vents as air is exchanged. This reduces the potential for carbon monoxide poisoning if there is a leak.
Enclosed Space: When the furnace is in a sealed basement or crawlspace or utility closet, then leaked carbon monoxide will build up to dangerous levels until the problem is solved.
What does this have to do with furnace efficiency?
Probably nothing, in our opinion.
But an HVAC company’s popular blog post raised the carbon monoxide issue and recommends a 95% furnace installed with a concentric vent for enclosed spaces. The concentric vent – a vent within a vent – pulls in combustion air through one vent and exhausts combustion gases – gases containing carbon monoxide – through the other. The view is that this setup creates better airflow and more effectively removes those gases from your home than an 80% in which those hot gases rise out of your home due to their heat. The suggestion is that some CO might leak even if the furnace isn’t broken.
Since this view is “out there,” you might want to discuss this potential safety concern. The issue is, “Does a 95% furnace with a concentric vent more effectively exhaust combustion gases than an 80% furnace?” We think not, but a technician will tailor the answer after inspecting your home and where the furnace will be installed.
We’re not going to give either a point here. It’s just an issue to be aware of.
Warm Climate Score: 80% 6, 95% 1
Cold Climate Score: 80% 5, 95% 2
Bottom Line: 80% AFUE Furnace vs 95% AFUE Furnace
The risk of carbon monoxide poisoning is best alleviated by installing a CO detector near the furnace and making sure it has fresh batteries or is plugged in. Check/Test it as recommended in the owner’s manual.
If your carbon monoxide detector goes off, call 911 and leave the house.
If the whole family is feeling sick and/or drowsy, leave the house and see if that helps. If it does, call 911 or the fire department. CO might be the culprit.
The family in Ohio felt ill for several days before they were all found dead. The tragedy was avoidable.
In either case, first responders, usually firefighters, will show up with a highly sensitive CO detector. They’ll know immediately if there is an elevated level of CO in the house. If so, they will shut off the gas valves to all gas appliances.
Your next step will be to call an HVAC company to check the gas appliances and their vents. The problem will be found and fixed.
Get an annual furnace check-up going forward. It’s the best way to be sure the vent and furnace heat exchanger are in good working condition.
If you’re told the furnace heat exchanger is cracked and leaking CO, but the detector has never gone off, get a second opinion. While not common, a few unethical HVAC technicians try to scare homeowners into buying a new furnace with the ol’ cracked heat exchanger scam.
Here’s a useful map for deciding between an 80% and 95% furnace. The map was developed a few years ago when the US Department of Energy was threatening to force standards on consumers for minimum furnace efficiency.
Homeowners in the North would have been forced to install a furnace with 90% AFUE efficiency or higher. Those in the SE and SW of the country would have been able to install an 80%, which is the current national minimum, or more efficient model if they chose.
The proposed changes were challenged in court, and the regulations never went into effect.
However, the map does show useful boundaries where it is worth considering a 90+ furnace.
80% furnaces are good for:
- The South
- Small Homes
- Part Time Locations (guest house, vacation home or outbuilding workshop)
- Vented Space
95% furnaces, or anything above 90%, are better for:
- The North
- Large Homes
- Older Homes with Poor Insulation