This guide has those answers and more.
You will also find a chart showing different efficiency levels and how that affects furnace output. For example, if a load calculation shows your home needs 100,000 BTUs of heat per hour (BTU/h) and you’re planning to buy an 80% AFUE furnace or boiler, it will have to be much larger than 100,000 BTUs. See details below.
What is AFUE?
Annual fuel utilization efficiency is a measure of how efficiently a furnace or boiler heats with the fuel it burns. Most gas furnaces and boilers have AFUE ratings between 80% and 98%.
It’s a little narrower for oil furnaces. Most have AFUE ratings between 81% and 87%. A few have ratings above 90% AFUE.
How much of the heat created makes it into your home? Many assume that it all does, but furnaces and boilers that burn fuel are not 100% efficient.
Measuring AFUE: How a Furnace Works – Boilers Too
Most furnaces burn natural gas (NG) or propane, i.e. liquid propane (LP). Oil furnaces burn fuel oil.
Fuel combustion: A line brings the fuel to the furnace or boiler from a pipe or tank. The fuel is under pressure. A valve in the equipment opens when the igniter gets hot, and fuel is released into the burner where it ignites and creates heat.
Heat transfer: The heat rises and travels through one heat exchanger in standard-efficiency furnaces and boilers and through two heat exchangers in high-efficiency unit. Only a few oil furnaces have a secondary heat exchanger, and those super-efficient oil furnaces are very expensive.
Air circulation in a furnace: The blower fan pulls cool air over the heat exchanger(s), and heat is transferred into the air and pushed through the ductwork in your home.
Exhaust: However, not all the heat produced by a furnace or boiler transfers through the heat exchangers. Some of it is lost out the vent along with the dangerous combustion gases that contain carbon monoxide.
Furnace AFUE: This represents the percentage of heat that is transferred into the ducts.
Boiler AFUE: In a boiler, most of the heat is passed into water circulating through the equipment. Some heat is lost out the exhaust vent. The hot water carries the heat through pipes to radiators where the heat naturally radiates into the air in your home. Radiant floor systems use tubing beneath the floor to radiate the heat.
Heat Input vs Output and Measuring AFUE
These are the terms the furnace and boiler manufacturers use to discuss how much heat the equipment makes and how much heat is transferred into your home’s ducting or radiant heat water piping. The terms are vitally important to getting the right furnace size for your home – or boiler.
Furnaces and boilers have varying size gas valves and one or more sets of burners. Heat input is the amount of heat measured in BTUs that the equipment can produce.
Furnace Sizes: Residential gas furnaces start at about 40,000 BTU/h. The largest are about 140,000 BTU/h. The term “BTU/h” stands for British Thermal Units per hour of heat created.
Boiler Sizes: Residential boilers range from about 50,000 BTU to 200,000 BTU. Most are in the 60,000 to 150,000 BTU. In boiler terminology, that’s 60 MBH to 150 MBH, which stands for “thousands of BTUs per hour.”
The larger boilers heat water as well, called “domestic hot water,” since it is used for washing, laundry, etc. The water that flows from the faucet doesn’t come directly from the boiler. Rather, a pipe circulating hot water passes through the hot water tank to heat it.
This is the amount of BTUs that get transferred into the air (furnaces) or water (boilers).
AFUE – How it is Measured
The annual fuel utilization efficiency is determined by dividing the heat output by heat input. A chart follows, but here’s a simple example:
80,000 BTU output divided by 100,000 BTU input = .80 or 80% AFUE. Gas furnaces start at 80% AFUE and are very common.
What Size Furnace Do I Need?
That is best determined by a load calculation.
What is a load calculation?
The purpose of a load calculation is to determine how much heat your home needs in the coldest weather. Dozens of factor are considered including your home’s size, layout and your local weather. It also factors the amount of insulation your home has and its type of windows, doors, roofing, etc.
A physical test can be done to determine how airtight your home is. That’s a huge factor in how well it keeps in heat in winter and prevents the infiltration of warm air in summer.
The factors are gathered and used to determine the BTUs needed – in other words, what furnace size is right for your home.
Equipment Size, Efficiency and Heat Output
Remember, it’s the amount of heat the equipment transfers into your home that matters, not how much heat it creates.
This chart shows standard furnace sizes along the left column. The top row is common furnace efficiencies.
Gas furnaces are made with 80% efficiency and then it jumps to 90% and higher. The reason for the jump is explained next.
First, here is the chart.
You have furnace choices based on how many BTUs of heat your home needs – Choices with one caveat: 80% AFUE furnaces are not sold in the north. This map shows minimum efficiency requirements for HVAC equipment.
I need a 60,000 BTU furnace
When homeowners say this, they, of course, mean they need a furnace with output of 60,000 BTUs (60K BTUs) or very close to that number. The same idea holds for the 100K and 120K BTU examples below. If your load calculation shows you need 60,000 BTUs per hour maximum output, then here are your furnace options. Not all are exactly 60,000 BTUs, but they all would be acceptable. It’s better to go slightly too large than too small – though going way too large is a bad idea.
As you can see from the chart, your best options that get you close to 60K BTU or slightly over are:
- 80,000 BTU furnace with 80% AFUE (South only)
- 70,000 BTU furnace with 90% AFUE (better a little large than a little small)
- 70,000 BTU furnace with 92% AFUE (might be a little too large)
- 60,000 BTU furnace with 97% AFUE (probably large enough)
- 60,000 BTU furnace with 98.5 AFUE (probably a better choice than the 97% furnace)
I need a 100,000 BTU furnace
- 130,000 BTU furnace with 80% AFUE (South only)
- 110,00 BTU furnace with 90% AFUE (almost “perfect”)
- 110,000 BTU furnace with 92% AFUE (the best fit)
- 100,000 BTU furnace with 98.5& AFUE (slightly undersized, but probably fine)
You’d probably be OK with a 100,000 BTU furnace with 97% AFUE, but it might not quite keep up when weather is near or below 0 degrees F.
I need a 120,000 BTU furnace
- 140,000 BTU furnace with 90% AFUE (a little too big, but OK)
- 130,000 BTU furnace with 92% AFUE (almost perfect)
- 130,000 BTU furnace with 95% AFUE (might be a little large, but you’d get about 3% savings on energy)
- 120,000 BTU furnace with 98.5% AFUE (just about right, and super-efficient)
Our Gas Furnace Reviews and Prices Guide has complete information on Sizing a Gas Furnace and comprehensive details that will assist you in your furnace buying decision.
Why do gas furnaces jump from 80% to 90% AFUE?
Because when efficiency rises above 80%, condensation forms in the flue gases as they cool – these are gases that are exhausted. That’s right – water is a byproduct of combustion. But it’s not just the water that is a problem. Nitrogen and carbon dioxide are also part of the exhaust – and those elements combine with water to form nitric acid and carbonic acid.
Both acids are highly corrosive and damage any flue or vent pipe made of any affordable metal. And the gases are too hot in the 80% to mid-80% range for PVC. The plastic vent would melt. Remember that an 85% furnace would have flue gases containing 15% of the heat made, and they’d be extremely hot.
The acids would also destroy the steel heat exchanger at the core of the furnace.
80% AFUE furnaces: Therefore, manufacturers make 80% furnaces that can be manufactured with a steel heat exchanger and be vented through galvanized steel without corroding the steel.
90% AFUE and higher furnaces: These furnaces have a regular steel primary heat exchanger because no condensation occurs there. They use a secondary heat exchanger to transfer more heat out of the furnace and into your ductwork. As heat is removed from the exhaust gases in the secondary heat exchanger, the gases cool and condensation occurs. Therefore, secondary heat exchangers must be made from non-corroding metal, most commonly stainless steel.
OK, that was a bit of a digression into how manufacturers boost annual fuel utilization efficiency, or AFUE, and what they have to do to keep the furnaces from being eaten up with corrosion.
But it is important for homeowners to understand when a furnace salesperson explains which furnaces are and are not a good option for their home and why.
Should I Buy the Highest AFUE Furnace?
No. Maybe a better answer is “not necessarily.”
Why is that true? Because as the AFUE, or furnace efficiency, increases, furnace cost goes up. There are two reasons for this.
First, furnaces that are 90% efficient and higher have a secondary heat exchanger. More parts/equipment means higher cost.
Secondly, the most efficient furnaces are two-stage and variable capacity furnaces. This means they heat in stages, either two stages (low/high) or modulating stages between about 40% and 100% of capacity. Staged heating, especially variable capacity heating, requires equipment that is more expensive than single-stage furnaces.
This is another topic discussed at length in our Gas Furnace Reviews and Guide.
What AFUE is Right for You – Tips for Buying a New Furnace
Here is our pro advice for buying a furnace or boiler. It starts with climate, one of the most important factors in determining boiler or furnace efficiency.
#1 Climate Matters when Buying a Furnace or Boiler
If you live in a warm climate, paying more for a 90% furnace, especially if it is a 2-stage or variable capacity furnace, won’t pay for itself for many years. Do the math.
80% AFUE furnaces in warm climates: This is an ideal choice. The furnace will cost less. You’ll have single-stage and two-stage models to consider. A two-stage furnace costs a little more for the gas valve, but you’ll get heating that is a little more comfortable and balanced.
80% AFUE furnaces in cold climates: They’re not available, as noted. But let’s say you could save $1,500 on an 80% AFUE furnace vs. a 98.5% furnace. Your annual heating bills would be about 18% higher. If you’d pay $800 per year with the 98.5% furnace, you’d pay $944 with an 80% furnace, or $144 more per year. Your $1,500 savings would be gone in about 10.5 years. But then again, if you’d invested that $1,500, you might be far ahead.
So you can see, the government didn’t ban 80% furnaces in the north to save you money but as part of its green energy directives that some feel limit freedom.
High-efficiency furnaces in warm climates: Paying $1,500 more for a 98% variable capacity furnace when your annual heating bill is $150 = 10 years to pay you back (plus the lost opportunity cost of the money you could have invested or had in interest-bearing savings). It’s probably not worth it.
High-efficiency furnaces in cold climates: Paying $1,500 more for a 98% variable capacity furnace when your annual heating bill is $700 = Just over 2 years to pay you back. Your savings in the third year and beyond will keep a lot of money in your pocket over the 15-20 years the furnace will last.
#2 How Long Will You Live in Your House?
If your furnace has to be replaced now – it just can’t be repaired anymore – what should you replace it with?
Moving Soon: If you plan to move in the next five years, then the cost-effective choice is to buy a less efficient single-stage furnace, with one exception. That exception is this: If you live in a very cold climate – The Northern Plains, Upper Midwest, New England, then buying a furnace above 90% efficiency might help you sell your home.
You have many options in this range. All the major manufacturers including Trane, Carrier and Lennox make quality furnaces with efficiency ratings at or above 95% in single-stage, 2-stage and variable capacity options.
Staying Put: If you plan to live where you are long-term, then it makes sense to spend more on efficiency. You’ll be paid back for the extra you spend, and if you choose a furnace with stages of heating, you’ll enjoy improved indoor climate control in your home.
#3 Types of Fuel Available for Furnaces and Boilers
In most urban and suburban areas, natural gas is the most common fuel. In rural areas, propane is popular, but heating costs are higher. In the Northeast, heating oil is used in more than 25% of homes, as the graph shows.
Where you have options, it makes sense to talk with several HVAC companies in your area about which heating source and fuel is best for your home. Generally, natural gas heat is the cheapest form. Where both heating oil and propane are available, but natural gas isn’t, prices vary, so do your due diligence to determine which fuel and furnace/boiler type will yield the lowest operating costs.
Wondering What Other HVAC Acronyms Mean?
We’ve done guides on these heating and air conditioning acronyms too: