Furnace Types and Efficiency: What AFUE Means

What “furnace” means, and what it is not

In the federal regulations, a furnace is a product designed to be the principal heating source for the living space of a residence. The definition covers electric central furnaces, forced-air central furnaces and gravity central furnaces, and it covers boilers too. A forced-air central furnace burns gas or oil, and a fan or blower moves the heated air through the ducts. An electric central furnace makes heat with electric resistance heating elements instead. A gravity central furnace depends mainly on natural convection, the rising of warm air, to circulate the heated air.

A boiler is the kind of furnace that supplies low pressure steam or hot water for space heating, not heated air. A heat pump is not on that list of furnace kinds. Heat pump vs furnace covers how the two compare. How the blower, ducts and registers work together is covered in forced-air heating.

Furnace types by fuel, and what Florida homes use

Gas furnaces burn natural gas or propane, and oil furnaces burn home heating oil. Our reading: a street without a gas main cannot have a natural gas furnace, so ask the gas utility about your address first.

How electric furnaces compare with gas for running cost depends on local fuel prices, and gas furnace vs electric strip heat covers that choice.

Central heating in Florida gives the share of Florida homes by main heating fuel, from the 2020 federal survey. In that survey, EIA's Florida sample had no household that used fuel oil or kerosene as its main heating fuel. An EIA analysis of the 2015 survey says natural gas furnaces are the most common main heating equipment in every climate region except the hot-humid Southeast. Our reading: in a hot-humid state like Florida, electric heat, from a heat pump or an electric furnace, is more likely than a gas furnace.

Furnace types by stages and installation

Furnaces also differ in how they fire. A single-stage furnace has one firing rate. A two-stage furnace has a low and a high rate. A modulating furnace can change its rate in small steps. In its 2023 rule, DOE's analysis of manufacturers treats AFUE and the combustion system, meaning single-stage, two-stage or modulating, as separate factors. Our reading: the AFUE number and the number of stages are two separate things, so ask for both.

ENERGY STAR describes an upflow furnace as one that discharges air upward, with the blower below the heating element. A downflow furnace discharges air downward, with the blower above the heating element. It describes a horizontal furnace as one designed for low headroom, with air moving across the heating element in a horizontal path. ENERGY STAR also requires product literature to state the orientations to which its certification applies. A weatherized furnace is a different kind: it is designed for installation outdoors and comes with its own venting system. ENERGY STAR does not certify weatherized furnaces.

How furnace efficiency is rated: AFUE

DOE's Federal Energy Management Program (FEMP) defines AFUE as the ratio of useful energy output to energy input, as a percentage, over the entire heating season. As an illustration, an AFUE of 90% means that 90% of the fuel's energy becomes useful heat and the other 10% is lost.

ENERGY STAR's specification defines AFUE as the ratio of annual output energy to annual input energy. For gas and oil furnaces, it says the ratio does not include electric energy. Our reading: the electricity that runs the blower is not counted in the AFUE of a gas or oil furnace. FEMP says installation problems such as oversizing, poorly designed distribution systems and leaky ducts result in efficiency losses. Duct leaks are covered in ductwork in Florida. Our reading: AFUE rates the furnace by itself, so it cannot tell you what the whole heating system delivers to the rooms.

What “high efficiency” means, and what a condensing furnace is

FEMP says every model that meets ENERGY STAR's furnace specifications is a condensing furnace. It says this technology takes advantage of heat that normally leaves in the furnace's flue gas. Flue gases are the exhaust from burning the fuel, and condensing means the water vapor in that exhaust is cooled until it turns to liquid. A Department of Energy Building America page says high efficiency gas furnaces use sealed combustion and a condensing process with a secondary heat exchanger. In its 2023 rule, DOE recalls an earlier analysis that paired a non-condensing 80-percent AFUE level with condensing levels of 90 to 98 percent AFUE. Our reading: a furnace rated at 90% AFUE or more is a condensing furnace, and a furnace at the 80% level is not.

FEMP says furnaces with sealed combustion have supply lines that bring outdoor air directly to the combustion chambers. It says the supply lines, combustion chambers and flues are sealed from the inside of the home, which keeps exhaust gases from leaking or being back-drafted into occupied spaces. Back-drafting means exhaust flowing back into the home instead of out through the flue. DOE's 2023 rule lists the condensate removal parts that can be involved: a drain, a pump, a drain pan and a neutralizer. Condensate is the liquid that forms from the exhaust, and a neutralizer is a device that treats it before it drains.

The federal minimum and the ENERGY STAR level

Under the federal regulations, a non-weatherized gas furnace manufactured on or after November 19, 2015 and before December 18, 2028 must have an AFUE of at least 80%. The same regulation sets 95% for non-weatherized gas furnaces manufactured on and after December 18, 2028. The Department of Energy calls this a final rule. Its page also says planned dates for future regulatory milestones are preliminary and subject to change. Our reading: confirm the current status of the 2028 level with the Department of Energy before you rely on it.

ENERGY STAR's key criteria page lists 95% AFUE or greater for gas furnaces in its South region and 97% in its North region. Florida is on the list of South states. ENERGY STAR defines its South as states with population-weighted heating degree days under 5,000. A heating degree day, in ENERGY STAR's older specification, is found by subtracting the day's average temperature from a balance temperature of 65°F. Only positive results are added up over the year.

EPA's cover letter for the Version 5.0 specification says furnaces manufactured as of July 31, 2026 must meet Version 5.0 to bear the ENERGY STAR mark. The earlier Version 4.1 specification, effective February 1, 2013, set 90% for gas furnaces in the South and 95% in the North. Our reading: a furnace made under the older ENERGY STAR specification may carry a lower AFUE than a newer one, so read the rating of the exact model.

How much efficiency does a short Tampa Bay heating season reward?

The National Weather Service's Tampa climate summary of 1991 to 2020 normals lists 434 heating degree days a year against 3,928 cooling degree days. Cooling degree days count the degrees above the same 65°F balance temperature. So the two figures compare the year's heating need with its cooling need. The same summary lists a normal of 0.8 days a year with a low at or below 32°F. Our reading: ENERGY STAR's North starts at 5,000 degree days for a state, a different measure from one station's normal, but the gap is large.

FEMP says ENERGY STAR asks for more efficient furnaces in the northern region because of its colder climate and greater number of heating degree days. It says the warmer southern region, where space heating requirements are less, has a special ENERGY STAR logo for furnaces at the lower efficiency level. Our reading: how much a higher AFUE matters depends on how many hours the furnace runs, and in much of Tampa Bay that is few. The fuel's local price and the condition of the ducts matter too, and neither is in the AFUE number.

Mobile and manufactured home furnaces

The federal regulations define a mobile home furnace as a direct vent furnace designed for use only in mobile homes. A direct vent system supplies outdoor air, or air from an unheated space, straight to the furnace for combustion. The Department of Energy's mobile home training material says these furnaces are direct-vented, sealed combustion units that need an outdoor air source for combustion. It says mobile homes require furnaces designed and approved for use in mobile homes.

HUD's construction standard for manufactured homes requires heat-producing appliances to be listed or certified for residential use by a nationally recognized testing agency. It also says an appliance's installation must conform to the terms of its listing and the maker's instructions. Each fuel-burning appliance must carry a permanent marking naming the fuel or fuels for which it is listed. For mobile home gas furnaces, the federal minimums match other gas furnaces: 80% now, and 95% for those manufactured on and after December 18, 2028. Our reading: for a mobile or manufactured home, ask to see the listing mark on the quoted furnace and the fuel it is listed for.

Safety: furnaces that burn fuel are professional work

EPA names leaking furnaces among the sources of carbon monoxide, a colorless, odorless and toxic gas. The gas-smell rule and the ignition sequence are covered in furnace won't ignite. Installing, converting, venting or adjusting a furnace is work for a licensed contractor, and the page gives no steps for it. Permits for this kind of work are covered in HVAC permits in Florida.

What to ask before a furnace is chosen

These questions come from the documents above, and none of them calls for you to touch the furnace.

1. Which fuel is this furnace listed for, and is there a gas main or a propane tank at my address?
2. What is the AFUE of this exact model, and is it a condensing furnace? ENERGY STAR says a model's certified rating must be used in all of the maker's literature.
3. If it condenses, where will the condensate drain, and how will the exhaust be vented? ENERGY STAR itself names system venting and condensate removal as contractor training topics.
4. Is it single-stage, two-stage or modulating?
5. Which orientation is the rating for, and will the furnace be installed that way?
6. If this is a mobile or manufactured home, is the model listed for that use, and can I see the mark?

The Florida Department of Business and Professional Regulation says to verify a contractor's license with the license number, the contractor's name or the business name. For repair-or-replace questions, see furnace replacement in Florida. For the wider heating choice, see central heating in Florida.