Furnace System GuideA two-stage furnace runs its burner at two output levels instead of one. It spends most of its time on a lower setting for steadier heat, then switches to full output on the coldest days. This page explains how two-stage operation works, what efficiency these furnaces reach, and how the major makers frame the design.
How Two-Stage Operation Works
A two-stage furnace has a gas valve that fires at high or low. Carrier, a furnace manufacturer, states that its two-stage models adjust between high and low heat settings based on a home's needs. Running mostly on low produces longer, gentler heating cycles than a single-output burner.
Manufacturers usually pair two-stage burners with better blowers. York, a furnace manufacturer, describes a model as a 97 percent AFUE Two Stage Variable Speed furnace, and states that its gas furnaces feature two-stage operation, variable speed technology, and smart controls to achieve a precise temperature.
The combination of a stepped burner and a variable-speed blower is what lets a two-stage furnace hold temperature more evenly than a single-stage unit.
The Efficiency Two-Stage Furnaces Reach
Two-stage furnaces are commonly sold in the high-efficiency range. Payne, a furnace manufacturer, lists its PG97VTA model with an up to 97.0 percent AFUE rating, and states that a variable 25-speed blower motor and a two-stage gas valve provide enhanced levels of even temperature control.
Rheem, a furnace manufacturer, also builds two-stage models such as its R972V series alongside its single-stage and modulating furnaces. That places two-stage designs in the middle of most lineups, above single-stage and below modulating flagships.
Because a two-stage burner runs longer on low, the design can improve comfort even at a given AFUE, since AFUE measures fuel-to-heat efficiency rather than how evenly the furnace holds temperature.
Where Two-Stage Sits in a Lineup
Two-stage furnaces occupy the middle tier of most brand lineups, between the base single-stage unit and the top modulating model. American Standard, a furnace manufacturer, lists two-stage options below its modulating Platinum flagship, and notes that a furnace with an AFUE of 90 or higher qualifies as high efficiency.
For a buyer, a two-stage furnace is a step up in comfort and steadiness over single-stage without the price of a fully modulating unit. It is a common middle-ground choice.
The right fit still depends on climate and home size. In a colder climate the high stage sees more use, while in a milder climate the furnace runs mostly on low.