Furnace type
Two-stage furnaces
A two-stage furnace has a low gas rate and a high gas rate. It runs low fire most of the time and steps up only when the house needs full output, which means longer, quieter, steadier cycles. It is the most common choice for detached homes at the mid price tier.
Short answer
Two-stage means two gas rates, typically about 65% and 100% of input. The control chooses low fire when it can carry the load and steps to high fire when needed (on Carrier's Comfort Heat controls, adaptively from past cycle history). The result is longer runs at lower output on mild days: steadier temperatures, quieter operation and less start-stop wear. AFUE is the same 95%+ as single-stage; the benefit is comfort, not efficiency.
How the stages work
On a call for heat the control starts in low fire or decides the stage from history and demand (Carrier's two-stage furnaces, for example, use Comfort Heat logic to choose low or high fire and how long to run each). Most heating-season hours are spent in low fire, which is where the comfort comes from.
Thermostats and staging
Two-stage furnaces can run with a single-stage thermostat using the control's adaptive logic, or with a two-stage thermostat that calls W2 for high fire. Some controls also switch to high fire when W2 is energized regardless of the adaptive setting. The wiring and thermostat type are part of the installation set-up, and this site's model pages list each furnace's actual terminals.
When two-stage earns its price
The premium over single-stage pays off where heat loss varies: large houses, open layouts, or homes where mild-weather heating dominates the season. For a tiny, steady load, the low stage may rarely be needed, and single-stage would do.
Related
- ServiceFurnace Purchase AdviceIndependent-minded furnace buying advice before you sign anything
- ServiceGas Furnace InstallationNew furnace installation sized from a heat-loss calculation, not from the old furnace
- ServiceGas Furnace ReplacementReplace an aging or failed furnace with the right high-efficiency furnace for your house
Questions people ask
Is a two-stage furnace more efficient than a single-stage?
Not by AFUE: both are certified 95%+. Two-stage furnaces can save a little gas by running longer at lower output with fewer heat-up losses, but the main benefits are comfort and quiet.
How do I know if my two-stage furnace is using low fire?
On many controls the display shows the current mode (for example low-heat or high-heat on the 3-digit displays documented on this site). A technician can also read the operating history.
Does a variable-speed blower mean two-stage?
No. Blower type and burner staging are separate. A furnace can have a variable-speed blower and a single-stage burner, or a two-stage burner with a fixed-speed blower. Check both fields on the product data.
Need this looked at?
Call BC Wide with the brand, model number and anything on the display. Or
Request service online with this page attached.