Single-stage vs two-stage vs variable-speed furnace: what is the difference?

What each type does

Goodman describes a single-stage gas burner as one constant flow: it is either on or off. Bryant says a single-stage furnace runs at 100% capacity whenever it runs.

A two-stage furnace has a high and a low setting. Bryant and Carrier both say the low setting is typically about 60% to 65% of full capacity, and Bryant adds that the exact low-stage output varies by equipment.

Modulating furnaces adjust the burner in small increments. Carrier says its modulating furnaces adjust between 40% and 100% of capacity in 1% steps. Lennox says modulating units offer dozens of small output levels, where two-stage units are limited to two.

Variable-speed and modulating are not always the same thing

Goodman uses the terms for different parts. It calls variable-speed a blower motor feature, which adjusts airflow, and modulating a burner feature, which adjusts the flame. A furnace can have a variable-speed blower with a single-stage or two-stage burner, so ask a contractor exactly which parts a model has.

Carrier says a modulating furnace always pairs with a variable-speed blower. DOE's 2011 Building America HVAC guide for contractors notes that two-stage gas valves, two-speed draft fans and variable-speed blower fans are available on mid- and high-efficiency furnaces and can lower electricity use.

Comfort and noise

These are manufacturer claims, not independent test results:

• Carrier says single-stage on-off cycling can create noticeable temperature swings, and that two-stage gives more consistent heat by running mostly on the lower stage.
• Lennox says homeowners often notice indoor temperatures staying within a degree of the thermostat setting with modulation, and that modulating furnaces are quieter because they rarely run at full blast.
• Bryant says a furnace on a lower stage gives less abrupt airflow, but it does not publish a specific decibel reduction.
• Goodman's comparison guide does not address noise.

Efficiency and electricity use

Staging changes how the furnace runs. It does not change the AFUE label by itself, since AFUE is a separate rating (see 80% vs 96% furnaces). Bryant says a lower stage can reduce fuel used when full output is not needed, but it publishes no universal savings percentage. Carrier and Lennox give no figure on the pages we read.

The electricity side has DOE data. A DOE Building America article says variable-speed (brushless permanent magnet) blowers keep their advantage in multi-speed systems that run many hours in low-fire mode, but that the heating advantage roughly halves from test conditions to installed conditions. For cooling, it says the two motor technologies have essentially the same power consumption installed. Goodman quotes DOE as saying a variable-speed motor running continuously at half speed may use up to 75% less power; that figure applies to the blower motor, not the whole furnace.

Humidity and air conditioner staging

Humidity claims mostly concern cooling. Lennox says two-stage air conditioners run longer at lower capacity and so remove more humidity. Trane says two-stage systems remove moisture more efficiently than single-stage, with a low stage around 65% of capacity, and variable-speed systems run from 25% to 100%. Carrier says its variable-speed AC runs from 30% to 100% and claims up to 400% more moisture removal than a single-stage 14.3 SEER system in what it calls worst-case cool, damp conditions, based on Carrier's own testing.

On sizing, DOE's guide says many older furnaces and central air conditioners are oversized, and that variable-speed equipment can reduce the problems of oversizing because it adjusts output. That does not replace a proper load calculation. See what size furnace or AC you need and what SEER rating to get.

Trade-offs to weigh

• Single-stage: one speed, more on-off cycling, and a louder blower at full capacity, per Carrier.
• Two-stage: a low stage for mild days and a high stage for cold ones; Trane describes it as a middle ground.
• Variable-speed or modulating: the finest control by manufacturers' accounts. Carrier says its variable-speed AC has a higher installation cost and a poorly installed one will struggle to perform.
• Installation matters for all three. DOE's guide says poorly installed or damaged ducts can cancel out the savings of an efficient fan motor.
• For price differences, see new furnace and AC cost in Omaha.

Ask for quotes on more than one type

To compare options with one independent local contractor, get a free quote. We are a referral service and do not install equipment.

If your furnace has stopped working, start with furnace repair in Omaha.

Sources

• Furnace Features Comparison Guide (Goodman)
• What is a 2-Stage Furnace? (Bryant)
• Modulating vs. Variable-Speed Furnaces: Comparison Guide (Carrier)
• What Is a Modulating Furnace? (Lennox)
• Single Stage vs Two Stage vs Variable Speed Air Conditioner (Lennox)
• Compare Single-Stage, Two-Stage, and Variable-Speed AC Systems (Carrier)
• One-, Two-, Multi- and Variable Speed HVAC Systems Explained (Trane)
• HVAC: A Guide for Contractors to Share with Homeowners, Building America Best Practices Vol. 14, 2011 (U.S. DOE / PNNL)
• Furnace Blower Performance Improvements, Building America Top Innovation (U.S. DOE)