One firing rate, two, or many: what the stage count buys
A two-stage low fire is 70 percent of high fire on the Rheem R96V series, not a gentle trickle. What that means for comfort, and what the thermostat has to be.
What this means
A two-stage furnace has two firing rates, and on the Rheem R96V series the low one is 70 percent of the high one on every model. A modulating furnace changes its rate in steps across a wider span. A single-stage furnace has one rate and meets a mild day by running for less of it.
Equipment and model context
Product classes compared; figures belong to specific matched systems
- Rheem R96V two-stage variable speed gas furnaces
- Lennox SL280UHNV two-stage and SLP98UHV modulating furnaces
- Carrier 58MVC variable speed step-modulating condensing furnaces
Figures are taken from the published literature for the three ranges named and are not general properties of every furnace. Another manufacturer's two-stage furnace can have a different low fire fraction, and the thermostat behaviour described belongs to the ranges cited.
Criteria compared
- A quotation offering three furnaces at three prices with staging as the difference.
- A two-stage furnace that spends most of the winter on high fire.
- A modulating furnace behind a thermostat that only has one heat wire.
Decision factors
- Low fire is a fraction of high fire, not a separate small burner, so the turndown is limited.
- The thermostat class decides how the staging is reached, and a retrofit can leave it timed rather than measured.
How far a two-stage furnace actually turns down
Rheem publishes both firing rates for every R96V model. On the smallest model the two rates are 42,000 and 29,400 BTU per hour; on the largest they are 112,000 and 78,400. Divide one by the other and the ratio holds across the range: low fire is 70 percent of high.
That is a turndown of about 1.4 to 1. Set against the picture of a furnace idling gently through a mild afternoon, 70 percent is a lot of heat, and a well-insulated house on a 40 degree day can still be over-served by it.
The consequence is that low fire is not a substitute for correct sizing. An oversized two-stage furnace short cycles on low fire in the same way a correctly sized single-stage one runs steadily, and the staging buys less than the marketing implies.
What modulating adds, and what step-modulating means
Carrier documents the 58MVC as a variable speed step-modulating condensing furnace. Step-modulating is the useful word: the firing rate moves through a set of steps rather than continuously, so it sits between two-stage and true continuous modulation.
Lennox's SLP98UHV is documented with heat modes listed separately for single-stage, two-stage, and communicating thermostats. That the modes have to be listed three times is itself the finding: what a modulating furnace can do depends on what is telling it what the room wants.
The comfort argument for modulation is a longer, lower burn that holds supply air temperature steadier and runs the blower more of the time, which distributes air better through a house with rooms far from the plant. It is a real difference, and it is largest in the houses where staging was needed least.
The thermostat is part of the purchase
The R96V literature permits either class of wall control and names the two stage one as preferable. Lennox documents second-stage recognition periods for the SL280UHNV on a single-stage thermostat, so the control steps up on elapsed time rather than on a room signal.
So a staged furnace behind an old wall control still stages. What it loses is any relationship between when it steps up and how cold the room is, which is the thing the second stage was bought to provide.
For a replacement where the thermostat is being kept, that turns a staging upgrade into a partial one. The wall control is a small line on the quotation and a large fraction of what the furnace can deliver.
Where the money is better spent
Two things sit under the staging decision and change the outcome more. The first is sizing: a furnace matched to the calculated heat loss runs longer cycles at any stage count, and one that is not will cycle whatever staging it has.
The second is the duct system. Rheem publishes minimum external static pressure figures from 0.18 to 0.28 inches water column alongside a maximum of 1.0, and Lennox publishes 0.8 inches in heating and 1.0 in cooling for the SLP98UHV. A duct system outside those bounds constrains a modulating furnace and a single-stage one alike.
A correctly sized single-stage furnace on a duct system inside its published range will outperform an oversized modulating one on a restricted system, and it will cost less. Staging is the last decision in that list rather than the first.
| Arrangement | Firing rates | What the thermostat has to do |
|---|---|---|
| Single-stage | One | Call for heat |
| Two-stage | Two. 70 percent low on the Rheem R96V series | Call for the second stage, or leave it to a timer |
| Step-modulating | A set of steps, on the Carrier 58MVC | Supply demand information for the full range |
| Communicating | As the equipment allows | Exchange demand rather than close a contact |
| The decision | Why it comes first |
|---|---|
| Correct sizing to the heat loss | Cycling is a sizing problem before it is a staging one |
| Duct system inside the published static range | A restricted system limits every stage count |
| Thermostat class | Decides whether staging responds to the room |
| Two-stage over single-stage | A 1.4 to 1 turndown on the range quoted here |
| Modulating over two-stage | More steps, longer burns, steadier supply air |
| Communicating equipment | Only useful if the whole system is communicating |
Stage count is one of four inputs and the only one a sales conversation reaches. The other three are settled before the equipment is chosen, and they set the ceiling on what staging can achieve.
- House heat lossThe demand the furnace is trying to match
- Furnace input ratingHow far above that demand the equipment sits
- Lowest firing rateThe smallest output the furnace can hold
- Thermostat classWhat information reaches the control
Questions people ask about this
How much lower is low fire on a two-stage furnace?
On the Rheem R96V series, 70 percent of high fire on every model, so the smallest unit runs 42,000 BTU per hour against 29,400. That is a turndown of roughly 1.4 to 1 rather than the gentle trickle the description often suggests.
Is a modulating furnace worth the extra cost?
Its advantage is longer, lower burns and steadier supply air, and it is largest in a house that is correctly sized and has a duct system inside the manufacturer's published static pressure range. Where either of those is wrong, the extra steps have less to work with.
Do I need a two-stage thermostat for a two-stage furnace?
Not to make it work. Either class of wall control is permitted on the R96V, with the two stage one named as preferable, and Lennox documents timed second-stage recognition periods for the SL280UHNV on a single-stage control. What a single-stage thermostat cannot do is tell the furnace how far the room is from setpoint.
What does step-modulating mean?
Carrier uses the term for the 58MVC, describing a furnace that changes its firing rate through a series of steps rather than continuously. It sits between a two-stage furnace and a continuously modulating one.
Will a two-stage furnace stop my system short cycling?
Not on its own. If the furnace is oversized for the heat loss, low fire on a 70 percent turndown can still exceed what the house is asking for, and the cycle pattern returns. Sizing is what the cycling question is actually about.
Evidence recordHow this page was checked
service manual · checked 2026-09-07
Every technical claim above was written from primary documentation held in the HVAC Bench evidence record: Rheem, Lennox and Carrier technical literature. Where a source limits a definition to certain models, test conditions, or product classes, that limit is repeated here rather than generalised.
- Documentation class
- service manual
- Scope of the definition
- Confirm against the exact model manual
- Last checked
- 2026-09-07