Single Stage Vs Two Stage Vs Variable Capacity Furnaces

A common question for homeowners planning a new furnace is what’s the difference between single-stage, two-stage and variable-capacity furnaces?

The differences are explained in detail below. Here is a summary.

How single-stage vs two-stage vs variable capacity furnaces differ:

• Single stage furnace features: Runs at 100% capacity all the time. Furnaces cost less, are less likely to break, repair costs are low, efficiency is low to average.
• Two stage furnace features: Run at 70% capacity most of the time, but can run at 100% too. Furnaces are moderately priced, less likely to break, have average repair costs, and efficiency is low to high.
• Variable capacity furnace features: Run at any capacity from 40% to 100%. Furnaces cost more, might be more prone to break, have average to high repair costs, and efficiency is very high.

Variable-capacity furnaces are also called modulating furnaces.

This table summarizes the differences between single stage, two stage and modulating furnaces.

*(1) Single-stage and two-stage furnace can have a “High” operation cost because 80% efficient models are available.

Heating Stages Explained

This is a quick summary of the three furnace types we’re discussing. How many stages a furnace has is determined by its gas valve type.

• Single-stage furnaces have a gas valve that opens at 100% capacity. If it is a 100,000 BTU per hour furnace, it is always burning at the 100,000 BTU rate.
• Two-stage gas furnace valves can open at either about 70% or 100%. That means that the 100,000 BTU furnace can operate at 70,000 BTUs per hour or at 100,000 BTUs per hour.
• Variable-capacity gas valves modulate between about 40% and 100% capacity. A modulating 100,000 BTU furnace can operate at any heating level between 40,000 and 100,000 BTUs.

What are the Advantages of Two-stage and Variable-capacity Heating?

Comfort and efficiency.

A More Comfortable Indoor Climate

The more stages a furnace runs on, the better temperature balance and air filtration you have.

Single-stage furnaces create temperature swings of up to 4 degrees. Most have single-speed blower motors, so they blow at full speed (and noise!) all the time.

Air that is not fully warmed is blown into your living spaces at the start and end of the cycle.

Two-stage furnaces run at the low stage most of the time, which is 65% or 70% depending on the furnace model. Cycles are longer and produce lesser temperature swings of one to two degrees.

Most have multi-speed or variable-speed blowers. They start blowing at a low speed until the furnace is fully hot, and then ramp up. Less cool air is pushed through your ducts.

Variable-capacity furnaces run at the lowest capacity necessary to maintain balanced temperatures in your home. They all have variable-speed blowers too, so you rarely feel cool air coming from the ducts or any change in temperature. They are the quietest furnaces too.

Air Filtration: The longer cycles produced by running at less-than full capacity means air gets moved through the filter more times during the day. This helps remove more dust, debris and allergens from the air.

If you have an air purifier in your furnace, the effect is even greater. If anyone in your household has allergies, asthma or other breathing issues, an air cleaner is a great idea. They are explained in our Guide to Air Purifiers.

Humidity Control in All Seasons: If you have a humidifier attached to your furnace, longer cycles will cause it to run more, and that will mean more comfortable humidity levels in winter.

In summer, if you have central air conditioning, a furnace with a variable-speed blower will remove more humidity. This is especially true if you have a two-stage or modulating air conditioner or heat pump. This is fully explained in our Single Stage vs Two Stage vs Variable Speed Heat Pump & AC Guide.

Efficiency: Lower Energy Use and Cost

This is closely related to comfort.

Furnace efficiency is measured by AFUE, annualized fuel utilization efficiency. It’s exactly like gas mileage – a measure of how much heat you get for the fuel burned.

It measures how much of the heat that is created gets transferred through one or two heat exchangers into the ductwork of your home.

Furnaces are either 80% AFUE or 90% and up. The reason is that units with a single heat exchanger achieve a top AFUE of 80%. An 80% AFUE furnace costs less and is a suitable choice for warm climates where the furnace isn’t used very much.

When a secondary heat exchanger is included, 90% or more heat gets transferred. These units are required in cool climates. They cost more, but the extra cost is quickly recouped in lower heating costs.

This is explained in full in our comprehensive Gas Furnace Buying Guide.

80% AFUE furnaces are available in single-stage and two-stage models only. 90% and higher furnaces are offered in all three types.

Single-stage furnaces waste energy when they run at full capacity, stop, run at full capacity and so on. They heat past the thermostat set point, and that hurts both indoor comfort and efficiency. It would be like driving your car at 100mph, then stopping. Starting again at 100mph. Fuel economy would suffer.

Most 90%-plus single-stage furnaces are 90% to 95% efficient. A few are higher. The Carrier Performance Boost 90 furnace offers an outstanding 96.5% AFUE.

A two-stage furnace runs at 70% capacity most of the time it is heating. This gives you better efficiency. The heat cycle is longer, but more consistent.

The most efficient two-stage furnaces are 95% to 97% efficient. The current most efficient two-stage furnace is the Lennox SL297NV with 97.5% AFUE.

A modulating gas valve is designed for longer cycles at lower capacity. It delivers the best efficiency because the furnace never makes more heat than needed to keep your house very close to the thermostat set point.

Variable-capacity furnaces range from 95% to more than 98% efficient. The current most efficient model is the Lennox SLP98V gas furnace with 98.7% AFUE, but several others are close.

The efficiency gap between single-stage, two-stage and variable-capacity furnaces has gotten much smaller in the last decade. You can now have very high efficiency with any type. The buying decision often comes down to furnace cost vs. the level of climate control you desire.

Gas Furnace Costs

There are three ways to look at cost, and all are important. They give the “big picture” of how much a furnace will cost you over its lifetime.

• Initial price: The cost of buying the gas furnace and have it installed
• Operating cost: The cost of running the furnace – it’s efficiency
• Repair costs: The cost of repairing it and the likelihood it will break

Initial cost: Single-stage furnaces cost the least because they have a single heat exchanger and overall design that is less complex. Two-stage 80% furnaces are slightly more expensive. Those with 90%-plus efficiency cost more due to a secondary heat exchanger. Variable-capacity furnaces are the most complex, so most expensive.

The table above gives cost ranges for the furnace only.

Installation cost ranges from about $600 to more than $1,800 based on factors specific to each job.

Operating cost: As noted, 80% furnaces aren’t very efficient, but their lower cost makes them a financially sound investment in warm climates.

When it comes to high-efficiency furnaces, the gap is only about 8% between them. The most efficient air conditioners are 100% more efficient than the least efficient!

Repair cost: Single-stage and two-stage technology is well established and durable, if not perfect. And when those furnaces require repair to functional parts like the blower motor or gas valve, repairs are less expensive.

Variable-capacity furnaces are newer, so their durability hasn’t been established. One thing is known: When their functional parts need repair, costs are significantly higher.

For this reason, some installers warn customers to stay away from modulating furnaces for now. A two-stage furnace offers nearly the same climate control and efficiency with lower initial cost and potential repair costs.

Which Furnace Type is Right for You?

The best approach is to choose an efficiency level that is right for your climate.

In warm climates, your best options are single-stage and two-stage 80% AFUE furnaces and single-stage 90%-93% furnaces. If you spend more on a more efficient furnace, it might be 10-20 years before you recoup the extra cost with lower energy bills.

In cool and cold climates, high-efficiency gas furnaces pay for themselves in less than 10 years – less than five years in some cases. You just have to decide whether it is worth it to pay more for the added climate control of a two-stage or modulating furnace.

There’s a section in our Gas Furnace Guide on selecting a furnace for your climate. Be sure to scroll up the page to see our Climate Map that marks the US climate zones.

Got Questions?

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Written by Rene Langer

Rene has worked 10 years in the HVAC field and now is the Senior Comfort Specialist for PICKHVAC. He holds an HVAC associate degree and EPA & R-410A Certifications.