Single Stage vs Two Stage vs Variable Capacity Furnaces (Complete Analysis)

Single Stage vs Two Stage vs Variable Capacity Furnaces

If you live in a place where you experience steep and freezing-cold winters, a furnace is an absolute necessity. Thanks to their easy and quick heating process, furnaces can be great companions to help you out of the teeth-shattering hair-raising winter.

If you’re a new furnace buyer or a shopper of any kind, really, it can be daunting to get into the market and navigate between the countless types, models, sizes, features, etc.

But it’s best to start from the basics, and the basics start by enlisting the different kinds of furnaces, including those that vary in their capacity.

In fact, if you’ve already done some research, you might have come across the distinct single-stage, two-stage, and variable capacity furnaces.

Determining which one you need of the three is often the most important challenge most homeowners have to face. Before we delve into the feature and quality specific details, here’s an answer to get you started:

A single-stage furnace is one that constantly runs to its fullest capacity, i.e, at 100%, while a two-stage furnace is one that runs at 70% capacity while having the potential of running at 100%.

Variable Capacity or modulating furnaces have the ability to operate at a range of 40 to 100% capacity. 

These differences can largely amount to how comfortable you feel in your home, and whether your furnace is able to create the right conditions for you taking into account the surroundings. However, differences can also be found in terms of cost and efficiency rates among the different models.

Quick Comparison Single vs Two vs Variable Capacity Furnace

Single-stage furnaces, for instance, are known for having a low price along with cheap repair costs. The efficiency guaranteed by such furnaces is very less, however, which makes them a little less appealing.

Another feature of single-stage furnaces that don’t give them much credit is their loud and constantly running blower motors. These motors blow at full speed throughout the duration in which the unit is switched on, and can cause quite a bit of disturbance.

On the other hand, two-staged furnaces have moderate prices for both parts and replacement, and their efficiency usually varies from low valued efficient units to those that exhibit high efficiency and cost-saving tendencies. 

Efficiencies are determined on the basis of both how frequently the model is used, as well as the stage at which it is used, i.e, 65%, 75%, or even 100%.

Unlike single-stage units, these furnaces run at a slow but consistent speed so as to ensure that the heating cycle is lengthier and there are fewer temperature swings.

Then come the variable capacity or modulating furnaces, which can be termed as the most expensive models amongst them all.

A variable capacity furnace owner will have to indulge in high repair and replacement costs, but he or she will also be able to experience startlingly low return costs thanks to the high efficiency of these units. Their high efficiency owes itself in part to their operation.

These models are known for being able to manage indoor-outdoor temperatures and regulate them in a quiet manner. In terms of lifespan and durability, however, Variable-capacity furnaces are known to be a little weak in the area, while single-stage and two-stage models come with strong and lasting builds.

Single-stage vs Two-stage vs Variable Capacity furnaces: An Elaborate Differentiation

Now that you understand the basic distinctive aspects of the three furnace types, it is time to delve deeper and take a look at other factors pertaining to their performance.

Hopefully, this will help give you an insight into what kind of furnace would be the most suitable in meeting your personal, home, and lifestyle needs.

The Number of Heating Stages

Heating stages are the main distinguishing qualities between furnaces- so much so, in fact, that it is in the name itself. Yep, if you haven’t figured it out already, single-stage or two-stage furnaces are named that way with reference to the number of heating stages that a model has.

The heating stages are under the control of the gas valve, which predominantly varies between the three models. The difference, then, comes down to the fact that:

Single Stage Gas furnaces have gas valves that are capable of opening up to a full 100% capacity. What this implies is that when a furnace has a designated capacity of, say, 12,000 BTU- the gas valve will open up fully and push 12,000 BTUs out of the unit constantly throughout the course of its running.

Similarly, two-stage gas furnaces have gas valves that open at two different stages or two different capacities. These capacities could be 100% or 70% capacity.

Depending on the mode of choosing, the gas valve could let out a full 10,000 BTU worth of heating in a 10,000 BTU furnace (at 100% capacity), or it could open up enough to heat at about 7,000 BTU for the same furnace (at 70% capacity).

Unlike single-stage or two-stage gas furnaces, Variable-capacity furnaces have gas valves that allow them to operate at any heating level between 40% and 100% capacity of the furnace. In a 10,000 BTU furnace, thus, the heating stage could vary at any number between 4,000 and 10,000 BTUs depending on the need of the hour. This is why these are also called modulating furnaces.

Temperature Balance

When it comes to gas furnaces, whether they are able to create a good and stable temperature in the given surroundings is the big question. Good temperature balance is the key to good heating.

Single-stage furnaces are particularly disadvantageous in the way that their temperature swings have a wide berth of difference. Temperatures can differ frequently up to 4 degrees apart, and this can be quite taxing when all a homeowner wants is to be comfortable.

Two-stage furnaces are more apt in this area, as they make it a point to operate consistently at the lower heating stage as opposed to the full capacity. This significantly reduces temperature swings and spaces them only 1-2 degrees apart.

Variable-capacity furnaces are great when it comes to reducing temperature swings, as they run at the lowest capacity needed to maintain warmth.

Air filtration

In terms of air filtration, two-stage and variable-capacity furnaces have shown more promise.

This is because of their long cycles, which ensure that air is kept moving constantly through the ducts and filters, allowing more circulation and cleansing of the air. Things like dust, debris, pet dander, etc are successfully removed.

Single-stage furnaces, which choose to work at full capacity don’t have as many cycles and can be negligent in filtering the air appropriately.


The next state of distinction which inarguably affects most customers today is the cost. The cost of a product is usually a total estimate of the numbers it draws from your wallet throughout its lifetime, from the moment it is bought. 

Therefore, costs are divided into 

Initial costs- referring to the price of buying the furnace unit and installing it; 

Operating cost- the cost incurred in the duration of running the product. These are also called return costs and have a lot to do with the efficiency of the product. The higher the efficiency, the lesser operating costs on things like electricity, etc; 

Repair costs, which include all the money it would take to repair or replace a certain component when it malfunctions or breaks.

Single-stage furnaces have very low initial costs compared to the other models. This is because of the manufacturing, as this type of furnace has a simple and structured build without many intricacies.

Additionally, its single heat exchanger does not increase the number on the price tag by a large amount. These models have exhibited low rates even in terms of repair costs, as their technology is quite sturdy yet easy to examine and fix.

The repair costs for single-stage furnaces, thus, are also less expensive. However, these furnaces really take the hit when it comes to operating costs. As you know, single-stage units have very low efficiency. Naturally, their return and operating costs are higher.

In terms of initial costs, two-stage gas furnaces are more pricey than single-stage, especially so when highly efficient models are involved, as this requires them to have an extra heat exchanger.

Repairs are also easy and quite cheap to manage as they have been proven to have strong frames. Most two-stage furnaces are energy efficient, so you can expect their operating costs to be quite low, although this varies from model to model.

Variable Capacity furnaces are said to be integrated with high and complex technology, and so you can expect them to be on the expensive side of things.

Owing to such build, their repair costs also tend to be higher in case of any faulty functional parts. Due to their high rate of efficiency, however, one can expect that these models are more generous when it comes to returning costs on electricity.

An overall comparison: Which furnace is the best?

When you’re trying to answer questions where you expect one product to reign supremely over the others, you must understand that the answer depends solely on your preferences and needs.

Single-stage furnaces are generally compatible with similarly small, single-level homes where temperature variance is very less. Thus, there is a lesser need for modulating furnaces.

Two-stage and variable capacity furnaces, on the other hand, can be very useful in houses that span over a large area or have multiple floors, as temperature changes may be drastic in different parts of the house. These furnaces will be able to help modulate and balance the temperatures.

Variable capacity and two-stage furnaces are also great options for all-year-round heating- especially during the fall or spring months. Single-stage furnaces generally give out full-blown heating at all times and are likely to only be helpful in the coldest of months.

Single-stage furnaces are also not the most favorable in terms of energy saving. They absorb more power for their start-ups and shut-downs compared to two-stage or modulating furnaces, which only switch speeds to adjust to the surroundings. Notable, single-stage furnaces are also more prone to damage.

Two-stage or modulating furnaces don’t operate at full capacity all the time, and so exert less energy and lower the rate of wear-and-tear caused to the unit. Modulating furnaces are great for ensuring that heat is distributed as evenly as possible in large homes with multiple floors.


All three furnace types have their own advantages and disadvantages over the others. In modern-day homes, dual-stage or variable-capacity furnaces are considered to have more benefits than single-stage units.

Still, aspects like climate and the home design can make it so that a single-stage unit is the most suitable for you. A careful evaluation of all these factors will lead you to the right choice.

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