2-Stage Furnace Disadvantages

You’ve likely come across the single-stage vs. two-stage if you’re shopping for a new furnace. To be specific, you’ve probably come across tons of articles, videos, and ads claiming that dual-stage furnaces are the best thing since sliced bread. But are they? Are 2-stage furnaces significantly better than single-stage models? Do you stand to save substantially? Are they vastly superior?

Unfortunately, the stats don’t say so. Recent research findings show that two-stage furnaces are only slightly better than their single-stage counterparts and may not be better at all in some circumstances. Read on to find out the common downsides of a two-stage furnace so you can make an informed decision.

What is a Two-Stage Furnace and How Does it Work?

A two-stage furnace operates at two levels. First, it can work at full throttle, i.e., 100%. However, it can also operate at a “LOW” setting, typically 70% to 80% of full capacity.

For example, if you purchase a 50,000 BTU dual-stage furnace, it may operate at 75% and 100%. This means it will give off the full 50,000 BTUs when running at full capacity and 37,500 BTUs when in “LOW” mode.

Single-Stage vs. Two-Stage Furnaces: What are the Differences?

The only notable difference between the two is the two-stage operation. Whereas single-stage furnaces are either off (at 0% heat) or ON at full throttle (100% heat), two-stage models have a third setting known as “LOW,” in which the furnace gives off 70% to 80% of usual heat.

All the other components are the same, including combustion mechanisms and heat transfer. However, efficiency levels and run time/frequency may vary.

Advantages of Two-Stage Furnaces

Two-stage furnaces are advantageous for many reasons. The most commonly cited advantages over single-stage models include;

  • Increased flexibility: Two-stage furnaces can run at full speed (100%) or below full capacity depending on the weather conditions 
  • Notable savings: You’re guaranteed at least a few dollars in energy savings at the end of the heating season.
  • Typically quieter: Finally, two-stage furnaces generally produce less noise than similar-sized single-stage models.

Disadvantages of Two-Stage Furnaces

Unfortunately, two-stage furnaces aren’t the “lifesavers” they’re meant to be. The following are three significant reasons you should think twice about buying a two-stage furnace.

  1. Don’t necessarily keep you more comfortable

Let’s begin by debunking the claim that two-stage furnaces keep you “more” comfortable than traditional single-stage furnaces. Recently, homeowners have been duped into purchasing two-stage furnaces on claims that two-stage units guarantee greater comfort.

Advertisers particularly love to remind homeowners that they can run their two-stage furnaces at 70-80 percent, thereby allowing the furnace to run longer, thus reducing temperature swings.

Unfortunately, it all sounds practical until you observe a two-stage furnace in action. Let’s consider an example to understand better why you may not necessarily feel “more” comfortable.” Assume that you have a 100,000 BTU furnace.

At full thrust, i.e., the furnace will likely raise the indoor temperature by about 50°F. So, for example, if the air entering the furnace at the return air plenum is 50°F, the air coming out of the supply plenum will be 120°F. Now, imagine that you have a two-stage furnace that “steps down” to 75% capacity once you reach the thermostat setting.

 Your 100,000 BTU furnace would put out 75,000 BTUs worth of heat during the “LOW” setting. Field tests show that at 75,000 BTUs/hour, you’d still raise indoor temperatures by 40-34 percent. Thus, if air at the return plenum is 50°F, the air entering your ductwork out of the supply plenum will be between 110°F and 113°F.So, the first question is whether the 7°F temperature drop would make you feel “more” comfortable. Unfortunately, research doesn’t support this claim. Various tests show that the difference isn’t discernible.

Next, does the temperature drop significantly reduce temperature swings? Unfortunately, field tests show otherwise. ASHRAE field tests conducted in 2006 show that two-stage furnaces only run 11% longer than similarly-rated single-stage models.

Essentially, this means that if your single-stage furnace cycles off every 10 minutes, a similarly-rated two-stage model will cycle off every 11 minutes – or a minute after the single-stage model cycles off.

It tells you everything you need to know – that two-stage furnaces aren’t significantly “more” comfortable than similar-sized single-stage models. The difference is minuscule and not worth the hype.

  • Don’t necessarily save energy

Another common misconception is that two-stage furnaces are more energy-efficient than similar-sized single-stage models. This, too, isn’t entirely true. On the contrary, two-stage units may be more expensive to run in some circumstances.

For evidence, let’s refer to the “Residential Two-Stage Gas Furnaces: Do They Save Energy” study published on the ACEEE website. The study analyzed the fuel and electricity consumption of different AFUE levels, controls, and motor types.

 In addition, it discusses findings from the DOE and ASHRAE. Generally, the findings show the same level of fuel consumption. For example, the Department of Energy (DOE) tests show fuel savings of about 3.4% for two-stage furnaces with BPM motors, while the ASHRAE tests show an even smaller reduction (0.4%) in fuel use for the same motors.

Meanwhile, the DOE tests show a 2.1% reduction in fuel usage for PSC motors, whereas ASHRAE tests show a 1.4% drop in fuel use for the same motors. Interestingly, the tests show that you may use up to 1.2% more fuel if using a two-stage BPM motor furnace compared to someone who uses a single-stage PSC motor.

The electricity consumption rates are just as interesting. The DOE tests show that two-stage furnaces consume 2% more electricity than comparable single-stage models. Meanwhile, the ASHRAE tests show an even bigger increase (11.4%) in electricity usage for two-stage furnaces.

Incidentally, the longer run times were the main reason for the increased fuel usage. The longer the furnace runs, the more electricity the fan draws to keep running. A few things have changed since then. Notably, the DOE introduced the national efficiency standard in 2014 and the FER (Fan Efficiency Rule) in 2019, which significantly impacted furnace fan efficiency.

For instance, FER requires that manufacturers only make furnaces with more efficient electronically commuted motors (ECM), leading to 46% more efficient motors than older models. However, we all know that many of the furnaces in use today were made before 2019. Indeed, the majority were made before 2014. Even among the new ones in stores across the country, a significant number were mostly built before 2019.

  • More expensive to purchase 

Finally, two-stage furnaces are also significantly more expensive than their single-stage counterparts. Although the exact costs vary from one store to the next, and depending on brands, you may need to fork out several hundred more dollars to acquire a two-stage furnace. According to compare it, a mid-efficiency single-stage furnace costs in the region of $2,000 to $3,000, including installation.

The prices range from $1,000 to $1,500 without installation, while labor costs $1,000 to $2,000, depending on several factors, including your geographic location and furnace model. Meanwhile, high-efficiency single-stage furnaces cost $4,000 or more, including installation. Again, installation typically costs about as much as the appliance itself.

Two-stage models are significantly more expensive. For instance, a mid-range two-stage furnace costs between $2,500 and $4,000, including installation. The furnace alone will set you back between $1,500 and $2,000. It means that both the appliance and installation charges are higher for two-stage furnaces.

You need to set aside about $500 more to acquire a similar-sized two-stage furnace and may spend up to $3,000 on installation costs. The prices are even higher for higher-efficiency models, typically costing between $4,000 and $5,000. This means you may spend $1,000 to acquire and install a two-stage furnace. Many homeowners are prepared to bear the extra costs hoping to recover the investment from energy savings.

But, as we’ve seen, it is rarely practical. Assuming 3% energy savings for a family that spends $300 on heating per year, that’s about $9. Even if you push it to $500/season (which most families don’t spend on heating), you can still only save $15. It would take about 70 years to recover $1,000 at that rate. So, it’s not worth the effort.

Other Drawbacks of Dual-Stage Furnaces

More worryingly, inconsequential saving and the likelihood that you may never recover the extra investment aren’t the only things wrong with dual-stage furnaces. You also want to consider the following;

  • Are two-stage furnaces really quieter?

The short answer is yes. However, a closer examination reveals that the difference isn’t much. Instead, a crucial question you should always ask yourself when evaluating furnace noise is where the noise comes from. Furnace noises typically come from broken/loose (grinding) parts or the blower motors.

For this reason, all new furnaces, devoid of broken parts, are very quiet. However, the units become notably noisier as the various parts become loose and others break down. Moreover, all modern furnaces use advanced, highly-quiet fans for minimal- noise output.

So, there isn’t much difference between a new single-stage and dual-stage furnace noise-wise. You may argue that furnaces produce less noise at 75% capacity – which is true to a degree. But the difference is only worthwhile if the rest of the furnace, including the blower fans, are in top shape.

  • Expensive parts/repairs 

Finally, two-stage furnaces are a lot more challenging and even more expensive to repair. The increased challenges and costs arise from the scarcity of two-stage furnace parts and the complexity of the furnaces. As expected, the two-stage burner is a lot more complex than traditional single-stage burners as it can “step down” heating and later raise heating levels as necessary.

It takes a few more controls to achieve such an operation. Thus, technicians require advanced skills to locate and fix the problem if something goes wrong. Parts scarcity is highlighted in many industry forums and blogs. For instance, some homeowners note that their dual-stage furnaces require unique thermostats that aren’t easy to find.

Additionally, during repairs, you’ll notice that most stores only stock standard parts for single-stage furnaces. As a result, you may need to order parts online or travel to the city to find the necessary parts.

What are Some of the Best Two-Stage Furnace Brands?

Some of the most popular furnace brands also double as the best two-stage furnace brands. They include;

  • American Standard: American standard makes durable single and dual-stage furnaces that typically last 14 to 20 years. Their furnaces are rated 80% to 97.3 AFUE.
  • Carrier: Carrier is best known for the brand’s insistence on quality installations. They make excellent furnaces and insist on professional installations by Carrier-approved technicians.
  • Goodman: Goodman furnaces are also excellent. However, the company stands out for something else – low prices. They’re the brand of choice if you want a reliable furnace at a great price.
  • Amana: Finally, Amana makes some of the most durable residential furnaces. While standard furnaces last 15 years, Amana models rarely live under 20 years. Some can live for 25 to 30 years with few repairs.

FAQs

Is a two-stage furnace worth the extra cost?

Not really. Although two-stage furnaces are slightly more efficient than their single-stage counterparts, the difference is rarely worth the extra $1,000.

Are two-stage furnaces less reliable?

Unfortunately, some studies say so. The reports show that dual-stage furnaces break down more frequently, thus leading to higher maintenance costs. The downtime can cause inconvenience.

Are two-stage furnaces quieter?

Yes, to a degree. Naturally, furnaces make loud noises when running at full capacity as the fans rotate faster and longer. So, you can notice a noise reduction when the appliance is running at 70% to 80%.

Does a two-stage furnace need a special thermostat?

Yes. Most two-stage thermostats require special thermostats. Although you can use the regular thermostat, for the time being, two-stage furnaces typically require more wires than a regular thermostat can support.

How long does a 2-stage furnace last?

15-30 years. There’s no big difference between single and two-stage furnaces regarding durability.

Do two-stage thermostats run more often?

No. In fact, it’s the opposite. Two-stage furnaces run longer (per cycle), thus less often than single-stage models. Generally, a two-stage furnace runs ten cycles for every 11 cycles by a single-stage furnace.

Conclusion 

Now you’re better informed about two-stage furnaces. They’re great for many reasons. In fact, they’re generally more efficient than single-stage furnaces. However, manufacturers and advertisers tend to overhype them. So, don’t be misled.

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