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How Efficient Is My Furnace? A Full Analysis

Is your gas or electric furnace efficient? How can you tell? And how much will you benefit from a new furnace installation?

How Efficient Is My Furnace? A Full Analysis

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Joshua Rodriguez


December 14th, 2020

Buzzwords are everywhere. High-efficiency, Energy Star rated, SEER, HSPF, AFUE extravaganza! Did anything in that last sentence make sense to you? It's ok if your answer was “no.”

Efficiency matters in HVAC equipment, but cutting through the noise to figure out what matters and what doesn’t is difficult. So that’s what this article is all about. We’re going to do a handful of things to help you out in this article:

  1. Show you how to figure out your furnace’s efficiency
  2. Explain how efficiency is measured in furnaces
  3. Talk through what aspects of a furnace contribute to its efficiency
  4. Help you figure out what that will mean for you in terms of cost savings
  5. Explain the difference between comfort and efficiency (they’re related but not identical)
  6. Give you some tactics to choose your ideal furnace

Sound like something that would be beneficial to you? I hope so. If so, read on.

How to Check Your Furnace’s Efficiency

To find out your furnace’s efficiency, first check the vents. If your furnace vents out of a metal pipe and/or chimney flue, it’s a low-efficiency or “standard” furnace. If it vents out of a plastic PVC pipe, it’s high-efficiency. If you want to get more detailed, call the installer who put in the unit or look up the model number online. Below, we discuss what efficiency means in terms of mechanical operation and cost savings for a home.

What Is AFUE and Why It Matters

AFUE stands for Annual Fuel Utilization Efficiency. It’s a measure of how much of your fuel (natural gas, propane, oil, etc.) is converted into heat by the furnace. It’s measured in percentages.

A “standard” modern furnace is 80% AFUE. This means that 80% of the fuel burned by the furnace is converted into heat for your home. The rest is vented out of your home, usually through a chimney flue or PVC drain pipe.

For a furnace to be considered high-efficiency, it has to be at least 90% AFUE. The most advanced modern furnaces go up to about 98% efficiency.

RELATED: SEER, AFUE and HSPF Ratings in HVAC: Why They Matter

To convert this to your utilities, imagine that 80 cents on the dollar are being used to heat your home. If you upgrade to a 90% furnace, that’s 10 more cents per dollar that are being converted to heat.

This also leads to increased comfort. Additionally, it means less gas is vented into the atmosphere, which is more environmentally friendly.

Importantly, AFUE isn’t the only thing that matters when considering the efficiency of a furnace. It’s an important metric, but is incomplete on its own.

Does Cost Increase With AFUE?

Yes, the cost will increase. Near the end of this article, we’ll talk about how to make the right decision for your home, because the extra cost isn’t always worth it.

But a high-efficiency furnace will cost more, both because of the equipment and occasionally due to installation considerations.

Because more fuel is converted into heat in a high-efficiency unit (this is due to the unit having two heat exchangers), the remaining gas isn’t actually warm enough to rise out of a chimney flue. It may necessitate the addition of a PVC ventilation pipe to handle the condensation that will be generated by the cooling gas.

This is generally a small cost in the grand scheme of things, but knowing what to expect going in is important.

RELATED: Cost of a New Furnace

Napkin Math

So let’s say you’re considering a new furnace. Your top two options are 80% and 96% efficient. Your current system might be 70-80%, but because of its age, it’s operating at a lower level.

Based on the “cents to a dollar” calculation we’ve mentioned a few times, it’s not impossible to calculate your savings per year. So if you spend $100 on heating costs, the difference between 80% and 96% is roughly $16.

In reality, this is a bit trickier, because not all of your winter utility costs are for your furnace. Other equipment or fees are involved in your total bill. However, it’s still possible to approximate your savings.

A 96% furnace will cost more. But it will also be more comfortable and environmentally friendly. $16 for every $100 might not be enough to justify the extra cost for the unit, but those other factors might contribute.

It’s often not until years 7-10 that homeowners start seeing savings by purchasing the more expensive equipment, and for some who don’t use their furnaces as much, it might be 10-15 years. However, for those people, the savings are very real in those years.

We’ll discuss this a bit more in the last section below, but this is how you can start figuring out which furnace will be best for you.

Furnace parts

Windows, Insulation, and Everything Non-HVAC

Of course, having new, tight windows helps the efficiency of your entire home. The same is true of insulation, since a lot of heat escapes out of the roof of your home.

However, that doesn’t mean a new furnace won’t help, regardless of your situation. Good windows or bad, crummy insulation or brand new, if you upgrade from a furnace that’s only operating at about 70% efficiency to one that’s 95%, that’s 25 more cents of every dollar that now goes toward heating your home.

But which matters most? Window companies will tell you windows are most important. Insulation companies will tell you insulation is most important. HVAC companies...well, you get it. So which one is right?

It depends. I know, that’s an unsatisfying answer. But it truly does depend on the status of your home. When each was last installed, and how well-maintained they’ve been in the years since, will determine where you’re losing the most comfort.

Do Air Conditioners or Heat Pumps Contribute to Furnace Efficiency?

Technically, it’s the other way around. I’m happy to explain what I mean by that.

Your air conditioner won’t affect your furnace meaningfully. A heat pump can provide supplemental heat that means you don’t have to run your furnace as often in the fall and spring, but in a strict sense it doesn’t change the efficiency of your furnace.

What matters, though, is the blower fan, which is part of a new furnace installation. This is what moves the air throughout your system for ALL equipment, not just the furnace.

So upgrading the furnace to include a more advanced blower motor and blower fan can actually increase the efficiency of your air conditioner and/or heat pump.

This is one of the quirks about having an HVAC system. When we call it a system, we really mean it, because it’s often working together in ways that people don’t realize. The health and maintenance of one piece of equipment can absolutely affect others. Heating and cooling are not completely separate processes in most homes.

Ductwork and vetilation in HVAC

Ductwork, Filters, and Your Ventilation System

We mentioned that thinking of your HVAC equipment as a system is important, and here’s another reason why: your ductwork and your filters affect the efficiency of

It doesn’t matter if you have a cutting-edge 98% efficient furnace. If your filter is clogged, your actual efficiency is garbage.

This is true of a lot of parts in the system. Imagine that your system is too powerful for the ductwork, which isn’t wide enough in certain places to handle the airflow. What happens then? Well, you’ll start to get leaks, and you’ll have backups in your airflow that will create hot and cold spots in the house.

Long-term, these things affect efficiency in ways that can amount to thousands of dollars in costs! You’ll also be a lot less comfortable in your own home.

The solution here is fairly simple: change your filters regularly, invest in a quality filter (we often recommend 3-inch, MERV 11+ filters for a variety of reasons), and have regular maintenance done on your HVAC system.

And that’s it. Do those things, and you’ll be optimizing the efficiency of your system.

But if you don’t...well, don’t say I didn’t warn you when the system breaks down years before you should be having major problems with it!

Gas vs. Electric Furnaces: Which One Is Better?

Better or worse is contextual here. A lot of it relates to the cost of fuel sources where you live.

For example, I used to live in Colorado, where it’s very sunny. A lot of people would install solar panels on their home and power their homes with electricity. As a result, they paid almost nothing in electrical costs. So an electric furnace made a lot more sense.

In the southern United States, electric is much more popular as well, mainly because heat pumps are able to provide efficient heat through most (or all) of the year. In the mildest climates, you might never have to use a furnace in the winter months.

Here in Ohio, it gets cold, and the cost of natural gas is far lower than the cost of electricity. There are times when homes don’t have access to a natural gas line, so they have no choice and need to use electric, propane or oil, all of which have various drawbacks. But for the majority of our customers, natural gas is going to be by far the most cost-efficient option.

Both gas and electric come in a variety of models, so you’re not really sacrificing choices to pick one or the other. Mostly, it comes down to your individual situation, which often relates to where you live.

ALSO READ: Switching to a Gas or Electric Furnace - Pros, Cons & Costs

Happy home with a furnace

Single-Stage, Two-Stage and Variable-Speed Furnaces

We’ve written about this at length in the past, so I’ll link to our article below. But in brief, furnaces can have a single stage (“On”), two stages (usually 70% power or 100% power), or multiple stages (the number varies depending on brand and model).

This is very important when it comes to comfort. It can also affect efficiency. As you might suspect, the variable-speed furnaces are more efficient, because they can perform a lot of heating at lower levels, which doesn’t waste fuel and energy.

RELATED: Single-Stage, Two-Stage and Variable-Speed Furnaces: Differences and Benefits

You probably don’t need your furnace at full blast on a fall day, but in the dead of winter, you will. So on those coldest days, efficiency is roughly the same between these types. But on the fall day where it’s just a little bit chilly, being able to have your furnace run at half capacity is going to save you a lot of money long-term.

This is also the best way to keep your home uniformly comfortable throughout the year. The price is higher, but for many, it’s an easy decision due to the amazing levels of comfort that variable-speed furnaces provide.

Find the Best Furnace For Your Home

I talk with a lot of folks every week. And how do I help them find the right furnace for them?

I ask the right questions. Sure, maybe you’ll be ok if an HVAC salesperson comes into your home and immediately gives you a recommendation. But what do they know about your particular needs and goals?

Probably nothing.

So I ask questions to get to the heart of the matter. What questions? Let’s run through a few:

  • How do you currently feel about your utility bills? Are they high in the summer? In the winter? The answers may help determine where you need a bigger upgrade in equipment.
  • What is your current budget? Sure, the more expensive equipment can sometimes pay for itself over time, but if you can’t afford it now, that doesn’t matter.
  • How long are you expecting to live in your home? If the answer is under five years, there’s rarely a good argument for top-of-the-line equipment. You won’t be around to get the long-term cost savings from it. But if this is a long-term home for you and your family, the longer you plan to stay, the more it makes sense to consider high-efficiency and models that are variable-speed.

There are other questions to ask, but that’s a good start.

RELATED: Will A New Furnace Save Me Money?

RELATED: The Complete Guide to Home Furnaces

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