How to choose the Perfect Furnace for your Home – The Ultimate Guide
How to choose the Perfect Furnace for your Home – The Ultimate Guide
October 14,2020 This article is written by AC Direct and Michael Haines, a licensed HVAC and furnace contractor.
How do I choose the right furnace and what do I look for?
Let’s face it. Buying or replacing your existing furnace isn’t the most exciting purchase you’ll make in your lifetime. Nowhere near as fun as buying a new car, new furniture or other of life’s necessities. But, not near as expensive either.
A new furnace is a big decision however, and one that will affect your comfort and wallet for many years to come. So today we are going to show you how to pick just the right furnace for your home, climate, and budget without overdoing it or buying a furnace that isn’t up to the task at hand.
In this guide we will show you the most popular features and what they mean to you as well as the latest technological advancements in terms of comfort and energy efficiency.
There is an overall furnace selection summary based on climates at the end of this article but be sure to read this article and educate yourself on the latest furnaces that are available to you.
How do you calculate a furnace size? How do I size a new gas furnace?
Pay close attention here as this is a critical step in selecting a new furnace and one that is often calculated incorrectly. When a homeowner is calculating the size of their new furnace they most often use calculators that take climate and square footage into account.
This can be used to get a general idea but by no means is a definitive sizing solution. The AC Direct square footage and climate furnace calculator will provide you with an approximation but we suggest a more exact method discussed below.
Why is proper furnace sizing so important?
The goal in sizing your new furnace is to avoid getting one that is too small or too large. A furnace that is too small presents an obvious problem; your furnace will run excessively and never heat the home properly.
An oversized furnace has its own set of issues, however, and these include:
Short-cycling due to an overkill of heat delivery. This means your furnace will provide too much heat and come on and off much more frequently than necessary. This creates comfort issues and wastes fuel.
The constant on and off cycling creates excessive wear and tear that will lead to a substantial reduction in the life of your furnace as well as additional wear-related repairs.
The HVAC industry estimates that upwards of 40% of furnaces are the wrong size, sometimes dramatically so.
The most accurate and dependable method of determining the proper furnace capacity is to run a manual J load calculation. This is what contractors exclusively use when sizing a heating and/or cooling system for a customer. It takes not only your climate and square footage into account but many other factors as well.
These include:
- The insulation values of your walls and attic
- The number and quality of your windows.
- Which way you home faces
- The age of your home
- The type of construction used to build your home (frame, block, brick, etc.)
- The number of stories your home has
- Your ceiling height
These factors all have a tremendous influence on how much heating is required for any given square footage home.
Our current price for a manual J load calculation is $80 but as a licensed HVAC contractor who fully understands the long term price you will pay for an improperly sized furnace, I would NEVER base my furnace purchase on a best guess even though my best guess is very informed, although inaccurate at best.
A properly sized furnace will struggle on the very coldest extreme nights to keep up because it is sized for cold nights but not the absolute coldest ever. If it is sized for the extreme 1% of cold weather, it will be oversized and short cycle the vast majority of the time.[1]
A properly-sized furnace may be just a degree or 2 short of satisfying the thermostat setting on a night or 2 each winter.
We are not implying you need to get the sizing exactly correct. Furnaces are offered in broad sizing increments of about 20,000 BTU’s, so if you home needs 88,000 BTU’s of heat, then a 100,000 BTU heating output furnace would be the right size.
A 100,000 Btu furnace that is 95% efficient (95 AFUE) produces heat using the following formula:
100,000 X .95% = 95,000 BTU net output.
A 100,000 BTU furnace that is 80% efficient produces 100,000 X .80% = 80,000 BTU net output.
You just need to be within a range of about 15,000 BTU’s net furnace output as compared to your home’s BTU heating requirement.
Stick to a manual J load calculation if you are sizing a new furnace for the first time or if you suspect that your existing furnace is one of the 40% of furnaces in the U.S. that is improperly sized. It is $80 well spent!
You can sometimes have a local heating contractor give you a sizing calculation as well that will cost you nothing.
What is a furnace and how does it work?
There are 2 common types of furnaces; gas and electric. This guide will be about forced air furnaces, or any furnace that is connected directly to a central duct system which is used to distribute the heated air throughout a given space.
Electric furnaces are the least complicated and easiest to install but are also the most expensive to operate when compared to a similar capacity gas furnace.
Electric furnaces are typically used in extreme southern climates where the temperature rarely dips below the low to mid 40’s. Learn more about electric furnaces here
This guide is going to show you exactly how to select the right gas furnace for your home based on your climate and household heating needs.
What does variable speed mean?
A variable speed furnace blower enhances comfort and energy efficiency in both heating and cooling modes. It works to precisely adjust the fan speed of the furnace based on the current demand the furnace is experiencing.
When the furnace is in a low demand heating phase, the blower will sense communication readings from the thermostat and automatically adjust the furnace fan speed to the most energy efficient and advantageous speed which, in turn, provides precisely the amount of heating required at the moment.
The variable speed feature saves energy and improves comfort year-round. The variable speed blower inside your gas furnace will also act as the blower for your central air conditioning.
The variable speed blower will optimize the air flow needed for the cooling demand based on the current load that is placed on your air conditioning system. This means that the blower will operate in a lower and more energy efficient mode about 80% of the time.
And, if you live in a humid summer climate, the variable speed blowers achieve a 400% improvement in humidity removal in the cooling mode. This feature alone means they are very popular and functional in moist summertime climates.
Plus, if certain rooms in your home don’t seem to get enough heating and/or cooling, a variable speed blower is more powerful and better able to overcome the airflow resistance that long duct runs always create. You will feel a noticeable increase in air flow in rooms throughout your home.
The Department of Energy (DOE) is considering enacting regulations that would require all residential furnace manufacturers to upgrade all furnace blowers to variable speed. Time will tell if this legislation actually takes place.[2]
These variable speed blowers are driven by an electronically commutated motor (ECM).
Variable speed furnace and air handler advantages include:
- Optimized and finely-tuned air delivery in both heating and cooling modes
- 4X better humidity removal in the air conditioning mode
- Improved air flow from the much more powerful variable speed blower
- The variable speed blower uses about 70% less electricity than a standard blower
- It can best optimize the staging benefits of a 2 stage and modulating furnace
- Saves money year-round on both heating and cooling
- Variable speed blowers are available on 80% efficient furnaces as well
- They are considerably more quiet than ordinary furnace blowers
Propane Vs Natural Gas Furnace – What is the difference?
Most modern forced air gas furnaces can operate with either natural gas or propane by switching out the gas valve at the time of installation to convert the furnace from natural to propane gas.
You will need to select the optional propane conversion kit when choosing a gas furnace online if you need it to operate on propane gas.
Propane is a fuel that is in liquid form and delivered to your home and stored in an above-ground or below-ground propane fuel tank. If a truck comes to your home periodically and refills your tank, you have propane. You will receive a bill separate and apart from your utilities.
Natural gas is constantly supplied to your home via an underground piping system from your municipality. So, you are interconnected right into the natural gas delivery system.
Your monthly natural gas bill is most likely part of your overall utility billing. Think of it in the same way your water is provided to you. It is automatic and measured by a meter for billing purposes.
You generally do not have a choice when it comes to natural gas because either your municipality has the underground piping delivery structure, or they don’t. In this case, propane is your only gas option.
And in just about all markets across the US, natural gas is less expensive than propane. That, and the fact that propane delivery requires considerably more work and expense, makes natural gas the go-to choice wherever it is available.
If you don’t like the idea of having gas appliances on your property, then an all-electric heat pump is a good choice in most climates.
Should I buy the highest efficiency gas furnace?
The highest efficiency gas furnace is also the most expensive and thankfully, for most US residents, we don’t need the most efficient furnace available. We are going to go into greater detail about this within this guide.
What we are going to do in this furnace buying guide is discuss the various levels of efficiencies you will find across the web and help you determine when and why you should buy a particular furnace for your home while getting the most value possible from every dollar you invest in your new furnace.
Here are the most common furnace efficiency options
Furnaces, first and foremost, are rated according to the efficiency level at which they can convert fuel to heat, which is known as the Annual Fuel Utilization Efficiency or AFUE. The current federal minimum is 78% and this ranges as high as 98%.[3]
A 20 year old furnace could easily have an effective AFUE of just 65%.
An 80% efficient furnace converts 80% of the fuel it consumes into heat while the other 20% is not converted to heat in the combustion process and is exhausted out of the flue pipe.
On the other extreme, a furnace with a 98% AFUE will convert 98% of the consumed fuel into heat and expel just 2% of the fuel byproduct through the exhaust flue.
This means a 100,000 BTU furnace with an AFUE of 80% actually produces 100,000 BTU’s X 80% =80,000 BTU’s net output of heat.
The same 100,000 BTU furnace with an AFUE of 98% produces 100,000 BTU’s X 98% = 98,000 BTU’s net output.
Higher efficiency furnaces not only cost less to operate but they are much more earth-friendly in that they emit considerably less toxic emission into the atmosphere.
A 98% furnace will use 22% less fuel than an 80% efficient furnace but the savings don’t end there.
There are other comfort and energy saving benefits to consider when choosing a new furnace.
You’ll see 3 basic types of furnaces online and these are:
- single stage
- 2 stage
- modulating
What is a single stage furnace?
A single stage furnace operates at 100% capacity whenever heat is called for by your thermostat. Single stage furnaces are almost always 80% efficient.
Single stage means the furnace operates in the high-speed mode when it is on. This is fine in mild winter climates where energy efficiency and superior comfort is not as big a concern in the winter.
A single stage furnace cannot adjust its heating output based on current demand. It is either 100% on or in the “off” position. There is no in-between.
What is the benefit of a 2 stage furnace?
A 2 stage furnace is like 2 furnaces in one; it operates in a lower, energy efficient mode on the low demand days or nights. This saves fuel and produces a more even temperature throughout your home.
On the higher demand days and nights, it operates at a higher, more powerful stage to keep up with the demand and reduce the run time required to satisfy the thermostat temperature setting. You simply set the desired temperature and walk away. The furnace and the thermostat do the rest for you.
2 stage furnaces will operate in the lowest and most energy efficient stage about 80% of the time.
What is a modulating furnace?
The third and most efficient type of furnace is called a modulating furnace. Think of the gas pedal in your car. When you need extra power for passing or merging, you apply the exact amount of power needed for the current situation (you modulate the gas pedal).
And, when you are cruising or coasting, you apply much less pressure to the pedal as the demand for fuel is much lower. Or think of a water faucet. The handle allows you to “modulate” the water flow and temperature based on exactly how much hot water you need at any given moment.
A modulating furnace heats your home in exactly the same way as the gas pedal in your car determines the speed of your vehicle.
The furnace senses what the current demand is based mostly on the temperature differential between where your desired temperature setting is and the current temperature inside you home.
If your thermostat is set at 74 degrees and the temperature inside your home is just 48 degrees, the furnace senses the high demand and ramps up to its most powerful setting to provide the highest level of heat to fill in this gap as quickly as possible.
On the other hand, if the furnace has been running and the home is being heated, then the demand is much lower. So, if the thermostat is set at 74 degrees and the interior temperature is 72 degrees, this is a very low demand situation.
The furnace will come on at the lowest possible (and most energy efficient) setting and will satisfy the demand and shut off.
So, a modulating furnace serves 2 primary benefits: improved comfort and dramatic energy efficiency.
A furnace that delivers air in an overkill manner will cycle on and off quite frequently which leads to wider temperature variations from room-to-room and more equipment wear-and-tear.
The lower air delivery with a 2 stage or modulating furnace leads to slightly longer run times which allows the heated air to be more evenly distributed throughout the entire space.
And even though the run times are a little longer, the system still uses substantially less fuel and electricity while maintaining a more even and stable interior temperature.
How do single stage, 2 stage and modulating furnaces compare in comfort, energy savings, noise levels, and initial cost?
Comfort comparison
First Place - The modulating furnace easily wins this prize. And this is because it can adjust its output to meet the demand without the constant on and off cycling common to single stage furnaces.
When the furnace is first turned on during a cold period, it will operate at the highest speed to quickly meet the desired temperature setting of your thermostat.
The rest of the time it just cruises along, supplying just enough heat to get the job done so your home remains much closer to the temperature setting of your thermostat without all the wild temperature swings single stage systems inevitably experience.
Second Place – The 2-stage furnace finishes in second place behind the modulating furnace and well ahead of a single stage furnace. A 2 stage furnace is not as flexible as the modulating furnace in terms of comfort but is considerably more effective at creating a comfortable environment than a single stage furnace.
As mentioned earlier, a 2-stage furnace is like having 2 furnaces in 1. On the low demand days, it operates in a lower and more energy efficient stage. This, in turn, allows for more balanced air delivery and less temperature swings throughout the operating stage.
So, compared to a single stage furnace it is a huge step up but doesn’t reach the occupant comfort levels common to modulating furnaces.
Third Place – No guesswork here, the single stage comes in a distant third. These furnaces are typically used in low demand heating climates such as Florida and Texas. Therefore, not as much is expected from them due to the low demand and greatly reduced operating time.
Will an 80% single stage furnace keep you warm in a high demand climate? Yes. They will keep you warm in a very cold climate as well. But think of an off-road automobile. It will get you to your destination, but without the comfort, finesse, and style of a road-worthy sedan.
Energy Savings
First Place – The modulating furnace easily takes the top spot here as well. This is attributable to its ability to efficiently deliver heat and consume fuel based solely on current demand (modulation) as well as the much more efficient variable speed blower.
Combined, these 2 advanced features can cut your annual gas and electric consumption by 30-40% when compared to a standard single stage 80% efficient furnace.
Second Place – The 2 stage again takes the consolation prize for energy savings. A 2 stage furnace is more functional than a standard single stage and 2 stage furnaces generally have an AFUE of 92% or higher.
So, the annual fuel savings compared to an 80% furnace can run into hundreds of dollars saved (based on your consumption). The variable speed blower is usually an option on a 2 stage furnace so choosing this feature can be a wise choice based the year-round benefits of variable speed function.
Third Place – The bronze for energy savings once again goes to the single stage furnace. Again, the single stage 80% furnace has its economical place in warmer climates, and this means energy use is not as big a deal as the initial installed cost, which is coming up next.
However, if you live in a northern climate where the temperature dips into the 20’s or lower every year, you should completely remove a single stage 80% furnace from your options menu.
Furnace Noise Levels
First Place – Here, the contest is a little closer. A variable speed, modulating blower takes the checkered winner’s flag, but the 2-stage furnace is in a close race.
The modulating furnace wins mainly because its lower stage settings are whisper quiet especially when combined with the lower settings required of the variable speed blower during low stage operation.
Second Place – The 2-stage furnace earns a respectable silver award with the caveat that it is outfitted with a variable speed blower.
And because it will operate in the lower-stage setting roughly 80% of the time, this reduces the noise levels and lowers the air volume needed meaning the blower is much quieter as well during the low stage operation. Overall, noise is not really an issue with the modulating and 2 stage contestants.
Third Place – The single stage furnace takes the bronze once again. Keep in mind that even 80% furnaces are much quieter than the models from even 20 years ago.
But if noise levels are a concern be careful with an 80% single stage furnace as you may be disappointed if you are looking for a quiet furnace or it is located in place where noise has been an issue with a previous furnace.
If nothing else, opt for a variable speed 80% furnace. To reduce noise levels without spending more than necessary.
Initial Cost
First Place – No surprise here! The single stage 80% model is the clear and unadulterated winner. In the local retail marketplace, an 80% single stage can be about $3000 installed. The AC Direct price for a 100,000 BTU 80% single stage furnace is $841.66.
Second Place – It seems as though the 2 stage furnaces are a consistent runner-up in this marathon just as it is in this initial cost contest. Expect an installed price for the furnace and labor from a local contractor of about $4500-$5500. The AC Direct price for a 100,000 BTU 96% 2 stage variable speed furnace is $2073.66.
Third Place – Here’s where the elite status of the modulating furnace comes home to bear. It finishes this contest in a distant third. This is why it’s only the best choice in cold climates where you have extreme operating time and spend considerable chunks of your income paying down large gas bills.
A local HVAC contractor will charge about $5500-$7000 for a top of the line modulating furnace installed. The AC Direct price for a 100,000 BTU 97% modulating variable speed furnace is $2344.05.
Furnace Installation
You’ll need to check your existing furnace (if you are replacing one) to determine its “configuration”.
There are 4 type of furnace configurations as it relates to installation:
- Upflow
- Downflow
- Horizontal Left
- Horizontal right
You’ll see the terms “supply and return air” in this part of our guide so let’s clarify.
A forced air heating and /or cooling system uses a network of supply and return ducts to circulate the heated and cooled air throughout each room of your home.
A main supply duct is connected to the supply air side of your furnace so that air is delivered or “supplied” to each room of your conditioned space. A return duct(s) pulls or “returns” the air from within your home to the furnace where it is heated and then delivered into the supply ducts and supplied, or delivered, back into your home.
What is a horizontal furnace?
Most furnaces these days are “multi-positional”, meaning they can be installed in a variety of configurations. A horizontal furnace is designed to be laid on its side, typically for use in spaces such as an attic, crawlspace or perhaps hung from a ceiling in the garage.
A horizontal furnace will deliver or “supply” the heated air from either the left or right side (depending on what your particular installation requires.) This means the return air comes in from one side and is then heated and supplied back into the duct system from the other side of the furnace.
Most horizontal furnaces can be installed to be either horizontal left or horizontal right depending on which side you lay it.
Can any furnace be installed horizontally?
Most of the time furnaces can be installed horizontally but to be sure you don’t get stuck with the wrong furnace always confirm the furnace is horizontal compatible as delivered to you.
What does upflow furnace mean?
An upflow furnace means the furnace is installed in an upright position and delivers the heated air into a duct system mounted above the furnace. The most common upflow furnace installations include inside of a closet, on a platform in the garage, or a basement installation with a duct system located above the furnace.
If your furnace stands upright and has a supply air duct system attached to the top, then it is an upflow furnace. The return air ducting will be attached to the very bottom of the furnace or to the side of the cabinet toward the bottom.
What is a downflow furnace?
A downflow furnace is installed in an upright vertical manner and delivers or “supplies” the heated air out the bottom into a duct system located beneath the furnace.
The return air is connected to the top of the furnace and is delivered in a downward manner where it passes across the furnace heat exchanger and is heated and continues into the supply duct system under the furnace.
Common installations include within a closet when the home is on a crawlspace or basement with the duct system beneath the floor of the home. Mobile homes often use a downflow furnace installed in a hall closet as the supply ductwork is frequently mounted within or beneath the flooring.
Again, even if a furnace is advertised as “multi-positional” just confirm that it is suitable for your application. Don’t assume anything.
This is especially true if you need a downflow furnace as downflow furnaces have to be factory assembled in such a way that they are not usually field-convertible from upflow into downflow. So you may need a dedicated downflow furnace depending on the manufacturer.
This guide has walked you through all of the latest furnace technological features. The intent was to show you why and when you should or should not consider each of these features for you next furnace purchase and installation.
Summary
If you live in a very cold northern climate here are your 2 best bets for a gas furnace:
- A modulating furnace is the very best choice
- A 2 stage furnace is your second best choice
If you live in a milder winter climate:
- A modulating furnace is the best choice
- A 2 stage furnace is the second best choice
If you live in the southern part of the US:
- A 2 stage furnace is your best choice
- A variable speed 80% AFUE furnace is the second best choice
If you live in the extreme south (South Florida and South Texas):
- An 80% variable speed furnace is your best choice
- A standard 80% non-variable furnace is the second best choice
- An electric furnace is an option here as well
If you have questions or need sizing advice give us a call. Experts are standing by...
References:
[1] https://www.startribune.com/problems-with-over-sized-furnaces/229071341/
[2] https://www.achrnews.com/articles/136459-fer-standards-require-electronically-commutated-motors
[3] https://www.ehow.com/info_10047368_90-vs-95-furnace-efficiency.html
