What efficiency values are available?
Just like when we described air conditioners, the efficiency value is where we like to start the conversation. Furnaces are all described by their Annual Fuel Utilization Efficiency (AFUE). The most basic and common type of furnace on the market is the 80 AFUE, commonly referred to as an 80% furnace (we will get into why this is more or less a misconception to describe them by finite % later). A homeowner would know if they currently have an 80 AFUE furnace installed based on the type of venting that is presently coming out of the front of their furnace. If you notice that the flue stack is made of metal, that furnace would be in the 80 AFUE, or lower, category. I say “or lower” because a good handful of furnaces still operate on natural draft instead of having a draft inducer motor installed – which operate at lower efficiencies than 80 AFUE. All furnaces being produced today will have an inducer or draft motor installed. Hence, a mechanical squirrel cage is spinning to create a draft to mechanically rid the home of the harmful flue gas produced from burning natural gas or propane to the exit point, typically through a chimney. Your next option would be a 90+ AFUE furnace, which will operate similarly but differently to get higher efficiency values. Quite a few 90+ AFUE furnaces are currently operating in homes today, and you would know you currently have a “high efficiency” furnace installed in your home based on the present venting. These types of systems use PVC venting typically installed toward a side wall termination, forgoing any need to use the home’s existing chimney. There are situations where a 90+ AFUE furnace can be vented out the roof. Still, they are case by case if that would even be an option for a homeowner and would undoubtedly require a professional to come by your property to investigate. It’s also important to know that 90+ AFUE furnaces are also considered condensing gas furnaces, meaning that the flue gases come out of the unit at lower temperatures thus, they condensate and drain the water back into the unit for it to drain out. This acidic water will need to be dealt with, whether using a floor drain in the basement or a condensate pump when the furnace is operating in heating mode.
So, I have an 80 AFUE furnace, and I would like to have a 90+ AFUE furnace installed.
From the installation standpoint, these are only sometimes easy conversions for homeowners due to the difference in venting. Remember, we must install PVC pipes to a side wall. If your mechanical room is interior to the home with finished ceilings all the way around that room, it may be a tricky process – but not always impossible. It takes a professional to come to do a walk-through of the home when there are a lot of finished spaces to go around. Now, in the case, you have an unfinished basement, it is much easier to find the termination point of the venting as the amount of options we have to avoid decks on the back of the home, far enough away from windows, etc., is going to be much higher. Regardless of the likelihood that it would be “easier” to complete the venting installation, we still ask that we take these situations on a case-by-case basis. Maybe a professional would not be required to come out to the home BUT, we would still like to participate in looking at the project using a Facetime-like app to verify all that we need to see.
The truth – downfalls of an 80 AFUE to 90+ AFUE furnace installation and operation
Let’s preface and stress this – no HVAC system will make it to the life expectancy of 15-20+ years without proper maintenance being completed on them. In the case of 90+ AFUE furnaces, they create acidic condensation, so they are going to require maintenance to get the same longevity out of them that would be present with a well installed 80+ AFUE. This corrosive water is going to wear on all the drain ports that are tied to the pressure switches/inducer motor, and they need to be sure they are clean and free of any kind of dirt and debris to ensure the furnace operates and doesn’t fill full of water during peak usage seasons. Also, having someone make sure that all the drain lines are working properly and are seated correctly is going to be a huge step to make sure that we don’t have any water in locations that there shouldn’t be – specifically around the electrical components like the control board found in the bottom compartment of the furnace.
Also, the math just isn’t there quite yet to be in the homeowner’s favor from a “savings” standpoint for most small to midsized homes (larger homes, especially with those that have two systems, this math would not be applicable – or at least the timeline to “return on investment” is shortened). Yes, I know it’s easy to get wrapped up in the idea of efficiency, seeing around a supposed 16% savings, but here in Kansas City in the Winter of 2023, we still have some of the lowest natural gas rates in the country at $0.3740 per CCF. Look at this calculation of an average home here in Kansas City – around a 1500 square foot home that would require 60k BTUs of heating to be output into the space to hold temperatures. Moving away from 80 AFUE into a 96 AFUE furnace means the annual savings are estimated at $97/year. It’s important to note that moving a customer from an 80 AFUE furnace to a 96 AFUE furnace will cost at least another $1500 to install that furnace. This means it would take 15 YEARS for the savings to be actualized – the furnace is on the backside of its life at that point, you no longer have a parts warranty, etc. This calculator is sponsored by Wrightsoft, one of the leading engineering software in the world, and doesn’t even begin to consider the unit’s degradation and a loss of efficiency due to mechanical wear down.
Base System – Existing 80 AFUE Single-Stage with a fixed-speed blower motor
Investment 1 – Replacing base system furnace for an 80 AFUE two-stage furnace with a variable speed, constant air flow motor.
Investment 2- replacing base system for a 96 AFUE single-stage furnace with a fixed-speed blower motor.
Investment 3 – replacing base system for a 96 AFUE two-stage furnace with a variable speed, constant air flow motor.
Simply put, we want to tell you the truth, and we do not believe that under the current cost structure set forth by our utility companies, there is a “need” to convert people to “high efficiency” furnaces. We will need to have some considerable natural gas price increases take place for us to change our minds, based strictly on the math and ensuring our homeowners get the most bang for their buck – not what makes the most bucks for us.
I have PVC already coming out of my furnace – what does that mean for me?
Great! That means a lot of the solutions for getting the water to an appropriate place and the vent route have already been determined by a previous installation company; thus, your installation cost would be less than that of someone that does not have the venting already installed. Your installation costs are still going to be higher, relative to a household with an 80 AFUE furnace, due to the higher cost of the equipment. And much of the construction process can be left off the proposal to install your new furnace.
Ok, if I am not going to invest in the AFUE value – what should I be buying?
We cannot stress this enough, find a contractor that is goal driven – not one that is just pushing their bias onto your situation. If someone wants to invest in their comfort, here is where we start to talk about the differences in the furnaces.
Three different types of gas valves can be purchased when going through the process of having a new furnace installed in your home. The first is the single-stage gas valve, which only gives you one kind of heat. This gas valve will kick on at its max BTU output, run until it satisfies the thermostat, and then turn off until the temperatures start to drop in the house again and the thermostat recognizes the drop in temperature. Then, the process starts working until the thermostat tells the furnace not to. With the usage of a single-stage gas valve, you will experience the most significant swings in temperature, and it’s more likely (not an absolute) that you will have areas of the home that are less temperate or conditioned to your liking than others. These gas valves would do the least amount of control over humidity – they can only deliver the highest energy levels into a space, leading to a drop in available humidity. Lower humidity will cause you to “feel” colder inside of the home, and viruses thrive in a climate with a lack of humidity – thus, virus-based illnesses spike in the wintertime. We always recommend installing a whole home humidifier with these systems to offset some of the drying effects from these gas valves.
Your next option is going to be the two-stage gas valve. This gas valve will have a middle ground on which it can create heat for the home – around 65% of its BTU output, or 100% in cases where it is needed. These valves shine when we try to have smaller temperature swings across the home. The high stage will satisfy the thermostat, but we only need to hold that temperature for comfort once that temperature is achieved. Now we don’t need all of the furnace’s capabilities to do such; we can depend on the lower stage to do that for us. Also, we want long run times out of a furnace for the sake of creating even temperatures across the home since it takes a long time to get the energy to be able to “sink” into hard surfaces like drywall, hard countertops, etc., so we can achieve radiant effects inside the home. Also, the longer the run time, the easier we are on the oh-so-precious heat exchanger (your heat exchanger is tubing that runs in the heart of your furnace. This is the entry point of the flames produced by the furnace and heating the metal, to which air goes across that hot metal to supply hot air to the living spaces). These are constructed with metal and metal that is constantly being heated up, then cooled, and then heated, etc., leads to brittle metal; a crack in a heat exchanger is a catastrophic failure of the unit and could put homeowners at risk of dangerous gases being in the breathing air. A two-stage gas valve will make the furnace run longer for overall comfort, for longevity of the unit’s lifespan, and won’t dry the home out quite as much since less energy is being delivered to the space.
The ultimate control over your heating process is achieved using a modulating gas valve, which can only be found on 97+AFUE furnace models. This gas valve doesn’t have stages; it just ramps up its BTU output up and down based on humidity and temperature within the home. These are very intelligent systems, using several sensors found on the outdoor unit, on the furnace, and typically in conjunction with a communicating thermostat. You will have the most control over indoor humidity and temperature throughout the home and have the most extended run times out of your furnace.
Your blower motor is the motor at the very bottom of your furnace that does all the mechanical work to move your air through the ductwork system in your home, whether it be a heating season or a cooling season.
Found commonly with single-stage gas valves is the ECM fixed-speed blower motor. This is a blower motor that is going to operate only at one speed, in tandem with the gas valve. This blower motor will be set up by the installation technicians when it is first installed, and not very often is that speed changed from that point onward. It will be moving as fast as programmed, or it’s off. Although this blower motor will be more efficient than other blower motors that were used previously, it still has its shortcomings.
With a two-stage or modulating gas valve, you will receive a variable-speed blower motor which, I think it’s more important to think of this motor as a constant air flow motor. Above your furnace, typically, an evaporator coil is a ductwork system used to move the air around the home. Typically, this ductwork system is improperly designed, making it hard to move air fluidly. Not being able to move air quickly and having a build-up of pressure on the furnace’s blower motor is what we know to be “static pressure”. A variable speed, constant air flow motor is designed to recognize this increase in pressure and will work harder to move air adequately to the settings we program to it. Sure, we will address the duct work concerns we can address with you, but not all can be fixed when the ducts are covered by drywall. When trying to create a very comfortable home, we always recommend going with a variable speed motor to slow down the heating or cooling process to ensure we conditioned air going to all locations of the home. Also, these will be the most efficient blower motor from the standpoint of overall electrical consumption.
Other things to consider about purchasing a furnace.
Properly sizing the furnace for the size and construction of the home is essential.
Ok, now you understand a little about how the gas valve and blower motor type can create comfort in the home. But, an oversized furnace is the most detrimental to a home’s comfort and will shorten the furnace’s lifespan. All furnaces should be installed per a load calculation – software that will help a contractor understand the building’s size and construction to ensure the air conditioner is properly sized.
If it is oversized, the furnace will give the home TOO MUCH heating TOO FAST, thus, never allowing for constant energy to be delivered to the living spaces to heat the “home.” And the furnace will “short cycle,” meaning it will kick on and off very often. Remember, that would be very hard on the heat exchanger, and the rapid/constant expansion and contraction can make the metal brittle.
Obviously, an undersized furnace is not what you want either; you’ll just never have enough furnace to be able to achieve the temperature set points you would desire. It’s a fine science, at times, to be able to get it correct for your home.
Sometimes it will be in your best interest to invest in a 90AFUE unit, even when your current setup is 80AFUE
In some situations, we must ignore our understanding of the cost of high-efficiency furnaces and put the customer’s long-term at the forefront of the conversation. There are a lot of 80 AFUE furnaces out there today that are currently being vented into an old brick-and-mortar chimney. Per code, we need to install a chimney liner with your retrofit furnace installation project, as the chimneys will result in condensation forming, running down the flue, and more than likely will rust out the metal flue with time. Thus, exposing the living environments potentially to harmful gases being present. Installing a chimney liner is not an inexpensive portion of the project that would need to be completed. We take these situations on a case-by-case basis to ensure we keep our employees safe and sometimes look to hire this process out to a professional chimney company – an added cost to the project. Also, for individuals with larger homes, we can show the math to be in your favor if you plan on staying in your home for a considerable amount of time.
If indoor air quality concerns you, buying the right equipment will help.
Another topic to talk about is how the different furnace types can help with your overall indoor air quality. Opting for a variable-speed blower motor can help keep the air cleaner since these units utilize the fan on your furnace more often since the run times are longer, circulating the stale air in the home. At lower humidity levels in a home, viruses will flourish in those environments. Increased run times by the blower motor will also mean that your humidifier can come on more times, allowing a better chance for it to do its job and further increase humidity levels within the home.
Since the fan is running for more extended periods of the day as well, your air has a better chance of being filtered using your furnace filter, or if you elect for additional products found with Respicaire’s line, they will perform better under these circumstances.
Local utility rebates, federal tax credits, and the all-mighty AHRI certificate
You may be aware that there are several ways to offset some of the costs associated with installing a furnace in your home. Here in Kansas City, Spire has a program that will award instant rebates to homeowners for installing “high-efficiency furnaces. As we discussed in air conditioners, the governing body that determines the efficiency values is the AHRI. These units work as systems, and their certificate proves their efficiency value. The furnace must be rated a 92 AFUE or higher for Spire rebates.
For federal tax credits, please consult with your tax professional to determine if these would be of value to your tax situation. We can ensure the units will hit all the minimum requirements set forth by the CEE.