Furnaces

Gas furnaces are the most common and most efficient furnaces at the moment, feeding off municipal gas lines that provide fuel at a very competitive price. This gas is fed into a burner, where several jets feed the controlled flames that heat air, which is then circulated through the house.

They are cheap to run and very convenient, but if you are ecologically minded you might want to consider other options. 

There are areas of the USA where natural gas is not readily available, and that’s where oil furnaces come in. However, if gas is available it is preferable, since oil is about 10% less efficient. We will discuss annual fuel utilization efficiency (AFUE) ratings later on.

In very loose terms, an oil furnace works like the combustion engine of a car: gas is sprayed into a combustion chamber and lit up by a spark, to heat water or air depending on the type of furnace.

The cleanest of these options, electric furnaces rely on heating elements to create the warm air that will circulate through your home. The furnace cost is lower than the previous options, and they also tend to be smaller in size, which can be very convenient.

While they are technically the most efficient of the three, since 100% of the energy is transformed into heat, the cost of electricity makes them a less attractive option compared to the gas furnace, which is less efficient yet much cheaper to run.

Let’s talk about annual fuel utilization efficiency (AFUE), one of the ways in which a furnace’s efficiency is calculated.

In the simplest possible terms, a furnace converts fuel into heat. The big question is, how well does it do its job – what percentage of the fuel is effectively turned into heat, and how much is lost? That, in a nutshell, is the meaning of a furnace’s AFUE rating. 

If a furnace’s AFUE rating is 90%, it means that 90% of the fuel is converted into heat, while 10% is lost as exhaust. The ratings scale range from 30% to 100%, but if your rating is 30% you should get that furnace replaced ASAP – it is wasting twice as much fuel as it is using for heat!

On average, gas furnaces are more efficient than oil furnaces, and electrical furnaces are the most fuel efficient of all. Electrical furnaces have a 100% AFUE rating, which means they are 100 percent efficient at turning electricity into heat – no energy is wasted at all. Modern gas furnaces have AFUE ratings of 88-98% on average, while oil furnaces score a little lower with an average of 80-90% annual fuel utilization efficiency.  

A furnace’s AFUE rating is a great place to start deciding if a given furnace is the right one for you, but it is not the only factor you should take into consideration. For example: if you are trying to save money in bills, electrical furnaces are not the best option – even though they have perfect efficiency, electricity is much more expensive than gas, so your bills would be higher with electricity (even if they are the most efficient furnaces) than with a slightly less efficient gas furnace.

There are several factors to keep in mind when choosing a high efficiency furnace – we already went over AFUE rating and type of fuel. Now let’s talk about sizing.

When designing any kind of HVAC system, or a heat system in particular, it is important to have a professional carry out the load calculations to make sure you get a furnace that emits the right amount of heat. This is measured in BTU (British thermal units), and if you’ve ever shopped for a furnace you will remember each option had a prominently displayed BTU/hour number, usually ranging between 10,000 and 150,000. 

It is pretty obvious why you don’t want a too-small furnace: it will run too often, wear itself out sooner, and it may not even be enough to heat your home appropriately on especially cold days. A too-large furnace is also not recommended: if it heats too much or too quickly, the thermostat will make it shut down often to keep your home from being too warm. This is called “short cycle”, and is actually quite wasteful in terms of energy consumption, not to mention it will also shorten the lifespan of your furnace.

The right size furnace for the space you want to heat will ensure optimal efficiency and the longest life span. But how do you find out the right size?

While a proper load calculation is best left to the experts, there are many tools online that can help you get a ballpark number. These formulas take into account the area of your home, number of floors, location and insulation levels, and they will give you an approximation of how many BTU/hour your furnace has to be able to provide.

Old school furnaces had two settings for their blower: on and off. This is called a single stage furnace blower. When the temperature drops below the set threshold, the thermostat starts the blower, to heat up the house. When the air inside the house reaches the desired temperature, the thermostat turns the blower off. This is not very efficient, and results in uneven temperatures and an excessive use of energy.

Next generation furnaces came with multi-speed blowers, which have 2 blower settings instead of just “on”. This type of furnace is popular right now, and conveniently priced. Multi-speed blowers are the current industry standard, and they do partially solve the issue, but they are still not the most energy efficient.

The newest development (and by “newer” we mean it was developed in 1985) is the variable speed blower. Based on an electronically commutated motor (ECM), a variable speed blower keeps the air circulating continuously, so the rooms are always at the right temperature. Since it doesn’t have to stop and start, there is less wear and tear on the device, and greater fuel and energy efficiency. Variable speed blowers are quieter, require less maintenance and keep the air in the house fresher and cleaner. The downside? They are more expensive up-front, of course, but an investment worth considering if this is a place where you will live for a long time, or if you suffer from allergies or asthma.