What is a Two Stage Furnace, How Does it Work and is it Right for You?

What is a two-stage furnace?  Contractors like to throw around technical words and specifications when they are making a sale, and HVAC contractors are no exception.  We sometimes get asked about two-stage furnaces and whether or not they are worth the extra money, and as you might imagine, the answer is, it depends.  There are several factors to consider when making the decision about whether or not to buy a two-stage furnace, including the average temperature for your region and your budget.  One thing is certain, two-stage furnaces are far superior to their single-stage counter parts.  If you’ve read some of our articles then you know that All Systems Mechanical is a small, U.S. Veteran-Owned business in Southern California, and we pride ourselves on giving straight, honest answers and have built our reputation on it.  In this article we will explain what a two-stage furnace is, how it works, the benefits of buying one and hopefully help you answer the question of whether or not a two stage furnace is right for you and your home.

What is a Two-Stage Furnace?

When someone refers to a one-stage (aka single stage) or a two-stage furnace, they are actually talking about the burner section inside of the furnace, or more specifically the valve that controls it.  Your average, every day, old fashioned furnace is a single-stage or one-stage furnace.  What this means is that the valve that controls the amount of fuel that enters the burner in your furnace has two different positions: open and closed.  When this valve is open, fuel (usually natural gas or propane) travels through the valve to the pilot light where it ignites, and burns inside the burner section of the furnace.  The now heated gases then heat up the heat exchanger, which is where the air from your home is actually heated (for more information on what a heat exchanger is and how it works, see: Furnace Heat Exchanger).  When the valve is in the closed position, the heat is off because no fuel is being provided to the burners and heat exchanger.

A two stage furnace actually has three positions to this valve: full open, partially open and closed.  What this allows your furnace to do when it turns on is move this valve to one of it’s two open positions (hence the term, ‘two-stage’).  This allows your furnace to operate at full blast, like your old fashioned furnace, or in a low position that only provides about 60 to 65% of the heating power of the full-open position.  Therefore a two stage furnace has two heat settings that are used automatically, high heat and low heat.

How Does a Two-Stage Furnace Work?

A two-stage furnace works by using a different set of programming than a normal one-stage furnace.  Simply put, when it is only moderately cold inside, then the furnace’s control board opens the fuel control valve to the partially open position, and heats the house to the desired temperature before turning off.  If the house is cooling a lot quicker than normal, for instance on a miserably cold day in the dead of winter, then the control panel opens the valve to the full open position, where the furnace runs full blast to heat your home.

How does your furnace do this?  Well much of the technology is proprietary, but the gist of it is that the position of your valve is based on how often the thermostat is kicking your furnace on and off, and the difference between the temperature set on the thermostat and the actual temperature of the house.  If it is only moderately cold, for instance, then this would not require a full open position on the furnace.  You can kind of think of the operation of a two stage furnace as being kind of like the gas pedal in your car.  If you are driving on the Autobahn in Germany, then you will probably need to floor the gas pedal.  However, you don’t need to floor it when you are driving through the suburbs of Santa Clarita, California (although it seems as though people still do sometimes).  Just like your gas pedal, a two stage furnace can vary the amount of fuel that is used.

For more information on furnaces and how they work, try the Wiki page: Furnace.

What are the Advantages of a Two Stage Furnace?

There are three advantages to having a two stage furnace: it saves energy, it heats your home more completely and it filters your air better.

How a Two Stage Furnace Saves Energy

This part is probably pretty obvious.  Your two stage furnace saves on energy costs because it doesn’t burn as much fuel.  Going back to the Autobahn example, how would the average MPG of your car look if you only drove your car using two gas pedal positions, flooring-it or no gas at all?  It wouldn’t look very positive, would it?  Your two-stage furnace saves gas because it doesn’t dump as much fuel into the heat exchanger when it’s only moderately cold outside.  If it’s only 40 or 50 degrees out, then a two stage furnace will run in the partially open position and only use about 60% of the energy of it’s single stage counterparts.  A one stage furnace, however, can only run in the full open position, even if it’s only 50 degrees outside.


A Two Stage Furnace Heats Your Home More Evenly and Removes Cold Spots

The reason your home gets cold spots is one of three reasons: improperly positioned ductwork and air registers, a furnace that is too large for your house, or if it is a one-stage furnace.  Improper ductwork means that heat is not being evenly distributed throughout the extremities of your house.  An improperly sized furnace will constantly turn on and off because it is so powerful that it heats up your house in a matter of seconds and then kicks off again, allowing your house to cool before this heat is evenly distributed.  The third reason is because of a one stage furnace.  Just as with the example of using a furnace that is too big for your house, a single stage furnace only has one setting; full blast.  As such, it will heat your house rapidly and then turn off allowing your house to cool down, causing cold spots before it kicks on again.

Where a two stage furnace helps you out is that on a moderately cold day, the furnace kicks into low heat and runs continuously for several minutes and for it’s whole heating cycle.  Furnaces (and air conditioners) are made to operate for a specific period of time each time that they turn on.  This is how modern furnaces are so efficient.  If a furnace is too large, it kicks off before it is allowed to run through it’s whole cycle, which means that air is not evenly distributed and circulated throughout your house and your insulation is not properly warmed up, causing cold spots.  For more information on how to properly size a central air conditioner, try: What Size Central Air Conditioner Do I Need for my House?

Two Stage Furnaces Filter Your Air Better

Because a two stage furnace will run more often, it will be circulating more air through your ductwork and thus increase the filtration of your house.  If a furnace is one stage or is too big for your house, again it will kick off more often and spend less time filtering the air in your house.  More air moved means more air filtration.  For more information on the differences between a one stage and a two stage furnace, try this short video: Single Stage vs Two Stage Furnace.

Final Thoughts on Two Stage Furnaces

Whether or not a two stage furnace is for you depends on your own personal needs.  I would say, without a doubt, that a two stage furnace is well worth the few hundred extra dollars that you will have to pay for it.  It is a useful feature that saves you money each year on your energy costs, removes cold spots and increases filtration.  For more information to help you in your quest for a new furnace, try our: Lennox vs Carrier Furnace Review, the troubleshooting article: Furnace Not Blowing Air? Furnace Troubleshooting, or just visit our ASM Air Conditioning Blog.  If you live in the Santa Clarita or Los Angeles, or if you have more questions please feel free to give us a call and we’d be happy to help you.

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