The best place to start may be "what is variable speed?"
"For any furnace, providing maximum comfort means achieving the proper amount of airflow. This is true for both heating and cooling operations. Unlike conventional single speed motors, a variable speed motor runs at a wide range of speeds. Using intelligent technology, it continually monitors incoming data from your heating and cooling system, and it automatically makes the adjustments necessary to meet your comfort needs. The motor varies the amount of air circulated, compensating for factors like dirty filters by increasing speed. Put simply, it delivers just the right amount of air necessary to provide the desired level of heating and cooling comfort." Danny Heineman
As you will see in the chart above, this feature enhances every part of your heating and cooling system. Both Comfort and Savings, even your Health, is affected by having a variable speed blower in your furnace or air handler. The easiest way to explain all this is going to be by taking these one at a time, so get ready.
We will start with the most important of the three, which may not be the one you would expect. It is your Health. There is an insane amount of data that will back up my claim, but all you need to know is that while you can see "dust" in the air and settled on surfaces, about 99% of the particles in the air are smaller than 1 micron (1 millionth of a meter). The human eye can only see particles bigger than 10 microns in size. So next time you are looking at all the dust in your air when the sun is shining in your window, think, "Wow, that is only 1%?!" and then call me.
How variable speed effects your health is really very simple. Tell me, what is your furnace doing when it is 68 degrees out side? That's right, nothing. What is your filter doing while your furnace is doing nothing? More nothing. That is where variable speed comes in; when the furnace is not heating or cooling and the fan in is constant operation, a variable speed furnace will continue to slowly and inexpensively circulate air throughout your home. This allows your filters to capture more contaminants (because air is constantly passing through them), so you can breathe easier.
*Note: I have a Carrier Variable Speed Furnace with the Carrier Air Purifier, read my experience on the What's in My Home page.
From a Comfort stand point, variable is a god send. Literally, a gift to man sent from heaven. The hilarious thing is that it is just so simple. The element that it all boils down to is constant air flow, period. That is it. Most people do not turn on their fan 24/7 because they believe it will cost them a lot of money (on a standard blower they are correct) and more often than not, it is just too loud. What variable blowers do is ramp the blower down so you do not notice it running, nor does it cost you any more money than standard furnace operation.
Sorry, back to Comfort. Here is what variable speed will do by providing constant air flow. The greatest thing is the even heating and cooling you get. Instead of having the upstairs hot and downstairs cold, variable combines those two environments to create an equal and balanced air temperature. When I turn off my fan in my home the upstairs will generally be about 7-8 degrees warmer than my downstairs; once I turn it back on, the spread becomes 1 degree if not the same!
It also solves the age old issue of Cold Blow. This is the burst of cold air that comes from the ducts before the hot air gets to you, just like the cold water at your faucet. Well, just like plumbers have circulation pumps that keep hot water in the pipes at all times so when you turn on the faucet you just get hot water, we do the same thing with variable speed. When the furnace turns on, the ducts are already tempered with warm air flowing through them constantly, so you do not feel that blast of cold air in between heating cycles!
Next is Savings. Not only is the blower itself more efficient at about 8-10x that of a standard blower, but it makes everything it is attached to more efficient as well. You will consistently get better efficiency ratings on heat pumps and air conditioners that are attached to a variable speed system. Not only that, your furnace will operate better and your filters work harder for you.
Perhaps the most abstract, but exciting, feature that variable speed offers is the use of latent BTUs to evenly heat your home. When you touch a light bulb it is hot, your audio/video system is hot, your refrigerator pumps out heat like crazy, and you emit 400 BTUs per hour yourself, not to mention Solar heat gain! Just think if there was a way to take all that heat being generated and distribute it all throughout the house, creating less need for your heating system to turn on! Of course, you know the answer.