How Your Furnace and Air Conditioner Work

Today we’re walking through the process of how a furnace or an air conditioner works. If you know the different components, as well as the features and functions of what the components are doing then you’ll know why one has a benefit over the other. 

Now if your furnace is in a closet it typically sits up on a platform and sucks air from underneath the furnace, the platform typically has a grill on the front or side of it where the air is pulled from the house into the furnace closet. If it’s in the garage the furnace or air handler will still sit on a platform but the intake will be cut into the floor next to the furnace going up in through the attic then into the home and the grill is typically in the ceiling. 

Let’s say that it’s winter time, the cold air that’s in your home is being sucked in through the blower goes over the heat exchanger, heated up by the gas burners, and it warms up the air. The air now goes through the duct work and comes through the supply registers back into the home. To make it easy, fire makes air hot, it’s pretty simple. 

We’re using the laws of thermodynamics. The first law is, energy is never created nor destroyed it only changes forms. A great example of the first law is the sun’s energy coming down to earth hitting plants and causing those plants to grow with its energy through photosynthesis. In a furnace the energy from the combustible gas turning into fire then hits the metal in the heat exchanger and the air from your home is flowing over that metal where the heat is transferred into it. So the furnace is pretty simple, in the winter time the blower is sucking cold air from the house, into the furnace, over the gas burners, warming the air up and returning it back into the house.

 Now if it’s summer time obviously the air in your house is going to be hot if the air conditioner isn’t running. We’re going to take that same furnace or air handler we were pulling the cold air in with and we’re going to use that blower to pull the hot air in, in the summertime. The heat in the air is held in the particulates, sometimes that’s the moisture in the air or other things, nitrogen, oxygen, whatever is in the air. So we have hot wet air passing through the filter, the filter cleans the air so that the dirt doesn’t build up on the evaporator coil that sits above the furnace, the evaporator coil has pipes that go back and forth all through it and the substance inside those pipes is extremely cold in fact most refrigerants are so cold that they boil at only forty degrees. Now we’re taking the energy from the hot molecules in the air, sucking it over the evaporator coil which takes the heat right out of it and absorbs it into the refrigerant lines, now the air molecules can continue on into the supply duct, past your registers, and back into the house. Those molecules now have a reduced amount of energy or heat in them and it feels cold. There’s really no such thing as ”cold” though, it’s just energy or no energy.

Ok let’s go back to the evaporator coil, now we’ve absorbed all this heat into the refrigerant lines and you’ve got to get rid of it somehow, if you don’t you’ve got hot air going over hot coils and nothing is going to happen. As more and more heat is absorbed into the evaporator coil the refrigerant inside expands and by the time it gets all the way to the end of the piping it’s ready to go outside as a fully evaporated gas, as it travels outside it goes into something called a condenser which typically sits on the ground although sometimes it’s placed on the roof. The easiest way to remember these items is that in the evaporator coil the refrigerant is evaporating into a gas and as the refrigerant goes outside the heat is removed in the condenser and the refrigerant condenses back into a liquid. To break it down, in the evaporator coil the system is absorbing heat and the refrigerant evaporates, at the condenser heat is being removed and the refrigerant condenses.

 Now let’s talk about the condensing unit, for a thorough discussion of your condenser read our article here. Inside the condensing unit is the compressor, it’s the heart of the whole system, compressing the refrigerant but also sending it back and forth from the inside portion to the outside. The compressor is taking a low pressure hot gas with all the heat from your house compressing it into a very tight space so that it gets hotter. The outside air, even though it can be a hundred degrees, is going over the really hot coils exhausting that heat outside. The thing that’s pulling the outside air over the hot condenser coils is called a condenser fan, it sucks air over the condensing coils and blows it out the top of the condensing unit rejecting the heat that was absorbed from inside your home into the outside air. Another way to think of it would be to imagine your house is a canoe with a hole in it and your air conditioner is your bucket. You’re trying to bale out your heat or water faster than it can come in through your walls and windows, so that your boat doesn’t sink, or so you can stay comfortable in your home. 

That is how your furnace and air conditioner work. 

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