Heat and Air Conditioning - a Simple Explanation


Heating and Air Conditioning for Homeowners

You found it! Finally, a simple explanation of how your furnace and Air Conditioner (AC) work to keep you comfortable. I went googling for this for a client and could not find a simple explanation without technical jargon, even after an hour's search. So, if you are looking for a technical, scientific approach to the subject, I suggest going elsewhere.


There are several clever ways to heat and cool the air in a building. We will explore the most common ones found in homes in Southern California. There are other systems which rely on temperature differences in the natural surroundings, these save a lot of energy, are considered "green". For simplicity sake, we will leave those for another day.


We'll start with the controls. There really is no mystery to a thermostat. It is a switch, same thing as a light switch on the wall! The only difference is, it has it's own thermometer built in, and it can be set to heat, cool, or "decide" on it's own whether to heat or cool the house. As the temperature passes the degree you set, the thermostat "calls" for either heating or cooling from the system(s). Now, there are some much more complicated, more energy saving devices out there. The oldest is called a set-back thermostat, because you can tell it to set the temp back to a  lower or higher one at certain times of the day or days of the week. This prevents your system from heating or cooling when you're not home.

One step, or a giant leap, further are devices like the Nest, which you can watch and control over the internet with a mobile device as well. These can be self programming, based on your system use over time. Still, all of these devices perform one simple function, they turn the system on when needed and off when not needed. Like I said, simple.





There are two types, or really one that can be installed horizontally or vertically. They operate similarly in both cases. The most common furnace by far is gas fired, whether natural gas or propane it works on the same principle. The furnace has burners, it automatically lights them when the house gets colder than the temp set on the thermostat. The air required by the furnace to burn gas is obtained from it's surroundings OR it is vented or ducted in from somewhere else, like the attic, crawl space or the building exterior. Somewhere outside the building "envelope". This air combines with the gas to burn, the hot air moves through a chamber and then out the flue.

It's very important that this air is kept separate from the house air, you do not want to be breathing exhaust gas! It has a fan to circulate air over the hot chamber. This fan pulls air from the house into the furnace, over the hot chamber and then pushes it back into the various rooms of the house to keep you comfortable. That's it. Simple. There are a lot of computerized controls and safeguards as well, we don't have to go into those to understand how a furnace heats a house.

Air Conditioner

The most common way to cool the air is a split system. One part is outside the building, typically on the ground nearby but sometimes on a roof. This part is called a condenser, and it's connected to the other part that is installed as an added part to your furnace called an evaporator.

There are a couple of pipes connecting the two parts together, and refrigerant - "stuff" - gets circulated between the outdoor and indoor parts of the system. The evaporator, which is inside ducting attached to the furnace, looks sort of like a radiator in a car. Stuff circulates through it and warm house air is pushed or pulled through it by the furnace fan. The stuff inside the coil pulls heat out of the air, making it colder and you more comfortable. It also pulls moisture out of the air, more on that later. The stuff circulates through the pipe to the outdoor part, the condenser, where there are similar coils and a big fan. The condenser cools the warm stuff in the coil with the fan and then sends it back to the evaporator near the furnace to start the whole process over. There's a bunch of physics we could complicate this explanation up with, but why? The principle is simple. An air conditioner takes heat out of a building and dumps it outside! Simple as that.

Now, back to the evaporator in the furnace, where does all that moisture that gets pulled out of the house air come from? Think of a cold surface like an iced beverage glass on  a hot day. The glass "sweats". Moisture in the air condenses on the outside of the glass. Same thing happens at the evaporator coil in the furnace, so there is a pan under the coil.  The moisture is collected and must be disposed of somewhere, so it gets piped to a safe place, like somewhere outside the house. A pipe that only drips when you run the AC is normal. Sometimes, the installer runs this pipe to a bathroom sink drain, where you will see a hose or tube connected. Now you know what that hose is for. By the way, on a humid day I have measured the flow of water from my own system at one gallon per 2 hours of cooling. You might consider collecting the water and using it to water your plants, it's very pure water.

Package Systems

There is another type of system where the furnace, evaporator and condenser are all packaged in one box. Bigger than a furnace but smaller than a furnace and an AC condenser together, these are typically installed on a roof or, rarely, on the ground next to the building. They require direct supply and return ducts into the house through the attic or crawl space. These systems work exactly the same as the furnace and split system outlined above. Advantage is smaller size overall and maybe easier installation.

Heat Pump

Typically installed in the hall or closet ceiling of a condo unit in a multi-story building, this is a different system with some similarities. There is no furnace to make heat, no gas burners at all, the unit runs entirely on electricity. The part that's similar is, the system is split just like the AC above, and works in the same way. The amazing part of a heat pump is, to make heat a valve is switched from cooling to heating mode, and the coils at the outside draw heat from the surrounding air and "pump" it inside to the same coil that works to cool the air in AC mode. What's amazing is, the system will do this efficiently even when the outside air temperature is lower than the building inside temperature. This works efficiently down to about 40-45 degrees. Free heat! The only cost is electricity for the fans and to move the stuff from the outside through pipes to the inside unit and back. Heating and cooling with the same system, just by adding an electronically controlled valve!


There are other systems in use, but they are so rare as to hardly rate a mention. Water borne heat pumps on very large condominium buildings (think towers), geothermal, etc. For more on these, I suggest googling them. Hey, I had to stop somewhere!
All these systems have one major thing in common, they are designed to move heat, thermal energy, from one place to another. That is just how simple heating and air conditioning is. I hope I've solved some mysteries for you.


While furnaces and ACs are simple, they do need maintenance to continue working and to do that efficiently. 


The biggest enemy of these is dust, dirt and hair that gets sucked into the system, clogs up the filter and fan blades, and makes the system work harder to do it's job. This costs you more money to operate it. Filters are cheap, I suggest you look at the filter, either in your ceiling, wall or at the furnace, and change it when it starts to look dirty. Filters are NOT designed to make the air cleaner for your lungs, they are designed to protect the system and keep it clean. I suggest using the cheapest flat filter you can find. The pleated type reduces the efficency of the system and is not needed unless you have respiratory issues. Then you should use whatever type system or filter your doctor recommends and bear the additional cost.


For safe, efficient use of your furnace/AC, I recommend yearly service by a qualified technician. The tech will clean and examine the system as needed. He should also examine the chamber where gas is burned. This area can develop cracks or holes, letting the exhaust gas mix with the house air. Not good! This is a service that is beyond the scope of a general home inspection and should be performed by a specialist. The optimum time of year to get this done is either spring or fall, before the cooling or heating season. The techs are not as busy, you may even get a better deal.
I hope I've kept this simple enough for everyone to understand, but comprehensive enough to be truly useful. If you have any comments or questions, feel free to email me at gary@dewittinspections.com I'm here to help.