Breaking Down the Parts of a Air Conditioning System
Technicians just starting in the field have many questions about the process required to troubleshoot an AC unit. In this series, I’ll break down the major parts of an AC system. But first, let’s go through a simple service call to figure out why the AC in question is not working. Then we can get into more details in this series once we know what’s going on.
To successfully troubleshoot an AC unit, let’s start at the thermostat and go all the way to the outdoor unit turning on and the blower turning on, forcing air into the rooms of your house.
When your house reaches a point where the AC needs to come on, a series of components work in a specific order to produce cold air. So, go ahead and turn on the air conditioner. Set the temperature down below what the temperature of the room is now.
Taking this step will make two switches inside the thermostat close: the Y and the G terminal. Y is for cooling – it turns on the outdoor unit, and G is for the air handler’s blower fan.
At this point, I always check the filter to make sure it’s clean. Without a clean filter, your system can’t breathe in, so it won’t be able to breathe out, sending air into the house.
The Air Handler
Let’s go to the air handler first and see what’s going on there.
At the air handler or furnace, the control board is what’s calling the shots. It receives the signal from the thermostat for Y and G to energize the terminal block. If you put your meter leads on the C and Y terminals, you’ll get 24 volts. Between C and G, you’ll get the same.
G is going to send the signal to the relay switch on that same board. The closed switch tells the blower motor to come on. It allows the 120 volts from the correct blower tap to start turning the blower wheel. The blower motor on these units will have a capacitor on it. See my video below outlining the steps to test it. On models made after 2019, blower motors became a little more advanced and energy-efficient. Digitally commutated motors like this don’t use a capacitor.
The only other thing going on up at the air handler is the cold evaporator coil has refrigerant moving through it. There’s a metering device at the coil, but we’ll address that in another segment in this series.
Some furnace and coil combos have a condensate safety switch wired into the control board. The air conditioner creates condensation that drains out to the side of the house. This switch provides a safety device that stops the air conditioner from producing any more condensation should the drain clog up. See my video on this topic as well, below.
The Air Conditioner
Now let’s get out of this hot attic and head out to the air conditioner! Technicians must be safety conscious at the AC. Two hundred forty volts flowing through your body is no fun but regularly happens to people who aren’t qualified to work on it.
Let’s see what should be happening at the air conditioner when you take the panel off. That Y signal from the air handler connects to the contactor, which pulls in, allowing the 240 volts from the house on to the compressor and condenser fan motor. The compressor will pump the refrigerant to and from the outdoor coil and the indoor cold coil we talked about earlier. The condenser fan motor keeps the outdoor unit cool by sending the heat from inside the house out of the AC unit’s top.
From here, the AC will provide about 18 to 22 degrees cooler air than is going into the return side of the system. If it’s not and the air is reaching that temp split, you may need to check the refrigerant charge and start doing some more in-depth troubleshooting, which is just what this series will explore.
Troubleshoot an AC Unit: Improving Your Skill Set
As a new technician, you don’t have to be intimidated by all kinds of moving parts and thermodynamics. Yes, when you get down to the details about it, you’ll need to have a greater skill set, which means more training – and hopefully, this series will provide that for you.
Thanks so much for stopping by, and we’ll see you at the next blog post!