How does an HVAC system work?

Have you ever wondered how an HVAC/R system works?

I’ll be the first to admit this isn’t the most attractive topic at first glance and the more you investigate it the less appealing it can seem. AC heating and refrigeration systems are complex machines and it can take years to get a firm grasp around all the concepts involved. For this article we will stick to the basic component arrangement and look briefly at how they are intertwined.

A substantial portion of homes today use what is called a split system, that is, there is a unit on the outside of the home and a unit on the inside. These 2 pieces of equipment are connected to each other with 2 copper lines and at least one control wire. Both units will have a separate source of high voltage power and there will be at least one thermostat connected to the indoor unit.

The indoor unit also has a drain line connected to it which carries away condensate from the evaporator coil. Drain lines are often equipped with an overflow safety to prevent damage to the home should the drain line ever become clogged. In some locals this is a code requirement if the indoor unit is located on a second floor or in an attic. The thermostat controls the cooling, heating and fan functions. On more sophisticated systems the thermostat can control other functions as well.  Duct work is attached to the indoor unit and to each area to be conditioned.

For more information on these topics search the forum for system types or thermostats and be sure to look over the article titled component definitions for more detailed explanations. Moving on to some different system configurations.

If you were to take the indoor unit and the outdoor unit mentioned above, and put them together in one box, you will have what is called a package unit. Package units are normally installed on the side of a home or building as well as the roof. If you were to shrink the package unit into a miniature box you would then have a unit suitable to be installed in a window, hence the name, window unit.

Another type of split system that is very popular today, and for good reason, is the ductless mini split system, also called a ductless system. These systems are some of the most efficient on the planet (regarding mechanical cooling that is), they are very quiet, and they can be installed in places where the conventional system will not fit.  A mini split indoor unit is most often hung on a wall with one in each room to be conditioned, while there are also models that can be installed in the ceiling, attic or crawl space. Ductless mini split systems are very popular among online retailers, but you should be very cautious about buying them. I always tell people that ask, buy 2 of them if you are going to get one, that way you have parts available when the cheap unknown brand breaks and the manufacturer is nowhere to be found.

Now we can get into some of the details about how air condition and refrigeration systems work.

We will start this exploration at the heart of the system which is the compressor. The compressor in the examples discussed in this article, is in the out-door unit. It consists of a motor and a pump, both of which are encased in one sealed housing. We call this a hermetic compressor.

During the cooling cycle the compressor pumps a liquid refrigerant to the indoor section called an evaporator coil. The liquid is made to evaporate as it enters the coil while air from the home or building is made to pass through the coil area. The coil looks similar to a radiator. Just like when water evaporates and carries heat away, when the refrigerant evaporates it also carries heat away. The heat energy in the air is absorbed by the liquid as it changes into a gas. The result is a drop in the temperature of the air leaving the coil.

The gas is then sucked back to the compressor which compresses the gas and pushes it into the condensing coil.  Air is caused to move through the condensing coil area while the gas can slightly decompress. When the gas is decompressed in the coil it condenses into a liquid and releases heat while the air carries that heat away. This results in a rise in the temperature of the air leaving the condensing coil. During the heating cycle a heat pump utilizes a reversing valve which changes the path that the refrigerant takes after it leaves the compressor. So, the evaporator and condensing coils change roles and the same compressor can be used to heat or cool.

At this point we have identified the 5 major components which performs most of the mechanical work required for air conditioning and refrigeration systems, but we have yet to review two other forms of heating commonly used in homes. Those are the electric heat system and the gas furnace system. We will start with the electric heat system since heat pumps utilize electric heat in conjunction with the compressor heat. An electric heat system uses heat strips like those found in standalone space heaters. The heat strips are located inside the air handler and they are usually set up to come on one at a time, as needed, depending upon how cold it is in the home or for how long they have been on. Electric heat is very costly to run but it does work very well.

A heat pump system will have electric heat in addition to the heat provided from the compressor for two essential purposes. We call this auxiliary heat and emergency heat. First, the heat pump must go through what is called a defrost cycle when the outdoor coil has begun to build up a layer of ice, which will eventually stop the air flow across the coil.  During a defrost cycle the reversing valve switches into the cooling mode and this is where the electric heat strips are used. If the heat strips were not running during a defrost cycle, the air temperature coming out of the ducts would be very cold and the temperature in the home would quickly drop.

During the defrost cycle the compressor is cooling the air inside and the heat strips are heating the air. What we end up with is usually about 65 -75-degree air coming out of the vents. After the defrost cycle is compete the reversing valve switches back to heating and the heat strips shut off.  It is worth noting here that the out-door unit’s fan motor will shut off for a portion of the defrost cycle and when it comes back on, a large flume of steam can often be seen shooting up from the top of the unit.  As well, the switching of the reversing valve can be very loud and sometimes sound as if something is broken. If the noise subsides and the system resumes heating as normal, then there is likely nothing to be concerned about.

If your system was installed correctly it will also be configured for “emergency heat.” This is used when the outdoor temperature stays well below freezing for days on end in which case the heat pump will run non stop. Heating, defrosting and then heating again. This happens because the heating capacity of the system is too low during these extreme cold days, and the system can not keep up with the heat loss.

The final topic for this article is the gas furnace system. As with the above systems, there is an indoor and an outdoor section but the heat source here is, you guessed it, gas. The most common types are natural gas, propane and oil. There is a section we call the burner compartment where gas is drawn through the chamber and air is passed over the chamber after it has warmed up. This chamber is called a heat exchanger. The exhaust from the exchanger consists of spent fuel as well as unspent fuel and it is very important that the vent system is installed correctly and in good condition. Equally important is that the gas pressure is properly set, and the temperature rise across the furnace is within the manufacturers specifications.

There is allot more to consider for each of these topics and we hope that you will find the information you are looking for in our resource articles.  If you have questions that are not already answered in our forum feel free to reach out to us.