An OutLine of the Heat Pump Cycle in an HVAC System

We are going to track through all the processes and the individual components of the heat pump portion of our split stage cooling and heating system so that you understand how home cooling works.

We will throw in  some helpful tips and tidbits along the way. I  use the term heat pump in the classic sense. Heat pumps are really cool heat engines using the laws of thermodynamics, and the energy requirements for creating phase changes in refrigerants.

The refrigerant moves from liquid to gas and from gas to liquid to transfer energy out of your house to cool it and transferring heat into your house in the heating season. 

compressor info condenser info expansion valve info air handler info info on evaporator coil info on fans info on fans condensing unit info

I don’t really like the above diagram that well; but I cannot draw, so I just swiped  it from Martin Holladay a wise fellow over at Green Building Advisor Here, we are going to discuss all the critical components of the heat pump cycle of your split heating and air conditioning system. This is a subset of our orientation  of  the entire split system here. 

I am going to go through the critical parts of this diagram so you have a better chance to see and fit these basic concepts of components into something that looks familiar to you.

Heat pumps work on the principal of the refrigeration cycle as described in the laws of thermodynamics. If you would like more detail on the the laws of thermodynamics and heat engines and refrigeration cycles, you can look over the nerd’s view. Actually, this is important and has a nice video there that can help you understand the natural science of it all.  It helps to understand the basic principles so that you are less intimidated by experts and you are better able to spot BS for sale.

We are going to start our orientation with the big visible pieces at your home reference them to the above diagram and then describe them as a component and then break them down into their critical sub-components.

The condensing unit

The condensing unit sits outside in the environment. The condensing unit is a major unit of your HVAC system. We discuss the condensing unit in all its configurations here. Right now, we are addressing just the heat pump function here as a combination  of critical sub-components. The compressor is in here, plus the condensing coil. The outside fan is on the top of the unit and facilitates the extraction of energy across the condensing coil.

We have to clarify the name.

Many call the condensing unit a heat pump- particularly to separate it from what some companies call an air conditioner which just uses the heat pump cycle to cool. These same companies designate a heat pump as a unit which can either cool or heat depending on what you want. This is rather arbitrary but just so we know what is happening. A heat pump is a general term for a device utilizing the refrigeration cycle and the laws of thermodynamics to to compress a refrigerant from a gas to a liquid to extract heat.

We will be elaborating on that more after we get our bearings here. But the item below is often called a heat pump… or a condensing unit or even the air conditioner… the names can be  confusing, but the process is straightforward. Just relax and make your way through the following parts list and you will have this whole idea well understood.

This is a typical Condensing Unit. You can see the coolant lines entering the house. These end up connected to the evaporator coil....

This is a typical Condensing Unit. You can see the coolant lines entering the house. These end up connected to the evaporator coil….

need to see the heat pump cycle diagram again

Lets meet the Air Handler

The next picture is of an air handler in an attic. Your might be vertical in a closet or in the garage. The air handler is extremely important to the cooling function of your house and the configuration of it can be quite complex. We discuss air handler technology and what you get for your investment here– but for right now- we are looing at the air handler as a component in our heat pump cooling or heating and cooling system.

The air handler contains the inside fan and the evaporator coil. You can see the refrigerant lines coming in from the condensing unit. The white plumbing at the bottom and the nice catch basin under the air handler is there to catch the condensate collecting on the evaporator coil. We discuss that when we get into the actual heat pump cycle description. 

Air-Handler in attic

Here is an air handler lying horizontal. Yours might be upright in a closet or garage or… You can see the refrigerant lines coming in. The evaporator coil is right behind it. The drain line for the condensate collected is the white PVC line at bottom of image.

There you go. The entire heat pump cycle takes place between these two visible units connected with refrigerant lines. Lets start our discussion of the actual heat pump cycle. We are talking about the cooling cycle here.

need to see the heat pump cycle diagram again?

The Compressor

The Compressor is in the outdoor condensing unit. We are in a cooling cycle in this drawing so refrigerant as a gas is returning to the compressor. This refrigerant is no longer cold as it has been warmed in the air handler by the hot air in your house passing over the refrigerant in the evaporator. OK no more backing up in this story.

Forward! The warm refrigerant enters the compressor where it is pressurized and hot now because it wants to change state to a liquid but has all this energy to dump first.  Make sure you see a nerd’s eye view of phase change properties so that this makes clear sense. 

need to see the heat pump cycle diagram again?

The Condenser

The Condenser is our condensing coil in the condensing unit. We discuss the condensing unit in detail in a bit but lets complete our cooling cycle here. This condenser is an elaborate series of copper tubes build to maximize the heat conducting surfaces between refrigerant and the outside air.

You can see the general principle by looking at the image of the evaporator coil inside the air handler- the critical difference is that the evaporator coil is configured for exchanging energy as efficiently as possible with the air traveling through your plenum and the outside condensing coil is build to just dump energy or gain energy as effectively as possible into the heat sink which is the great outdoors.

The condenser is hot because the refrigerant has entered here and wants to get rid of a lot of energy. The outdoor fans draws outside air over the condensing coil to efficiently allow the heat to evacuate the refrigerant through the heat exchanging coils.

need to see the heat pump cycle diagram again?

The Expansion Valve

The Expansion value is the next stop. We have hot liquid refrigerant under pressure headed back into the house to the air handler. This refrigerant passes through the expansion value where the refrigerant has room to expand and lose pressure. The refrigerant now wants to become a gas.  It will become a gas as fast as it can absorb the necessary energy to complete the task. This happens in accordance with the physical properties of our refrigerant according the laws of thermodynamics. Make sure you venture over to the nerd.s view of the heat pump

need to see the heat pump cycle diagram again?

The Evaporator Coil

The evaporator coil as pictured below is another heat exchanger where to refrigerant becomes quite cold as it turns into a gas and the copper coils and the strategic fins allow the refrigerant to absorb energy from your plenum. You air is being cooled because the refrigerant needs energy to become a gas. The refrigerant leaves the coil as a warm vapor; it then cycles back to the outdoor condenser. And that is where we started this whole trek. We have to imagine the coolant lines in this diagram. They are visible in the image of the heat pump sitting in a yard. There is also an image of an evaporator coil to represent  the evaporative coil inside the air handler portion of the inside unit.  The copper lines come into the air handler and are connected to the expansion value and the expansion value connected to the evaporative coil.

The refrigerant treks through the copper tubing - loses pressure and changes phase requiring a great deal of energy to do so which you are happy to give up- because this process makes your home cooler. Note the water catching tray at the bottom and the fitting for drainage.

The refrigerant treks through the copper tubing – loses pressure and changes phase requiring a great deal of energy to do so which you are happy to give up- because this process makes your home cooler. Note the water catching tray at the bottom and the fitting for drainage.

need to see the heat pump cycle diagram again?

Refrigerant Lines

The copper refrigerant lines You don’t really see them here, but you can see the insulation and copper line entering the house from the condensing unit above and you can see the copper and insulated lines entering the air handler in the next image. The copper refrigerant lines are what deliver the refrigerant around the heat pump collection of parts described above. We discuss the refrigerant in detail here.

need to see the heat pump cycle diagram again?

Inside and Outside Fans

Fans are Extremely Important in this heat pump diagram. The inside fan is inside the air handler and drives the amount of air across the evaporator coil by the speed of the fan. We go into all the details of inside fans when we discuss the air handler separately. The outside fan is in the condensing unit and it just runs to draw air across the condensing coil so that it can get rid of energy into the outside air heat sink that was picked up from your home during the cooling cycle.

need to see the heat pump cycle diagram again?

OK, that was the cooling cycle.
Lets look Briefly at the heating cycle

Heat pumps are also used to extract heat from the environment and bring that energy into your house to warm your house.This is the cycle running in reverse and we will leave that to your imagination. Just a hint.. In a heating cycle the outside condensing fan would be acting as the evaporator coil and the fan would then be moving cold ambient air across the coil and the coil would be extracting heat from this ambient air because energy demand for the phase change of the refrigerant from liquid to gas.

A few Last Caveats that relate to the Heat Pump cycle.

This equipment really needs close attention and regular maintenance. Proper maintenance of your HVAC system is critical to proper performance and to longevity. This equipment is really expensive and problems can accumulate quickly into expensive issues. A Clogged filter will eventually bypass particulates that will then collect on damp evaporator coils. This is a path to poor performance of your heat pump cycle and a first step towards system failure. We cover maintenance issues here.

You might have seen the condensate catch basin portion of the evaporator coil image above. The evaporator coil basically removes heat from your home in the cooling cycle and this energy lose causes condensation which needs to be drained from the house.   You can see the drain line in the air handler image above. You need to keep an eye on your condensation collection system. The line can clog and the overflow can leak or stain the outside of your house. This is part of the job of a technician performing regular maintenance on your HVAC equipment.

Moisture is a critical issue in your home. North Texas is a humid community. It is a critical issue in having a healthy air in a healthy home. Your home structure and your heating and cooling mechanical system determine if you have a problem with moisture or not. We share a perspective on moisture in your home here.

Feel free to add your thoughts below. Ask a question. We will generate some answers.

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