Heating & Cooling

Does a Heat Pump Bring Outside Air Indoors?

Heat Pump

We‘ve grown rely on heat pumps on a daily basis to heat in cool our homes. Without our heat pumps, we would have to deal with sweltering heat in the summer and cold temperatures in the winter. Despite how reliant we are on them, many people do not know how our heat pump operates.

One of the common questions we hear is “does my heat pump draw air from the outside into my house?” No, heat pumps do not draw in air from the outside. It does draw in air, but it comes so from the interior of the house through the air return vent. As air is drawn in through the air return it passes through the heat pump system where it is either heated or cooled depending on the thermostat control settings.

Granted, it’s a bit more complicated than that but you get the gist of it. We will go over how the heat pump works and where the air comes from in more detail. We will also answer some other questions you may have about your heat pump and return air.

People often think that heat pumps draw outside air into the house because of the outside fan. However, that is not the function of the heat pump.

In today’s modern airtight houses, to improve indoor air quality you may hear mold inspectors or HVAC contractors talk about improving indoor air quality by introducing outside air to the heat pump system.

However, heat pumps do not do this. If you need to introduce outside air into your heat pump system, you will need to install an air exchanger that works with your heat pump system. If you are interested in learning more about air exchangers check out our article, Do You Need an Air Exchanger with a Heat Pump?

First, let’s look at how a heat pump works and then we will talk more about air exchangers and how you can use them to bring outside air inside to improve your home’s indoor air quality.

How Heat Pumps Work

A heat pump can be found in two types; the package system (also called a self-contained heat pump) and the split system. Let’s briefly look at both.

  • In a package heat pump system contains all the components including the compressor, heat exchangers, and fans are all inside one central system that is installed on the exterior of the house.
  • A split heat pump consists of 2 separate units typically referred to as the air handler (indoor unit) and the condensing unit (outside unit). The two units are connected through copper tubing where the refrigerant travels from through the system in a continuous looping cycle.
    • The outside condensing unit contains the compressor, condenser coil, and a fan.
    • The indoor air handler unit has an evaporator coil and a blower fan. Air handlers can be installed inside the house, attic, basement and in some cases even a crawl space.

In heat mode, refrigerant leaves the outside compressor as a high temperature, high pressure, superheated vapor and travels to the heat exchanger located in the indoor air handler.

Interior air from the air return then blows over the evaporator coil. The evaporator coil then heats the air and distributes it back into the house through the supply ducts. The refrigerant cools into a liquid form and travels back to the outside heat exchanger to repeat the process.

In cooling mode, the opposite occurs. refrigerant leaves the compressor as a high pressure, high temperature, superheated vapor and travels to the outside condenser coil. The outside fan draws outside air into the condensing unit and across the condenser coil.

The refrigerant transforms into a high pressure, lower temperature, liquid state that then travels to the indoor evaporator coil which cools the coil inside the air handler. Warm interior from the air return then blows over the cold evaporator coil which cools the air and then blows the cooled air through the supply ducts back into the house. 

For more information on this process, check out the video below: 

What Does the Outside Fan Do

So you may be wondering what the outside fan does if it’s not drawing in outside air into the house? Good question. Let’s explore this.

The fan is located on the top of the unit. If you’ve ever placed your hand over the outside fan you’ll feel it blowing upwards. Since the fan is blowing upwards, it draws air in across the outside condensing coil fins then immediately expels it.

In cooling mode, the outside condenser coil will heat up hotter than the outside air. This creates a heat transfer where hot refrigerant transfers heat to the surrounding air and expels it outside.

In heating mode, the outside condenser will become colder than the outside air. This creates a heat transfer where the cold refrigerant absorbs heat from the surrounding air which heats the refrigerant up.

Refrigerant can drop as low as -37 degrees Fahrenheit, so even in extreme cold as low as 0 degrees Fahrenheit, the condensing coil can extract the heat that is present in the surrounding air.

The outside air never actually enters into the ducting system. This is true for both self-contained heat pumps and split heat pumps. So while it may look as through outside air is entering the system, it actually is not.

Where Does the Air in the Ducts Come From?

The hot and cold air from the supply ducts comes from the return air vent that is located inside the house. The blower fan inside the air handler blows in the direction of the supply ducts. As the fan blows air through the supply ducts it draws filtered interior air in from the return vent.

The heat exchanger coil is positioned between the return air coming in and the blower. As the air passes over the heat exchanger is it heated or cooled before entering the supply ducts.

Do Heat Pumps Exchange Air?

The heat pump draws air through an air return located inside the house. The heat pump never draws air from outdoors.

You may be wondering why the heat pump doesn’t just draw the air it needs from the outside instead of from the inside. The reason is really quite simple. During periods of extreme heat or cold, drawing air from the outside would cause the system to work much harder and potentially wear out faster, not to mention more expensive.

This is easier on the heat pump components to recondition the interior air as it doesn’t have to work as hard to heat or cool interior air as it does the outside air. 

This also helps with controlling indoor air quality and humidity. Since interior air humidity levels are more stable than outside air, it can regulate the interior comfort levels easier.

Which Vent is the Return Air Vent? 

Airfilters Lg

The heat pump system needs a large amount of airflow into the system to operate properly. Therefore the air return vent is much larger than the supply vents that are located in the floor or ceiling of your home.

Air returns vary in size, however, will always be larger the supply vents in the floor or ceiling.

Where is the Return Air Vent Located?

The return air vent is typically located near the thermostat. The reason the return air vent is located near the thermostat is to help the heat pump operate more efficiently.

The interior air around the thermostat will closely match the temperature reading on the thermostat. If the return was located too far away from the thermostat, the air coming into the heat pump may be drastically different than the thermostat reducing efficiency and accuracy. 

The return air vent will be larger than the floor or ceiling vents. The return air vent will draw air in rather than blowing air out. The return air vent should also have an air filter installed. 

Should the Return Air Vent Have an Air Filter Installed?

Yes, you should always have a clean air filter installed inside your return air vent. The purpose of the filter is to help keep dust, debris, and other air contaminants from entering the heat pump system.

If you tried to run your heat pump system without an air filter the interior coil will get very dirty and not function properly. Dust and debris can also clog your condensation discharge pipes causing water to back up in the heat pump system and eventually leak. 

The air filter also helps to control indoor air quality by removing dust, debris, and other contaminants (such as mold spores) from the air you’re breathing inside your home. 

Related Searches

Is it ok to cover a return vent with furniture? No, your heat pump needs an unobstructed flow of air to operate properly. Starving your heat pump of air can cause the system to overwork with can lead to mechanical breakdown. 

What will happen if my air vent is installed backward? Does it really matter which way the air filter is installed? It may not appear so but actually yes it does. The air filter is designed for air to pass through it in a particular direction. Installing it backward is like trying to paddle upstream.

If your air filter is installed backwards, air will have a harder time passing through the filter and entering the heat pump. The heat pump needs a large volume of airflow and reducing that volume will force the system to work harder causing higher utility bills.

Is a dirty air filter bad for my heat pump? Yes. A dirty air filter reduces the airflow through the filter and starves the heat pump of what it needs most and that’s airflow. Heat pumps require large volumes of air to work efficiently and keep energy costs low.

Without airflow, the system could breakdown prematurely and not provide airflow. If you have a situation where you have no airflow from your supply vents, chances are you have a dirty air filter.


Hubert Miles | Licensed Home Inspector, CMI, CPI

Hubert Miles is a licensed home inspector (RBI# 2556) with more than two decades of experience in inspection and construction. Since 2008, he has been serving South Carolina through his company, Patriot Home Inspections LLC. As a Certified Master Inspector, Hubert is dedicated to providing his expertise in home inspections, repairs, maintenance, and DIY projects.