Should You Put Filters in Your Return Vents?


Air filters or not something that you think about on a daily basis. However, they are important to your overall indoor air quality and the health of your heat pump system.

One of the questions we are asked a lot is, “Should you put filters in your return vents?” Yes, air filters are an important part of your heat pump system. The air filter helps to keep your indoor air clean by removing airborne contaminants, like allergens, from the air. The air filter also serves as protective equipment that protects the evaporator coil inside the heat pump from becoming dirty, which can cause longterm damage to the heat pump. 

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If you run your heat pump system without an air filter, dust, dirt and other debris will collect on the evaporator coil inside the heat pump. During operation of the heat pump, condensation develops on the evaporator coil. This condensation is part of the heat exchange process and has to drain away from the heat pump. 

If your evaporator coil is dirty, this dirt and debris gets trapped in the condensation and can clog your discharge pipe. If the discharge pipe becomes clogged, condensation can not flow properly and could back up causing your heat pump to stop working, or worse could cause water damage to your home.

Now that you know that you need an air filter, let’s examine why return air vents need filters, what type of filter you need, and if every room needs a return vent.

If you’ve ever thought, I’ll just run my heat pump without an air filter, don’t. It’s not recommended. You can do real harm to your heat pump. See our article Can You Run a Heat Pump Without an Air Filter.

Which Vent is the Return Vent

In most heat pump setups, the return air vent will be located near your home’s thermostat. The thermostat should be located as close to the center of the space as possible to create a balanced system. 

The return air vent will be located on the wall, in the ceiling, or in the floor on some older homes. The return air vent is generally much larger than the supply vents located in your floor ceiling. This is because you usually only have one return air vent per thermostat. 

If you have a multi-zone system, you have one heat pump being controlled by 2 thermostats. The 2 thermostat control damper doors positioned inside the ductwork that open and close based on demand. In a multi-zone system you’ll have two thermostats with one air return vent for each thermostat.

If you were still unsure as to which vet is your return vent, there is a simple task you can do to help you make that determination. It’s called the tissue test. With the heat pump on, use a piece of tissue and place it up next to the vent. If the tissue blows away from the vet this is a supply vent. However, if the tissue is drawn inward towards the vent this is the return vent. This is what you’re looking for.

Why is the Return Air Vent Near the Thermostat

The return air vent is generally located within 10 feet of the thermostat. The return air vent is located near the thermostat so that the air drawing into the heat pump system closely matches the temperature displayed on the thermostat. 

The thermostat is reading the ambient temperature of the space. Thethermostat is directing the performance of the heat pump. Heat pumps will generally look to obtain a 15-20 degree temperature differential. The return drawing into the heat pump needs to match the ambient temperature for proper operation. 

The return air vent comes in a variety of shapes and sizes but most are bigger than the supply air registers. The size of your return air vent is in direct correlation to how much return air your heat pump needs. Here are some common sizes for return air vents:

What Type of Air Filter Do I Need

The MERV rating on air filters stands for Minimal Efficiency Reporting Value. The MERV rating starts at 1 which is low-efficiency up to 16 for  high-efficiency. Most air filters will have a MERV rating printed on its packaging. If no rating is present it should be assumed that it’s a low-efficiency air filter. 

Air filters with a MERV rating of 13-16 are high-efficiency air filters that are not suitable for some standard heat pumps. Air filters with a Merv rating of 8-12 are a great choice because they are suitable for most heat pumps and will do a great job at capturing a wide array of airborne contaminates. 

Here are the 4 common types of air filters. They are:

  • Disposable flat fiberglass: these air filters typically have a MERV rating of 4 or less. They are inexpensive and are generally replaced every 30 days. These air filters are not recommended unless you are only concerned by cost. 
  • Disposable pleated filters:These air filters Are available in a variety of MERV rating from 4-12. These filters will typically very in price based on the MERV rating and how many contaminants the air filter will capture. These air filters are typically used for a period of 30 to 90 days. 
  • Electrostatic filters: electrostatic air filters are interesting in that they can be either disposable or permanent. Permanent electrostatic air filters are washable and can last up to eight years. Electrostatic air filters you static electricity that charges the material causing air contaminants to stick to the fibers. Plated disposable filters are also considered to be electro static filters and actually become more effective as they age because of the build up of particles which helps block more particles. The opposite is true with washable electrostatic filters because as a builder occurs on the filter to filter becomes less effective. 
  • High-efficiency pleated air (HEPA) filters: HEPA filters are thicker than the air filters that are installed at the air return. These are generally located inside the ductwork. Since these air filters are somewhat harder to maintain, it is generally recommended do you have your heat pump on my service contract. 

Does Every Room Need a Return Vent

No, every room does not need to return vent. In most modern homes you would only have one return air vent. However in some older homes, it is common to have a return air vent and a supply air vent in each room. 

If you own an older home that has return air vents in each room, you likely have a central location where the air filter is installed that is closer to the heat pump system.  Most heat pump systems (particularly split systems) will have a filter slot at the end tag where the air return duct connects to the air handler. This is likely where your air filter is located 

For more information on changing your air filter see our article How to Locate & Replace Your Heat Pump Filter: Step-by-Step Guide. This step by step guide will help you locate and change your air filter quickly.

Can I Block the Return Vent? 

Most people find the return air vent to be unattractive and will ask if they hide the return air vent with furniture or a picture. No, The return air vent is a very important component to your heat pump system and cannot be obstructed. If you block the return air vent it can prevent your heat pump system from working properly. The heat pump system needs the return air in order to blow air through the vents.

The way a heat pump works, the blower fan turns on and draws air through the return across the evaporator coil and then out the supply ducts. Without the return air flowing freely, you will have little to no airflow from your vents. This will create a strain on your heat pump system which could cause mechanical failure overtime.

If they return air vent is unattractive to you, you can purchase decorative grills that can improve the appearance with out obstructing the flow of air through the return air vent. 

In Closing

Maintaining your air filter and return air vent will go along way to making your heat pump last for many years. Heat pumps that have been kept clean and well maintained can last up to 20 years or more. 

The heat pump system as one of the most expensive mechanical components inside your home. It’s also a system you do don’t want to fail when you need it most. It only makes sense to ensure that your heat pump system is well maintained and running smoothly. 

If you need, set a monthly reminder on your phone to alert you when to check and change your air filters as needed. Purchase several air filters and keep them on hand so that when it’s time to replace it you can do so in an instance.

Sources:

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Hubert Miles

I've been conducting home inspections for 17 years. I'm a licensed Home Inspector, Certified Master Inspector (CMI), and FHA 203k Consultant. I started HomeInspectionInsider.com to help people better understand the home inspection process and answer questions about homeownership and home maintenance.

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