Let’s start with the bottom line – The air returning to your furnace or air handler must have an air filter to clean the air before it causes dirt-related issues in your HVAC system.
Do I Need a Filter in the Return Air Vent?
So, the answer to the question, “Are return air vents supposed to have filters?” is –
Yes, the return air vent leading to your furnace or air handler for a heat pump or AC should have an air filter. To say it a different way, if your system has one large return air vent that is designed for a filter, you should keep a filter in it at all times, and the filter should be changed when it gets dirty enough to hinder airflow. Some of the major problems of hindered airflow are explained below. Here’s an entire page on the subject.
Filter in the cabinet? The exception is if there is already an air filter in the cabinet of the furnace or air handler. If that’s your setup, then the filter is located on the return air side of the system, so that air is filtered before it enters the furnace or heat pump / AC air handler. You don’t need filters in both the return air vent and the cabinet of the furnace or air handler unit.
This return air vent filter guide gives full details for why you should have a return air vent filter, what will happen if you run your furnace or AC without a filter and what your filter options are (with costs).
Filter Options and Costs
If you buy into our answer to the question – should you put filters in return air vents – then let’s jump right to filter types and options.
Terminology Talk: The term “MERV'' is used frequently in our discussion of air filter types. While the name isn’t much help – Minimum Efficiency Reporting Value – the number is: The higher the MERV number, the smaller the particles, and the more of them, the filter stops or traps, preventing them from getting into the air you breathe in your home.
The top MERV ratings for various filter types – and what pollutants they trap – are discussed on our Highest MERV Ratings for Different Types of Air Filters page. The page includes charts showing what pollutants each MERV rating stops – and the percentage of particles of that type and size are trapped.
So, here MERV and non-MERV filter types for your return-side airflow, whether it is the return air vent or furnace/air handler cabinet.
Spun Fiberglass Filters
There are a few spun aluminum filters in this category too, but most are fiberglass. These are the cheapest return air vent air filters, ranging from a few bucks to about $10.
They have a MERV rating of up to 4, which isn’t very good.
Best Use: If your home is very clean, without pets, and the environment around you isn’t very dusty, these might be enough. Try one, if you want to spend as little as possible. If you’re happy with the results, great. But if anyone notices breathing issues or your home gets dusty more quickly than you’d like, upgrade your return air vent furnace or AC filter.
These are the best-selling of all return air vent filter types. And you have a bunch of options in terms of MERV, from about 5 or 6 depending on the brand all the way to 16. The higher the MERV, the more they catch, and the more they cost. Pleated return air vent filter cost is $10 to about $40 for 1” filters. Thicker filters may cost more.
As the name implies, the material is pleated, accordion-style, to increase the total surface area of the filter. If you ripped the filter material out of its frame, it might be twice as wide, or more, than the frame. Pleated return air vent filters are available in 1”, 2” and 4” versions mostly, but some 5” and 6” filters are sold too. However, most return air vent filter brackets are designed for 1” filters, and you would have to have the setup modified to accommodate a thicker filter.
The average cost to modify a return air vent to house a thicker filter, if it can be done at all, is $250-$400. When modifying ductwork for one of these filters, cost rises to about $500-$600.
Best Use / Pro Advice: We recommend a MERV rating of 8-10 for most homes. The rating is enough to stop most common air pollutants found in homes, but it isn’t so great that it restricts airflow, something that can be detrimental to your indoor comfort and the HVAC system. Below, we talk about the dangers of running your AC or heat pump or furnace without a filter. Dirt builds up in the system and causes multiple problems. Most of the same problems can occur if the filter’s MERV rating is too high.
When a higher MERV filter might be better: If you have breathing issues like asthma or COPD, or if the air around your home is very polluted, you might try a filter up to 13 MERV. But if you experience hot/cold spots in your home, or it doesn’t feel like much air is coming out of the supply vents, or your indoor coil freezes up (read about that problem below), switch back to a lower-MERV filter.
There is additional research on these topics here:
- Recommended MERV Rating for Your HVAC System
- When you Shouldn’t Use a High MERV Filter
- Our Guide to Furnace Filter Types page on Pick HVAC
- How Often Should I Change My Vent Filter?
You’ve got disposable and washable options here. MERV ratings are usually around 8 or 10.
Air passing through the filter creates an electrostatic charge, like rubbing wool socks on carpet does. The charge then attracts and holds small particles of dust, pollen, mold spores, etc. Larger particles are stopped in the conventional way – they’re too large to pass through the mesh of the filter.
Best Use: First, we recommend the washable type, but only if you don’t mind the maintenance required every few months. Most are cleaned by rinsing them off with a hose.
In the Filter Types guide, electronic air filters are discussed. They cost $70-$100 for the unit. As the name implies, they have a plug, so you’ll need a standard 110-120 volt outlet nearby. This vent filter type creates an electrical charge that attracts pollution particles. MERV rating is about 8 for most filters.
UV germicidal lights don’t trap particles. They are designed to kill bacteria and many viruses. If you find that members of your household are often sick, then you might consider one of these in addition to your standard return air vent filter. There are good options available, and we’ve reviewed them on our Top 6 HVAC UV Lights for DIY page.
What About HEPA Filters?
HEPA filters start at 16 MERV and rise to 20 MERV. This high level of fine filtration makes them too restrictive for residential HVAC systems – they would bog down airflow to the point your system would overwork and fail mechanically. And not enough air would get to most rooms to make them comfortable. That’s the short answer. There is more in our HEPA vs MERV Guide.
We’ve covered the need for a return air vent filter and what your filter options are.
The rest of this page goes into more detail about why you need a filter in the return-side of your HVAC system and what can happen if you don’t use a filter.
Basics of Supply and Return Ductwork
We won’t make this long.
Your heating and air conditioning system has two “sides” to it, the supply side and the return side.
Supply Side: Supply air is air that has been “treated.” To be “treated” means to be heated during a heating cycle or cooled and dehumidified during an AC cycle.
Return Side: Return air is air drawn from various rooms in your home back to the furnace or air handler to be treated.
Which vent is the return vent? It is usually the largest vent in your home, and it will be located near the thermostat. It’s located near the thermostat to ensure that air going into the return vent is about the same temperature as the actual air temperature shown on the thermostat.
The tissue test: If you’re still unsure, get a single square of tissue, small piece of ribbon, a feather, etc. Turn on the system. Once the blower fan is going strong, hold the tissue next to the vent you’re wondering about. If the tissue is blown away from the vent, the vent is a supply vent. If the tissue is sucked toward the vent, it is the return vent.
This image shows airflow during a heating cycle with the HVAC equipment in the basement. The equipment could be on the main floor, in the attic, on the roof or outside. But the airflow process is the same: Treated air is pushed out into your home through the supply ducts. Untreated air, aka return air, is pulled back into the equipment to be treated through the return air vent and ducts.
See the blue Return Air label? That is where the vent filter would be installed – unless, as we noted, the filter is installed in the furnace cabinet. In the case of a heat pump system, the filter would be in the return air vent or in the air handler cabinet. In this case, the return air vent is on or near the floor. But many times, the return air vent is on the ceiling or high on a wall.
Why Do I Need a Filter on the Return Air Vent?
Dirt is the enemy of HVAC equipment and of good IAQ, or indoor air quality. Here are potential equipment and air quality problems caused by unfiltered air. Where to begin? It’s a bit of a list, but the notes will be brief and hopefully explanatory enough that you will be motivated to keep an air filter in good condition in your return air vent or furnace/air handler cabinet.
Blower motor: If the motor gets really dirty, it’s rotation will be impeded. This will cause low airflow, and treated air won’t get to the furthest reaches of your home. The dirt and grime might also cause the motor to work too hard and burn out. According to the Pick HVAC, the average cost of blower motor replacement is around $850, so a simple furnace filter replacement of less than $30 can save you a lot of cash.
Indoor evaporator coil: This is an AC and heat pump part. If you have a furnace but no air conditioner, then it doesn’t apply. Heat is absorbed and dispersed through this coil. If it gets gummed up with hair, dirt, lint and other debris, all of which will cause mold growth on the coil, the coil’s efficiency will drop significantly. The result will be higher energy costs. Oh, and the evaporator coil could freeze up and become covered in ice, preventing the system from working at all and causing a nasty leak when it eventually melts. The cost to have an HVAC technician service a frozen and/or dirty evaporator coil ranges from about $150 to more than $300 depending on the scope of the problem.
Drain pan/drain line: During an air conditioning cycle and also during heating if the furnace is a condensing type (90% AFUE or higher), condensation will form inside the unit. The equipment has a drain pan where it accumulates and enters a drain line. If you end up with a bunch of dirt and debris in the pan or line, it will become clogged. Eventually, the accumulated water will leak. Depending on where the equipment is located, you might have a wet basement floor, water damaging the drywall overhead or water leaking out of your vents.
Ductwork: When the coil stops doing its job, moisture isn’t removed from the air, and the result might be mold in your ductwork. That’s a problem for air quality.
Heat exchanger: The heat exchanger is the heart of the furnace. It’s where heat is transferred out of hot combustion air into your ductwork to warm your home. If the system gets clogged with dirt, the heat exchanger might overheat and crack. If it cracks, the combustion air might escape into your living spaces. The combustion air contains carbon monoxide, and breathing carbon monoxide leads to serious illness or death.
Air quality: We’ve touched on it. Nobody wants mold spores circulating through return air and supply air. Plus, if the air in your HVAC system isn’t filtered, then you and those in your home are subjected to air containing basic pollutants like dust, lint, dust mite debris (you can guess what that is), pet hair, pet dander and many others that cause allergies or trigger episodes of asthma.
Here is more information on the Problems of Running a Furnace or Air Handler Without a Furnace.