Dangers of Blocked Cold Air Registers If your HVAC system is designed this way, blocking off a single return vent can wreak havoc on the system and lead to: Pressure imbalances that let allergens and pollutants get pulled into your air supply from unconditioned areas.
You should never block your air return vents. Period.
In the heating cycle, blocked cold air returns can cause the heat exchanger inside the blower compartment to store too much heat and eventually crack. If the cracks are too large, the furnace can emit carbon monoxide into your home's air. Such a condition normally requires a complete system replacement.
No this won't create any form of a fire hazard. It would be like blocking any other vent in the house. Blocking a vent may create an imbalance within your system, but seeing as you've only created a small obstacle, I don't foresee any noteworthy problems.
Return vents are usually larger than supply vents and are in large rooms in the home. The return vents in your home help equalize the pressure that changes as your HVAC system heats or cools your home. These vents should never be fully closed because closing them off restricts airflow and strains the system.
Parts of Your Home Are Hotter or Colder
If there's not enough return air, your living room can be 10℉+ warmer or cooler than, for example, the bedrooms.
The Most Terrifying Threat: Fire
Think blocking vents is a harmless offense? Think again. Overworking your system isn't just a strain; it's a fire hazard waiting to ignite. Your furnace, pushed beyond its limits, becomes a fiery risk.
There Are Dangerous Long-Term Effects On Your System
The pressure built up within your system due to closed vents can also result in long-term damage to your system. Leaks can be caused or made worse inside your HVAC system ducts, which can greatly impact the efficiency of your system.
Operable cold air return vents have a lever that enables you to open or shut the vent depending on the time of year. It is a small lever that you just push up or down to control louvers, similar to the variable dashboard vents in a car.
Even one blocked vent in the home can cause an inefficiency in air flow, which means your HVAC system will likely have to work harder to achieve the desired temperature. The result may be a home that is riddled with uneven temperatures from room to room.
The Importance of Cold Air Returns
The cold air return vents distributed throughout your home ensure that air efficiently moves in a continuous loop through your HVAC system. Without cold air returns, your system would struggle to pull in the air it needs.
Dirty filters, leaky ducts, and poor insulation can cause cold return vents. Leaky ducts often cause cold air to blow even after cranking the heat up. 20-30% of air, especially hot air, is lost because of leaky ducts. Cracked, holey ductwork needs sealing or replacement.
Broken heating or cooling elements
On the heating side, if you block off the return, the exchanger can overheat. “In extreme examples, it can crack and fail,” Smith says. “They're all designed to have that proper airflow and bad things can happen when that's thrown out of whack.”
Instead, there should be at least one return vent in every room, with two or three being ideal. If your house has only one return vent, this is not a problem — make sure to keep the doors open in every room so the air can circulate properly. Make sure there are no furniture, draperies, rugs, etc. blocking return vents.
Block Off the Vent: You can block the vent by placing a piece of furniture at the opening or purchasing a magnetic damper that's strong enough to stick to the steel on the wall or ceiling. Plastic sheeting can be placed over the opening as well. You can also install a vent grille filter.
There's a good reason why many furnaces shut off when their vents are blocked. For most new systems it is a built in safety mechanism. Without proper ventilation, carbon monoxide can build up inside the house, which is incredibly dangerous as it can cause illness and potentially be fatal.
Contrary to what most people think, closing the vents in a room doesn't divert that air to other rooms. What's actually happening is that air will leak from the ductwork, sending cool air into areas such as interior walls and attic spaces.
Avoid blocking air vents with pillows, furniture, plants, or pictures. When the air vents are blocked it means that heated or cooled air needs to squeeze by what is blocking it to reach the rest of the room.
A Leading Cause of Fires
The Consumer Product Safety Commission reports that approximately 1,200 fires a year are caused by portable electric space heaters.
So how much space does an air return vent need to work properly? Home Inspection Insider explains that an air return vent needs 6-12 inches of space in front of it, and recommends that you do not put large, bulky furniture like couches and bookshelves in front of an air return vent.
You can, but it won't have any benefits. A University of Saskatchewan study in 1972 found that heat in your car is lost to the outside cold air after four hours. Plugging in your car overnight, therefore, will give you no extra boost and will only eat into your power bill.
If your HVAC system is designed this way, blocking off a single return vent can wreak havoc on the system and lead to: Pressure imbalances that let allergens and pollutants get pulled into your air supply from unconditioned areas.
Reduced efficiency: Your HVAC system will work less efficiently if you block a return air vent. This will make the units use more energy and will raise your utility costs.
Dropping a mattress directly onto a floor vent will completely restrict the airflow and prevent the HVAC system from heating or cooling the space. This, in turn, can cause the HVAC system to work overtime, leading to premature wear and tear.