Blocked or Clogged Air Filters Blocked return vents are common causes of insufficient return air. It is crucial to keep return air vents unobstructed to ensure efficient operation of your HVAC system. Furniture placement, dust buildup, or closed vents can obstruct airflow.
Inadequate return air to a furnace will cause the heat exchanger to get hotter. Depending on the amount of air being restricted, this can cause the heat exchanger to crack and leak combustion gasses into the air flow. This happens far too commonly in residential units.
To fix this, you could look at enlarging the existing return grill or adding an additional return grill. You would need at least another 176 square inches of free space or roughly 270 square inches of filter space. So something like a 14x20 should put you at that threshold.
However, most HVAC contractors use a rule of thumb to determine the number of air vents needed. For example, for a standard residential HVAC unit, you'll generally need one supply vent and one return vent for every 100 to 150 square feet of living space.
From dirty air ducts, blocked damper valves, damaged duct runs, to an insufficient numbers of return air vents, there are a variety of signs that may indicate a furnace air flow issue in your home.
By installing more return vents throughout your home, they can trap the air and remove it, improving air circulation. A drawback with this method is that your ductwork needs enough space to support additional vents. Smaller homes that can't add more vents need to pursue other options.
If your boiler or furnace is in an enclosed space and relies on indoor air for combustion, you run the risk of incomplete combustion, which produces toxic byproducts like carbon monoxide.
You might notice hot and cold spots or uneven temperatures from room to room. Early identification of these issues is key; check if some areas feel stuffy or stagnant. Consistent discomfort in specific rooms is a clear sign of poor air circulation.
You don't need an HVAC return vent in every room. In fact, many homes only have a single, centralized return duct. However, in houses with one return vent, air can only circulate properly if all the interior doors remain open.
As mentioned, dirty filters are the most common cause of a furnace blowing out cold air when the heat is on. But some other common reasons for cold air blowing out when the heat is on include: your thermostat being set incorrectly, your furnace having overheated, or your condensate line being clogged.
Because the return air vent works to suck air from a room and back into the ducts, you can try this simple test to see if it is working: take a tissue or thin paper and hold it in front of the return air vent, about 6” away. If the paper is being pulled toward the vent, you know it is doing its main duty.
Undersized return ducts can cause up to 20% of the conditioned air in your home to escape. This occurs due to air pressure and not a hole in a duct. The excess pressure is caused by exhaust air staying in a room longer. The room may become uncomfortable and stuffy.
Due in large part to the short cycling issues mentioned above, components on an oversized heating and/or air conditioning systems are more likely to fail sooner than those on a properly sized system. As a result, you may end up having to replace your HVAC system sooner than expected.
If you have a dirty or clogged furnace filter, the cool air from inside your home will struggle to travel through it and into the furnace. This means you have insufficient air to heat and redistribute. The result is your heater blowing cold air.
Leaving a 6- to 12-inch clearance around your return air ducts can keep your heating and cooling units operating smoothly.
Larger AC units may cool your home faster, but that means the cooling cycle will end faster and they will shut down faster – only to cycle back on again before very long. This rapid cycling creates a lot of wear and tear on the system, shortening its life and increasing the maintenance needed to keep it running.
Homes measuring 1,000-1,500 square feet: You'll need a 2-ton unit, which corresponds to a cooling capacity of 24,000 BTUs per hour. Homes measuring 1,500-2,000 square feet: You'll require a 3-ton unit, equivalent to 36,000 BTUs per hour. Homes measuring 2,000-2,500 square feet: A 4-ton unit is necessary.
A return air drop replacement will run around $300-$400, including installation. In rare cases where a second drop needs to be added, the cost can be up to twice that amount.
The earliest HVAC systems featured a large, single return vent placed somewhere in the middle of the home, but this is not the most effective system. Instead, there should be at least one return vent in every room, with two or three being ideal.
Common Issues with Return Ducts: Clogs and Leaks
The blockage usually comes over time, by a buildup of dust and debris that just accumulates until it begins choking the system. To alleviate this, perform consistent air duct maintenance, as well as utilize the help of a professional with professional HVAC maintenance!
This can be caused by a loose fan belt, or a dirty motor. Solution: First, clean the blower fan and the area around it. It has to deal with a lot of air, so it naturally becomes dirty over time. If that doesn't fix it, the fan belt probably needs to be replaced.
Improve Airflow with a Booster Fan
Installing a booster fan in your HVAC system ductwork will increase airflow to rooms that are struggling to get warm air. A booster fan is a DIY trick that homeowners can use if they don't want a complete overhaul of their ductwork.
When temperatures drop below zero or even below freezing for extended periods of time, sometimes your furnace can't keep up. If your home is not heating up to the temperature you've set, try lowering it a few degrees. The temperature of the air coming out of your vents doesn't get warmer by turning up your thermostat.