HVAC systems account for a large chunk of energy bills. So, it makes sense to attempt to reduce energy consumption by shutting the supply vents in unoccupied rooms. However, completely sealing off vents in functioning HVAC units does not produce the desired result.
In this article, I provide a detailed description of the repercussions of completely closing off your vents. I also include safe alternatives that can help you save on energy costs.
Closing vents strains the HVAC system by raising static pressure. Conditioned air takes longer to reach a room. The pressure causes duct leaks, so hot air is lost before it reaches a room. Heat builds in the furnace, making basements too hot. Rooms with closed vents become cold sinks, attracting heated air.
Closing Vents in Unused Rooms During Winter
Winter months are synonymous with high energy bills. So, it isn’t out of place to seek out ways to reduce your energy consumption. However, instead of heating other parts of the house faster, which is the prevailing misconception, sealed vents would only wreak havoc to your HVAC unit.
Pressure in Ducts Increases
HVAC systems are installed with close attention to the size and layout of a house. This way, the heating and cooling system provides adequate conditioned air for the whole house.
By closing a vent(s), you increase pressure in the ductwork, because the system would try to make up for the sealed register. This happens because the air that is supposed to be released through the duct is backed up into the ductwork and, this results in increased internal pressure.
The closed supply vent would cause the system to work extra hard to move the required amount of conditioned air. If the vents are left closed for long, the pressure causes severe damage to the system like duct leaks, overheating, and an eventual breakdown.
Heat Is Lost Through Ducts Before Reaching Rooms
One of the internal pressures that increase is static pressure, which is both a resistance to the forward movement of air and also a force that presses out against the walls of the ducts. Increased pressure on the ductwork causes cracks and breaks, particularly by seams and connections. The result is the air leaks we mentioned in the previous section.
The increased velocity pressure of the air (it has to increase in order for the air to still be supplied to the rooms at the correct rate), would force more air out of these cracks and faster. This means that some of the heated air that was supposed to be supplied to a room is actually lost to the unconditioned spaces, like the attic or crawl space.
Heat Builds in Furnace
An increased pressure forces the furnaces to work extra hard to warm the house. This coupled with air leaks would make the furnace run for longer periods.
To maintain a safe internal temperature in the furnace, components like the heat exchanger require a certain amount of return airflow. If the furnace continues to run and the system is unable to get the required return airflow, it would cause heat to build up in the furnace. If this continued to happen, the heat exchanger would overheat.
A major cause of overheated furnaces is restricted airflow. So, if you close multiple vents in your home, your furnace would continue to perform according to design but the return air would not match the amount of heat the furnace is producing. If this happens for a prolonged period, the furnace might start producing humming sounds and would stop at intervals, usually before it finishes a cycle.
Thermostat is Confused
If the temperature in your home is controlled by a single thermostat, the poor performance of the HVAC system might cause the thermostat to misbehave.
This would be prevalent if the system features leaky ducks and system components that have slowed down due to strain.
Because the system isn’t performing optimally, the temperature in the house might be uneven, meaning that some rooms would be warmer than others. If the thermostat is set to a particular temperature, it might turn on and off at intervals because of the irregular temperature in the house. This confusion is because there are most likely areas in the house that have gotten to the desired temperature and others that have not.
Heat Will Not Be Redirected as Desired
The common reasoning behind closing vents in unused rooms is that the system would direct the much-needed warmth to other parts of the house, causing your home to heat up faster. However, this misconception is far from reality.
If the pressure in the ductwork leads to duct leaks, it would definitely redirect air but in this case, the air would be sent to an unconditioned room, which is usually anywhere the unit is situated. That’s why you might notice that the air in your basement/attic is sweltering even during winter. If this happens, it’s a clear indication that heat is being lost to the basement because of duct leaks.
So, while you wish to reduce your energy consumption by closing the vents in unoccupied rooms, your action would result in air loss. Loss of conditioned air would cause your HVAC system to take longer to heat your home to a comfortable level.
Unused Rooms Become Cold Sink
Additionally, the room that you are not heating is not actually in isolation from the rest of the house just because it is not allowed access to the HVAC system. There are still cracks and seams in the door, walls, and ceilings, through which air can move. And the air will move. All the heated air you are trying to keep in the used areas of the house will naturally gravitate towards the cold, unused room down the temperature gradient.
System Efficiency Deteriorates
HVAC systems come in different specifications and designs. Your HVAC system is usually installed with close attention to the size and the ventilation needs of your home. Thus, the number of ducts installed is according to what the system requires for effective air circulation.
Like I stated earlier, closing vents increases the static pressure within the system. An increase in pressure has so many adverse effects on the performance of HVAC units. It causes the system components to slow down, making them take longer to release conditioned air into your space. If this continues for a prolonged period, the strain on the unit could cause individual components to break down resulting in the need for constant repairs.
Another effect of increased pressure on the system is leaky ducks. If this happens, the system would have to work overtime to heat your room, as it would have to make up for lost air.
All this would cause your system to slow down and you’d notice that its performance is no longer as good as it used to be. For instance, if it used to take the unit 40 minutes to warm your home, it might increase to 1 hour. This would ultimately result in an increase in energy consumption, so closing your vents is not a good way to run an energy-efficient HVAC system!
Safe Alternatives to Closing Vents
If your primary reasoning behind closing the vents in unoccupied rooms is to reduce energy consumption then there are other alternatives you should consider.
So far, I’ve established that it isn’t safe to completely shut off your vents. However, if you must, it is recommended that you only shut them partially, typically not more than 70%. This way you do not completely restrict the airflow from the duct and the slight change in air movement wouldn’t cause harm to your system.
A more costly solution would be to add a zone control unit to your HVAC system. This would give you control over what parts of your house get conditioned air. It allows you to assign different temperatures to various zones in your home.