The ductwork is one of the most important parts of a central HVAC system, and this is true for systems that use either a furnace or a heat pump. The ducts are what supply heat to every room and also serve to pull cooler air back into the system so that it can be heated.
If your ductwork isn’t properly designed or has air leaks, your heating system won’t be nearly as effective and will struggle to evenly distribute heat around your house. In this article, we’ll explain all you need to know about the ductwork in your home and how it can affect both airflow and heat distribution.
Different Parts of a Ductwork System
The central part of your ductwork and heating system is known as the air handler, which is located either above or next to your furnace. The air handler is where the blower or fan is housed, and it also contains the AC evaporator coil if your home has central air conditioning. If your home uses a heat pump for heating or cooling, its indoor coil is also located inside the air handler.
There are also two other primary parts of any central HVAC system: the supply ducts and return air ducts. The large metal chamber where the hot air exits the furnace is known as the supply plenum, and it connects to the main supply duct known as the trunk. Depending on the size and layout of your home, there will typically be at least a few smaller branch ducts that connect to the central trunk. These branch ducts are what supply hot air to the supply vents in every room. In homes with only a basement and main level, the same trunk and branch ducts will supply both floors. In homes with upper floors, there is usually a secondary trunk located underneath the floor on the upper level.
Central heating systems also have a large return air plenum, which is located on the opposite side of the furnace from the supply plenum and is where the air filter is usually located. When your heating is on, the blower pulls cool air in through one or more return air vents that are also situated on a wall near the floor. However, some larger homes may also have one or more return air vents in the basement ceiling. The cool air being pulled into the system flows through the return duct, through the air filter and then into the furnace.
Air Volume and Circulation Rates
For a heating system to heat effectively, there always has to be a specific volume of air circulating through the duct system. The volume of air that an HVAC system circulates is measured in cubic feet per minute (CFM), and the CFM that a heating system requires depends on the type of furnace and how many BTUs of heat it produces.
High-efficiency condensing furnaces have the highest airflow requirements and need approximately 150 CFM of airflow for every 10,000 BTUs of heat they produce. All conventional furnaces sold in the US now are induced-draft units, and these only require 130 CFM of airflow for every 10,000 BTUs of heat. Natural-draft furnaces have the lowest airflow requirements at only 100 CFM for every 10,000 BTUs. However, it’s almost unheard of to find a home with a natural-draft furnace these days since they were essentially phased out in 1987 when the US Department of Energy set the minimum AFUE standards for gas furnaces.
Heat pump systems have much higher airflow requirements. Although the specific requirements can vary a bit from manufacturer to manufacturer, most heat pumps require 400 CFM of airflow for every 1 ton or 12,000 BTUs of cooling/heating they produce.
If your HVAC system doesn’t move a sufficient volume of air for any reason, it will heat far less effectively and your home will be less comfortable. Insufficient airflow will also lead to higher energy costs and often result in cold and hot spots in various parts of your house. Furnaces can also be extremely prone to overheating if the HVAC system doesn’t have sufficient airflow.
Why Duct Size and Layout Are So Important
The volume of air that an HVAC system can circulate depends mostly on the power of the blower and the size and layout of the ductwork. This makes ductwork design and installation one of the most essential factors in how effectively an HVAC system works. If your ductwork is too large, your HVAC system will have major difficulties pushing air through the ducts. This typically leads to very poor airflow at the vents that are further away from the air handler. As a result, the rooms supplied by those vents will typically be constantly colder than the more central parts of the home.
Ductwork that is too small or too narrow can be an even bigger issue. When ductwork is undersized, it leads to a major increase in static pressure. This can also happen if the ductwork is poorly designed and has too many bends where the air has to suddenly change direction. Dirty ductwork can also greatly increase the static pressure in an HVAC system, which is why experts recommend getting your ducts professionally cleaned at least once every five years.
Another issue that will lead to much higher static pressure is if lots of the supply vents are closed or the airflow coming from your vents is obstructed by furniture or other objects. You’re usually okay to have a few vents closed, but you should never close more than 25% of the vents or else it will make your heating system much less effective.
Static pressure refers to how much resistance the air flowing through the ducts meets. As static pressure increases, it makes it much more difficult for the air to circulate through the ducts since there is more resistance. High static pressure is a serious problem since it forces the entire heating system to work much harder, which will often lead to the blower and the heating unit wearing out faster. Your HVAC system will also make far more noise when the static pressure is too high, and the issue can also cause blowouts in the ductwork and lead to major air leaks.
It’s also just as important that the return air ducts are properly sized and the home has a sufficient number of return air vents. If the return ductwork is too small, there won’t be a sufficient volume of air being drawn back into the system. This will not only greatly reduce the effectiveness of the heating system, but it also creates a pressure imbalance that will cause some rooms to be poorly ventilated and feel stuffy.
The condition of the air filter in your HVAC system is also a major factor in terms of airflow. A dirty filter can drastically restrict the volume of air flowing into the system, which results in it putting out less heat. Your furnace can also be at risk of overheating if you let the air filter get too dirty. These issues are why HVAC manufacturers typically recommend changing the air filter no less than once every 90 days.
Expert Home Services
If you need any HVAC, plumbing or electrical service in the Milwaukie area, 3 Mountains Plumbing is the one to call. We install all types of residential electrical equipment including EV chargers, whole-home surge protectors and smart thermostats.
Installing a smart thermostat is a great way to make your home more comfortable in summer and winter and will also help keep your cooling/heating costs lower. Whether you need drain cleaning, electrical repairs or any other home service, contact 3 Mountains Plumbing to get the professional assistance you need.
