One of the advantages to having a central heating system is that you can add to the system to service new or expanded areas. For example, if you add a room or enlarge a kitchen, providing the necessary heat for the additional space can often be accomplished by adding an additional duct run to the existing system.

When considering the addition of a new duct run, there are two things to keep in mind: furnace size, and the size and layout of the duct system. For all but the very simplest of small duct extensions, you’ll need the help of an experienced heating contractor to make all of the complex calculations required to size and balance the system.

First, you need to evaluate the size and condition of the existing furnace to determine whether it has sufficient capacity to handle the additional load. For example, if you’ve added 100 square feet to your kitchen by bumping it out into the garage and your existing furnace has always been able to heat your home without effort, chances are that adding a duct can be done without any additional strain on the system. On the other hand, doing a 600-square-foot addition and hoping to heat it by adding duct runs onto a system that was barely able to heat the original home isn’t going to work. You’re probably looking at a new system for the entire house, or at least for the addition.

The second consideration is where you’ll tap into the duct system for the new duct. Duct systems are carefully sized to provide adequate airflow from the furnace to each of the ducts in the system. A duct run may start at the furnace with a 10-inch-diameter duct, then step down to 8 inches and then to 6 inches as the duct run branches off and gets further away from the furnace itself. Depending on the size of the area you’re trying to heat and the distance away from the furnace, you may be able to extend right off the end of the 6-inch duct, or you may need to go further back and tap into the 8-inch or even the 10-inch duct.

For long duct runs or runs that will serve a large area, such as handling that 600-square-foot room addition, you’ll usually need to go all the way back to the furnace itself to begin the new run. In that situation, the new duct will be tapped into the furnace plenum — a large box attached directly to the furnace that distributes air into the different duct runs — to ensure that the maximum amount of air volume is available for the new ducts. …CONTINUED

Directing all that heated air into a new duct run will obviously rob air volume from the other runs, and here’s where you can run into some problems. A single small duct run won’t have a huge effect on the system, but several larger ones will. As the airflow is redirected, those rooms farthest from the furnace will suffer the most, and in some cases the airflow will be reduced to the point of being ineffective for heating that space.

Adjusting all of these airflows to all of these different spaces is called balancing the system, and it’s often a tricky undertaking. Setting up and balancing a duct system is a matter of knowing the amount of airflow being produced at the furnace, as well as the size of the spaces being heated, the diameter of all the ducts in the system, and the total length of each of the duct runs. Under- or oversizing the duct runs can result in poor performance throughout the entire system.

Another factor that your heating contractor will take into consideration is the energy efficiency of the original house, as well as the addition. Homes with good insulation, good windows and doors, and a low amount of air infiltration are simply easier to heat. As a result, the ducts serving each space can be smaller.

Remember that energy efficiency is always part of the calculations for a new or extended heating system. So if you’re considering adding on to your home, now is also the perfect time to make some weatherization upgrades as well.

Remodeling and repair questions? E-mail Paul at


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