A furnace and duct system is a carefully designed arrangement. The furnace needs to be of sufficient size to heat all the air within a particular home, and the ducts need to be large enough to deliver an adequate volume of air to each room. If the duct going to one room is too small, the room won't get enough heated air to sufficiently raise its temperature. If the duct is too large, the room may get overheated, or the large duct may rob enough air from the rest of the system that the other ducts won't have sufficient volume to heat their assigned rooms.

This relationship between furnace size and duct size is known as balance. A properly balanced system will operate more cost-effectively, and will provide adequate heat to each room without under- or overheating it. So, 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 will typically need the help of an experienced heating contractor to make all of the complex calculations required to size and balance the system.

One of the first things the heating contractor will take into consideration is the overall energy efficiency of the home, as well as the remodeled space. Homes with good insulation, good windows and doors, and a low amount of air infiltration is simply easier to heat, and as a result, the ducts serving each space can be smaller. Homes with poor energy efficiency require larger ducts to overcome the heat loss and keep the spaces sufficiently warm.

The next consideration is the furnace, which needs to be of sufficient size to provide an adequate amount of heated air for the volume of the home and amount of heat it's losing. If the furnace is large enough, then a new duct run can typically be added to the system pretty easily. If the furnace is too small, then you need to either upgrade the size and/or efficiency of the furnace or improve the home's energy efficiency through better insulation, better windows, or other means.

Finally, you'll need to determine how large of a duct will be needed for the new space, and where in the system the new duct will be inserted. An existing duct run may start at the furnace with a 10" diameter duct, then step down to 8" and then to 6" as the duct runs branch off. 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 a new duct right off the end of the 6" duct, or you may need to go further back and tap into the 8" or even the 10" duct in order to get sufficient air flow.

For long duct runs or runs that will serve a large area, such as handling a big room addition, you will usually need to go all the way back to the furnace itself to begin the new run.  In this case, 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.

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 probably won't have a huge effect on the system, but several larger ones will. As the air flow is redirected, those rooms farthest from the furnace will suffer the most, and in some cases the air flow will be reduced to the point of being inadequate to heat that space.

Adjusting and balancing all of these air flows to all of these different spaces can be a tricky undertaking, requiring a knowledge of the amount of air being produced at the furnace, the size of the spaces being heated, the diameter of all the ducts in the system, the total length of each of the duct runs, and the amount of heat loss. 

Under- or oversizing the duct runs can result in poor performance throughout the entire system and can put a lot of strain on the furnace, so these calculations are definitely best left to the pros. Even if you would like to do the physical work of installing the ducts yourself, plan on hiring an experienced heating company to run all of the airflow and heat loss calculations so you can be assured that the system will always be working at its best.

Remodeling and repair questions? E-mail Paul at paul2887@hughes.net.