Improving Your Home: Adding to duct system requires careful calculations

If you’re remodeling or adding on to your home, you might be wondering how to heat that new space. Or maybe you’ve simply got a room or two that just don’t get as warm as the rest of the house, and you’re wondering how to get things a little cozier. For both situations, you have a variety of different options, from individual wall heaters to radiant floor systems, but if your home already has a central heating system, chances are that expanding that system will be the easiest and most cost effective choice.

A central heating system is a very easy thing to visualize. A furnace uses electric coils or the combustion of some type of fuel to heat the air within it, and a fan then pushes the warmed air into a big box, called a From the plenum, the air moves into individual tubes, called which lead to the individual rooms. As the fan pushes air out of the furnace, it also needs to draw more air in, so a

is also provided within the heating system. The return duct pulls air from the house and directs it back to the furnace, where it’s reheated and redistributed in an endless loop.

What many people don’t realize, however, is that 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, but not so large that it wastes money in initial installation cost and ongoing operational expenses. Also, the ducts need to be the right size to deliver an adequate volume of air to each room. If they’re too small, the room won’t get enough heated air to sufficiently raise its temperature; too large, and the All ducts need to be the right size to deliver an adequate volume of air to each room. /Shutterstock

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 A properly balanced system operates more cost-effectively, and will provide adequate heat to each room without underor over-heating it.

If you’re thinking about adding

to your existing duct system to heat a new space or augment existing cold rooms, you need to keep in mind the 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 home’s energy efficiency. Homes with good

insulation, good windows and doors, and a low amount of air infiltration are easier to heat, so 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.

Next, the furnace will need to be evaluated to see if it can provide enough air volume for the proposed duct addition. If so, then a new duct run can typically be added to the system pretty easily. If not, then you need to either upgrade the size and/or efficiency of the furnace, or improve the home’s energy efficiency.

Finally, you’ll need to determine how large of a duct is needed for the new space, and where in the system the new duct will be inserted. An existing duct run may start at the plenum 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. This ensures 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 one or more 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 over-sizing 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 for you. That way you can be assured that the system will always be working at its best. _________________________ Have a home repair or remodeling question for Paul? He can be reached by email at improvingyourhome@ykwc.net.