By Douglas J. Burgasser, P.E.

We have fielded a number of questions regarding duct design for forced air heating and cooling systems. For example, people ask us how duct systems differ between modern homes and old homes. We are also asked what it means when a house is advertised as being “air conditioning ready”.  This blog is intended to answer some of those questions.

First, it is important to understand that forced air heating systems were around long before air conditioning. Therefore, many older homes were constructed before there was a thought of air conditioning a home. The duct system was designed strictly to provide circulation of heated air.

In years past the prevailing thinking was that the supply runs in each of the rooms should be located towards the interior of the room, and the returns should be located at the outer perimeter of the rooms. The problem with this design is that the heated air was not provided to those areas of the room where it was needed the most, namely along the exterior walls. This resulted in less than uniform heating in the various rooms.

Thankfully, since warm air rises, the heated air tended to distribute itself throughout the majority of the house. Many old farmhouses did not have supply runs to the second floor and relied on warm air rising through transfer openings or through the stairwell. The very fact that warm air rises to the upper floors also explains why the lack of return runs on the second floor of an old house did not result in unacceptable heating of second floor rooms.

For the last 40 to 50 years residential forced air systems have been designed differently than their predecessors. Now, each room will have at least one supply run (larger rooms will have 2 or 3 supply runs), and these are located along the outer perimeter of the rooms. They are usually positioned beneath windows or near exterior doors. This is because the exterior walls, the windows and the doors represent those areas where heating (and cooling) is needed the most.

Also, in modern systems return runs are located at the interior of the major rooms. Return runs are not necessary for each and every room, but there should be a generous distribution of return runs throughout the house. Returns are usually located at a higher elevation on an interior wall.

Some builders choose to install a small number of centrally located return runs when constructing a house. These would be located in halls or central areas, rather than in the individual rooms. There are both advantages and disadvantages to this ducted return air design. This type of installation usually saves a small amount in terms of construction cost and prevents the wall cavities of the house from being negatively pressurized. Standard return air systems use return plenums in the basement and wall cavities throughout the house (not actual ductwork) to draw air back to the heating and cooling equipment. However, this can draw in unconditioned air from exterior walls and dust and dirt along baseboard floor trim. Unconditioned humid air in the wall cavities of an air conditioned home can result in mold and mildew growth in the wall cavities. On the negative side, the central ducted return air design does not allow for uniform distribution of air flow from all of the bedrooms, offices, etc.; especially during the air conditioning season.

It is more difficult to cool a house than it is to heat a house. This is because cold air is heavier, and it does not naturally rise up through a house. Instead, cool or cold air needs to be forced into the various rooms and air needs to be circulated through the rooms. Further, the primary sources of heat during the summer are the windows and warm ceilings near the attic on the upper floors of a house. This is why it is more difficult to cool a second floor, and it is also the reason why the location and distribution of return runs becomes more important. Placing the returns at a high elevation in the various rooms (especially on the upper-most floors) allows for warmer air to be drawn off the upper portion of the room during the cooling cycle. This allows for much more uniform air flow, and more uniform cooling.

Some builders will even go so far as to install returns at the floor level and near the ceiling level in the major rooms. This is very beneficial. The lower returns can be opened in the winter. The high returns are used in the summer (usually by closing a damper in the low returns) so that the warm air is drawn off the top of the room. This is a desirable design.

Oftentimes you will hear that a house that does not have an air conditioning system is “air conditioning ready”. All that this typically means is that the duct system is a modern duct system, with a generous number of supplies and returns throughout the house. Essentially, any modern home with a forced air system should be “air conditioning ready”. Sometimes a plenum or coil will already be installed in the ductwork above the furnace as well, such that the system is ready for an outdoor condenser/compressor unit (air conditioner) to be piped to the coil.

If you are looking for ways to decrease temperature differences throughout the house during the summer, it is a good idea to operate the fan or blower on your cooling system continuously during the air conditioning season, rather than only when the air conditioner is actually operating. This allows for greater air circulation rates and more uniform temperatures throughout the house.

Since cold air is more difficult to distribute throughout the house than warm air, it is beneficial to adjust the dampers on the duct system accordingly, depending on the season. Dampers that feed the second floor rooms should be fully open during the cooling season. Dampers that are located in the branches for the first floor rooms can be partially closed. This combination allows for more air flow to the second floor. Since warm air naturally rises to the second floor it can be beneficial to partially close the dampers for the second floor heat runs and fully open the dampers for the first floor heat runs during the heating season. Similarly, if looking for ways to cool the second floor better, you can block off or close a few returns on the first floor during the summer to force more of the warm air near the ceilings of the second floor back to the air conditioning equipment.

In most cases the dampers for the supply duct branches can be easily accessed from the basement space. The following photographs show examples of the dampers in the opened and closed positions. Care should be taken not to close too many of the dampers and you should start with all dampers fully open. If closing too many dampers, the air flow through the system can be reduced and this can cause problems in terms of frozen air conditioning coils or overheating heat exchangers. A heating contractor can check for adequate air flow as part of annual servicing and can help you balance the system optimally.