Figure 1. When outlets are spread around the entire house, but the return inlets are put mostly or entirely on the first floor, it costs less to do but leads to an unequal flow path for the second floor. It also prevents air from reaching the remote part of the system with enough pressure to provide comfort.


Why is the second floor always warmer than the first? Many homeowners ask this question, especially after the first warm day in spring or in early summer. They want to know why their air conditioning system isn’t providing cooling to their second-floor bedrooms.

They are reminded of what it was like last summer. It may have been difficult to get a good night’s sleep with the heat buildup on the second floor. An attic fan may have been tried, but damp night air just raises the humidity level in the whole house.

The simple answer to the question is that warm air migrates to the highest level of a building, whether it’s a house, gymnasium, church, factory, or warehouse. The laws of physics dictate that since warm air is lighter, it will rise and sit at the high point of whatever enclosure it is in. Conversely, cool air is heavier and will therefore migrate to the lowest levels of a building.

In the modern home, summer cooling has become a necessity. We have come to expect a comfortable indoor environment all year. So with a modern air conditioning system, why is it warm upstairs?

In order for the upper floor to be near the temperature of the lower floor(s), the natural inclination of warm air to stratify upstairs has to be overcome. The system for delivering air for cooling and heating consists of a furnace blower and ductwork that convey air to all parts of the building that need to be conditioned.

A split-system cooling unit and an electric or gas furnace generate the cool and the warm air, respectively, that keeps us comfortable year round. Residential furnace blowers typically have a limited amount of pressure that they can exert to move air throughout the house. This creates a challenge for the system designer to use this limited capacity to ensure that air is put where it is needed.

Figure 2. In order for the most remote part of the system to have an equal flow path, the return air system must be extended to be closer to the remote area of the house, the second floor.

AIRFLOW RESISTANCE

When the installer of the heating and cooling system puts in the equipment and ductwork, the least-costly approach is often used. Poor selection of ductwork and fittings can make a system operate as though it has a lot more ductwork than it actually does. For instance, a collared connection to a supply main is equivalent to an extra 30 feet of ductwork, compared to a 45-degree or other low-loss fitting. This added resistance reduces the blower’s capacity to deliver enough air for comfort, let alone to overcome natural stratification.

In addition, cooling and heating load calculations probably have not been done. Air distribution, in this case, is an estimate or, at worst, a guess.

When the blower is running, air moves through ductwork and out of diffusers or registers into rooms. The air then moves across the room to a return grille and back to the blower at the furnace.

The key to this working properly is to make the flow path to the most remote room as equal, or close to equal, to the flow path to the closest room on the system. This can be done by making the supply-room-return path as long for the closest part of the system as for the most remote part of the system.

Figure 3. Some townhouses and condos have a crawl space and an attic-mounted furnace. When the builder puts the return in the ceiling of the second level, the first floor will not heat without blocking off the stairs.

A common and fundamental mistake is made when the outlets are spread around the entire house, but the return inlets are put mostly or entirely on the first floor (see Figure 1). It costs less to do this, but it leads to an unequal flow path for the second floor and prevents air from reaching the remote part of the system with enough pressure to provide comfort. In order for the most remote part of the system to have an equal flow path, the return air system must be extended to be closer to the remote area of the house - the second floor (see Figure 2).

An example of the reverse situation is shown in Figure 3. Some townhouses and condominiums have a crawl space and an attic-mounted furnace. When the builder puts the return in the ceiling of the second level, the first floor will not heat without blocking off the stairs. The solution for this and for most heating situations is to put return inlets on the lower floor at 40-60 percent of total airflow (see Figure 4).

The low return will pick up the coolest air and return it directly to the furnace for reheating.

This practice is applicable regardless of the heating source’s location for tall structures, such as gymnasiums, auditoriums, warehouses, or industrial plants.

In most homes this can be done by using the back of a closet, or a space next to it to place a cavity or a duct that is connected to the furnace return duct and to a high sidewall grille at the high point of the house. By locating half or more of the return system on the second floor, the flow path is much closer to being in balance, and air can move from the supply to the room to the return in a more balanced fashion. In addition, the high return location picks up the warmest air in the house and delivers it quickly to the furnace.

Figure 4. Here’s a solution: Put return inlets on the lower floor at 40-60 percent of total airflow. The low return will pick up the coolest air and return it directly to the furnace for reheating.

BALANCED FLOW APPLICATION

Why isn’t this fairly simple approach used for all multilevel homes? Primarily because of the cost to do it. It may also be that implementing the balanced flow approach is not well understood.

From personal experience with design and installation of residential heating and cooling systems, I have applied this approach to new and repair projects. I installed a system in a new, super-insulated home and put virtually all of the returns in the second-floor corridor and bedrooms.

In another case, I was asked to fix a system in a nearly new home that was so cold on the second floor during the winter that the new owner’s family slept on the first floor all winter. We discovered that a supply plenum extended two directions from the basement furnace. A tap was made to rise to the attic for supply to the second-floor ceiling outlets. We discovered that in the attic, the supply duct was stubbed up and taps were made with flex duct runs up to 30 feet to the room outlets.

We corrected the distribution system by removing the flex and replacing it with rigid round fiberglass duct. [It is generally recommended as a guide that flexible air duct branch takeoffs (run-outs), flexible air ducts direct to boots from the plenum and flexible ducts in radial duct systems be limited in length to 25 feet.]

In addition, we placed a scoop in the basement supply plenum to force more air to the second-floor system. The final step was to extend the return to the second floor. We did this by using a closet to extend the first-floor stack up to a high point on the second floor.

Make the flow path to the most remote room as equal, or close to equal, to the flow path to the closest room on the system.

Another example is from my own home. The system installed in the home was for heating only. It had a crawl space supply from a garage-mounted furnace. The first floor had floor supplies and the second floor had ceiling supplies. There was a ceiling return on the first floor and a floor return on the second floor.

We wanted to add cooling to the house, so I had the plenum made for a new cooling coil. When the system was started, it could not cool the second floor very well at all. I was fortunate to have a nook above the return system that I used to extend the floor return up to ceiling level. After blanking off the floor inlet, cooling was able to fill the entire house and keep us pretty comfortable in hot conditions.

The lesson for homebuyers, homeowners, and builders is that the second floor doesn’t have to be hot. It can be made pretty comfortable by applying a balanced flow path approach to it. It will cost more, but a good night’s sleep is worth it.

Older homes that were built without a cooling system are a challenge to retrofit. One method that is effective is to install the cooling unit separately in the attic space. By putting the outlets in bedrooms and in the stairs, cold air is delivered effectively to the upper floor, and flows downstairs to fill the lower floor with cooler and heavier air as well. This separate system can function effectively with any heating system.

Modern heating and cooling equipment has evolved to variable-speed blower operation and modulating gas furnace heat output. Electronic controls have made the simple systems respond faster and more accurately. Zoning areas in the house is possible with electronic controls. These developments have made it possible to be comfortable in the spring and fall.

Even with high-tech equipment, the system design needs to use a balanced approach if the second floor is going to be comfortable.

Publication date:09/29/2008