Air Distribution in Forced Air Systems

No other part of a heating and air conditioning system is more important and more overlooked than the duct system which distributes the air effectively and purposefully throughout the house. The duct system creates comfort, allows the equipment to breathe properly, creates efficiency, provides longevity to the equipment and makes the HVAC equipment a successful HVAC system. If the duct system isn't designed properly it will cause unbearable comfort, deteriorate efficiency gains of the equipment and destroys vital components of the equipment over time.

Yet without question the duct system is the most overlooked and ignored part of an HVAC system. New construction HVAC contractors, builders and new homeowners rarely if ever give attention to this part of their house. Replacement HVAC contractors rarely if ever give any regard to the duct system when replacing equipment. If the system failed during the first five years or if there are comfort problems in the house the air distribution system should be addressed immediately. The cost to correct a faulty air distribution system after construction can be costly. What ends up happening is attempts are made to mask the problem rather than correct it.

So when does the duct system have it's final day in court? Hopefully right here and now with you.

Without knowing what makes a successful duct system it's impossible to know what to look for when making an assessment. Let's first of all take a look at what makes a great air distribution and what goes into the design and installation.

An air distribution system is about creating even temperatures throughout the house. Hot air rises and colder air falls. So to effectively heat and cool a house supply air is required at the ceiling level for cooling and the floor level for heating. Without this basic system set up comfort will be a sacrifice. All supply air located in the ceiling is preferred for cooling only but creates cold floors and high levels of stratification for heating. All supply air located in the floor is excellent for heating but the blower will never push the air high enough for satisfactory cooling. Rarely is an air distribution system installed with the basic set up for both heating and cooling. Even in the largest most expensive homes the air distribution system is rarely of the same input and design as other more noticeable features such as fireplaces, flooring, bathrooms, kitchen cabinets, etc.


Keeping the same basic principle in mind let's see how an excellent air distribution system really works. In cold winter months the greatest area of air infiltration comes from windows and exterior walls. Locating air diffusers on exterior walls below windows provides the best location for excellent heating. As the warm air discharged from the diffuser it wipes the exterior wall offsetting drafts and cold air. Cooling air distribution from the ceiling is perfect as the colder air discharges from the diffuser and wipes the warmer air and gently disperses over the room like a blanket. As the cool air reaches lower levels it has cooled the upper levels of warm air creating an even temperature throughout. However it is not generally practical to install a duct system that provides an ideal condition for heating and air conditioning as the method described. A single story home with the furnace located in the basement or crawl space and supplying air to the exterior walls with an air conditioning system located in the attic supplying cool air from the ceiling areas is perfect. But we don't all have a single story house making this only an example of what would be ideal. Everything else becomes a compromise. It is the degree of that compromise that makes the difference between a successful efficient comfort system or a system that becomes intolerable.

An effective compromise is a duct system with a trunk in the exterior or interior wall that has both high and low supply diffusers. The lower diffuser is usually located at 4" to 12" off the floor with a high supply diffuser located 6" to 12" below the ceiling level in standard 8 foot high rooms. During winter operation the ceiling diffuser is closed while the lower floor diffuser is open and vice versa for summer operation. The supply trunk for this set up can be located in an exterior or interior wall. The problem with locating this trunk in the exterior wall studding is the loss of insulation and building structural integrity. Locating the duct in the exterior wall is difficult and can seriously affect structural integrity as the access requires removal of bottom and top plates in the studding, a practice seriously frowned upon by any codes and building inspector. So for practical purposes the duct should be located in an interior wall. Return air ducting is then located on floor level on the exterior wall.

Somewhere along the way installers reversed this process and installed the supply air in the floor level at the exterior wall and then low and high returns in the interior wall. This won't work effectively and here is why. It is far more difficult to pull air into an intended direction as it is to push the air.

A well design and installed air distribution system also includes proper and adequate return ducts and diffusers located in opposite position in each room. Ideally each room will have a return diffuser located in the floor opposite to the location of the supply air diffuser. This will direct the air pattern to properly flow across the room. When return diffusers are located on the same wall or within close proximity to the supply diffuser there will be short circuiting of the air flow. This short circuiting avoids sweeping the room and conditioned air is returned to the equipment without properly cooling or heating. The greater the distance of the supply air diffuser to the return air diffuser in any given room will provide the greatest comfort level and ventilation. Why do we want the return air located in the floor? There are very rare situations where we want return air located in the ceiling areas except in air conditioning only systems. The best approach to designing a duct system intended for heating and cooling is to place priority of the design towards heating in most areas of the country. The majority areas of the country have a longer time period and larger capacity requirement for heating than for cooling. The heating system is larger in capacity and will operate for longer periods over the course of a year than the cooling system. As with most items and concepts in HVAC design one size doesn't fit all and no one particular design concept fits every application. So if you are located in Florida, Georgia, southern California, Arizona, Nevada and Texas you will want a design concept that enhances the performance of the cooling rather than heating. For the other majority areas of the country your design concept will favor the heating.

A well designed duct system will also keep duct lengths no further than a total of 80 feet of cumulative ducting which includes the total length of the supply and return ducting. The location selected for the furnace or blower is critical to the overall performance of the duct system. If the central heating equipment is located in one end of the house rather than in a central point the furthest run of ducting will be too excessive causing the last run out to suffer comfort due to inadequate air flow. A well designed duct system places the HVAC equipment centralized to the house.

It is important to understand the following information to fully realize the importance of sizing of ducts. We all have an outside water fixture for a hose. Let's assume for this example we all use 50 foot hoses 5/8" in diameter. Most homes have a water regulator keeping water pressure at 40 pounds. When we open the valve supplying the hose we all get the same flow rate. Only a certain flow rate will travel through a 50 foot hose at that pressure. Unless we change the water pressure or increase the size of the hose we can't get anymore flow. It's just a simple engineering principle of flow versus pressure and resistance. We can't change the pressure so our only solution for providing more flow is to increase the diameter of the hose or shorten the length. On your forced air system there is a blower. Like the water it can only produce a certain flow rate at a certain amount of pressure. Through a certain size duct we can only deliver a certain air flow measured in cfm or cubic feet or air per minute. If we increase the fan speed to satisfy those areas that are too small the remaining areas will get too much flow. And if the duct is simply beyond design limitations we can never provide enough fan speed to deliver the required amount of air. When we exceed fan limits we lose air flow and use too much energy in trying to overcome the resistance created by the small duct size. The fan can't produce the required air flow. Now if this happens in only one or two areas then only those rooms suffer comfort due to lack of air flow. If there are too many areas or the entire system is undersized the minimum air flow won't be achieved for comfort or efficiency and the safe operation of the equipment. We lose comfort which is the first symptom. We use more electrical energy from the fan as it has to overlabor to attempt to achieve an air flow that it can't. Breakdowns begin to occur and before too long the entire system fails. In the interim the energy efficiency was lost as the equipment couldn't deliver the full capacity. A rare problem? No, not rare at all. It's a problem that happens in over 50% of every HVAC system. Unless the duct system is designed for the standard resistance of the blower providing adequate air flow, true comfort and energy savings will never occur and the equipment has a short term life. The only solution is the same as for the hose. We can either make the duct larger or reduce it's length. We can't reduce the length because we need that distance to distribute the air. The only solution is to increase the diameter of the duct. And that can be very costly to do after the house is already constructed. But without the right size ducting there is no other alternative. And if the ducting isn't corrected it will destroy any replacement system and efficiencies. So it is critically impotent to run the right size ducting for any system regardless of whether it is a new or replacement application. 

The ideal duct system also places importance on the amount of supply diffusers per room. Each diffuser has a rating of air flow and the distance at which air will travel beyond the diffuser is called the throw. A diffuser should typically not be handling more than 400 cubic feet of air for a residential application. And a well designed duct system will not provide more than 200 cubic feet of air per minute in any diffuser. Why? Noise and even air distribution. If you have a large living, dining or family room a single diffuser will never distribute the air out in a sufficient pattern. Attempting to do so in a single diffuser causes nuisance air distribution as the single diffuser creates discomfort near that diffuser with intense air flow and insufficient conditioned air at points furthest from the diffuser. The intent and purpose is to blanket the room with gentle air movement at no more than 400 to 500 feet per minute of air flow with a throw of no more than 12 to 14 feet. This design will provide comfort and even room temperatures. The intent and purpose is not to pump the required amount of air into any room with the least amount of materials and labor. How many times have any of us been in a restaurant located near a supply diffuser that makes being there objectionably uncomfortable similar to a wind tunnel? This is exactly what we don't want especially in our homes.

What else makes a well designed duct system is adequate overall air flow? Newer more energy efficient homes are very tight and the resultant heating and cooling requirements provide a feeling of stuffiness and discomfort. In these applications such as ICF or Insulated Concrete Formed Homes or homes with over R-19 in walls and R-25 in ceilings the heating and cooling requirements become marginal. The net result for air flow is also minimal and ineffective for comfort. As duct systems are sized on the basis of cooling because air flow requirements are greatest, most homes have air flow requirements on the average of .40 to .75 cubic feet of air per square foot while newer energy efficient homes have .25 to .33 cubic feet of air per square foot requirements. This minimal air flow requirement is simply too low. Avoid making this mistake in any HVAC design and layout. For minimum air flow for proper circulation every application should use no less than .40 cubic feet of air per square foot. Regardless of the actual load calculation this requirement of .40 cubic feet of air flow should supersede the cooling load calculation. In other words if a Manual J8 load calculation determines anything less than 1,000 square feet per ton, adjust the equipment selection so the cooling requirement equals no less than 1,000 square foot per ton. 1,200 to 2,00 square feet per ton are common calculations for ultra energy efficient homes but back that number down to 1,000 square foot per ton for satisfactory ventilation and cooling regardless of the load calculation. At these light load levels adjusting the cooling load downwards will not effect energy conservation or performance but will actually elevate the ventilation characteristics and create a healthier more comfortable indoor level. Other factors to consider are indoor air pollution. Fresh air needs to be introduced in houses requiring light cooling loads of more than 1,000 square feet per ton therefore raising cooling and heating loads. For many reasons designing a house too tight and energy efficient may work against your goals after a certain level and the resultant light ventilation and heating/cooling loads are negated by the additional outside air requirements. For this reason designing a house at or above R-19 for walls and R-25 for roof areas reaches a point of negligible return in energy conservation.

The ideal duct system cannot overcome problems where multiple systems should have been utilized. Designs that generally require more than one system are as follows: A house with two stories and more than a total of 1,600 total square footage and an open stair case. By utilizing two systems the second floor system is able to effectively provide sufficient cooling when the first floor has no requirement. Also the second floor duct system can be high supply diffusers for the additional cooling requirements and the fact the floor is already conditioned. The first floor system focuses more on heating with low supply diffusers. This type of layout and design works well for almost all applications. The alternative would be to use one system where compromise and discomfort would be the net result. A single system could be installed with a two stage compressor and dampers with electronic control system. However the cost for the two stage system, dampers and controls and the extra size of the ducting and labor exceeds the cost of two separate systems.

Three story townhouses are impossible to effectively heat and cool with one system regardless of the type of ducting, the design or the quality of the installation. There are no substitutes for multiple system requirements, only compromises and sacrifices.

Another component of an effective duct system is one which is properly balanced. To properly balance air flow typically a contractor or homeowner will adjust the damper at the diffuser. Before long dampers are closed off in many parts of the house restricting air flow to the point it becomes detrimental tot he proper operation of the equipment. The noise and whistling created by closing dampers on the diffusers becomes annoying. Balance dampers which are very inexpensive located at the take off of the main trunk 6 feet or more before the supply diffuser allow air flows to be accurately balanced without causing noise problems and whistling. Then dampers in the diffuser are not necessary and they can't be closed off resulting in conditions detrimental to the equipment.

Besides a duct system being properly sized and in the proper locations with the correct diffusers, the most important functional physical property is to not leak or transmit the energy contained within the walls of the ducts. The overwhelming majority of metal duct systems leak an average of 25% of all the air supplied or returned through them. That translates into losing 25 cents of every energy dollar spent. Add to this ducts that are located in attics, garages and crawl spaces which lack sufficient insulation losing an additional 25% energy. It's difficult to imagine losing this much energy in a duct system but the surveys conducted by the Department of Energy find 25% of all energy is lost through duct systems mostly due to leakage. Energy efficient duct systems are designed with an insulation value or R-6 or more when ducting is located in unconditioned spaces. Newer systems on energy efficient homes use a main trunk constructed of fiberglass duct and take offs are kept at a minimum of insulated flexible fiberglass ducting. This type of system results in total energy losses of less than 3%.

Knowing what is required to achieve an energy efficient duct system that provides maximum comfort with ultimate quietness will help you assess the condition of your own air distribution system. If it is a typical sheet metal duct system and replacement is not cost effective, seal all joints with a duct mastic or caulk that is rated for this application. Don't use duct tape unless it is UL rated for duct systems. Duct tape sold at home centers is not rated and is ineffective for any long term usage. What happens is the tape is not designed for wide temperature variances and soon loses it's bonding and effective sealing. Only aluminum duct tape with a UL rating should be used. In many installations duct mastic and high temperature silicone caulks are more effective.

The following is a real life example of the common problems associated with a typical poorly installed duct system. Everything about this duct system was wrong. It's this writer's experience and may help you. The house was a two story house. The second floor was 1/2 the size of the first floor with a total of 1500 square feet. The house originally had a heat pump that used a downflow air handler located on the first floor. The main trunk of the duct system was located in the crawlspace underneath the house with another main trunk run upstairs for the second floor. The house had a 3-1/2 ton heat pump installed that could not properly heat or cool the house. This was the first clue. A heat pump that large should have adequately cooled and heated a house twice the size of this house. But this heat pump would run continuously and couldn't keep the house comfortable. The second floor was always too hot with 6 to 10 degrees warmer than the first floor. In researching it was found the house had a 2-1/2 ton heat pump originally installed. So why couldn't this 3-1/2 ton heat pump keep up? First the crawl space was super cool in the summer months and toasty warm in the winter. In visual inspection there were all kinds of air leaks where air would actually be hitting me in the face and body when inspecting the crawl space. Standard duct tape was hanging from the ducts that had dried out and had no adhesive qualities. The joints of the main trunk had come apart at taped seams and metal ducting was leaking at all joints. The flex duct was sagging or had come completely off it's hangers as they were incorrectly fastened or not fastened at all. In the second floor there was conditioned air coming out of the light and electrical outlets. The light outlets on the eaves of the first floor overhang also had conditioned air coming out of them. Heating and cooling costs were out of sight. In renovating the house the ducting in the chase to the second floor was never properly connected to the second floor horizontal duct run. It was six to eight inches separated due to settling in the house. Where the same trunk connected to the main trunk in the crawl space it had completely separated. This explained how the

conditioned air was escaping through air and electrical fixtures on the second floor and exterior. In addition the horizontal ducting on the second floor was inadequately sized. So even connecting the main trunks correctly would have not accomplished proper comfort to the second floor because they weren't capable of handling the air flow required. Due to the problems associated with the size of the ducting, it's location and the associated locations of the supply diffusers, connecting, insulating and repairing the duct system would not have accomplished the goal of achieving adequate comfort and energy conservation. To bring this system to a point where it would be energy efficient and provide adequate comfort wasn't possible using the existing ducting and grossly oversized heat pump. First the duct system was tremendously undersized as it was marginally sized for a 2-1/2 ton heat pump when the house was built. Now to attempt to use the existing duct system for a 3-1/2 ton oversized heat pump was useless. This house being only 15 years old at the time had 3 heat pumps installed with the third one ready to fail at any moment due to the abuse it had sustained with the undersized ducting. Yet the original owner never understood why they were having so many system failures and enormous energy bills. Most of the problem they blamed on defective equipment and HVAC contractors who were incompetent. The previous homeowners had the same attitude that most homeowners have when faced with this typical situation. The final solution was to remove entirely the existing system and install two separate systems, one for the first floor and another for the second floor. There were already plans to completely gut and renovate the house and add an addition to the second floor. So the entire existing system and ducting was removed. The final design called for the installation of two 1-1/2 ton heat pump systems with a total of 3 tons capacity. The new house is 30% larger than the original with 50% more glass area. New high efficiency windows were installed throughout the house with double insulation, low e and argon and there was 50% more glass area. All the walls and ceiling areas had new insulation installed from the original R-11 to R-19 and with radiant barriers. Insulation of R-19 was also installed to the crawl space area where there was no insulation previously. The house today is 30% larger than originally, has two smaller more energy efficient 1-1/2 ton heat pumps at 14 SEER with variable speed, better insulation, 50% more window area using more energy efficient windows and energy management thermostats. The temperature is maintained at 74 degrees all year long. Previously the house was only used on week ends throughout the year and had less space and less glass. The net result is a  house that uses 15% less energy than previously. This is what energy efficiency can accomplish. But to the subject we are addressing in this article, the new duct design has created an environment where the temperature difference between the first floor and second floor is one to two degrees. The house also has the versatility to keep the temperature and air flow constant in each room and no discomfort. At times there may be cooling required on the second floor while a call for heating can exist on the first floor at the same time. While an immediate view of this dual functionality may seem energy inefficient, the net result has been a house that has extremely low energy usage wile maintaining a high degree of comfort. The first floor air distribution system is designed for primarily heating with floor diffusers and the second floor provides high wall supply diffusers for more attention to the cooling. Due to heat stratifying to the second floor from the first floor through the open stair case, there is rarely a time when there is a call for heating on the second floor system. Equally the first floor system has substantially less requirement for cooling in the summer months. Attempting to accomplish the goal of energy efficiency with a single system would have never achieved these results. A single system using a single stage compressor would have required the system to either heat or cool at 100% capacity every time it operated. Remember we have many times when heating is required on the first floor while there is no requirement on the second floor and opposite for cooling. If I had decided to use a single system with a two stage compressor and zone dampers it would have been more costly than two single systems. Beneficially if there should ever be a time when a system fails there will always be a second system available. So if the heating system fails on any system there will still be another to provide at least a tolerable level thus eliminating a time when there is an emergency service call required. The problem could sustain itself until another day or if it occurred on a week end it could wait until the following Monday without serious loss of comfort. Further if a single system would have been considered the difficulty and extra room and labor required to install ducting from the first floor to the second floor would have added substantial cost and sacrifices in comfort. It would have been very difficult to install second floor ducting with high wall supply diffusers from the first floor air handler. The lesson learned is that ultimate energy efficiency and comfort can be achieved only with the right design concepts and duct installation.

In recent years many new homes are being designed with vaulted ceilings. The most popular and difficult of all home designs is the vaulted ceiling which escalates along with the stair cases. These open staircases accompanied by vaulted ceilings can be real nightmares if not approached correctly with an adequate air distribution design. If the vaulted or open ceiling area is confined to a single room and the area is not joined to the second floor areas, the duct design should only incorporate the area where comfort is required. In other words don't attempt to cool any area higher than eight feet unless that ceiling area is part of other living areas or stairs. Heating and cooling ducts are critical in their location for vaulted ceilings in untreated areas. By limiting the cooling diffuser to an eight foot area then only that cubic volume of air needs to be taken into account for the cooling load requirement. For example if you have a great room with a 16 foot vaulted ceiling that is confined to only that room condition only to a 7 or 8 foot area. Let's say the great room is 800 square feet. In doing the cooling requirement there is only 1 ton or 12,000 btus of cooling required if the cooling supply diffuser is located an the eight foot level. But if the cooling diffuser is located at the ceiling or 16 foot level the cooling requirement doubles to 2 tons or 24,00 btus. There's nobody living up in that ceiling area so by simply placing the diffusers at the level where comfort is required the cooling requirement is cut in half. The same is equally true for the heating load. By locating the heating diffusers at floor level remembering that heat rises and the cooling diffusers at an eight foot level remembering that cool air falls you will decrease the size of the heating and cooling system and the energy required to heat and cool the room substantially. But what about all the hot air up in that ceiling area? Doesn't that need to be conditioned? No and if left alone it will actually create a type of insulation  barrier to the roof. Again the return air should also be kept at the lowest level possible. Locating the return diffuser below the 8 foot level is important as well for reducing load requirements. Return air located at the ceiling level would also cause a doubling effect and increase the system capacity requirements to serve the great room.

If the great room in our example above is attached to an open staircase then the design consideration changes completely. There are many designs of beautiful open stair cases accompanied by vaulted ceilings so uncomfortable homeowners can barely live properly. Unfortunately there are too many times where the HVAC system and resultant duct design does not address this application for consideration. Attempting to deal with this type of design with a single system and single ducting systems is a poor compromise. If the homeowner has sufficient finances to invest in this style house then they should also demand the appropriate HVAC system. There are more examples of failures in HVAC system design in this type of application than successes. This style house is generally upscale and yet the design of the HVAC system is of the lowest scale. Realizing the open second floor will naturally be warmer than the first floor on a constant basis a single system will never accomplish the goals of comfort. Only if the single system has two stages with zone dampers the level of comfort will be achieved. Two separate systems are amore cost effective method to provide satisfactory comfort levels. There will always be a 6 to 12 degree temperature differential between the first and second floor levels without proper design of a two stage or dual system. Generally the cost of the single two stage system and zone dampers is more expensive and less effective than two separate systems.

To assist in proper distribution of air flow use a variable speed motor. Besides saving lots of electrical energy the main purpose and advantage of variable speed motors is to provide constant air flow. Standard blower motors simply work by only maintaining a constant blower speed which does not take into consideration variances that occur normally in every HVAC system. Filters pick up dirt and as they do they also restrict air flow. Cooling coils load up with moisture as they dehumidify creating increased resistance to air flow. Cooler air changes weight as it is heavier requiring more resistance to air flow. Many factors vary air flow including minor design problems. By being able to monitor it's energy consumption the variable speed motor will change it's speed to maintain a constant air flow as resistance to the air flow changes.


What about the duct system design? Who can design a proper air handling system? Where can I get a duct system that's easy to assemble and install without the need for special machinery and tools? Is there a duct system that doesn't require intensive labor to install and insulate and that doesn't leak so much air at every seam and joint? Is there a duct system that can be designed exactly for my application where I don't have to pay a sheet metal shop an exorbitant sum of money? DESCO Energy will design and provide sizing of any duct system when you purchase an HVAC system. DESCO Energy will also provide a complete fiberglass duct system precut for your application. The duct system can be assembled like an erector set complete with your new HVAC equipment. Fiberglass duct material has less than 3% air leakage compared to 25% for sheet metal. Fiberglass ducting is easier to assemble than sheet metal and has no sharp razor edges and is more forgiving. Sheet metal requires the need for large machinery and specialized tools and is not forgiving. Fiberglass provides more efficiency, easier to work with, creates quiet operation by attenuating noise and less expensive than the equivalent sheet metal. If you can assemble cardboard boxes you can assemble fiberglass ducting. For more information refer to ducting in our Products section. 

In our library you will find numerous in depth articles on designing duct systems both in metal and fiberglass. DESCO Energy can provide a complete precut fiberglass duct system for your specific application as easy to assemble as cardboard boxes. Fiberglass ducting has no sharp edges, is easy to install, provides maximum energy efficiency and quietness. Easy to assemble and install with a minimum of tools available for inexpensive rental. You can also have a sizing layout and review of any duct system at no extra charge when purchasing new HVAC equipment. In addition you can find more information about the benefits of fiberglass ducting in our Products and Library section. DESCO Energy is the only online source providing the complete system including a complete efficient duct system designed just for your house. DESCO Energy is the most complete source of products and information for HVAC online.

For more information and assistance call our technical support associates toll free at 877-265-9764 or email at info@descoenergy.com

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