Posted on Dec 8 2010 by Image 1 of 4 This photo shows a mock-up of the de Keiffer bypass; spacing the head casing off of the wall around the door creates about 23 sq. in. of free space for air to return from a room (with the door closed) to the common space where a central return is located. If a forced-air HVAC(Heating, ventilation, and air conditioning). system is pushing more air into a space or room than return pathways can match, some spaces in the home become pressurized and others depressurized. This imbalance can cause problems: thermal comfort, moisture, and even combustion safety. Types of HVAC returns In an ideal world, every supply would have a companion return duct. This approach is called a dedicated return for each supply. But most older and even many new homes have one or maybe two central returns located in common spaces because it is less expensive and difficult to locate and run just a supply to each space. With central returns, here are the ways to provide return pathways: door undercuts, jump ducts, transfer grilles, and the deKeiffer bypass.

This is certainly the easiest and cheapest way to attempt a return pathway but it is rarely sufficient. For most situations, door undercuts need to be greater than 1.5 inches to be sufficient and that amount of undercut is very often not acceptable for aesthetic or privacy reasons.ac unit cool not cold This New Door Design Solves an Old Problemoutside heating/air conditioning units Perfect Balance Makes the Cutnew ac unit estimate New Green Building Products — March 2011 All About Furnaces and Duct Systems A jump duct is a large insulated flex duct that connects two ceiling grilles, one just inside the door to the supplied room and the other just on the other side of the door in the hallway ceiling. The sharp bend and variegated surface of the interior lining of the jump duct reduces sound transfer without compromising air movement.

Since this approach penetrates the ceiling plane, the jump duct must be carefully air sealed if it breaks into an unconditioned space like the attic. There are two types of transfer grilles, above-door and high-low offset. The above-door transfer grille simply has two grilles located above the door between the room and common space, one on each wall, with offset baffles to limit sound transmission. The high-low offset is located in an interior wall framing cavity adjacent to the door between the room and common space. One grille is located high on the wall on one side (usually the common space), the other low on the wall (in the room). The offset reduces sound transmission. Note that this interior framing cavity must be clean and without wiring penetrations (or seal these penetrations as you would if it were an exterior wall framing cavity). The de Keiffer bypass An HVAC engineer from Boulder, CO, Rob deKeiffer of the Boulder Design Alliance, came up with an elegant return pathway: the deKeiffer Bypass.

The door trim is spaced off of the wall on both sides enough to create around 23 square inches of free air space connecting the room and the common space. The framing cavity space above the door is sleeved with sheet metal to create a dedicated pathway connecting the two sides of the spaced-out door trim. Armand Magnelli’s (Livable Housing, Inc.) perspective Another approach is to install a product that combines a metal sleeve with an interior baffle and grilles on both sides of the wall, such as the Tamarak Return Air Pathway (RAP). Or one of Halton’s transfer grilles (these more commercial in nature). These products purportedly provide better noise reduction with good air flow and are relatively easy to install. The fit between standard stud spacing in interior partition walls. For an independent analysis of several approaches see the following review by the Building America Program. An advantage of using the products from Tamarak is that they are sized for CFM capacity, an important variable in balancing the airflow between rooms.

The issue with the products from Tamarak and Halton is that while they provide an attractive and effective packaged solution it may be difficult to convince local HVAC distributors to stock their lines of products forcing contractors to order them online. Some of the other solutions are fabricated from stock components available at most HVAC distributors. The use of any of these solutions may also improve performance of whole house ventilation systems by enabling better circulation of fresh air from bedrooms at night, a crucial time to deliver fresh air to occupants. Be great to learn what method you use and why? Your hvac duct system is the heart of your home's heating and cooling system. Without a healthy system you could be wasting a lot of money. Like the human body, if the heart isn't healthy you are in trouble. A duct system provides a controlled path for airflow throughout the home. The system can be made out of sheet metal, fiberglass ductboard, insulated plastic, or cloth.

Many times, a combination of these materials is used in the same home. There are three main parts to the system. A problem in any part results in a sick system. The first section is called return air. This part provides a path for air from the individual rooms to the inlet of the furnace or air handler. The air is usually cleaned in this section of the system. In a healthy system, fresh air is introduced here. In some cases moisture is required and added here as well.Next, the air passes into the furnace or air handler. This is where heat is added or removed to make the home comfortable. In most older systems, the filtering was performed here. This was not very convenient for the owner and caused service problems.Finally, this conditioned air enters the supply section of the hvac duct system. The purpose of this part of the system is to deliver conditioned air to the individual rooms. The two main types of supply systems are extended plenum and central plenum. A central plenum system is a system where all of the branch ducts start at the same location called a plenum.

The extended plenum system consists of a supply air plenum which is connected to the outlet of the hvac equipment. The main supply ducts are attached to this and typically run to the end of the home. The size of them is reduced as necessary to maintain adequate air flow. The branch ducts attach to the main ducts and carry the conditioned air to the individual registers.That is the basics of how a system works. Now we will look at some common problems and how they can be fixed. So, how will you know that your hvac duct system is sick? One of the first symptoms will be a whistling, wheezing, or popping noise. This is a sign that the system is working harder than normal to maintain your comfort. It is like the beginning of chest pains for a heart patient.As the condition gets worse, the next symptom will usually be an abnormally high utility bill. Many times this will go unnoticed until the unit is unable to keep you comfortable. This is where the situation gets really critical. Intervention is required to save your heating and air conditioning equipment.

If this is a new system (less than a month old), it is likely that the ducts are not properly sized or the system is not properly balanced. For an improperly sized system, there are a few things that can be done to minimize the effects. The first thing is to retrofit a zone system but this can be expensive. If too much air is being delivered to a room, a damper can be partially closed to lower the amount of air. Many registers have a built in damper to allow you to do this. If there is not enough air being delivered to a room, then a duct booster fan may be installed to increase the amount of air delivered to the room. If it has been operating for a month or longer, the system filter may need cleaning or replacement. If you have an uninsulated metal hvac duct system, you can do some easy upgrades to enhance your comfort as well as saving you money on your utility bill. You can begin by sealing the system which is applying a special mastic to the cracks and connectors to make them airtight.

This ensures that the conditioned air does not leak out into areas where it is not needed. After sealing the system, you should insulate the ducts if they are located in an unconditioned basement, crawlspace, or attic. This is an easy project that can provide a quick return on the investment. The installation of a hvac duct system is not overly complicated and the work does not require a huge amount of technical knowledge. The process starts with a proper design and then it is simply a matter of following the duct plan.With our air duct installation guide you can install your own metal duct system and save. You can also use our flexible duct installation guide for situations that require insulated ducts such as an attic or crawlspace.If you are not going to do it yourself, then your selection of a hvac contractor is very important. Our partners will assist you in this process. With their free service, you can get up to four quotes from local hvac companies without even leaving your home.