DAVID R. OLSON, PE, LEED AP
It seems that there is always one room in a house that is coldest in the winter and warmest in the summer. I know – I had that room for a while as a child.
Airflow is “self-balancing” in all air handling distribution systems. That is not to say that the proper amount of airflow gets delivered to each room, but the pressure drop of each zone from the source of flow is exactly equal with each branch and/or air device. Residential air distribution systems rarely have balancing devices installed within the supply ductwork. Consequently, the airflow delivered to each room within the house, or the zone in larger systems, is equal to the proportional share of airflow based upon equal pressure drop from the fan. The pressure drop from the fan to each supply air device (grille or diffuser) will be exactly the same. This occurs automatically by the system adjusting the airflow in each branch duct such that the resultant pressure drop in that branch is equal to all the rest of the branches.
Pressure drop is caused by air friction in the ductwork. It results from air molecules rubbing against the sheet metal or flexible ductwork and from turbulence introduced when the airflow passes through elbows or fittings. The pressure drop in a long straight run of duct may be the same as a very short duct that has one or more elbows in it. Duct fittings generate pressure drop due to resultant turbulence. The pressure consumed by various fittings is predictable based upon studies conducted and published by SMACNA and ASHRAE. The goal of a system designer is to provide a duct system that matches the airflow required in each room or space with a duct system that can deliver this quantity of airflow with a relatively equal pressure drop in all zones. First they must determine the amount of heated or cooled airflow required to offset the heat loss or heat gain of a particular room or area. Then they can determine an appropriate duct size to accommodate that quantity of air delivery. The duct size results in a certain velocity of airflow within the ductwork, based upon the pressure and total flow available. For instance, if a bedroom requires 180 cfm (cubic feet per minute) to maintain an equilibrium temperature when it is at a “design” heating or cooling condition outdoors, then a designer might determine that the room should be served by two 6” diameter ducts or one 8” diameter supply duct.
Supply ductwork is normally sized for the more demanding load on the space – heating or cooling. In many buildings the cooling load will dominate and will be utilized for duct sizing. Sometimes the opposite is true, and the heating load will be used as worst case. The determination of which must be made by the designer. Most heating systems deliver supply airflow to a space with a temperature between 90°F to 100°F. Assuming that the thermostat is set for 70°F in heating mode, and then the differential temperature between the space and the supply airflow is 20°F to 30°F. During cooling operation, the supply air temperature is normally about 55°F. With a room thermostat setpoint again assumed to be 70°F, there is a differential temperature of 15°F between the space setpoint and the delivery temperature. If the same total airflow from the supply fan is delivered during both heating and cooling function, the quantity of airflow necessary to deliver the appropriate quantity of btu’s is reduced during heating operation. In other words, there is less supply air delivery necessary during heating operation than during cooling operation for a given quantity of heating or cooling. Some HVAC control systems will actually reduce the heating supply air quantity automatically due to this increase in temperature differential and a desire to trim energy use.
When too much flow is pushed through an undersized duct, excessive noise and friction occur. The noise is annoying to room occupants and the excessive friction results in a reduction of airflow delivered to this zone. When/if all zones in a house are undersized, then the air velocity and pressure drop increases throughout the system. If all the ductwork is sized per the same criteria (ie: 0.1”wc/100’ of duct) then the system may work satisfactorily. However, the air delivery performance will require more energy (horsepower) to deliver the necessary airflow to a room or space. If the supply ductwork is sized for a more reasonable criteria (ie: 0.075”wc/100’ of duct) then quieter operation results and energy consumption due to transport energy is reduced.
Sometimes the ductwork does not get installed by the contractor in exactly the same manner envisioned by the designer. Perhaps there was a structural element that interfered with the nice straight run of ductwork, or perhaps the ductwork required one or more changes in direction that were not foreseen by the designer. The resultant pressure drop in this branch ductwork may be increased proportional to the balance of the system. When this happens, the airflow delivered to the branch with the additional fittings will naturally reduce since there is more pressure drop. Consequently, the airflow delivered to the remaining zones will increase somewhat. In commercial systems, balancing dampers are employed to artificially impose a pressure drop on the branch that has less natural pressure drop. The damper is adjusted by the balancing contractor to equalize the pressure drops in the various branch ducts, allowing each zone or room to receive its desired share of the total supply air quantity.
As stated earlier, residential systems rarely have balancing devices (dampers) installed – except for the operable damper incorporated in the supply air device (grille or diffuser). Since every air device has a damper incorporated in it, the occupant may adjust the supply air quantity from full closed to full open sat their discretion for each zone. However, the pressure drop imposed on the branch duct by the dampers in the grilles is virtually the same in each branch.
I referred earlier to the pressure drop in each branch duct being made up by friction loss from airflow within the duct and pressure drop due to fittings. The friction drop portion of the overall pressure drop is dependent primarily on the length of the duct. Therefore, the branch that is longest in overall length from the fan to the air outlet is impacted most by the friction loss component of pressure drop. If that long run of ductwork also has an extra elbow or two, as compared to other branch ducts in the system, then the pressure drop is further compounded – resulting in a reduction of supply airflow to the room or zone. The result is a room that may not have quite enough heating in the winter, and/or possibly has too little cooling in the summer. Thus, this room is cold or hot compared to other rooms in the house.
It may not be practical to install dampers in each branch duct in a residential duct system. This would be costly. In a retrofit application, it may be very difficult to access all branch ducts within the home. A simple solution may be to add an inline duct booster to the branch duct which serves the high pressure drop room. This simple device slips into the supply ductwork and is controlled by a differential pressure switch that is mounted on the main supply plenum. Thus, whenever the supply fan energizes, the pressure switch senses the increased pressure and energizes the booster fan. The result is an increase in the pressure available in the long run duct branch, tending to increase the amount of heating and cooled supply airflow to this zone.
When my children were young, my daughter used to complain that her room was always cold. It was actually not the furthest from the furnace, but the branch duct serving her room had the highest pressure drop – perhaps due to a more complicated duct route. I solved the problem by installing an electric inline booster fan in the supply duct delivering airflow to her room. There was a little bit of noise transmitted down the duct to her room, but there was also more conditioned airflow. Overall she was happier. As a dad, I was pleased to increase my child’s comfort for a very small cost and effort.