Hi Top Ten Reasons That Building HVAC Systems Do Not Perform as Intended!.

Hi Top Ten Reasons That Building HVAC Systems  Do Not Perform as Intended!.

The Heating, Ventilating and Air Conditioning Contractor (HVAC) and the Sheet Metal Journeyman’s roles in the construction of new and existing building projects can play a major part in the final comfort and energy efficiency of a building. There appear to be many issues prevalent in the HVAC industry that directly affect energy usage, sound and comfort levels in buildings. 

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This article will show several problem areas common to and recurring on projects. It is always easier to correct problems before the problems are built into the system. The knowledgeable contractor who understands the applicable standards will install these items per the proper standards or notify the engineer before performing work that could create a potential problem. This will reduce problems and reduce chances for an adversarial relationship to develop between the design engineer and the contractor once the project is completed. 

Issue # 1 Duct-work is not reinforced for the proper pressure classifications. The Sheet Metal and Air Conditioning Contractors National Association (SMACNA) is an example of the association that has published duct construction standards that designate the proper way to fabricate and install duct-work, duct accessories and air handling equipment. These standards are so prevalent that most construction documents and specification say to the effect “Install duct-work to SMACNA standards”. However, it is the very familiarity with the term SMACNA standards that has led engineers, contractors and sheet metal journeymen to forget to examine current SMACNA documents. These standards have been continually upgraded and improved over the years. The current SMACNA standard is the 3rd edition dated 2005. We still find many engineers and contractors specifying and installing duct-work exactly like they did 25 years ago SMACNA pressure classifications now specify duct-work reinforcing for specific duct pressures from ½ inch to 10 inches of static pressure. It is the design engineer’s responsibility to specify the systems design static pressure. If the design engineer does not specify an exact static pressure, but instead states “Install duct-work to SMACNA standards”, SMACNA standards compliance will have duct-work built to a 1 inch pressure class, with the exception of duct-work upstream of a VAV box which should be constructed to 2 inch pressure. This lack of clarification by the design professional or failure of the installing contractor to follow the required pressure classifications shown in the specifications is the main reason duct-work collapses or blows apart. The duct failure is invariably due to the fact that the proper pressure class was not specified or the duct-work was not constructed to the proper pressure classification. It is relatively inexpensive insurance for the installing contractor to check the pressure classification to verify that a reasonable pressure has been specified for the duct-work and that the actual current SMACNA pressure classifications are being followed in the field. 

Issue # 2 Failure to adequately seal ducts. Duct leakage refers to the fact that air inside a supply duct under positive pressure will leak out of the Pitts-burg or Snap lock seams, from the slip, drive or TDC connector joints or out of wall penetrations from damper rods, screws used in hanging ducts or any other wall penetrations. This leakage causes two problems. When air leaks out of a duct system, some areas at the end of the run may be short of airflow because the air has leaked from the system before it reaches its intended location. This lack of air can cause over heating in the summer or the inability to heat in the winter. In some cases the fan has the ability to supply enough air to overcome the leakage rate and still meet the room requirements. Excessive fan energy is used to provide the required flow to spaces. The fan energy increases as the cube of the air leakage. For example if a system has 10% leakage and the fan design was originally 20 Horsepower (HP), the new motor required to overcome the 10% loss will be 20 HP x (1.1)3 = 26.6 HP. In this case, the owner pays for an additional 6.6 HP fan operation for the life of the building. SMACNA has specified the duct sealing requirements for duct systems. If duct-work is not sealed, leakage rates of 25% on 2 inch plus duct-work can be expected. SMACNA seal classes of Seal Class A, B and C define the degree of sealing required to be completed. Seal Class C means that all Transverse joints (Slip, Drive, and TDC) connections need to be sealed. Seal Class B means that all longitudinal seams (Pitts-burg and Snap lock) and all joints as above need to be sealed. Seal Class A is the most stringent and means that all wall penetration (damper rods, screws and duct accessories) must be sealed in addition to all seams and joints as defined above. Failure to properly specify sealing or to actually seal ducts in the field are the primary reasons duct-work systems leak. 

Issue # 3 Failure to understand the need to seal return and exhaust ducts. Air will leak into the duct-work through the same openings described above. When excessive leakage occurs in exhaust ducts, it is sometimes impossible to obtain the required exhaust air at the needed location. We encounter many cases where the exhaust air flow at the fan is 15% over design and the exhaust flow at the register or hood location is 20% under design. This discrepancy is due to the fact that air is leaking into the duct systems along the duct route and not at the specified exhaust point. This can lead to stuffy, stagnant rooms or laboratory exhaust systems not performing as required. System operating costs are also increased by the amount as calculated in Item 2 above. 

Issue # 4 Failure to adequately pressure test duct-work to prove that duct sealing is effective. Duct-work pressure testing is a method to determine how well ducts are constructed to prevent air leakage. Duct leakage testing pressurizes a closed section of duct-work to a known pressure. The amount of leakage at a specific pressure is calculated by measuring the amount of air that is measured as it is blown into the closed duct system. SMACNA has defined duct leakage rates as Leakage Class 24, 12 or 6. These numbers simply mean that at 1 inch of test pressure ducts can be expected to leak 24, 12 or 6 CFM per square foot of duct surface. The mere presence of duct sealing material on a joint or seam is no guarantee that proper sealing has been achieved. Once the installing contractor has leak tested several sections of duct-work and has adequate documentation that the duct sealing procedures being used actually keep leakage within the specified ranges, the frequency of sealing can be reduced.

Issue # 5 Failure to understand SMACNA Duct Construction Standards in the fabrication and installation process. While SMACNA standards are almost always used as a project reference, experience has shown that proper techniques are not understood by both the engineers and contractors. Design engineers often do not allow sufficient room to install proper sized transitions or offsets. In many cases when there is insufficient room, journeymen overlook the applicable standard and install transitions or offsets that are chocked or do not have proper slope. When SMACNA standards on fittings are not followed, the entire system has imposed restrictions that the fan must overcome. This extra fan energy is an operating cost the owner bears for the lifetime of the project. In some cases, the addition of several improper fittings can cause air flow noise and lack of adequate air flow to occupied spaces. 

Issue # 6 Failure to properly install turning vanes. Air flowing in ducts has the same restriction whether it is flowing away from the fan or flowing to the fan. Vanes are required in all ducts where the air velocity is greater than 1000 feet per minute (fpm). While most contractors understand the importance of installing vanes in supply duct, a misconception seems to exist in the industry that implies return ducts and exhaust ducts do not require turning vanes. The previous six issues involved contractor related concerns in duct installation and fabrication. Many design engineers continue to unknowingly make similar missteps in preparing design documents and specifications. The HVAC contractor that understands the problems these issues create can stop potential problems that will show up during start up by informing the design engineer of the concerns before ducts and systems are installed. 

Issue # 7 Duct velocities approaching 2500 fpm may create noise and will cause increased pressure drops in fittings and duct-work. Duct velocity can be calculated by dividing the design air flow in a duct at any location by the area of the duct in that location. For example, if a fan is rated at 6000 Cubic Feet per Minute (CFM) and the duct size at the fan discharge is 24 x 12, the velocity is 6000 CFM / (24” x 12”) /144 sq inches per sq ft = 3000 fpm. When velocities are over 2500 fpm, it is good practice to notify the engineer and voice your concerns. It is much easier to solve a potential problem by increasing duct size before the duct is installed. Whenever potential problems can be addressed and corrected before they are found during the Test and Balance phase of the project, the design engineer and installing contractor remove a potential issue that has the chance to become adversarial. 

Issue # 8 The detrimental impact on fan capacity caused by fan system effect is a problem not fully understood by the design industry. System effect is the fact that fans are tested with a specific amount of straight duct connected to the inlet or discharge. Whenever installation conditions differ from the test installation, the fan will suffer reduction in capacity. In a scenario as such that the airflow leaving a fan does not reach a uniform flow profile for several duct diameters from the fan discharge. The distance required to reach uniform flow is dependent of the velocity of the air. The higher the duct velocity, the greater the distance required before this uniform flow is attained. All duct accessories (fire dampers, control dampers, sound attenuators, etc) are all tested with uniform flow conditions when their design pressure drop is determined. Whenever any one of these components is installed in a portion of duct-work where the flow is not uniform, the actual pressure drop will always be greater than the design pressure drop. When system effect imposed pressure drops are not counted in design calculations, fans have greater static pressure than calculated and the fans do not generate the required air flow. Elbows installed in the zone of non-uniform air flow also create a system imposed pressure drop. The American Movement and Control Association (AMCA) has documented the various additional pressure drops that should be added to the calculated pressure drops. System effect is a phenomenon that cannot be measured, but it is real and is one of the reasons many fans cannot develop the required capacity shown in manufactures catalog data. The owner and design engineer will all benefit if the contractor or journeyman will take a close look at fan inlets and discharges during shop drawing approval or the field measuring phase of the project. During this time it is advisable to determine if any duct accessory or elbow has been installed within the first 4-6 duct diameters from the fan inlet or discharge. If this situation exists, the design engineer should be notified and informed of the potential problem. 

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Issue # 9 Drawings with missing dimensional sizes are usually interpreted improperly. Undersized duct-work is usually installed. The problem is usually seen on VAV box inlets where duct sizes are omitted from the drawings. It has been known through experience that when inlet sizes are not shown on the drawings, the installing contractor invariably installs duct-work that is sized for the inlet size of the VAV box. In many instances the VAV inlet is sized for velocities that exceed 2500 fpm. While this velocity may be satisfactory for the 6 inch inlet length of a VAV box, the same problems shown in Item 7 will again surface on the ducts leading to the VAV box. The problems are compounded when extended lengths of inlet duct and numerous elbows are installed. Inlet static pressure to the VAV box can be reduced to the point that the box will be starved for air. To compensate for this, the fan speed is increased and the owner pays excessive energy over the life of the project just because one or two VAV inlets are undersized. 

Issue # 10 Most specifications and details call for four duct diameters of straight duct before the inlet to a VAV box, but the actual duct drawings do not have sufficient room to allow for this installation. When sufficient lengths of straight ducts are not installed, the air flow sensors on a VAV box do not read properly. This causes the VAV box to hunt and cause the VAV controller to be hard to control. Unstable control can cause noise and cause the fan to operate at higher static pressures than necessary. The following example shows the cost to operate two systems for the same sized building. While each building is the same size and has the same heating and cooling capacity, Building II will show the increase in energy costs because various items in the article listed above list exceeded those in Building I. 

Cost example: Assume a system with the following components: • 100,000 sq ft, 5 Story Office Building • Occupied 16 hours per day 6 days per week • $ .10 KW –hr • $12.00 KW demand Example 1 Example 2 Air Flow (Duct Leakage) 105,000 CFM 115,000 CFM System Effect Fan Discharge Elbow Placement .5 in 1.5 in Sound Attenuation Placement .25 in .75 in Duct-work Restrictions 2.5 in 3.5 in Missing Turning Vanes 1.0 in 1.75 in Total System Pressure 4.25 in 7.50 in HP = CFM x SP / (6356 x Fan Eff) Horsepower System I 105,000 x 4.25 / 6356 x .65 = 108 HP Horsepower System II 110,000 x 7.50 / 6356 x .65 = 200 HP Fan Operation 16 hours per day 6 days per week Electrical Cost $ .10 KW –hr 200 HP – 108 HP = 92 HP 92 HP x .746 KW Hr / HP = 68.6 KW-hrs 68.6 KW –hrs x 16 hours / day x 6 days / week x 52 wks per year = 342,500 KW –hrs / year 342,500 KW –hrs / year x $ .10 / KW –hr = $ 34,250 per year of extra electrical KW usage Increase in demand Charge 200 HP – 108 HP = 92 HP 92 HP x .746 KW Hr / HP = 68.6 KW –hrs x 1 hour = 68.6 KW demand 68.6 KW demand x $12.00 x 12 months = $ 9,878 per year Total cost per year = $ 34,250 + $ 9,878 = $ 41,000 The potential savings to eliminate restrictions and reduce static pressure over the 25 year life of the project is over $1,000,000.