High Cost of Poor TAB
May 19, 2008
Engineers and owners should be aware of the true cost of poorly balanced buildings, an issue that is prevalent in the competitive plan and specify/test and balance (TAB) market. Hidden energy, O&M, and other costs can go undetected for years until they are discovered in a retro-commissioning project. Without retro-commissioning or a TAB check, the extra costs are never found, and the owner pays high utility costs unnecessarily built into an operating budget.
Most TAB specifications refer to National Environmental Balancing Bureau (NEBB); Associated Air Balance Council; or Testing, Adjusting, and Balancing Bureau procedural standards. These three associations require an HVAC system's air and hydronic flows to be adjusted to +/-10% of design flow rates using instrumentation that has been accurately calibrated by a qualified firm using methods and equipment traceable to national standards.
Specifications typically call for the TAB agency to compare the total air flow of an air-handling unit as measured by a pilot reading and compare this flow with the sum of the registers at full flow.
These specifications require both air and hydronic systems to be adjusted to obtain flows with the least amount of static pressure. Procedural standards in all three associations require that at least one open pathway be maintained in each system, each branch within a system, and each terminal box within a branch.
While it is necessary to specify proper TAB procedures, ii is even more important to be certain that a certified professional is in charge and responsible for the TAB process. For example, NEBB insists that all certified professionals responsible for TAB projects meet stringent experience requirements, and must pass a very thorough written examination that shows proof of knowledge of the science invo1ved in TAB.
The applicant must then pass a practical examination that shows he/she has the practical skills to actually, perform the work. NEBB then insists that both the professional supervisors and firms sign NEBB's code of ethics form that slates the firms pledge to strictly follow procedures. This commitment to excellence is important because the increased energy costs ·the owner ends up paying due to culling corners in procedures are significant.
Failure to follow these simple procedural standards can be costly, and energy costs can be measured and predicted.
A typical 100,000 sq. ft office building may have 110,000 CFM of design air flow requirements and a design motor load of about 110 hp. Energy cost charges for a building that operates 16 11/day, 5 days/week, delivering 55 F air to the occupied spaces, with total electric costs of $0.08 per kWh and gas healing costs at $11 per therm energy can be determined based on changes to the system.
If the HVAC system is set up improperly and delivers 130,000 CFM from the fan to obtain 110,000 CFM in the occupied spaces, the extra motor costs can be calculated from the ran laws:
(130,000 CFM / 110,000 CFM)3 x 110 hp =
1.18 x 1.18 x 1.18 x 110 hp = 180 hp
(180 hp - 110 hp) x 0.746 kWh/hp x 16 h x 260 days/yr = 217,235 kWh/yr
217,235 kWh/yr x $0.08 kWh=
$17,400/yr or $350,000 over the 20-year life of the building
Failure to leave open paths to diffusers and terminal units results in an increase in total system static pressure. This is a common occurrence on many projects because it is much easier to simply increase the fan static pressure set-point to achieve the desired air flow than to eliminate the restriction or determine the reason the pressure drop is higher than design.
A 1-in. increase in total system fan static pressure will create the following costs.
100,000 CFM at 4 in. of static pressure = 98.4 hp
100,000 CFM at 3 in. of static pressure = 76.6 hp
(98.4 hp - 76.6 hp) x 0.746 kW/hp x 16 h x 260 days= 68,000 kWh/yr
68,000 kWh/yr x $0.08/kWh =
$5,500/yr or $110,000 over the 20 yr life of the project
The same type of analysis can be used to determine the costs of improperly set up exhaust systems, makeup air, reheating excess supply air, cost of unwanted reheat caused by high pump pressure-induced leaking reheat valves, and myriad other problems seen on many projects. The formulas and equations used to determine various costs can be found in NEBB's TAB Manual for Technicians and in NEB B's TAB l raining Course for Supervisor's. These manuals can be purchased online.
This chart shows the cost savings seen in a recent retro-commissioning project. The owner paid these extra costs for 3 years until the building was set up to NEBB standards. During the retro-commissioning process, air flows were lowered to the design requirements, restrictions in the duct-work were removed to reduce the static pressure, hot water leaking past control valves was eliminated by reducing the static pressure output or the healing pumps, occupied time sequences were corrected, and control loops were tuned to operate within a tighter range. The project cost were $100,000, which amounted to a 1.1 year payback.
|Issue||Yearly cost savings||20-year cost savings|
|Excessive fan air flow||$17,400||$350,000|
|Excessive reheat requirements||$25,000||$500,000|
|Excessive fan static pressure||$5,500||S110,000|
|Excessive exhaust makeup air costs||$25,000||$500,000|
|Excessive pump static pressure||$3,000||$60,000|
|Leaky reheat valves causing a 3 F air temperature rise||$14,800||$300,000|
Lost productivity costs are more difficult to determine. However, the same 100,000 sq. ft. office would have 700 occupants based on ASHRAE Standard 62 occupancy requirements. If the average salary and benefit.