By Dave Olson, PE, FASHRAE, LEED AP – PCD Engineering
Throughout my career I have conducted HVAC and plumbing design for many new and remodel projects, on virtually every sort of building type imaginable. When I am doing a re-adaptation of an existing building the first step for me is to conduct a thorough survey, focusing on HVAC, plumbing and fire sprinkler components. As part of that work, I document model numbers and serial numbers for all the existing equipment. Sometimes there are equipment installations with specific components that are less familiar to me. I wonder, is this equipment installed the way the manufacturer intended it. Wouldn’t it be nice if there was a copy of the original installation manual available for me to look at?
How many times have I walked into a furnace, boiler or water heater room and found copies of manufacturer’s literature wadded up, or squeezed between a furnace, duct or pipe, or a water heater and a wall. Many times it is grimy or burnt from its awkward place of rest – shoved where a well-intentioned installation contractor deposited it – thinking forward to the eventual service technician who would benefit from a quick reference to the intended operation of a device or system. Imagine if these documents were collected into a common collection, conveniently located within the mechanical equipment room?
On Tuesday, November 11, 1984, I attended a technical seminar presented by the Colorado Chapter of NEBB (National Environmental Balancing Bureau). The seminar topic was VAV balancing (variable air volume). At that time, VAV was a relatively new HVAC system type. We had several speakers that day, all but one were Test, Adjust and Balance Contractors from the Denver market.
The first speaker was Stan Simpson, then the Vice President of Brandt Engineering. Stan spoke a bit about the history of VAV. He talked about early developments of single duct induction terminals by Barber Coleman Company. The 1969 ASHRAE show had many manufacturer’s offering examples of their new VAV products. In 1970, Brandt Engineering introduced the first fan powered VAV terminal (Z-Box). The HVAC world was in a state of exciting new development – there was a great deal of energy to be saved over traditional constant volume air handling systems. Soon hot water coils were added to the fan discharges, and over-cooling of occupied spaces was mitigated – primary airflow could modulate from 0 – 100%, while the VAV terminal recirculated plenum air for the balance – providing 100% airflow to the rooms all the time. Lastly, Stan spoke about the Carrier Moduline Control Diffuser system. This terminal would include an integral thermostat positioned to sense return air temperature and was calibrated to modulate the supply air quantity in order to control space temperature. This system was quite popular and has remained available and in use ever since that time.
Next, Jim Winston of the RMH Group gave a presentation on VAV system controls. He spoke of the various developments which occurred to refine VAV user satisfaction. Jim discussed the first pressure dependent pinch down terminals, and the issues that occurred due to varying upstream duct pressures. This difficulty inspired manufacturer’s to develop pressure sensors within the primary air supply duct which would vary the delivered primary air supply – and enable the system to satisfy those terminals further down the duct system from the cooled air source. These new terminals were pressure independent – and significantly improved occupant comfort and space temperature control. Jim warned us to watch out for manufacturer’s who positioned the air-flow sensor downstream of the control damper. Multi-point pressure sensors were just being introduced to the market. Jim discussed the operational differences of series flow versus parallel flow fan powered VAV terminals. Jim advocated for parallel boxes since fan operation was not necessary during full cooling operation. He spoke of microprocessor controls which were coming onto the market – which did not permit building occupants to “tamper” with control settings. He mentioned that the microprocessor controls reliability was, “somewhat questionable still.” That would certainly change in the years to come.
Next Mr. Darold Fox of Air Right, Inc. spoke about common VAV balancing issues. He discussed the need to have VAV systems: 1) well engineered, 2) good equipment compatibility, and 3) good system installation. Darold also discussed the need for proper static pressure sensor location and he warned about the need for adequate inlet static pressure to the terminals. According to Mr. Fox, VAV boxes normally need twice as much static pressure at the inlet than most manufacturers will admit. He also stressed the need for a straight inlet duct condition and emphasized that engineer’s need to show this condition on their drawings. Fox suggested that designers call a TAB contractor to peer review their system designs prior to bidding, to try to weed out potential problems. Darold stressed the need for control system repeatability. He recommended that volume dampers be positioned at duct take-offs and not at the diffuser necks wherever possible – for enhanced acoustical satisfaction. He suggested the use of rigid sheet metal ductwork at the VAV terminal inlet. He recommended that both the minimum and maximum cfm for each zone be reflected within the contract drawing set. Mr. Fox warned that loose specifications led to significant variation in bid prices due to the lack of understanding of the engineering requirements.
Following Mr. Fox was Mr. Alex Ochinero of Ochinero Associates. Alex primarily discussed written balancing specifications for projects. He highlighted that balancing requirements should be included within the specs, and complained that most specifications that he had seen were way too vague. Much of the control discussion centered on pneumatic control components and the need to keep them clean during construction. He cautioned against using reverse acting thermostats where direct acting controls were necessary. Mr. Ochinero offered ten suggestions to improve the specification and use of VAV systems, preceded by the need to field inspect all manufacturers’ controls to determine compatibility with field mounted after-market control systems.
1. Investigate proper operation of the inlet static pressure sensor at all VAV terminals because 50% of all VAV terminals had issues with pressure sensors at the box inlet. He stressed that the TAB contractor must determine that proper signal and output is being generated for satisfactory control operation.
2. Determine that primary air controls will provide design flows without going to endpoint of possible flow. He emphasized the need to size the VAV terminals correctly, so that the box delivered suitable airflow on a design day and would still be able to overcome the system static pressure and close down to a minimum airflow condition. Most VAV boxes require minimum 1” w.c. static pressure at inlet for proper operation.
3. Determine that thermostat is calibrated and that all spring ranges are compatible and do not overlap.
4. Determine if 20 psig air is available for controls at furthest terminal unit. He warned that the designer should specify who should verify this condition.
5. Determine that primary static pressure sensor is located properly and that it is sending proper signal and controlling main fan properly.
6. Determine if SCR (3 speed switch) will allow the secondary fan to operate at design airflow. He suggested the need for backdraft dampers on parallel boxes to prevent backflow through the induction side of the fan powered terminal.
7. Determine if primary air damper effects flow of secondary air fan. Mr. Ochinero warned of back-spin caused by induced airflow, and that the fan will continue to spin backward if it energizes while the fan is spinning uncontrollably in reverse.
8. Determine airflows at all diffusers for primary and secondary air fans.
9. Determine that fan powered terminals do not force air out through return air grilles from return air plenum due to excess air pressure.
10. Record all determinations in writing, point by point.
Alex concluded his presentation by reminding the designers to specify who must calibrate the thermostats.
The last speaker of the event was a stately old gentleman named George Hightower of NEBB. George Hightower was a founding member of NEBB and is the namesake of an annual distinguished service award presented to an exemplary NEBB TAB Contractor. George made a number of recommendations during his presentations. Among these were to always use duct collars at air devices, always check ductwork for leakage, always check the manufacturers airflow data as it is not always accurate in their catalogs and specify that provisions be made to remove duct mounted pitot tubes for cleaning – as they will get clogged over time. Despite most of the seminar being focused on airside systems, he offered that hydronic systems should be designed for 5 – 10 psi of extra capacity over that calculated. Then Mr. Hightower advised the impacts of negative suction pressure at the inlet of a pump, and the resultant cavitation that will “eat” the impeller.
Mr. Hightower made a number of recommendations for avoiding issues with fan powered VAV terminals, and warned not to rely upon riding the fan-curve to accommodate the reduction of airflow caused by closing of terminals upon drop in loading. He emphatically warned, “it won’t work!”
Then George introduced the concept of “spec boxes” to the audience. He described this as a sheet metal box with a latching device located on a mechanical room wall where the installing contractor could deposit all warranties, specs, installation manuals, and other product and installation data during construction activities. He emphasized that this accessory should be part of the project specifications, and should be an early component installed by the mechanical contractor on each project.
I returned to my office and immediately wrote a section for our Section 15010 – Mechanical General Conditions. It read as follows:
Spec Box: As early as possible during construction process, Mechanical Contractor shall provide and install a sheet metal spec box in a conspicuous mounting location within the mechanical room where it will not interfere with ongoing construction or finished mechanical room layout.
1. As mechanical construction progresses, each mechanical equipment factory provided owner’s manual and warranties shall be removed from original packing carton and immediately placed in the spec box for future reference by Mechanical Contractor in preparing operation and maintenance manual.
2. At completion of construction, a copy of the operation and maintenance manual shall be located here for future use.
3. Spec box shall be constructed of sheet metal with hinged top and some form of latching device. Label front of spec box “Operating and Maintenance Data”. Spec box shall be minimum 16″ x 12″ x 4″ overall, secured to mechanical room wall in permanent manner.
In my opinion, use of a spec box on each project provides an added insurance that pertinent equipment operation and maintenance literature is available for the engineer and service technicians down the road. George commented that our specs should also require the O&M Manual to be completed and available for the use of the TAB Contractor when they arrive to complete their work. The spec box will still be on the wall long after the project is complete, allowing a safe and dependable lasting location for the invaluable O&M Manual.
Certainly today’s internet access changes the formulae for availability of product reference information. In my experience, original installation and product literature is often not found on the web, as companies who produced the original equipment may not exist, or perhaps do not support or possess the valuable technical literature. The cloud was not envisioned by George or any others in that room in November of 1984, but the message provided that day was solid and still relevant today.