By KENNETH M. ELOVITZ, PE, Esq., Engineer and In-house Counsel, Energy Economics Inc., and Adjunct Teaching Professor, Architectural Engineering, Worcester Polytechnic Institute

In 2009, Atul Gawande, a surgeon at Boston’s Brigham and Women’s Hospital, wrote a book called “The Checklist Manifesto: How to Get Things Right.” Gawande’s premise is that no matter how well-trained people might be, checklists aid recall and help avoid errors, especially in high-pressure or hectic situations.

HVAC engineers who serve as their firm’s project manager or project coordinator rarely face life-and-death or emergency situations. Yet a checklist with ideas like the ones presented here might help to avoid errors or oversights, improving project outcomes.

 

Consult the architect and structural engineer to confirm the roof structure is stiff enough for rooftop-equipment vibration and noise isolation.

Consider a 20-ton rooftop air-conditioning unit with a 7.5-hp supply fan turning at 970 rpm. For a fairly common 20-ft to 30-ft structural span, Table 47 in Chapter 48 of 2015 ASHRAE Handbook—HVAC Applications recommends vibration isolators with 1.5-in. static deflection.

Vibration isolators require rigid support to function properly. Rigid support means at least 10 times as stiff as the isolator spring or deflection no more than 1/10 the isolator design static deflection. To be effective, a 1.5-in. deflection spring has to sit on a structure that deflects no more than 0.15 in.—between 1/8 in. and 3/16 in.

Structural engineers often design to allow a roof structure to deflect up to 1/360 of the span under normal load. Therefore, a 30-ft span could deflect as much as an inch in the center of the bay. That structure would need additional stiffening for a vibration isolator to work effectively.

Besides vibration, there is noise. A 20-ton rooftop unit might have an Air-Conditioning, Heating, and Refrigeration Institute sound-power rating of 94 dB. A roof structure consisting of an insulated steel deck is largely transparent to noise, so much of that sound will penetrate to the occupied space below. Adding 3 in. of concrete to the structural bay under the rooftop unit will provide transmission loss and acoustical mass to cut noise transmission.

 

Consult the structural engineer regarding the weight of the pipes and ducts a structure will have to support.

In “43rd Edition Standard Specifications and Load and Weight Tables for Steel Joists and Joist Girders,” the Steel Joist Institute advises that uniformly distributed loads of up to 100 lb can attach to any point on the top or bottom chord of a joist with negligible effect on the joist. Heavier loads must attach at panel points (points where the diagonal braces meet the chord of the truss) or have welded struts that transfer the concentrated load to a panel point on the opposite chord. Joist manufacturers can design joists to accommodate heavy concentrated loads, but they need the load information before the joists are fabricated. Besides the size of the load, check with the structural engineer regarding the amount of eccentricity, if heavy loads are attached to the edge of a joist or beam.

 

Verify voltage characteristics with the electrical engineer.

A contractor friend says you can never check voltage often enough. Here’s why:

A 20-ton air-conditioning compressor designed to operate at 208 volts/3 phase has a power input of 22.3 kW and a rated current of 72.5 amps. The same compressor operating on 480 volts has the same rated power input and a rated current of 31.4 amps.

If a 480-volt compressor is connected to a 208-volt power supply, Ohm’s law predicts a current flow of 13.6 amps ([208 ÷ 480] × 31.4). It may be somewhat more because a motor will draw as much current as it needs to run its load, but it most likely will be considerably less than the rated amps. If the compressor does run, it probably will not deliver anywhere near rated capacity. That problem can be solved after the fact with a 208:480-volt step-up transformer, but at unplanned expense and with lifelong operating expense for the few percent of transformer losses.

If a 208-volt compressor is connected to a 480-volt power supply, the results can be more exciting. Ohm’s law predicts a current flow of 167.3 amps ([480 ÷ 208] × 72.5), or 2.3 times rated current. The circuit breaker might not trip right away because 2.3 times current does not look like a short circuit, even when scaled up for motor starting inrush current. Therefore, 2.3 times rated current will flow (at least briefly) through the motor windings. The resulting I2R heating in the windings is 2.32, or 5.3 times as much heat as the motor cooling is designed to dissipate. Because this is a hermetic refrigeration motor compressor, the motor does not get much cooling until the refrigeration has run for at least a few minutes. So, the motor burns up, and an expensive compressor has to be replaced, as does the contaminated refrigerant.

Check the voltage!

Giving the electrical engineer a list of the HVAC equipment needing power also can help. Chillers, pumps, air-handling units, and rooftop units are pretty obvious; an electric heater in the sprinkler room, a small induced-draft fan, or an electric damper motor here and there, however, can be easy to miss. The electrical engineer needs to hear from the plumbing engineer and fire-protection engineer about equipment needing power, too. One engineering firm includes a combination mechanical/electrical schedule sheet on every project. The sheet lists each piece of mechanical equipment and its electrical characteristics and accompanies both the HVAC and electrical drawings.

 

Coordinate with the plumbing engineer regarding condensate drainage.

Local codes might or might not let condensate be dumped by rooftop units onto roofs and flow into storm drains. I have seen sites where condensate had to be piped to the roof drain (at extra cost if not shown on the drawings).

Check local requirements for condensate from condensing boilers. Even though condensing-boiler condensate is no more corrosive than Coca-Cola, local codes might call for its neutralization before it is dumped down a drain.

Local codes and regulations might designate the drains that can accept condensing-boiler condensate. For example, neutralized boiler condensate does not fit the Massachusetts Plumbing Code definition of “clear water waste,” so it cannot be discharged into a storm drain. It also does not fit the definition of “sanitary sewage,” so it cannot be discharged into an on-site sewage-disposal (septic) system. For sites not served by a municipal sewer system, the regulations seem to require an industrial-waste holding tank, meaning owners have to hire an industrial-waste hauler to remove neutralized condensate. In such cases, it might be more practical to use a non-condensing boiler.

HVAC and plumbing drawings need to show clearly the division of responsibility for equipment connecting to both HVAC and plumbing systems. The list includes indirect water heaters and makeup-water systems for boilers, chillers, and cooling towers. Be explicit regarding:

  • Which trade provides the indirect water heater and which trade makes specific connections (boiler water, incoming city water, outgoing domestic hot water, and recirculated hot water).
  • Which trade provides backflow prevention required by a makeup-water system. Where does the plumbing piping end and the HVAC piping begin?

Check locations of plumbing vents. The plumbing code, the building code, or an applicable standard might require a specified separation between plumbing vents and outdoor-air intakes.

 

Discuss with the fire-protection engineer spaces that need heat to protect sprinkler pipes.

Section 8.16.4.1 of NFPA 13-2013, Standard for the Installation of Sprinkler Systems, calls for the protection of sprinkler pipes against freezing. Though identifying spaces that need heat to protect sprinkler pipes may be the primary duty of the fire-protection engineer, it does not hurt for the HVAC engineer to ask. Spaces the HVAC engineer considers unconditioned or unoccupied might need heat for freeze protection. An electric unit heater set at 50°F can be a cost-effective choice for a small room on an outside wall away from a heating hot-water supply line.

 

Check and coordinate to ensure firestopping gets done.

Pipe and duct penetrations through fire-resistance-rated walls, partitions, floors, and ceilings need firestopping to protect against the spread of fire. On some projects, each trade is responsible for firestopping its own openings. On others, all firestopping is assigned to a single specialty contractor. The question of who specifies firestopping and who does the work is far less important than checking and coordinating so the work gets done.

 

Pilots and the military are no strangers to checklists. Pilot Chesley “Sully” Sullenberger safely landed US Airways Flight 1549 on the Hudson River after bird strikes caused both engines to fail. He attributed his success partly to an emergency-procedures checklist. Developing and using project-coordination checklists might help HVAC engineers, too.