School Building Mechanical Engineering 101
- By Ernest MacFerran
- March 1st, 2011
The Hillsborough County School District, in Tampa, is the ninth largest in the country. We have created and implemented the following mechanical engineering systems for our school buildings to provide consistent and dependable indoor air quality, reduce construction, energy and maintenance costs, and always achieve green construction goals.
Reduce Fresh Air Ventilation.
The first thing we did was perform a certified test report to reduce fresh air ventilation in the schools, while achieving good IAQ and saving big money. According to national code, the amount of fresh air to be used is 15 cfm/student. Because of our test results, the Florida Building Code was recently changed allowing the amount of fresh air to be reduced to 7.5 cfm/student in cases of intermittent occupancy and good IAQ. To implement this code change we had certified, independent testing of eight schools that showed no difference in CO2 levels and no difference in the results of FCAT testing systems whether using 5 cfm(cubic feet per minute)/student, 7.5 cfm/student or 15 cfm/ student.
The reduction of fresh air will use less cooling tonnage, reducing the equipment mechanical and electrical construction cost by approximately $100,000 for each new or existing school. The energy saved will be approximately $10,000 per year, while still providing good IAQ and contributing to green construction.
Unfortunately, LEED will only recognize your new school if you use 15 cfm/student. If you use 20 cfm/ student LEED will reward you with extra credit. This provides no benefit to anyone except equipment suppliers. For example, instead of a 225-ton chiller you will need a 290-ton chiller.
Lastly, we chose to not use ventilation/demand limiting because it offers no chiller reduction savings, requires additional automatic control and hundreds of CO2 detector sensors which may or may not work. Also, the exhaust fan systems must modulate in unison to the modulating ventilation air to prevent the building’s negative pressure. In light of these changes, our district was recognized in 2009 by the Association of Energy Engineers with the regional (eastern USA) energy award.
Be Cautious of Cooling Tower Systems.
While cooling towers can be great for school buildings, they are not simple to control and require frequent and routine care. Unless you have the long-term maintenance commitment and craftsmen, we would recommend air-cooled equipment.
The system efficiency of a cooling tower/water-cooled system is not as appealing as advertised. Energy consumption must include the condenser pump motors, tower fan motors, the cost of domestic water evaporated and used for blow-down waste water and a realistic installation cost and maintenance cost.
Save Big Money by Ignoring the Low Bid.
Pre-purchasing HVAC equipment is common, but a unique evaluation of the bids is what is how our district benefits the community.
We require bids that include the manufacturer’s cost for an extended 15-year, bumper-to-bumper warranty, listed energy efficiency (EER or kw/ton) and the engineer’s first-year energy cost. All bidders know we will perform a 15-year comparative life-cycle cost analysis of all submitted manufacturers for each school, and make the selection accordingly; comparing energy consumption, maintenance cost and equipment first-cost. The chosen manufacturer will be the least owning and operating cost for the 15-year period, which is rarely the low bid.
Back to the Future — Saving Money With a Block of Ice and a Whirly Fan.
We started our ice storage design and construction in 2009 with five schools, and are now up to 11. Ice storage systems are not uncommon, and we are successful in saving money despite not using the local utility company’s off-peak power rates. It comes by using a partial ice storage system instead of a full ice storage system.
The operational result at one school (Bing Elementary School), were $14,400 cost savings compared to the previous year.
The extra construction cost for the school’s partial system over a conventional system was approximately $122,900. A simple payback of 8.5 years — better than a CD.
District’s Unique Pump Design Saves Money and Is Simple to Maintain.
The district’s typical pump installation (see illustration) is different than any implemented in the U.S. — lower installation cost, less energy consumption, simpler maintenance, longer-lasting un-insulated pump casing, requires easier preventive maintenance and contributes to green school building construction.
The district’s hydronic certified testing demonstrated that the age-old claim for a need of five-foot length of straight pipe into the pump’s suction is a myth, the expensive 25-year-old end suction diffuser is detrimental to the system and, the fact is, a standard, inexpensive, simple long-radius elbow provides good laminar flow and a very good low pressure drop. The application of the venture flow meter is always important, but it has been certifiably tested and proved to not need any straight section of pipe down stream of the venture, saving pipe and allowing a simple installation.
Good Use of Solar Hot Water.
The use of solar hot water has never been considered for air conditioning/de-humidification. The district’s independent study shows solar systems versus electric heat use less energy, still provide good IAQ, contribute to green construction and have only a 1.1-percent higher life cycle cost over 25 years than the electric system.
For this application, in Tampa, solar heating is abundant at the very time of year needed to dehumidify fresh air make-up. It should be noted that solar hot water systems are constant-air, while non-solar systems are variable-air systems. Solar systems are not the district’s standard but are being evaluated.
Other advantages to the solar system are that the air handler’s fan is constant/single speed; outside air requires no modulating; a solar system requires one supply diffuser (instead of the typical four); no VAV Boxes; the solar tank heater is intended for winter operation, but available to supplement solar heat; the common DX (direct expansion) system is not good with variable-air systems, but DX is perfect with solar systems; and controls, troubleshooting, testing and balancing are much simpler with solar.
We Are Saving Money and Protecting Tampa’s Sewage Treatment Plant.
A typical school produces approximately 170,000 to 190,000 gallons of condensate water per year from the air conditioning system. There are 216 schools in our district. Dumping this into the sanitary floor drains is a humongous burden on the sewer plant and amounts to approximately $91,800 to $102,600 per year cost to the district.
Consequently, the district strives to discharge the water into the storm water collection system or safely onto the grassy areas (with EPA’s support and approval).
Make Sure You Get the HVAC Equipment You Paid For.
When a new air conditioner is certified by the Test & Balance contractor to operate at 10 tons, but was designed as 15 tons, what do you do?
Rarely is equipment tested at design-ambient and design-room conditions. You just don’t know if it can produce 15 tons until design conditions occur. The way our district verifies the equipment capacity, is to submit these reported Test & Balance conditions back to the manufacturer to be rerun on the same computer selection program used to get the 15-tons figures. At these “non-design” conditions, the computer generated output should be 10 tons, or we have a problem (or more accurately, “Hello, Mr. Contractor, you have a problem”)
Save Money Making Happy School Kitchens.
The district had a problem with IAQ for our staff working in kitchens. We had not been air conditioning our kitchens, and the type of kitchen hood used has been the combination supply and exhaust air island hood. This has caused a continuous problem with IAQ (high temperatures and humidity) issues for the staff.
To fix that, the district is assessing the use of the canopy hood system, transfer air from the cafeteria to the kitchen, using no electric heat in the supply air duct, which will save energy, make good use of the kitchen’s floor space, use interlocked controls for the cafeteria and the kitchen fans so all fans either operate simultaneously or all are off — ultimately tempering the kitchen space to provide good IAQ for the staff at reduced cost.
Contract Directly With an HVAC Commissioning Agent.
Our district started commissioning HVAC systems in 2009. Prior to that, the builder submitted “perfect” final test, balance and commissioning reports, but many of the systems didn’t work most of the time, and there were problems with IAQ. Today, our district contracted HVAC Commissioner is our “enforcer,” who identifies design, installation, equipment and operational issues that prevent the HVAC systems from working like they are designed. The other big benefit is that there is no additional cost to the schools’ construction cost.
Save Money by Not Insulating Exposed Classroom Ductwork.
When a supply air duct is concealed above the ceiling, it must be insulated. If the supply duct is below the ceiling, exposed in the classroom, it doesn’t need to be insulated. This will save approximately $1,000 per classroom. The reason an exposed supply duct has always been insulated is the fear of condensation forming on the outside duct surface and dripping onto the classroom floor. This hasn’t happened in 10 years.
Save Money by Using One Grille per Classroom Instead of Four.
It has always been that air conditioning designs use multiple ceiling diffusers; usually four, sometimes two. We have determined that a constant air flow air conditioning system only requires one diffuser. With the single diffuser, when sized correctly, there is no issue with noise, no excessive pressure drop, no problem achieving a good air flow distribution, and will save approximately $1,000 per classroom.
HVAC Design for Kiln Rooms Is a Big Deal.
The district will be standardizing kiln room ventilation systems to avoid a potential dangerous condition and a unique IAQ challenge.
The problem is controlling the space temperature for the limited student and teacher occupancy. The kiln’s seven-kilowatt heater takes hours to build up to firing temperature of 1,000 degrees and takes hours to cool down. All of the time, fumes are being emitted, and if the room temperature exceeds 110 degrees, the fire sprinkler heads may discharge five gallons of water per minute.
The central air conditioning system turns off after school hours, so that is not the solution for the kiln room. Where the kiln room is located will determine the choice of ventilation system.
Use Standard, Simple Fresh Air Control.
As all classroom thermostats modulate the VAV (variable air volume) air handler, the fresh air also modulates. That is bad. By state law, each student and teacher shall receive a constant, prescribed amount of fresh air. With the modulating control damper in the return duct, the fresh air would automatically, always be constant air flow.
Calculate Duct Velocity Correctly.
Everyone knows how to calculate rectangular duct velocity, except most HVAC design engineers. (I was one and learned the hard way.) Engineers use the popular Trane Ductulator, and that is their mistake. Please, always use a hand-held calculator. It’s not Trane’s fault.
The fine print on the Ductulator says that the air velocity sizing results are for round duct only. Otherwise, the engineer’s duct velocities are always too large (by 14 percent) and the associated velocity pressure will be too high (by 30 percent), resulting in an oversized fan motor. Using the correct duct velocity will save construction cost, and provide green results.
Save Money With Waterless Urinals.
One waterless urinal would save approximately 3,700 gallons of water per year and contribute to green construction. To successfully use these urinals, the district’s custodial staff must be trained in the ways of maintaining waterless urinals and, also, the chronic problems with flush valve replacement and repair go away.
Due to State Codes, Districts in Florida and North Carolina Can Reduce Fresh Air Ventilation and Achieve Good IAQ (And Save Money).
Our district’s reduced fresh air will use less cooling tonnage, reducing the equipment mechanical and electrical construction cost by approximately $100,000 for each new or existing school. The energy saved will be approximately $10,000 per year; and we will still provide good IAQ for the students and staff.
Our Life Cycle Cost Analysis (LCCA) Is Simple and Results in the Proper HVAC System.
The simple improvements are using the correct amortization tables, incorporating the associated non-HVAC electrical, plumbing and building construction costs into the total HVAC construction cost; and the actual equipment first-cost adjusted to the Consumers Price Index.
We added a fourth sheet to the FDOE calculation form for the itemized non-HVAC construction costs.
The other big benefit: engineers submitting the school project’s LCCA are no longer able to skew the output with their input numbers to result in their favorite, comfortable system without being obvious. Good for the district.
Ernest MacFerran, PE, CIAQP, is the mechanical engineer for the Hillsborough County Public School District in Tampa, Fla. He is responsible for the planning, construction and maintenance of the district’s’ mechanical systems. He can be reached at 813/ 635-1256 or Ernie.MacFerran@sdhc.k12.fl.us.