In recent years, the building industry has been advocating more high-performance building systems to keep pace with client needs and global warming issues. Today, HVAC system design is often mandated to be more energy efficient while recovering waste energy wherever possible. You would think this would mean our rule-of-thumb values would reflect this effort, but they don’t. Instead, I believe we are going in the wrong direction. Equipment sizes for new construction projects are getting larger rather then smaller, and I will give you a couple of examples.
Real World Examples
The first project is a LEED® science building. For our area of the country, I’d estimate 320 sq ft/ton if this was done before 2001, and I’d assume this value would improve to 360 sq ft/ton based on energy codes, more energy-efficient equipment, and energy recovery today. Unfortunately, I’m seeing A/C loads being sized for 200 to 150 sq ft/ton.When I raised my concern to the design team, the response was, “Our historical data shows these are the new HVAC loads needed to satisfy this type of building.” I’m sure that their historical data is correct if this is how they are designing all their buildings, but have they gone back to check their past projects for actual peak demand? The answer was cleverly stated, but what I gathered from their answer was “No, we haven’t done that.”
We are currently retrocommissioning two projects to reduce energy. Both jobs (different engineering firms) had been designed for 2 cfm/sq ft. At each facility, it was the owner who raised the concern as to the HVAC design based on their excessively high utility bills. The first project was less then a year old, and the second project was two years old. We are now reducing each of these building systems to 1.4 cfm/sq ft.
Working with the pharmaceutical operations group, the first job has already been substantially retrocommissioned and their energy consumption has dropped without any complaints from occupants or the client’s health and safety person. We still have more to do to reduce the energy consumption, based on “less than two year” ROI. We are confident the same results will be achieved at the second building, a university research facility as we rebalance the central air and lab exhaust system to the 1.4 cfm/sq ft.
Oversized Pitfalls
There are several problems with current high-performance engineering approaches, the first problem being the client paying a higher first cost for HVAC equipment and distribution, as well as associated construction cost to build larger equipment rooms, vertical shaft space, etc. In addition, the electrical system size is influenced by this overdesign. A second concern is when high-performance engineering is projecting future master plan capacity based on exaggeration of infrastructure size. The building owner will soon learn after the first phase of the master plan is up and running that their energy bills are excessive.Retrocommissioning will be the answer here too, in an effort to bring operating costs under control without compromising building performance. This incident of master planning occurred at a college where the master plan had to be re-engineered to take into account their current chilled water capacity after another retrocommissioned job. Instead of a 3,000-ton central plant in the years to come, the client is confident that the infrastructure will not need to exceed the 2,000-ton cooling plant’s capacity based on actual system performance. The silver lining for this job has been the campus distribution piping, which was designed and installed to deliver 3,000 tons, resulting in significantly oversized chilled water pipe distribution that today equates to very low pipe distribution resistance, so pumping horsepower is less then initially estimated.
Another problem with conservatively engineered systems is the cost to go back and re-design and re-balance the central air and exhaust air systems. True, the retrocommissioning engineer could argue that the ROI is significant (one- or two-year payback) but it’s unfortunate that the retrocommissioning has to be done in the first place.
Some engineers are going to take exception to this column, but I’m not talking “perception is reality.” I’m talking about reality and not perception. Just think about today’s HVAC equipment technology: low kW/ton chillers, low-flow fume hoods, and fume hood occupancy sensors, to mention just three features. Take into account high-performance light fixtures, occupancy sensors, natural lighting design, etc. The A/C load has to be improving, yet designers are saying historical data shows these loads are increasing. If the overdesigned project is LEED certified and the client has to go back and retrocommission the job because of excessively high utility costs, what will that say for high-performance HVAC engineering?
My response? Keep doing what you are doing, because we’ll be waiting to retrocommission your client’s project while addressing global warming.ES
