Watch those reheat systems!

The plant floor isn't the only place where energy counts.

By Ben J. Sliwinski

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Typically, we’re concerned with the components of the manufacturing plant and process. But let’s not forget about the offices that might be attached to the plant, or the offices at company headquarters. In either place, a dollar is a dollar, and saving energy in these office spaces can be just as important as saving energy in the plant.

The HVAC systems in many modern buildings rely on variable air volume (VAV) distribution. Research at the University of Illinois Smart Energy Design Assistance Center (SEDAC) indicates that a significant fraction of these systems consume excessive amounts of energy for reheat (perhaps as much as 50% more than necessary).

What’s the problem?

Variable air volume distribution systems were initially viewed as a way to eliminate the energy waste that commonly occurs in constant-volume systems with reheat that were previously found in many commercial and industrial buildings. Reheat is the addition of heat to previously cooled air. Reheat is used sometimes for humidity control, and sometimes for occupant comfort. The latter use happens when zone cooling loads are low and too much cold air is supplied to a space.

In a VAV system, a damper modulates the amount of cool air supplied to the zone. As the zone cooling load decreases, the damper begins to close, supplying less cool air. However, the damper can’t be allowed to close completely because the cooling air usually includes ventilation (outside air), and minimum amounts must be maintained. If the damper reaches its minimum stop and the zone load continues to decrease, the zone temperature is dropping below the setpoint. Then the reheat coil activates and the incoming cool air is reheated to maintain the setpoint.

HVAC system designers estimate loads for each building zone and then size the VAV system for that zone. The objective is to size it so that when the VAV system supplies its minimum air volume, the zone loads provide all reheat, either for humidity or comfort purposes.

 Figure 1. Monthly natural gas consumption records for three typical buildings with variable air volume systems show significant natural gas consumption for reheat, particularly during the summer.
Figure 1. Monthly natural gas consumption records for three typical buildings with variable air volume systems show significant natural gas consumption for reheat, particularly during the summer.

Unfortunately, VAV systems usually include a reheat coil. The coil is supplied with hot water or steam, or in some cases might use an electric resistance heater. The reheat coil acts as a shim, a backup in case the designer’s load estimate isn’t exact. Occupant comfort and humidity control still can be achieved through judicious use of reheat.

In real life, however, the VAV system reheat coils often use large amounts of energy. The reasons include grossly inaccurate load estimates, improper system installation and sizing, or failures in system hardware and software. Figure 1 provides an illustration of the magnitude of the problem. Each of the data sets depicts seasonal gas consumption for a modern building (constructed within the past eight years).

While the usage varies, in each case the buildings are consuming significant amounts of natural gas during the summer for reheat purposes. Notice that summer usage is close to 80% of the winter peak usage. The cooling systems in these buildings waste significant amounts of electrical energy as well, but this is more difficult to assess simply from the graphs of utility bills. The buildings represented here have annual energy costs of about $4/sf. We estimate that about $2/sf of that cost is attributable to reheat system deficiencies.

 Figure 2. This 600-bhp boiler was being operated during summer only to supply the reheat coils
Figure 2. This 600-bhp boiler was being operated during summer only to supply the reheat coils

Every facility manager who operates office HVAC or plant space conditioning systems that incorporate reheat should be on the lookout for the problem shown in Figure 1. Plot the monthly gas consumption for each building or facility to see if you’re burning excessive natural gas during the spring, summer and fall. At the least, spend some time talking with the boiler plant operators. When we visited the plant shown in Figure 2, the first words out of the operator’s mouth were about the craziness of having to operate the boiler during the summer. In terms of energy efficiency, reheat waste is low-hanging fruit, on a par with lighting.

The University of Illinois Smart Energy Design Assistance Center is evaluating strategies for diagnosing reheat system problems in real time and determining methods for correcting the most likely sources of malfunctions or design deficiencies. We plan to achieve this either through upgrades to existing building automation system (BAS) algorithms and instrumentation, or through development of advanced wireless sensors and instrumentation that can be retrofitted to buildings that lack BAS.

Both approaches will integrate information about VAV performance with information about building energy consumption. Real-time methods are essential because reheat system failures frequently don’t result in occupant discomfort. Furthermore, owners and operators of buildings often lack the sophistication, time or interest to evaluate their utility bills in a meaningful manner. In the case of the buildings mentioned above, the owner was completely unaware of a problem of any kind in building performance.

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