Have you noticed that your boiler is working harder than it needs to, especially on hot muggy summer days? If your summer utility costs are almost as high as your winter utility costs, your reheating system may be at fault. If you have a Variable Air Volume (VAV) system, your boiler may be “reheating” air, an energy-intensive and costly process.

What is reheating? 

VAV systems reheat previously cooled air for occupant comfort and humidity control. Dampers control the amount of cool air supplied to a space by opening and closing slightly to allow enough cool air in a space. The damper can not close completely because outside air needs to be allowed in for ventilation. Because the dampers are left open, too much cool air is allowed into the space, further decreasing the temperature. When the temperature drops below a certain setpoint, the reheat coil activates, reheating the incoming cool air to maintain a comfortable temperature. Reheating is also used to control humidity, because reheating reduces the humidity of the incoming air. This reheating process is wasteful, since it means reheating air that has already been cooled. It is also costly, accounting for as much as 50% more energy than
necessary. Can reheating be minimized without losing humidity control and occupant comfort?

SEDAC has found that buildings can run happily in the muggy heat of central Illinois with little to no reheating. These buildings save a considerable amount of energy dollars, while maintaining comfortable temperatures and humidity levels. One building, for instance, saw its monthly utility bills drop from $15,000 per month to $9,000 per month. The gas company showed up and replaced the meter because they couldn’t believe the numbers!

Sometimes the cause of the inefficient reheating is oversizing—the HVAC system is designed for a much greater load than is actually needed. Other times the system was incorrectly installed, or there are software or mechanical malfunctions.

Solutions

With the help of a good HVAC contractor, there are a number of steps you can take to optimize your VAV system so that reheating is minimized:

  • Check your temperature control deadband. Set the reheat temperature at least 5 degrees lower than the cooling setpoint.
  • Use CO2 sensing and data logging to adjust outside air intake dampers.
  • Adjust and fine-tune individual zone VAV boxes to tailor the air flow to the space cooling load.
  • Adjust supply air temperature setpoint or implement a supply air temperature reset control.
  • Experiment with turning off the boiler or reheat systems during the cooling season.

For more information, read “Watch those reheat systems!” by former SEDAC staff member, Ben Sliwinski