Optimizing BMS Air-Side Economizers for Colder Weather: Harnessing Nature for Chilled Water Cost Savings

Posted by Daniel F Luna, P. E. on Oct 12, 2023 2:43:44 PM

Utilizing air-side economizers during the cooler seasons can significantly reduce chilled water costs by leveraging outside air for cooling, offering substantial energy savings for businesses in temperate climates.

As we slip into fall, our thoughts turn to pumpkin spice.  Our air-side economizers – a duct and damper arrangement with a control system that enables the heating, ventilation, and air conditioning (HVAC) system – however, think about saving chilled water costs.  If we can lower load for the chiller, we can save money due to less energy being used. If the outside air is cool and dry enough, we can open an economizer and let nature do the job instead of using the chilled water coil.  It also usually has the benefit of changing the air more quickly than during the winter or summer seasons.  

Less chilled water pumping also lowers the energy needed to move the water around. We control the differential pressure of the chilled water by slowing down the chilled water pump via a variable frequency drive (VFD), and less load raises the differential pressure.  The tower also must do less work because there is less heat rejected out to it, so the fans run slower.  This saves tower make-up water as well: less evaporation, less water needed.

EES two air handlers

Here is an illustration of two air handlers. The return air wet-bulb temperature is higher than the outside air wet-bulb temperature, and the outside air is cooler than the building air.  The economizer (left side) opens as much as needed to get the mixed air temperature as close to setpoint as possible.  In this picture, the air assist alone doesn’t do the job completely (the damper is at 100%), so there is some chilled water used.  As we get later into the fall, or earlier in the spring, the economizer can do the whole job and no chilled water will be needed.

This is a major cost savings in large air handlers in temperate and cooler climates.  It doesn’t do much in hotter climates, and is not worth the cost to install in these locations.

We find economizers in large air handlers as well as medium-sized ones.  One of the shopping malls we work with, for example, has an economizer on some of the newer roof-top units.  Even though they can implement this cost-saving feature, the original design team did not program it in.  We had to add it afterwards, resulting in a decent savings.

There does have to be some human interaction, however.  When the Canadian wild fires were billowing smoke down into the States and the air quality was terrible, running the economizer would have been a bad thing as we would have pumped in large quantities of very smoky air.  

As fall approaches, businesses have the opportunity to tap into the cost-saving potential of air-side economizers. By using the cooler outside air, it's possible to reduce the load on chillers, conserve energy, and decrease water usage in systems. However, it's crucial to monitor external conditions like air quality to ensure optimal and safe operation.

If you want to learn more about how you could save money through optimizing your air-side economizer, reach out.

Topics: energy control systems, dan luna, bms, HVAC, technology