Weather in Hillsboro, OR has been HOT recently, and this is the largest cooling load many of our projects have ever seen. To make sure the equipment is working as intended, Caleb Aring and I have been reviewing trends and performing off-season tests. At one Elementary School, we noticed the peak space temperature was late in the evening, 80+ F around 9:00 pm. At first glance this seems weird, so we made sure the RTU cooling worked correctly and the heat did not inadvertently turn on in the night.
Looking at trends, we know RTU cooling works great. Just before occupied hours, the economizer dampers open to 100% and cool the spaces down to setpoint. As the morning warms up, economizers return to minimum position and DX cooling is enabled to maintain the occupied cooling setpoint. Looking at the heating system, the boilers and RTU gas heat have not been used in several months. But why are spaces so warm at night?
The answer became clear when we compared the trend for an empty room to the outdoor air temperature. In the plot below you can see outdoor air temp peaks at 4:00pm, and without heating or cooling, space temp does not peak until 9:00 pm. It turns out that the spaces stay warm overnight because the building has a high thermal mass. The brick walls and tar & gravel roof are simply heated during the day and continue to release heat into the building throughout the night. On the reverse side of this, the building reaches its lowest temperature just before noon. It’s pretty amazing to see the time delay as heat moves through these surfaces.
I’ll be keeping a closer eye on this in the future to see if there are energy-saving opportunities available. With the right controls setup and an unoccupied building, I imagine you could provide even more economizer free cooling to prepare for those really hot days.