Vermont Energy Control Systems

Practical monitoring and control for the real world


The system now provides consistent Glycol temperature. The chart below shows the controls responding to changes in zone demands in the greenhouse.

Chart of temperatures from greenhouse sensors

At first, the temperature of hot water available from storage (magenta line) is a consistent 180℉, and the greenhouse office, bench, and slab are all calling for heat (it’s below zero outside). Circulator P15 (green line) is running at about 30%. Glycol supply to the greenhouse (yellow line) is stable at 120, which is the target temperature. Return water going back to storage is about 110℉.

At about 6:15, the available temperature in storage starts to drop. P15 speeds up to compensate, maintaining a steady 120℉ Glycol supply temperature.

At just after 8:00, the office is satisfied. A few minutes later, the slab is satisfied. At this point, the bench is the only zone calling for heat, and the target temperature is raised to 150℉. P15 ramps up to about 90%, and the Glycol temperature overshoots briefly. By 8:30, the Glycol temperature has stabilized and P15 is running at about 20%.

Finally, the bench zone is satisfied at about 9:40, and P15 shuts off.

As a separate note, it was determined that the heat exchanger was mis-plumbed with parallel flow rather than counterflow. When that’s fixed, return water temperatures will likely be even lower.