WSHP System Control and Optimization Part 1: Basic Control Overview
/By Chad Edmondson
Control of multi-unit WSHPs is typically quite simple compared to other HVAC systems. In some cases, individual control of units will suffice, such as a small office building with a ground source loop as a heat sink/source. However, most commercial size WSHP applications that include a boiler and cooling tower will require at least a rudimentary building management system (BMS). Control complexity increases with the number of integrated components, e.g. boiler, cooling tower, dedicated outdoor air system (DOAS), variable speed pumps, etc. How far you go beyond the bare minimum is also dependent on the needs and budget of the owner.
Basic Temperature Setpoint Control
Simple or complex, the key to controlling and optimizing any WSHP is making the most of the BTU storage in the water loop. Remember, this is a “battery” from which BTUs will be shared as needed by individual heat pumps to meet their corresponding loads. Properly sized heat pumps in a WSHP system have the capacity to meet corresponding zone demands as long as the loop temperature stays between 50°F and 90°F. This is called the “dead band” range. As long as the loop is in dead band range, neither a boiler or cooling tower is needed to help meet load, regardless of the season or outdoor temperature.
We recommend the following temperature control settings for a WSHP system with a cooling tower and boiler:
Dead Band Range: A loop temperature range of 50°F to 90°F is typically sufficient to maintain load without calling on the cooling tower or boiler, as long as individual units are appropriately sized for the load they serve.
Closed Circuit Cooling Tower Start Set Point: When the water loop temperature reaches a set point of 90°F, the system should be controlled so that the cooling tower dampers open and the spray pump is activated. This alone may be enough to cool the water loop without actually turning on the cooling tower fan.
Cooling Tower Fan START Set Point: If the water loop continues to rise, the cooling tower fan should be controlled to start rotating when the loop temperature reaches 95°F.
Maximum Loop Temperature Range: 50°F to 102°F is the operating range of the loop with either the boiler or cooling tower operating.
Cooling Tower OFF Set Point: Once the loop temperature is cooled to 80°F, all cooling tower components (fan, spray pump and dampers) should deactivate.
Boiler Operational Set Point: When the loop temperature drops to 50°F, the boiler should activate and remain active until the loop reaches 65°F. Recommended boiler set point is 60°F.
How Sophisticated Does the BMS Need to Be?
As mentioned early, small commercial systems with a geothermal ground loop are fine with simple WiFi enabled thermostats for unit control. This will allow you to control the heat pumps with an iPad or laptop and troubleshoot individual WSHP alarms. Small commercial systems with towers and boilers could also utilize this approach and just let the DOAS, boiler, cooling tower and pump operate based on their individual controls. However, a building will benefit from a BMS if any of the following are required (or desired):
• Centralized alarms for equipment faults
• Integrated control of DOAS, boiler, cooling tower and/or boiler
• Energy optimization (e.g. morning start-up, nighttime setback, etc.)
• Tower freeze protection
• Remote reset of individual units that may go off on an alarm
• DDC controls for monitoring status of all equipment
• Troubleshooting capability
Energy optimization may be as simple as automatic on/off control of units to correspond with periods of occupation. A BMS can gradually stage units on in the morning so that the entire system doesn’t start-up at once which could cause the utility to increase the owner’s electrical demand threshold. This approach is common in elementary school applications.
Larger educational, medical, governmental facilities, etc. will likely require more advanced BMS capability, with most if not all of the functionality listed above, along with enhanced, user-friendly graphics.
Next up we will look at cooling tower freeze protection for both open and closed loop towers.
