3.7.2 System Controls

• The spaces served by the system are located within 25 ft of an operable window.
• The openable window area is at least 4% of the floor space.
• Other requirements for natural ventilation specified in ASHRAE Standard 62.1-2007, Section 5.1 are satisfied.

• Cycle on call from any zone
• Cycle on call from the primary control zone
• Stay off
• Cycle zone fans only (for systems with fan-powered boxes) Restart fans below given ambient temperature.

The method of controlling the SAT. Choices are:

· No control – for this scheme the cooling coils are energized whenever there is a call for cooling

· Fixed (constant)

· Reset by warmest zone, airflow first

This control strategy resets the cooling SAT of a central forced air HVAC system according to the cooling demand of the warmest zone. The airflow first control approach tries to find the lowest supply airflow rate that will satisfy all the zone cooling loads at the maximum setpoint temperature. If this flow is greater than the maximum, the flow is set to the maximum and the setpoint temperature is reduced to satisfy the cooling loads. The airflow first strategy minimizes zone reheat coil energy (or overcooling) and central chiller energy consumption at the cost of possible increased fan energy.

· Reset by warmest zone, temperature first

This control strategy resets the cooling SAT of a central forced air HVAC system according to the cooling demand of the warmest zone. The temperature first control approach tries to find the highest setpoint temperature that will satisfy all the zone cooling loads at the minimum supply airflow rate. If this setpoint temperature is less than the minimum, the setpoint temperature is set to the minimum, and the supply airflow rate is increased to meet the loads.

The temperature first strategy minimizes fan energy consumption at the cost of possible increased zone reheat coil energy (or overcooling) and central chiller energy consumption.

Reset by outside air dry-bulb temperature

· Scheduled setpoint

· Staged setpoint (for single zone VAV and DX with multiple stages)

For baseline building systems 1 through 4, there is no SAT control. The cooling coil modulates based on the zone thermostat setpoint.

For systems 5 through 8, the air temperature for cooling shall be reset higher by 5°F under the minimum cooling load conditions using a reset by warmest zone, airflow first strategy.

For system 11, the supply air temperature setpoint shall be reset from minimum supply air temperature at 50% cooling load to space temperature at 0% cooling load.

For systems 9, 10, 12, and 13 (heating and ventilation and single zone systems), this input is not applicable.

A linear reset schedule that represents the SAT setpoint as a function of outdoor air dry-bulb temperature. This schedule is defined by the following data points (see Figure 9):

· The coldest cooling SAT

· The corresponding (hot) outdoor air dry-bulb setpoint

· The warmest cooling SAT

· The corresponding (cool) outdoor air dry-bulb setpoint

• Fixed (constant)
• Reset by coldest zone
• Reset by outside air dry-bulb temperature
• Scheduled setpoint

• The hottest heating supply air temperature
• The corresponding (cold) outdoor air dry-bulb threshold
• The coolest heating supply air temperature
• The corresponding (mild) outdoor air dry-bulb threshold

[figure title="Example of SAT heating setpoint reset based on outdoor air temperature (OAT)." id="example-of-sat-heating-setpoint-reset-based-on-outdoor-air-temperature-oat"][/figure]