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.

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

• The coldest cooling supply air temperature
• The corresponding (hot) outdoor air dry-bulb setpoint
• The warmest cooling supply air temperature
• The corresponding (cool) outdoor air dry-bulb setpoint

[figure title="SAT Cooling Setpoint Reset based on Outdoor Air Temperature (OAT) for Dry (B) and Marine (C) Climates" id="sat-cooling-setpoint-reset-based-on-outdoor-air-temperature-oat-for-dry-b-and-marine-c-climates"][/figure]

[figure title="SAT cooling setpoint reset based on outdoor air temperature for humid 'A' climates" id="sat-cooling-setpoint-reset-based-on-outdoor-air-temperature-for-humid-a-climates"][/figure]

• 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]

Same as the proposed design. However, setpoints and schedules can differ between proposed and baseline design:

· For HVAC systems that automatically provide occupant thermal comfort via means other than directly controlling the air dry-bulb and wet-bulb temperature, provided that equivalent levels of occupant thermal comfort are demonstrated via the methodology in ASHRAE Standard 55, Section 5.3.3, “Elevated Air Speed,” or Standard 55, Appendix B, “Computer Program for Calculation of PMV-PPD.”

· When necessary to model nonstandard efficiency measures, provided that the revised schedules have been approved by the rating authority.

Same as the proposed design However, setpoints and schedules can differ between proposed and baseline design:

· For HVAC systems that automatically provide occupant thermal comfort via means other than directly controlling the air dry-bulb and wet-bulb temperature, provided that equivalent levels of occupant thermal comfort are demonstrated via the methodology in ASHRAE Standard 55, Section 5.3.3, “Elevated Air Speed,” or Standard 55, Appendix B, “Computer Program for Calculation of PMV-PPD.”

· When necessary to model nonstandard efficiency measures, provided that the revised schedules have been approved by the rating authority.

Schedules for HVAC fans shall run continuously whenever HVAC zones are occupied and shall be cycled on and off to meet heating and cooling loads during unoccupied hours.

Hours with occupancy >5% are considered to be occupied hours and require continuously operated fans.

Exceptions:

· HVAC fans that do not provide outdoor air for ventilation shall cycle on and off to meet heating and/or cooling loads. This requires that outdoor air is introduced through some other approved means such as natural ventilation or another fan system.

· Where no heating and/or cooling system is to be installed and a heating or cooling system is being simulated only to meet the requirements described in Standard 90.1-2019 PRM, heating and/or cooling system fans shall not be simulated as running continuously during occupied hours but shall be cycled on and off to meet heating and cooling loads during all hours.

· HVAC fans shall remain on during occupied and unoccupied hours in HVAC zones that have health and safety mandated minimum ventilation requirements during unoccupied hours.

· HVAC fans shall remain on during occupied and unoccupied hours in systems primarily serving computer rooms

· For hotel guest rooms and high-rise residential, continuous operation is the default, however, the option to let the fan cycle with loads may be used when the following conditions are met and documented:

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 62.1-2016 are satisfied.

· HVAC fans shall run continuously whenever HVAC zones are occupied and shall be cycled on and off to meet heating and cooling loads during unoccupied hours.

Except:

· HVAC fans shall remain on during occupied and unoccupied hours in systems primarily serving computer rooms

· HVAC fans shall remain on during occupied and unoccupied hours in HVAC Zones that have health and safety mandated minimum ventilation requirements during unoccupied hours.

· Fan schedules may be allowed to differ when Standard 90.1-2019 Section G3.1.1(c) applies.

As designed

Heating and cooling systems with design air capacities greater than 10,000 cfm served by one or more supply fans are required to have optimum start controls.

Same as proposed

If optimal start controls are not required by the proposed design, but required in the baseline building, then this input will be set to “Any Zone”

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

20°F above zone temperature setpoint for systems 1 through 4 and 12 through 13. Same as maximum reset cooling SAT for systems 5 through 8 and 105°F for systems 9 and 10.

For baseline system 11, the heating supply air temperature shall be modulated to maintain space temperature and fan volume shall be fixed at minimum airflow.

The method of controlling heating SAT. Choices are:

· No control – the heating coil is energized on a call for heating, and the SAT is not directly controlled, but instead is dependent on the entering air temperature, the heating capacity and the airflow rate

· Fixed (constant) · Reset by coldest zone, airflow first

· Reset by coldest zone, temperature first

· Reset by outside air dry-bulb temperature

· Staged setpoint

· Scheduled setpoint

A linear reset schedule that represents the heating SAT or hot deck SAT (for dual duct systems) as a function of outdoor air dry-bulb temperature. This schedule is defined by the following data points (see Figure 10):

· The hottest heating SAT

· The corresponding (cold) outdoor air dry-bulb threshold

· The coolest heating SAT

· The corresponding (mild) outdoor air dry-bulb threshold