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C403.4 Heating and Cooling System Controls (Mandatory)
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C403.4.1 Thermostatic Controls (Mandatory)
The supply of heating and cooling energy to each zone shall be controlled by individual thermostatic controls capable of responding to temperature within the zone. Where humidification or dehumidification or both is provided, not fewer than one humidity control device shall be provided for each humidity control system.
Exception: Independent perimeter systems that are designed to offset only building envelope heat losses, gains or both serving one or more perimeter zones also served by an interior system provided that both of the following conditions are met:
- The perimeter system includes not fewer than one thermostatic control zone for each building exposure having exterior walls facing only one orientation (within ± 45 degrees) (0.8 rad) for more than 50 contiguous feet (15 240 mm).
- The perimeter system heating and cooling supply is controlled by thermostats located within the zones served by the system.
C403.4.1.1 Heat Pump Supplementary Heat (Mandatory)
Heat pumps having supplementary electric resistance heat shall have controls that, except during defrost, prevent supplementary heat operation where the heat pump can provide the heating load.
C403.4.1.2 Deadband (Mandatory)
Where used to control both heating and cooling, zone thermostatic controls shall be configured to provide a temperature range or deadband of not less than 5°F (2.8°C) within which the supply of heating and cooling energy to the zone is shut off or reduced to a minimum.
Exceptions:
- Thermostats requiring manual changeover between heating and cooling modes.
- Occupancies or applications requiring precision in indoor temperature control as approved by the code official.
C403.4.1.3 Setpoint Overlap Restriction (Mandatory)
Where a zone has a separate heating and a separate cooling thermostatic control located within the zone, a limit switch, mechanical stop or direct digital control system with software programming shall be configured to prevent the heating setpoint from exceeding the cooling setpoint and to maintain a deadband in accordance with Section C403.4.1.2.
C403.4.1.4 Heated or Cooled Vestibules (Mandatory)
The heating system for heated vestibules and air curtains with integral heating shall be provided with controls configured to shut off the source of heating when the outdoor air temperature is greater than 45°F (7°C). Vestibule heating and cooling systems shall be controlled by a thermostat located in the vestibule configured to limit heating to a temperature not greater than 60°F (16°C) and cooling to a temperature not less than 85°F (29°C).
Exception: Control of heating or cooling provided by site-recovered energy or transfer air that would otherwise be exhausted.
C403.4.1.5 Hot Water Boiler Outdoor Temperature Setback Control (Mandatory)
Hot water boilers that supply heat to the building through one- or two-pipe heating systems shall have an outdoor setback control that lowers the boiler water temperature based on the outdoor temperature.
C403.4.2 Off-Hour Controls (Mandatory)
Each zone shall be provided with thermostatic setback controls that are controlled by either an automatic time clock or programmable control system.
Exceptions:
- Zones that will be operated continuously.
- Zones with a full HVAC load demand not exceeding 6,800 Btu/h (2 kW) and having a manual shutoff switch located with ready access.
C403.4.2.1 Thermostatic Setback (Mandatory)
Thermostatic setback controls shall be configured to set back or temporarily operate the system to maintain zone temperatures down to 55°F (13°C) or up to 85°F (29°C).
C403.4.2.2 Automatic Setback and Shutdown (Mandatory)
Automatic time clock or programmable controls shall be capable of starting and stopping the system for seven different daily schedules per week and retaining their programming and time setting during a loss of power for not fewer than 10 hours. Additionally, the controls shall have a manual override that allows temporary operation of the system for up to 2 hours; a manually operated timer configured to operate the system for up to 2 hours; or an occupancy sensor.
C403.4.2.3 Automatic Start (Mandatory)
Automatic start controls shall be provided for each HVAC system. The controls shall be configured to automatically adjust the daily start time of the HVAC system in order to bring each space to the desired occupied temperature immediately prior to scheduled occupancy.
C403.4.3 Hydronic Systems Controls
The heating of fluids that have been previously mechanically cooled and the cooling of fluids that have been previously mechanically heated shall be limited in accordance with Sections C403.4.3.1 through C403.4.3.3. Hydronic heating systems comprised of multiple-packaged boilers and designed to deliver conditioned water or steam into a common distribution system shall include automatic controls configured to sequence operation of the boilers. Hydronic heating systems composed of a single boiler and greater than 500,000 Btu/h (146.5 kW) input design capacity shall include either a multistaged or modulating burner.
C403.4.3.1 Three-Pipe System
Hydronic systems that use a common return system for both hot water and chilled water are prohibited.
C403.4.3.2 Two-Pipe Changeover System
Systems that use a common distribution system to supply both heated and chilled water shall be designed to allow a deadband between changeover from one mode to the other of not less than 15°F (8.3°C) outside air temperatures; be designed to and provided with controls that will allow operation in one mode for not less than 4 hours before changing over to the other mode; and be provided with controls that allow heating and cooling supply temperatures at the changeover point to be not more than 30°F (16.7°C) apart.
C403.4.3.3 Hydronic (Water Loop) Heat Pump Systems
Hydronic heat pump systems shall comply with Sections C403.4.3.3.1 through C403.4.3.3.3.
C403.4.3.3.1 Temperature Deadband
Hydronic heat pumps connected to a common heat pump water loop with central devices for heat rejection and heat addition shall have controls that are configured to provide a heat pump water supply temperature deadband of not less than 20°F (11°C) between initiation of heat rejection and heat addition by the central devices.
Exception: Where a system loop temperature optimization controller is installed and can determine the most efficient operating temperature based on real-time conditions of demand and capacity, deadbands of less than 20°F (11°C) shall be permitted.
C403.4.3.3.2 Heat Rejection
The following shall apply to hydronic water loop heat pump systems in Climate Zones 3 through 8:
- Where a closed-circuit cooling tower is used directly in the heat pump loop, either an automatic valve shall be installed to bypass the flow of water around the closed-circuit cooling tower, except for any flow necessary for freeze protection, or low-leakage positive-closure dampers shall be provided.
- Where an open-circuit cooling tower is used directly in the heat pump loop, an automatic valve shall be installed to bypass all heat pump water flow around the open-circuit cooling tower.
- Where an open-circuit cooling tower is used in conjunction with a separate heat exchanger to isolate the open-circuit cooling tower from the heat pump loop, heat loss shall be controlled by shutting down the circulation pump on the cooling tower loop.
Exception: Where it can be demonstrated that a heat pump system will be required to reject heat throughout the year.
C403.4.3.3.3 Two-Position Valve
Each hydronic heat pump on the hydronic system having a total pump system power exceeding 10 hp (7.5 kW) shall have a two-position valve.
C403.4.4 Part-Load Controls
Hydronic systems greater than or equal to 300,000 Btu/h (87.9 kW) in design output capacity supplying heated or chilled water to comfort conditioning systems shall include controls that are configured to do all of the following:
- Automatically reset the supply-water temperatures in response to varying building heating and cooling demand using coil valve position, zone-return water temperature, building-return water temperature or outside air temperature. The temperature shall be reset by not less than 25 percent of the design supply-to-return water temperature difference.
- Automatically vary fluid flow for hydronic systems with a combined pump motor capacity of 2 hp (1.5 kW) or larger with three or more control valves or other devices by reducing the system design flow rate by not less than 50 percent or the maximum reduction allowed by the equipment manufacturer for proper operation of equipment by valves that modulate or step open and close, or pumps that modulate or turn on and off as a function of load.
- Automatically vary pump flow on heating-water systems, chilled-water systems and heat rejection loops serving water-cooled unitary air conditioners as follows:
- 3.1. Where pumps operate continuously or operate based on a time schedule, pumps with nominal output motor power of 2 hp or more shall have a variable speed drive.
- 3.2. Where pumps have automatic direct digital control configured to operate pumps only when zone heating or cooling is required, a variable speed drive shall be provided for pumps with motors having the same or greater nominal output power indicated in Table C403.4.4 based on the climate zone and system served.
- Where a variable speed drive is required by Item 3 of this Section, pump motor power input shall be not more than 30 percent of design wattage at 50 percent of the design water flow. Pump flow shall be controlled to maintain one control valve nearly wide open or to satisfy the minimum differential pressure.
Exceptions:
- Supply-water temperature reset is not required for chilled-water systems supplied by off-site district chilled water or chilled water from ice storage systems.
- Variable pump flow is not required on dedicated coil circulation pumps where needed for freeze protection.
- Variable pump flow is not required on dedicated equipment circulation pumps where configured in primary/secondary design to provide the minimum flow requirements of the equipment manufacturer for proper operation of equipment.
- Variable speed drives are not required on heating water pumps where more than 50 percent of annual heat is generated by an electric boiler.
VARIABLE SPEED DRIVE (VSD) REQUIREMENTS FOR DEMAND-CONTROLLED PUMPS
| CHILLED WATER AND HEAT REJECTION LOOP PUMPS IN THESE CLIMATE ZONES | HEATING WATER PUMPS IN THESE CLIMATE ZONES | VSD REQUIRED FOR MOTORS WITH RATED OUTPUT OF: |
| 1A, 1B, 2B | — | ≥ 2 hp |
| 2A, 3B | — | ≥ 3 hp |
| 3A, 3C, 4A, 4B | 7, 8 | ≥ 5 hp |
| 4C, 5A, 5B, 5C, 6A, 6B | 3C, 5A, 5C, 6A, 6B | ≥ 7.5 hp |
| — | 4A, 4C, 5B | ≥ 10 hp |
| 7, 8 | 4B | ≥ 15 hp |
| — | 2A, 2B, 3A, 3B | ≥ 25 hp |
| — | 1B | ≥ 100 hp |
| — | 1A | ≥ 200 hp |
C403.4.5 Pump Isolation
Chilled water plants including more than one chiller shall be capable of and configured to reduce flow automatically through the chiller plant when a chiller is shut down. Chillers piped in series for the purpose of increased temperature differential shall be considered as one chiller.
Boiler systems including more than one boiler shall be capable of and configured to reduce flow automatically through the boiler system when a boiler is shut down.
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