Title 24 · California Energy Code
What loop temperature dead band is required for hydronic heat pump water loops?
Your building’s water-loop heat pump system must be controlled so that the supply-water temperature at which you start cooling the loop and the supply-water temperature at which you start heating the loop are at least 20°F apart — unless a real-time optimization controller proven to save energy sets a smaller gap. **§ 140.4(k)7**
Last reviewed: July 6, 2026
What the code requires — 2-4 sentences
The California Energy Code requires a heat pump water supply temperature dead band of at least 20°F between the initiation of heat rejection and heat addition for hydronic heat pump (water-loop heat pump, WLHP) systems serving a common water loop. This requirement is stated in § 140.4(k)7 of the Energy Code. An exception permits a smaller dead band when a system loop temperature optimization controller determines the most efficient operating temperature in real time. § 140.4(k)7 .
For hydronic WLHP loops, keep at least a 20°F gap between the supply temperature at which you start cooling the loop and the supply temperature at which you start heating it — unless an approved optimization controller is managing the loop. § 140.4(k)7
Requirements in detail
What “dead band” means here
- Dead band (defined in context) is the supply temperature difference between the point at which central heat rejection devices (e.g., towers/fluid coolers) are initiated and the point at which central heat addition devices (e.g., boilers) are initiated. The code requires this supply-temperature gap to prevent simultaneous heating and cooling on the same loop. § 140.4(k)7 .
Who and what systems this applies to
- Applies to hydronic heat pumps connected to a common heat pump water loop with central devices for heat rejection and heat addition (typical WLHP loops). § 140.4(k)7 .
- The same language appears in the Mechanical Code Appendix E (adopting ASHRAE language) as E 503.5.5.2.3, which clarifies control and associated equipment conditions. § E 503.5.5.2.3 .
Numeric decision table (quick reference)
| Decision dimension | Required value / behavior | Code Reference |
|---|---|---|
| Minimum supply-temperature dead band between initiation of heat rejection and heat addition | At least 20°F | § 140.4(k)7 |
| Exception allowing dead band < 20°F | Permitted when a system loop temperature optimization controller is used to determine most efficient operating temperature based on real-time demand/capacity | § 140.4(k)7 (Exception) |
| Mechanical/appendix adoption and additional config guidance | Controls must be capable of providing the 20°F dead band; Appendix E (E 503.5.5.2.3) includes parallel requirements and ancillary equipment guidance (bypass valves, tower isolation) | § E 503.5.5.2.3 |
| Voluntary green-code adoption | Same 20°F dead band requirement in voluntary Green Building Appendix A6.207.2.4.7 | § A6.207.2.4.7 |
Implementation notes from the code text
- The code requires controls that are capable of providing the dead band and configured to provide it (i.e., the control strategy and setpoints must actually enforce the gap). § 140.4(k)7 .
- Appendix E clarifies freeze‑protection and bypass arrangements when towers or fluid coolers are in the loop (e.g., automatic bypass valves around towers in certain climate zones). These are equipment/configuration details you must consider when applying the dead band. E 503.5.5.2.3 .
Exceptions & special cases
- System loop temperature optimization controller: If a controller that optimizes loop temperature in real time (based on demand and capacity) is used, the code explicitly permits a dead band of less than 20°F. The controller must demonstrably select the most efficient loop setpoint in real time. § 140.4(k)7 (Exception) .
- Appendix/Mechanical Code considerations: Appendix E requires controls capable of and configured for the 20°F dead band and adds rules about bypass valves or shutdown of circulation for open/closed towers in certain climate zones. If your WLHP loop includes a cooling tower or fluid cooler, follow the Appendix E implementation guidance as well. E 503.5.5.2.3 .
- The Green Building voluntary appendix repeats the same dead-band requirement for projects following that standard. A6.207.2.4.7 .
If you need to rely on the optimization-controller exception, document how the controller determines and applies the optimized loop temperature (inputs, algorithms, monitoring) for plan review and field verification — the code requires justification that the controller operates to minimize energy use in real time (see the Exception language in § 140.4(k)7). § 140.4(k)7 .
Common mistakes
- Mistaking the dead band for an outdoor-air changeover differential. The WLHP dead band is a supply-water temperature gap, not an outdoor-air temperature changeover unless specifically stated elsewhere. Appendix E two‑pipe changeover rules (15°F outdoor differential) are a different requirement. E 503.5.5.2.2 .
- Treating the 20°F requirement as optional configuration language — the code requires controls to be capable of and configured to provide the dead band unless you use the documented exception. § 140.4(k)7 .
- Ignoring auxiliary equipment requirements for towers/fluid coolers (bypass valves, pump shutdown) that Appendix E ties to WLHP loop operation — these affect heat-loss and the ability to maintain the dead band in practice. E 503.5.5.2.3 .
- Lack of documentation for an optimization controller: reviewers commonly reject claims that permit a smaller dead band because the optimization strategy, sensors, and logic are not documented.
Worked example — concrete scenario
Scenario: A mid-size office building uses a common WLHP loop with a central boiler for heat addition and a closed-circuit fluid cooler for heat rejection. The design team must set control points that prevent simultaneous heating and cooling.
- Choose a heat-rejection initiation (cooling) supply setpoint: 75°F.
- Choose a heat-addition initiation (heating) supply setpoint: 95°F.
- Resulting supply-temperature dead band = 95°F − 75°F = 20°F, which meets the code minimum. § 140.4(k)7 .
If the project installs a validated loop temperature optimization controller that monitors real-time heat pump capacity and building load and demonstrates that a 12°F dead band yields lower total energy use, the exception allows using that 12°F setting — but the design submittal must include the optimization logic and verification approach to satisfy the exception. § 140.4(k)7 (Exception) . Also consider the Appendix E requirement to provide bypassing or pump shutdown for the fluid cooler per climate-zone guidance so that heat loss does not defeat control logic. E 503.5.5.2.3 .
Related provisions
- § 140.4(k)7 — Hydronic heat pump loop dead band requirement and exception (California Energy Code).
- § E 503.5.5.2.3 — Appendix E (Mechanical Code) language adopting the same 20°F dead band requirement plus equipment/bypass guidance.
- § A6.207.2.4.7 — Voluntary Green Building Appendix repeats the 20°F dead band and exception.
- Related pump/control requirements and exceptions (variable-flow and pump controls) appear in sections addressing water circulation and variable-flow controls; see the Energy Code sections that reference hydronic pump and flow-control requirements for systems serving WLHPs. § 170.2(c) / 140.4(k) family language in the Energy Code references these topics.
If you need precise plan‑review language or sample control narratives / sequences of operation demonstrating compliance or the optimization-controller documentation typically accepted by reviewers, I can draft those next.
Code references
Grounded in the retrieved California Energy Code — click a citation to read the verbatim passage:
§ 140.4 High relevance — show source text
2025 CALIFORNIA ENERGY CODE 125
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COMPLIANCE APPROACHES FOR ACHIEVING ENERGY EFFICIENCY
- Hydronic heat pump (WLHP) controls. 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 capable of providing a heat pump water supply temperature dead band of at least 20°F between initiation of heat rejection and heat addition by the central devices.
Exception to Section 140.4(k)7: Where a system loop temperature optimization controller is used to determine the most efficient operating temperature based on real-time conditions of demand and capacity, dead bands of less than 20°F shall be allowed. 8. High capacity space heating gas boiler systems. In Climate Zones 1 through 6, 9 through 14, and 16, gas hot water boiler systems for space heating with a total system input of at least 1 MMBtu/h but no more than 10 MMBtu/h shall meet all of the following requirements. A. Boiler system efficiency. Gas hot water boilers shall have a minimum thermal efficiency of 90 percent. Systems with multiple boilers can meet this requirement if the space-heating input provided by equipment with thermal efficiencies above and below 90 percent has an input capacity-weighted average thermal efficiency of at least 90 percent. For boilers federally regulated by combustion efficiency, the calculation for the input capacity-weighted average thermal efficiency shall use the combustion efficiency value. B. Hot water distribution design. The hot water distribution system shall be designed to comply with Items i and ii. i. Coils and other heat exchangers shall be selected so that at design conditions the hot water return temperature entering the boilers is 120°F or less. ii. Under all operating conditions, the water temperature entering the boiler is 120°F or less or the flow rate of supply hot water that recirculates directly into the return system, such as by three-way valves or minimum flow bypass controls, shall be no greater than 20 percent of the design flow of the operating boilers. Exception 1 to Section 140.4(k)8: Where 25 percent of the annual space heating requirement is provided by on-site renewable energy, site-recovered energy or heat recovery chillers. Exception 2 to Section 140.4(k)8: Space heating boilers installed in individual dwelling units. Exception 3 to Section 140.4(k)8: Where 50 percent or more of the design heating load is served using perimeter convective heating, radiant ceiling panels or both. Exception 4 to Section 140.4(k)8: Individual gas boilers with input capacity less than 300,000 Btu/h shall not be included in the calculations of the total system input or total system efficiency.
(l) Reserved.
(m) Fan control. Each cooling system listed in Table 140.4-I shall be designed to vary the indoor fan airflow as a function of load and shall comply with the following requirements:
- DX and chilled water cooling systems that control the capacity of the mechanical cooling directly based on occupied space temperature shall A. Have a minimum of two stages of fan control with no more than 66 percent speed when operating on stage 1; and B. Draw no more than 40 percent of the fan power at full fan speed, when operating at 66 percent speed.
§ 140.4 High relevance — show source text
Exception 1 to Section 140.4(k)4: Hydronic systems that use variable flow to reduce pumping energy in accordance with 140.4(k)1.
Exception 2 to Section 140.4(k)4: Systems serving healthcare facilities. 5. Water-cooled air conditioner and hydronic heat pump systems. Water circulation systems serving water-cooled air conditioners, hydronic heat pumps, or both that have total pump system power exceeding 5 hp shall have flow controls that meet the requirements of Section 140.4(k)6. Each such air conditioner or heat pump shall have a two-position automatic valve interlocked to shut off water flow when the compressor is off.
- Variable flow controls.
A. Variable speed drives. Individual pumps serving variable flow systems and having a motor horsepower exceeding 5 hp shall have controls or devices (such as variable speed control) that will result in pump motor demand of no more than 30 percent of design wattage at 50 percent of design water flow. The pumps shall be controlled as a function of required differential pressure. B. Pressure sensor location and setpoint. i. For systems without direct digital control of individual coils reporting to the central control panel, differential pressure shall be measured at the most remote heat exchanger or the heat exchanger requiring the greatest differential pressure. ii. For systems with direct digital control of individual coils with a central control panel, the static pressure setpoint shall be reset based on the valve requiring the most pressure, and the setpoint shall be no less than 80 percent open. Pressure sensors may be mounted anywhere.
Exception 1 to Section 140.4(k)6: Heating hot water systems.
Exception 2 to Section 140.4(k)6: Condenser water systems serving only water-cooled chillers.
2025 CALIFORNIA ENERGY CODE 125
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COMPLIANCE APPROACHES FOR ACHIEVING ENERGY EFFICIENCY
- Hydronic heat pump (WLHP) controls. 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 capable of providing a heat pump water supply temperature dead band of at least 20°F between initiation of heat rejection and heat addition by the central devices.
Exception to Section 140.4(k)7: Where a system loop temperature optimization controller is used to determine the most efficient operating temperature based on real-time conditions of demand and capacity, dead bands of less than 20°F shall be allowed. 8. High capacity space heating gas boiler systems. In Climate Zones 1 through 6, 9 through 14, and 16, gas hot water boiler systems for space heating with a total system input of at least 1 MMBtu/h but no more than 10 MMBtu/h shall meet all of the following requirements. A. Boiler system efficiency. Gas hot water boilers shall have a minimum thermal efficiency of 90 percent. Systems with multiple boilers can meet this requirement if the space-heating input provided by equipment with thermal efficiencies above and below 90 percent has an input capacity-weighted average thermal efficiency of at least 90 percent. For boilers federally regulated by combustion efficiency, the calculation for the input capacity-weighted average thermal efficiency shall use the combustion efficiency value. B. **Hot water distribution design.
§ 90.1 High relevance — show source text
[ASHRAE 90.1:6.5.2.2]
E 503.5.5.2.1 Three-Pipe System. Hydronic systems that use a common return system for both hot water and chilled water shall not be used. [ASHRAE 90.1:6.5.2.2.1]
E 503.5.5.2.2 Two-Pipe Changeover Sys- tem. Systems that use a common distribution system to supply both heated and chilled water are acceptable where in accordance with the following:
(1) The system is designed to allow a dead band between changeover from one mode to the other of not less than 15°F (8°C) outdoor air temperature.
(2) The system is designed to operate and is provided with controls that will allow operation in one mode for not less than 4 hours before changing over to the other mode.
(3) Reset controls are provided that allow heating and cooling supply temperatures at the changeover point to be not more than 30°F (17°C) apart. [ASHRAE 90.1:6.5.2.2.2]
E 503.5.5.2.3 Hydronic (Water Loop) Heat Pump Systems. Hydronic heat pumps connected to a common heat pump water loop with central devices for heat rejection (e.g., cooling tower) and heat addition (e.g., boiler) shall have the following:
(1) Controls that are capable of and configured to provide a heat pump water supply temperature dead band of at least 20°F (11°C) between initiation of heat rejection and heat addition by the central devices (e.g., tower and boiler).
(2) For climate zone 3 through zone 8, where a closed-circuit cooling tower (fluid cooler) is used, either an automatic valve shall be installed to bypass all but a minimal flow of water around the tower (for freeze protection) or low-leakage positive closure dampers shall be provided. Where an opencircuit 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 tower. Where an open-circuit cooling tower is used in conjunction with a separate heat exchanger to isolate the tower from the heat pump loop then heat loss shall be controlled by shutting down the circulation pump on the cooling tower loop.
Exception: Where a system loop temperature optimization controller is used to determine the most efficient operating temperature based on real-time conditions of demand and capacity, dead bands of less than 20°F (11°C) shall be permitted.
[ASHRAE 90.1:6.5.2.2.3]
E 503.5.5.3 Dehumidification. Where humidity controls are provided, such controls shall prevent reheating, mixing of hot and cold airstreams, or other means of simultaneous heating and cooling of the same airstream.
Exceptions:
(1) The system is capable of and configured to reduce supply air volume to 50 percent or less of the design airflow rate or the minimum outdoor air ventilation rate in accordance with Chapter 4 or ASHRAE 62.1 or other applicable federal, state, or local code or recognized standard, whichever is larger before simultaneous heating and cooling takes place.
§ 503.5.11.3. High relevance — show source text
APPENDIX E
point up to a maximum setpoint while the airflow is maintained at the dead band flow
rate.
(d) The second stage of heating consists of modulating the airflow rate from the dead band flow rate up to the heating maximum flow rate.
(3) Laboratory exhaust systems that comply with Section E 503.5.11.3.
(4) Zones where at least 75 percent of the energy for reheating or for providing warm air in mixing systems is provided from site-recovered energy (including condenser heat) or on-site renewable energy. [ASHRAE 90.1:6.5.2.1]
E 503.5.5.1 Supply Air Temperature Reheat Limit. Where reheating is permitted in accordance with this appendix, zones that have both supply and return or exhaust air openings more than 6 feet (1829 mm) above the floor shall not supply heating air more than 20°F (11°C) above the space temperature setpoint.
Exceptions:
(1) Laboratory exhaust systems in accordance with Section E 503.5.11.3.
(2) During preoccupancy building warm-up and setback. [ASHRAE 90.1:6.5.2.1.1]
E 503.5.5.2 Hydronic System Controls. The heating of fluids in hydronic systems that have been previously mechanically cooled and the cooling of fluids that have been previously mechanically heated shall be limited in accordance with Section
E 503.5.5.2.1 through Section E 503.5.5.2.3.
[ASHRAE 90.1:6.5.2.2]
E 503.5.5.2.1 Three-Pipe System. Hydronic systems that use a common return system for both hot water and chilled water shall not be used. [ASHRAE 90.1:6.5.2.2.1]
E 503.5.5.2.2 Two-Pipe Changeover Sys- tem. Systems that use a common distribution system to supply both heated and chilled water are acceptable where in accordance with the following:
(1) The system is designed to allow a dead band between changeover from one mode to the other of not less than 15°F (8°C) outdoor air temperature.
(2) The system is designed to operate and is provided with controls that will allow operation in one mode for not less than 4 hours before changing over to the other mode.
(3) Reset controls are provided that allow heating and cooling supply temperatures at the changeover point to be not more than 30°F (17°C) apart. [ASHRAE 90.1:6.5.2.2.2]
E 503.5.5.2.3 Hydronic (Water Loop) Heat Pump Systems. Hydronic heat pumps connected to a common heat pump water loop with central devices for heat rejection (e.g., cooling tower) and heat addition (e.g., boiler) shall have the following:
(1) Controls that are capable of and configured to provide a heat pump water supply temperature dead band of at least 20°F (11°C) between initiation of heat rejection and heat addition by the central devices (e.g., tower and boiler).
§ 207.2.4.7 High relevance — show source text
- Heating hot water systems.
- Condenser water systems serving only water-cooled chillers.
A6.207.2.4.7 Hydronic heat pump (WLHP) controls. 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 capable of providing a heat pump water supply temperature dead band of at least 20°F between initiation of heat rejection and heat addition by the central devices.
Exception: Where a system loop temperature optimization controller is used to determine the most efficient operating temperature based on real-time conditions of demand and capacity, dead bands of less than 20°F shall be allowed.
A6.207.2.5 Air distribution system duct leakage sealing. All duct systems shall be sealed to a leakage rate not to exceed 6 percent of the fan flow if the duct system:
A6.207.2.5.1 Is connected to a constant volume, single zone, air conditioners, heat pumps or furnaces; and
A6.207.2.5.2 Serves less than 5,000 square feet of floor area; and
A6.207.2.5.3 Has more than 25 percent duct surface area located in one or more of the following spaces:
Outdoors; or
In a space directly under a roof where the U -factor of the roof is greater than the U -factor of the ceiling; or Exception: Where the roof meets the requirements of Section 143(a)1C of Title 24, Part 6.
In a space directly under a roof with fixed vents or openings to the outside or unconditioned spaces; or
In an unconditioned crawlspace; or
In other unconditioned spaces.
The leakage rate shall be confirmed through field verification and diagnostic testing, in accordance with procedures set forth in the Reference Nonresidential Appendix NA1 of the California Energy Commission 2008 Building Energy Efficiency Stan- dards for Residential and Nonresidential Buildings.
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APPENDIX A6.1 — VOLUNTARY STANDARDS FOR HEALTH FACILITIES [OSHPD 1, 2 & 4]
A6.207.2.6 Variable air volume control for single zone systems. Effective January 1, 2012, all unitary air conditioning equipment and air-handling units with mechanical cooling capacity at ARI conditions greater than or equal to 110,000 Btu/hr that serve single zones shall be designed for variable supply air volume with their supply fans controlled by two-speed motors, variable speed drives or equipment that has been demonstrated to the Executive Director to use no more energy. The supply fan controls shall modulate down to a minimum of [2] / 3 of the full fan speed or lower at low cooling demand.
A6.207.3 Service water-heating systems and equipment.
A6.207.3.1 Certification by manufacturers. Any service water-heating system or equipment may be installed only if the manufacturer has certified that the system or equipment complies with all of the requirements of this subsection for that system or equipment.
§ 170.2 High relevance — show source text
Chillers that are piped in series for the purpose of increased temperature differential shall be considered as one chiller.
iii. Boiler isolation. When a hot water plant includes more than one boiler, provisions shall be made so that flow through any boiler is automatically shut off when that boiler is shut off while still maintaining flow through other operating boiler(s). iv. Chilled and hot water temperature reset controls. Systems with a design capacity exceeding 500,000 Btu/hr supplying chilled or heated water shall include controls that automatically reset supply water temperatures as a function of representative building loads or outside air temperature. Exception to Section 170.2(c)4Iiv: Hydronic systems that use variable flow to reduce pumping energy in accordance with Section 170.2(c)4Ii.
v. Water-cooled air conditioner and hydronic heat pump systems. Water circulation systems serving watercooled air conditioners, hydronic heat pumps or both, that have total pump system power exceeding 5 hp, shall have flow controls that meet the requirements of Section 170.2(c)4Ivi. Each such air conditioner or heat pump shall have a two-position automatic valve interlocked to shut off water flow when the compressor is off.
vi. Variable flow controls.
a. Variable speed drives. Individual pumps serving variable flow systems and having a motor horsepower exceeding 5 hp shall have controls or devices (such as variable speed control) that will result in pump motor demand of no more than 30 percent of design wattage at 50 percent of design water flow. The pumps shall be controlled as a function of required differential pressure. b. Pressure sensor location and setpoint. I. For systems without direct digital control of individual coils reporting to the central control panel, differential pressure shall be measured at the most remote heat exchanger or the heat exchanger requiring the greatest differential pressure. II. For systems with direct digital control of individual coils with a central control panel, the static pressure setpoint shall be reset based on the valve requiring the most pressure, and the setpoint shall be no less than 80 percent open. Pressure sensors may be mounted anywhere. Exception 1 to Section 170.2(c)4Ivi: Heating hot water systems. Exception 2 to Section 170.2(c)4Ivi: Condenser water systems serving only water-cooled chillers. vii. Hydronic heat pump (WLHP) controls. 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 capable of providing a heat pump water supply temperature deadband of at least 20°F between initiation of heat rejection and heat addition by the central devices.
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MULTIFAMILY BUILDINGS—PERFORMANCE AND PRESCRIPTIVE COMPLIANCE APPROACHES
Exception to Section 170.2(c)4Ivii: Where a system loop temperature optimization controller is used to determine the most efficient operating temperature based on real-time conditions of demand and capacity, deadbands of less than 20°F shall be allowed .
J. Reserved.
K. **Fan control.
§ 1704.2 High relevance — show source text
(10)Means for flow balancing for the building loop shall be provided.
(11)Supply and return header temperatures and pressures shall be marked.
1704.2 Circulating Pumps. The circulating pump shall be sized for the operating conditions and the heat transfer fluid properties.
1705.0 Valves.
1705.1 Where Required. Shutoff valves shall be installed in ground source-loop piping systems in the locations indicated in Section 1705.2 through Section 1705.8.
1705.2 Heat Exchangers. Shutoff valves shall be installed on the supply and return side of a heat exchanger.
Exception: Where the heat exchanger is integral with a boiler or is a component of a manufacturer’s boiler and heat exchanger packaged unit, and is capable of being isolated from the hydronic system by the supply and return valves.
1705.3 Central Systems. Shutoff valves shall be installed on the building supply and return of a central utility system.
1705.4 Pressure Vessels. Shutoff valves shall be installed
on the connection to a pressure vessel.
1705.5 Pressure-Reducing Valves. Shutoff valves shall be installed on both sides of a pressure-reducing valve.
1705.6 Equipment and Appliances. Shutoff valves shall be installed on connections to mechanical equipment and appliances. This requirement does not apply to components of a ground source loop system such as pumps, air separators, metering devices, and similar equipment.
1705.7 Expansion Tanks. Shutoff valves shall be installed at connections to nondiaphragm-type expansion tanks.
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2025 CALIFORNIA MECHANICAL CODE 349
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GEOTHERMAL ENERGY SYSTEMS AND AMBIENT TEMPERATURE LOOPS
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1705.8 Reduced Pressure. A pressure relief valve shall be installed on the low-pressure side of a hydronic piping system that has been reduced in pressure. The relief valve shall be set at the maximum pressure of the system design.
1706.0 Specific System Components Design.
1706.1 General. Heat pumps shall be in compliance with Table 1706.1, as applicable. Heat pumps shall also be listed and labeled in accordance with UL 1995 or UL 60335-2-40.
Ground coupled and water source heat pumps shall be listed in accordance with AHRI/ASHRAE/ISO 13256-1 for water-to air heat pumps and AHRI/ASHRAE/ISO 13256-2 for water-towater heat pumps. DX heat pumps shall be listed in accordance with ASHRAE 194. All heat pump equipment used in DX systems shall comply with AHRI 870. Heat pumps shall be fitted with a means to indicate that the compressor is locked out.
TABLE 1706.1
HEAT PUMPS
TYPE OF HEAT PUMP STANDARDS
Water-to-AirAHRI/ASHRAE/ISO 13256-1
Water-to-WaterAHRI/ASHRAE/ISO 13256-2 § 503.5.7.7 High relevance — show source text
E 503.5.7.7 Chilled-Water Coil Selection.
Chilled-water cooling coils shall be selected to provide a 15°F (8°C) or higher temperature difference between leaving and entering water temperatures and a minimum of 57°F (14°C) leaving water temperature at design conditions.
Exceptions:
(1) Chilled-water cooling coils that have an air-side pressure drop exceeding 0.70 inch of water (0.17 kPa) when rated at 500 feet per minute (2.54 m/s) face velocity and dry conditions (no condensation).
(2) Individual fan-cooling units with a design supply airflow rate 5000 cubic feet per minute (ft [3] /min) (2.36 m [3] /s) and less.
(3) Constant-air-volume systems.
(4) Coils selected at the maximum temperature difference allowed by the chiller.
(5) Passive coils (no mechanically supplied airflow).
(6) Coils with design entering chilled-water temperatures of 50°F (10°C) and higher.
(7) Coils with design entering air dry-bulb temperatures of 65°F (18°C) and lower. [ASHRAE 90.1:6.5.4.7]
E 503.5.8 Heat Rejection Equipment. Section E 503.5.8 through Section E 503.5.9 applies to heat-rejection equipment used in comfort cooling systems, such as air-cooled condensers, dry coolers, open-circuit cooling towers, closed-circuit cooling towers, and evaporative condensers.
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(3) Water temperature reset is not required where valve position is used to comply with Section E 503.5.7. [ASHRAE 90.1:6.5.4.4]
E 503.5.7.5 Hydronic (Water Loop) Heat Pump and Water-Cooled Unitary Air Condi- tioners. Hydronic heat pumps and water-cooled unitary air-conditioners shall have a two-position automatic valve interlocked to shut off water flow
when the compressor is off.
Exception: Units employing fluid economizers.
[ASHRAE 90.1:6.5.4.5.1]
E 503.5.7.5.1 Controls. Hydronic heat pumps and water-cooled unitary air-conditioners having a total pump system power exceeding 5 hp (3.7 kW) shall have controls, devices, or both (such as variable speed control) that will result in pump motor demand of not more than 30 percent of design wattage at 50 percent of design water flow. [ASHRAE 90.1:6.5.4.5.2]
E 503.5.7.6 Pipe Sizing. Chilled-water and condenser-water piping shall be designed such that the design flow rate in each piping segment shall not exceed the values listed in Table E 503.5.7.6 for the
appropriate total annual hours of operation. Piping size selections for systems that operate under variable flow conditions (e.g., modulating two-way control valves at coils) and that contain variable-speed pump motors shall be permitted to be made from the “Variable Flow/Variable Speed” columns. All others shall be made from the “Other” columns.
Exceptions:
§ 207.2.4.1. High relevance — show source text
Exception: Hydronic systems that use variable flow to reduce pumping energy in accordance with Section A6.207.2.4.1.
A6.207.2.4.5 Water-cooled air conditioner and hydronic heat pump systems. Water circulation systems serving watercooled air conditioners, hydronic heat pumps or both that have total pump system power exceeding 5 hp shall have flow controls that meet the requirements of Section A6.207.2.4.6. Each air conditioner or heat pump shall have a two-position automatic valve interlocked to shut off water flow when the compressor is off.
A6.207.2.4.6 Variable flow controls.
A6.207.2.4.6.1 Variable speed drives. Individual pumps serving variable flow systems and having a motor horsepower exceeding 5 hp shall have controls and/or devices (such as variable speed control) that will result in pump motor demand of no more than 30 percent of design wattage at 50 percent of design water flow. The pumps shall be controlled as a function of required differential pressure.
A6.207.2.4.6.2 Pressure sensor location and setpoint.
- For systems without direct digital control of individual coils reporting to the central control panel, differential pressure shall be measured at or near the most remote heat exchanger or the heat exchanger requiring the greatest differential pressure.
- For systems with direct digital control of individual coils with central control panel, the static pressure set point shall be reset based on the valve requiring the most pressure and the set- point shall be no less than 80 percent open. The pressure sensor(s) may be mounted anywhere.
Exceptions:
- Heating hot water systems.
- Condenser water systems serving only water-cooled chillers.
A6.207.2.4.7 Hydronic heat pump (WLHP) controls. 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 capable of providing a heat pump water supply temperature dead band of at least 20°F between initiation of heat rejection and heat addition by the central devices.
Exception: Where a system loop temperature optimization controller is used to determine the most efficient operating temperature based on real-time conditions of demand and capacity, dead bands of less than 20°F shall be allowed.
A6.207.2.5 Air distribution system duct leakage sealing. All duct systems shall be sealed to a leakage rate not to exceed 6 percent of the fan flow if the duct system:
A6.207.2.5.1 Is connected to a constant volume, single zone, air conditioners, heat pumps or furnaces; and
A6.207.2.5.2 Serves less than 5,000 square feet of floor area; and
A6.207.2.5.3 Has more than 25 percent duct surface area located in one or more of the following spaces:
Outdoors; or
In a space directly under a roof where the U -factor of the roof is greater than the U -factor of the ceiling; or Exception: Where the roof meets the requirements of Section 143(a)1C of Title 24, Part 6.
In a space directly under a roof with fixed vents or openings to the outside or unconditioned spaces; or
In an unconditioned crawlspace; or
In other unconditioned spaces.
§ 1.8 High relevance — show source text
For SI units: °C = (°F-32)/1.8 Note:
- Ethylene glycol shall not be used in one- and two-unit residential systems. In existing systems, where ethylene glycol is used, there shall be no direct or permanent potable water connections. Where a temporary potable water connection is required, a backflow preventer shall be installed.
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1201.0 General.
1201.1 Applicability. This chapter shall apply to hydronic piping systems that are part of heating, cooling, ventilation, refrigeration, and air conditioning systems. Such piping systems include steam, hot water, radiant heating and cooling, chilled water, steam condensate, condenser water, ground source heat pump systems, snow and ice melting systems, ambient temperature loops (ATL), and district ambient temperature loops. The regulations of this chapter shall govern the construction, location, and installation of hydronic piping systems. 1201.2 Insulation. Surfaces within reach of building occupants shall not exceed 140°F (60°C). Where sleeves are installed, the insulation shall continue full size through them.
Coverings and insulation used for piping shall be of material approved for the operating temperature of the system and the installation environment. Where installed in a plenum, the insulation, jackets, and lap-seal adhesives, including pipe coverings and linings, shall have a flamespread index not to exceed 25 and a smoke-developed index not to exceed 50 where tested in accordance with ASTM E84
or UL 723.
1201.3 Water Hammer Protection. The piping system shall be designed to prevent water hammer.
1201.4 Terminal Units. Terminal units, valves, and flow control devices shall be installed in accordance with the man ufacturer’s installation instructions.
1201.5 Return-Water Low-Temperature Protection. Where a minimum return-water temperature to the heat source is specified by the manufacturer, the heating system shall be designed and installed to meet or exceed the minimum return-water temperature during the normal operation of the heat source.
1201.6 Heat Transfer Fluid Quality. Heat transfer fluid used in closed loop hydronic systems shall be in accordance with IAPMO/ANSI H1001.1.
1201.6.1 Ethylene Glycol. Ethylene glycol shall not be used in one- and two-unit residential systems. In existing systems, where ethylene glycol is used, there shall be no direct or permanent potable water connections. Where a temporary potable water connection is required, a backflow preventer shall be installed.
1201.7 Heat Emitters. Heat emitters shall be installed in
accordance with the manufacturer’s installation instructions.
1201.8 Mechanical Devices. Where listed mechanical devices are used, the manufacturer’s installation instructions as to the location and method of installation shall be followed.
1201.9 Flexible Connectors. Listed flexible connectors shall be installed in readily accessible locations. 1201.10 Freeze Protection. Hydronic systems and components shall be designed, installed, and protected from freez
ing. The percent of glycol by volume shall be determined based on the freezing point of the solution and type of mixture in accordance with Table 1201.10 or the manufacturer’s specifications.
For SI units: °C = (°F-32)/1.8 Note:
§ 1715.7 High relevance — show source text
1715.7 System Start-Up. DX system start-up shall be in accordance with Section 1708.1 and the following:
(1) DX systems shall be pressurized using nitrogen for not less than 1 hour. There shall be no allowable variance to
the test pressure after being corrected for ambient temperature changes during the test. The test pressure shall not exceed 150 psig (1034 kPa) when pressure testing the compressor unit and indoor system components.
(2) DX systems shall have permanent type labels installed and affixed on the compressor unit with the refrigerant type and quantity.
(3) For DX systems, refrigerant liquid and vapor lines from the loop system shall be identified and tagged.
1715.8 DX Piping. DX Piping should be installed in accordance with approved plans and specifications, including provisions for cathodic protection.
Part V – Geothermal Ambient Temperature Loops (ATL).
1716.0 Ambient Temperature Loop (ATL) Distributed Energy Systems.
1716.1 General. An Ambient Temperature Loop (ATL) distributed energy system shall be installed in accordance with Section 1716.1.1 through Section 1716.6.3 and Section 1717.0. ATL systems shall comply with Part I through Part IV of this chapter, as applicable.
1716.1.1 Fourth Generation (4G) System Config- uration. A fourth-generation system configuration shall be a district geothermal energy system distributing hot water, cold water, or both to the conditioned space or building for a specific use. Where a geothermal energy source is used, such system shall comply with Part I through Part IV of this chapter, Chapter 11, and Chapter 12.
1716.1.2 Fifth Generation (5G) System Configu- rations. An advanced Ambient Temperature Loop (ATL) System or fifth generation (5G) ATL system shall also be capable of interacting with the electric utility system as well as other utility systems and systems compo nents.
The system components shall include, but not limited to, the following:
(1) Thermally diverse buildings with independent hydronic systems
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2025 CALIFORNIA MECHANICAL CODE 355
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GEOTHERMAL ENERGY SYSTEMS AND AMBIENT TEMPERATURE LOOPS
(2) Circulation loop
(3) Global control system
(4) Segment isolation capability
The system components may include, but not limited to, the following:
(1) Electric grid-interactive enabled buildings
(2) Hybrid components
(3) Other renewable systems
1716.2 Permitting. Permits required for the installation and application of an ATL distributed energy system shall be obtained as required by the Authority Having Jurisdiction.
1716.3 Ambient Loop Temperature Range. The operating loop temperature range of an ambient temperature loop (ATL) system shall be not less than the freeze point of the circulating fluid and not more than the maximum temperature as required by the manufacturer’s installation instructions for the attached heat pump equipment in accordance with Section 1716.3.1 and Section 1716.3.2. The ATL system shall use treated water as the heat transfer medium.
§ 0.293 High relevance — show source text
For SI units: 1000 British thermal units = 0.293 kW
- For mechanical cooling stage control that does not use variable compressor displacement the percent displacement shall be equivalent to the mechanical cooling capacity reduction evaluated at the full load rating conditions for the compressor.
E 503.5.4 Economizer Heating System Impact. HVAC system design and economizer controls shall be such that economizer operation does not increase the building heating energy use during normal operation.
Exception: Economizers on variable air valve (VAV) systems that cause zone level heating to increase due to a reduction in supply air temperature. [ASHRAE 90.1:6.5.1.4] E 503.5.4.1 Economizer Humidification Sys- tem Impact. Systems with hydronic cooling and humidification systems designed to maintain inside humidity at a dew-point temperature more than 35°F (2°C) shall use a fluid economizer where an economizer is required in accordance with Section E 503.5 through Section E 503.5.4.1. [ASHRAE 90.1:6.5.1.5] E 503.5.5 Simultaneous Heating and Cooling Limitation, Zone Controls. Zone thermostatic controls shall prevent the following:
(1) Reheating.
(2) Recooling.
(3) Mixing or simultaneously supplying air that has been previously mechanically heated and air that has
2025 CALIFORNIA MECHANICAL CODE 433
), Copyright © 2025 IAPMO, and may not be used for any other purpose or distributed to any other persons or parties.
APPENDIX E
point up to a maximum setpoint while the airflow is maintained at the dead band flow
rate.
(d) The second stage of heating consists of modulating the airflow rate from the dead band flow rate up to the heating maximum flow rate.
(3) Laboratory exhaust systems that comply with Section E 503.5.11.3.
(4) Zones where at least 75 percent of the energy for reheating or for providing warm air in mixing systems is provided from site-recovered energy (including condenser heat) or on-site renewable energy. [ASHRAE 90.1:6.5.2.1]
E 503.5.5.1 Supply Air Temperature Reheat Limit. Where reheating is permitted in accordance with this appendix, zones that have both supply and return or exhaust air openings more than 6 feet (1829 mm) above the floor shall not supply heating air more than 20°F (11°C) above the space temperature setpoint.
Exceptions:
(1) Laboratory exhaust systems in accordance with Section E 503.5.11.3.
(2) During preoccupancy building warm-up and setback. [ASHRAE 90.1:6.5.2.1.1]
E 503.5.5.2 Hydronic System Controls. The heating of fluids in hydronic systems that have been previously mechanically cooled and the cooling of fluids that have been previously mechanically heated shall be limited in accordance with Section
E 503.5.5.2.1 through Section E 503.5.5.2.3.
[ASHRAE 90.1:6.5.2.2]
Frequently asked questions
Can I set the heating and cooling setpoints 10°F apart if I have a smart controller?
Yes — the code exception allows a dead band of less than 20°F only when a system loop temperature optimization controller is used to determine the most efficient operating temperature based on real-time demand and capacity. You must document how the controller optimizes the loop for plan review. § 140.4(k)7
Does the 20°F dead band apply to individual heat pumps or only to the central loop?
It applies to hydronic heat pumps connected to a common heat pump water loop with central heat‑rejection and heat‑addition devices — i.e., the common loop supply temperature gap between initiation of central rejection and addition. § 140.4(k)7
Do I need special bypass valves when a cooling tower is in the loop?
Appendix E requires bypass or low‑leakage isolation for closed‑circuit or open‑circuit towers in certain climate zones (to control heat loss and freeze protection). See E 503.5.5.2.3 for the specific equipment requirements.
Is this requirement enforced in the voluntary Green Building appendix?
Yes — the Green Building voluntary Appendix includes the same 20°F dead band requirement and the same optimization-controller exception in A6.207.2.4.7.
If my WLHP loop is isolated by heat exchangers from the tower, does the 20°F rule still apply?
Yes — the dead band requirement covers hydronic heat pump systems connected to a common loop with central rejection/addition devices. If the tower is isolated by a heat exchanger, Appendix E still directs that heat loss be controlled (e.g., by shutting down tower-loop circulation) — see E 503.5.5.2.3.
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