ASHRAE Guideline 36 high-performance sequences

For decades, every mechanical engineer wrote their own sequences of operation, and every controls contractor programmed them differently. The result was enormous variation in how similar systems behaved, and many buildings that never achieved the efficiency their designers intended. ASHRAE Guideline 36, High-Performance Sequences of Operation for HVAC Systems, was created to change that by providing standardized, tested and programmable sequences for common HVAC systems.

This guide explains what the guideline covers, the key strategies it uses, and how owners and designers can apply it on Canadian projects.

Background

Guideline 36 grew out of ASHRAE research project RP-1455, which developed advanced control sequences for VAV systems. The first edition was published in 2018, and later editions have expanded the systems covered and refined the logic. Its sequences are written in precise, unambiguous language so that they can be implemented consistently and, increasingly, delivered as pre-programmed libraries by controls manufacturers.

The guideline has two main goals:

• Energy performance: sequences incorporate best practices such as resets, demand-based control and optimal start.
• Consistency and reliability: standard logic reduces design time, programming errors and commissioning effort, and makes buildings easier to operate and maintain.

Systems covered

Recent editions cover a range of common systems, including:

• Multiple-zone VAV air handlers, with various economizer and relief configurations
• Single-zone VAV air handlers
• VAV terminal units: cooling only, with reheat, parallel and series fan-powered, and dual duct
• Dual-fan dual-duct systems
• General zone logic: setpoints, modes, occupancy and window switches
• Plant sequences for chilled water and hot water in newer editions

Systems outside its scope, such as laboratory airflow control, data centre cooling, cleanrooms and many specialized plants, still need project-specific sequences. Check the edition you are referencing for the exact list of covered systems.

Key strategies

Zone requests and trim and respond

Instead of fixed setpoints at the air handler, Guideline 36 uses requests from zones. Each zone generates requests when it needs more cooling, more airflow or more heating, based on how far it is from setpoint and how hard its damper or valve is working. The air handler then uses trim and respond logic to reset its setpoints:

• When few or no zones request, the setpoint is gradually trimmed in the energy-saving direction.
• When requests exceed a threshold, the setpoint is adjusted to respond.

This is applied to duct static pressure, supply air temperature and, in plant sequences, chilled water and hot water setpoints. See static pressure reset with trim and respond.

Importance multipliers and rogue zones

Each zone has an adjustable importance multiplier for its requests. Operators can reduce the influence of a problem zone, for example one with an undersized box, so it does not drive the whole system. The guideline also tracks cumulative requests so operators can identify these "rogue" zones and fix the underlying problem.

Dual maximum VAV reheat logic

Traditional VAV reheat boxes reheat at minimum airflow, which can deliver very warm air at low velocity and cause stratification. Guideline 36 uses a dual maximum approach: the box first raises discharge air temperature at minimum airflow, then increases airflow up to a heating maximum. Discharge temperature is limited to avoid stratification, often to about 11 °C (20 °F) above space temperature. This improves heating comfort and allows lower minimum airflows, reducing reheat energy.

Ventilation calculations

Minimum outdoor air is calculated dynamically using the ASHRAE 62.1 Ventilation Rate Procedure, with zone ventilation requirements adjusted for occupancy where occupancy sensors or CO2 sensors are present. Zone minimum airflows can be reduced in unoccupied standby mode.

Economizer and relief control

Detailed sequences address economizer high limits by climate, minimum outdoor air control with dedicated dampers or airflow measurement, and building pressure control with relief dampers or return fans.

Built-in fault detection

Guideline 36 includes automated fault detection and diagnostics (AFDD) rules for air handlers, based on research into common faults such as simultaneous heating and cooling, stuck dampers and leaking valves. Faults are flagged with defined conditions and delays. See fault detection and diagnostics.

Zone groups and modes

Zones are organized into groups that share schedules and modes. Modes such as occupied, cool-down, setup, warm-up, setback, freeze protection and unoccupied are defined with clear transitions.

Benefits and trade-offs

Applying Guideline 36 on a project

1. Confirm owner support. Owners, especially portfolio owners, benefit from requiring Guideline 36 sequences consistently across their buildings.
2. Choose the systems and options. Select system types, economizer high limit options, relief strategy and ventilation logic in the design documents.
3. Provide project-specific parameters. Design airflows, minimums, setpoints and zone groupings must be scheduled.
4. Specify hardware that supports the logic. Airflow sensors for minimum outdoor air, accurate supply air temperature sensors, and VAV controllers capable of dual maximum logic.
5. Require the contractor to implement it as written and to document any deviations for engineer approval.
6. Commission against the guideline. Use functional tests that exercise requests, resets, modes and fault detection.
7. Train operators on how to read requests, adjust importance multipliers and interpret fault alarms.

Energy codes in Canada do not require Guideline 36 directly, but its strategies help meet NECB and ASHRAE 90.1 control requirements such as setpoint reset and setback. Our building automation work regularly includes specifying and verifying these sequences.

Common pitfalls

• Referencing the guideline without selecting options or providing parameters
• Mixing Guideline 36 logic with legacy custom logic in ways that conflict
• Inaccurate sensors, especially airflow at low VAV flows, undermining reset logic
• Contractors substituting simplified versions without disclosure
• Operators disabling trim and respond because they do not understand it

Key takeaways

• Guideline 36 provides standardized, programmable sequences for VAV air handlers, terminal units and some plants.
• Trim and respond resets driven by zone requests are central to its energy savings.
• Dual maximum reheat logic improves heating comfort and reduces reheat energy.
• Built-in fault detection helps operators find problems early.
• Success requires selected options, project parameters, capable hardware, rigorous commissioning and operator training.