What is ASHRAE Guideline 36 and how does it improve HVAC control sequences?
ASHRAE Guideline 36 — The Standard for High-Performance HVAC Sequences
ASHRAE Guideline 36 represents a paradigm shift in HVAC controls design — moving from custom, programmer-dependent sequences that vary wildly in quality to standardised, peer-reviewed sequences that deliver consistent, verifiable energy performance. First published in 2018 and updated in 2021, it has rapidly become the most consequential HVAC controls document since the BACnet standard.
### The Trim-and-Respond Philosophy
The guideline's central innovation is the trim-and-respond control methodology, which replaces arbitrary fixed setpoints with algorithms that continuously seek the most energy-efficient operating point. Unlike traditional PID loops that control to a fixed setpoint, trim-and-respond sequences:
1. Continuously poll all zones to determine which is most demanding 2. Trim the setpoint (static pressure, supply air temperature, etc.) in small increments toward a more efficient value 3. Respond by reversing direction when any zone indicates it is approaching a deficit 4. Result: the system operates at exactly the minimum energy state that satisfies all zones
This approach is applied to duct static pressure reset (Section 5.3), supply air temperature reset (Section 5.5), zone temperature setpoint adjustment, and chilled water plant control.
### Standardised Sequences Covered
The 2021 edition provides complete, implementation-ready sequences for:
Each sequence is documented with logic diagrams, setpoint tables, and tuning parameters — sufficient for a competent DDC programmer to implement without interpretation or improvisation.
### Code Mandate and Industry Adoption
ASHRAE 90.1-2019 Section 6.4.3.10 now mandates that DDC systems comply with Guideline 36 for many building types, effectively elevating the guideline to code status. Major BMS manufacturers have responded by developing Guideline 36-compliant application libraries:
- JCI Metasys: G36 Application Library with pre-engineered control programs
These pre-built libraries reduce commissioning time by 40–60% compared to custom-programmed sequences and eliminate the programming errors that plague bespoke DDC implementations.
### Australian Adoption
In Australia, Guideline 36 is increasingly referenced in project specifications, particularly for Green Star and NABERS-rated buildings where energy performance is independently verified. The trim-and-respond methodology aligns well with the NCC Section J performance requirements, and several Australian controls contractors now offer Guideline 36-compliant programming as a standard service offering.
ASHRAE Guideline 36-2021 Key Sequences Summary
Major control sequences defined in Guideline 36 with their primary energy impact mechanisms and the trim-and-respond parameters used.
| Sequence | Section | Primary Energy Mechanism | Trim Parameter | Respond Trigger |
|---|---|---|---|---|
| Duct Static Pressure Reset | 5.3 | Fan speed reduction (cube law) | Duct static pressure setpoint (-0.05 in. w.g.) | Any VAV damper > 90% open |
| Supply Air Temperature Reset | 5.5 | Chiller lift reduction; economiser hours | SAT setpoint (+0.2°F) | Any zone above cooling setpoint |
| Zone Temperature Setpoint | 5.9 | Heating/cooling load reduction | Zone setpoint towards ambient | Occupant override or demand |
| Economiser Changeover | 5.4 | Free cooling vs mechanical | Changeover setpoint | OA conditions vs return air |
| Chilled Water Plant Reset | 6.0 | Chiller lift reduction | CHW supply temp (+0.5°F) | Any valve > 90% open |
| Heating Water Plant Reset | 7.0 | Boiler efficiency improvement | HW supply temp (-0.5°F) | Any valve approaching closed |
🔑 Key Takeaways
- ✓ASHRAE Guideline 36's trim-and-respond methodology replaces fixed setpoints with continuous optimisation, proven to reduce HVAC energy by 30–50%
- ✓The 2021 edition covers VAV AHUs, single-zone units, fan coil units, and central plant control with complete implementation-ready logic diagrams
- ✓ASHRAE 90.1-2019 mandates Guideline 36 compliance for many building types, elevating the guideline to de facto code status
- ✓Major BMS manufacturers now provide Guideline 36-compliant program libraries that reduce commissioning time by 40–60% and eliminate programming errors
- ✓For Australian Green Star and NABERS projects, specifying Guideline 36 sequences provides a verifiable path to demonstrated energy performance