A/C Temperature Settings for Efficiency and Comfort
Choosing Optimal Settings to Heat and Cool Your Spaces
With all the available settings for heating, ventilation, and air conditioning (HVAC) systems, it might be confusing to work out the best way to keep occupants comfortable while minimising electricity costs. We’ll explain some of the most relevant terms, settings, and the potential savings that are up for grabs by tweaking these to match the application.
The background and advice given in this article mainly encompasses knowledge of the HVAC&R Nation Skills Workshop: Set Points and Control Bands, a publication of the Australian Institute for Refrigeration, Air Conditioning and Heating (AIRAH).
Why Adjust the Settings?
Currently, common practice across commercial office buildings, shopping centres, and public facilities is to keep set point temperatures at 22-22.5°C with narrow control bands, for the entire year. This presents a large potential to improve thermal comfort alongside energy efficiency by tweaking the settings to fit the purpose.
Did you know that changing the set point by 1°C can reduce its associated conditioning energy usage by an average of 10%? General optimisation of the settings can mean immediate savings of up to 20% of the HVAC services. Doing so can cut down on issues such as heating and cooling conflicts between base building and tenant systems or between adjacent mixing boxes and air outlets.
These optimisations can be applied against a variety of equipment sizes, for split, package, and small room a/c systems found at smaller sites, as well as typical commercial buildings with more complex, centralised HVAC systems incorporating chillers, AHUs, FCUs, and VAVs. These can incorporate small wall or remote control thermostats as well central Building Management Systems (BMS) with greater control programs and relays.
The 3 most important controls affecting both thermal comfort and efficiency are the temperature set point, the dead band, and the proportional band.
- Temperature set point: The target temperature for a space to be maintained by the HVAC system.
- Dead band: The range for which neither cooling nor heating is required. By widening the dead band, there is a greater duration where neither heating or cooling occurs.
- Proportional band: Also known as throttling range, it is used for modulation control of the HVAC system. By widening the proportional band, the system is able to run at less than full capacity more often.
Seasonality and Adaptive Thermal Comfort
By considering adaptive thermal comfort, which is to say that humans naturally acclimatise to higher Summer temperatures and lower Winter temperatures, then space set point temperatures should also vary accordingly. Recommended comfort ranges which also provide decent energy efficiency are given as:
- Winter: 20-22°C
- Summer: 24-26°C
A set point for a space would be chosen between the above ranges, along with a dead band to cover the space between the regions, and a proportional band to keep it within the region of comfort while heating and cooling. For example this might mean a set point of 23°C, 2°C dead band, and 1°C proportional bands.
More application specific examples are discussed with more detail in the relevant section below.
Good communication between the facility managers and staff/occupants/customers is especially important while optimising the HVAC control settings, as this enables cooperative progress in selecting them appropriately. This minimises the main risk of an increase in comfort complaints if input is taken initially and feedback is closely monitored during the exercise.
Temperature steps of 0.1°C are best for fine tuning system efficiency, so consider how well your controllers and sensors are able to provide for this flexibility. The temperatures at the edges of the control band also need to be considered for their comfort levels when optimising each space.
Application Specific Advice
For offices and similar purposes, good efficient settings would be a set point of 22°C, a 2°C dead band, 1°C proportional heating band, and a 2°C proportional cooling band. This has been graphically displayed below to provide a visual understanding of the HVAC settings.
Figure 2: A set point of 22°C, 2°C dead band, 2°C proportional cooling band, and a 1°C proportional heating band for a comfortable, energy efficient office space. [Image source: HVAC&R]
The above office example might be stretched for further energy efficiency targets with settings such as 23°C set point, 3°C dead band, and 2°C proportional bands.
For transient spaces encompassing food courts, malls, foyers, and shopping centres, requirements for heating and cooling can be significantly relaxed. Appropriate seasonal bands are 16-18°C in Winter, and 26-27°C in Summer. Implementing such a control strategy translates into savings of up to 50% compared with optimising the same space for office work.
Additional Methods for Improving Occupant Comfort
- Increasing the air movement in the building helps to reduce the apparent temperature. Options such as openable windows and ceiling or desk fans increase feelings of comfort when it might otherwise feel too hot.
- The ability to open windows, turn on a fan, or adjust a thermostat also gives a feeling of control to tenants and thus allows for toleration of a wider range of temperature levels.
- It’s important to monitor humidity to ensure it doesn’t get too high, and integrate a dehumidifier component to better balance thermal comfort and air quality.
- Encouraging staff to dress appropriately for the season can mean savings of 10-15%.
When making changes, ensure there is a good feedback loop between facilities managers and space occupants to work together in achieving a good balance. Make gradual, periodic adjustments by 0.3°C while monitoring the feedback to allow occupants to acclimatise to the changes and minimise the perceived impact.
Your specific settings may vary, as there are a number of variables to consider for each particular space, but these recommendations remain a good reference point to work from regardless.
General variables for consideration:
- Building envelope: The effectiveness of walls and windows in terms of insulation and natural ventilation.
- Type of air distribution: Is air distributed evenly or are some occupants more directly affected by the air outlets?
- Types of occupants: What is the dress code like and culture towards energy efficiency?
If you’re having trouble with equipment consistently delivering the desired settings, consider the types of controllers being used, which cover P (Proportional), I (Integral), and D (Derivative) to match to the system needs. P works most efficiently for stable systems, while PI can be used to reduce offset problems, PD to reduce overshoot issues, and PID to overcome both. For more information on controllers, the AIRAH publicationgoes into greater detail.
For more information around general HVAC recommendations, maintenance, upgrades, and troubleshooting, please see our article on Running HVAC Systems Effectively.
Dealing With Complaints
Once settings have been optimised, changes to set points and bands should be a last resort. Some of the checks to be carried out following a complaint are given below:
- Spot check and log space temperatures
- Check control valves and damper positions
- Review the air flow, heat load, temperature sensor locations, and air outlets with respect to the complaint location
- Review the units supplying the area for any conflicts such as from base building against tenant units