There are 6 variables that drive an occupant’s thermal comfort:
- air temperature
- surface temperature
- humidity
- clothing insulation
- activity level (metabolic heat)
- air velocity
The problem? These factors simply don’t tell the full story behind the maintenance involved in optimising temperature in office buildings, commercial buildings, primary schools or university campuses.
Thermal comfort depends on other factors as well, like climate and occupant preferences – for example, occupants in Denmark may be more susceptible to humidity and heat levels than occupants in Portugal.
The biggest challenges for Commissioning Agents and Facility Managers lie in finding a temperature that is satisfactory for the majority of their occupants, all who may have slightly different preferences, and doing so without sending the energy bill through the roof.
Here to tell us the full story behind getting thermal comfort right in buildings is Brenna Buckwalter from Copper Tree Analytics (before BuildPulse).
Occupant-Controlled Temperatures
Giving occupants control over their own thermostats can be a double-edged sword: occupant complaints will likely go down, but energy bills typically rise. Occupants often have a ‘not my bill, not my problem’ mentality, leading occupants to carelessly leave lights on or excessively raise and lower temperature.
To combat this, a good rule of thumb is to utilise thermostats in individual offices, accompanied by an occupancy sensor in order to avoid excessive heating or cooling when the room is vacant.
In an open office or classroom, refer to ASHRAE guidelines and keep the set point temperature at 20.5° C for heating and 23 °C for cooling. If complaints are low, a verbal response may be all that is necessary to solve the issue, as thermal comfort is so subjective that it is impossible to keep everyone happy. If complaints are high, however, Peter Turek, a commissioning agent at DLR Group, suggests that a survey be conducted. Ask occupants if they are receiving enough airflow and ventilation and if they feel their space is stuffy or humid to get a pulse on the issue.
Using Data to Manage Energy and Comfort
Besides simply lowering the temperature, Facility Managers and Commissioning Agents can lower energy costs by installing occupancy sensors, using insulation to stop the heated or cooled air from leaking out, and taking advantage of their existing Building Automation System by adding energy management capabilities.
Take Advantage of the Building Automation System
Collect temperature and humidity trend data – it’s available in your existing systems and, as the old adage goes: you cannot manage what you don’t measure.
Calculate the inside and outside of the setpoint deadband
Rank the zones by time inside or outside of the setpoint and if they are in the “Hot” or “Cold” zones. In winter, if spaces are consistently “Hot” verify that the heating valve that serves the zone is not leaking or that the occupant has not bought a space heater to work; if a space is consistently cold, check airflow to make sure that too much air is not being delivered, and confirm that the temperature of the air entering the space is no more than 30 degrees above the ambient air temperature for proper mixing in the space, (See High-Performance VAV Systems – ASHRAE ).
In summer, if spaces are hot, verify that blinds or natural shading is still effective and that proper airflow is being delivered to space. If spaces are cold, they are most likely being over-cooled. Confirm the pitots have not kinked or become loose and the box is getting a good measurement of the airflow to space. Calibrate as needed.
Compare the ambient room temperature to outdoor temperature
If the room temperature follows the same arc as the outdoor air temperature, check that space is getting an appropriate amount of airflow: too little impacts the spaces’ ability to heat, and too much can overcool the space and force simultaneous heating and cooling at the zone level.
Look for rapid changes in temperature
Cycling of equipment is detrimental to the equipment lifetime and negatively impacts occupant comfort. If you are working with a VAV system, this is sign of over-ventilation for both heating and cooling, which can also be caused by too low a discharge air temperature setpoint at the parent AHU.
If your building uses a heat pump system, and the change in temperature is rapid in one direction, increase the minimum run time of the unit so that float time is increased, versus cycling the fan and compressor every 5 or 15 minutes. In either case, this will prolong the lifetime of the equipment and improve comfort as the occupant is not being blasted with Cold and or Hot air, four or five times an hour.
Check Your Sensors
Location, location, location. If a sensor has not changed by more than one or two degrees over the course of a day, it has external influences that were not part of the design intent. For example, a bookshelf may have been placed in front of the sensor. If it does not change at all, then the sensor has been disabled and the value should not be trusted. Replace or calibrate the sensor as required.
Open Conversations Across Departments
Even energy-efficient buildings have their challenges. One issue that Turek noted is that “a lot of times, the good design intent is overridden by operators so that they don’t have to deal with occupant complaints.” Ideally, during the training period, the design engineer should explain the intent and answer questions for the operations staff to eliminate or reduce the practice of overriding or disabling set points.
In the case that this doesn’t occur, the key to solving this problem is to get organisation’s management on the same page as facility managers by making occupants aware of the consequences of energy waste on their organisation. Though occupants may not directly pay the energy bill, it is important that they know that energy costs affect them in the long run, because “that investment in energy savings is returned back into higher salaries or other benefits for employees.”
Check here for more ideas on how to convince upper management that energy management is a smart investment.
Invest in Occupants
With up to 90% of our time being spent indoors (especially during the winter months, when energy bills tend to reach their peak in most of Europe), it’s important that occupants are comfortable and happy in their office and school environments, despite the cost.
The cost of neglecting occupant comfort is actually higher, however: one study from the University of Helsinki and Lawrence Berkeley National Laboratory found that “a typical office could save 300 EUR per employee per year by maintaining office temperatures within” an optimal range.
Another study, conducted by PLOS One, found that in schools, “subjects exposed to a difference of 1 °C in indoor T differed by 13 points” on test scores. Small changes can produce truly significant results.
Managing a space full of occupants with different temperature preferences can certainly be a challenge for Facility Managers and Commissioning Agents who are under pressure to lower, or even maintain energy costs. Focusing on keeping a majority satisfied and at their most productive level is the best way to approach setting temperature in communal spaces.
For more information on thermal comfort, view these resources from ASHRAE, Rocky Mountain Institute, PLOS One, and the Lawrence Berkeley National Laboratory.
This article was first published on the Copper Tree Analytics blog by Brenna Buckwalter. It has been republished here with kind permission from the author. Some minor changes have been made to reflect DEXMA style considerations.
