- Controls determine when equipment runs and how hard. An HVAC system with good hardware and bad programming wastes energy and annoys occupants just as reliably as a failing compressor.
- Start with the basics: correct schedules, sensible setpoints and setbacks, and honest sensors. These deliver more than advanced features nobody maintains.
- Overrides are the most common way schedules quietly stop working. Audit them regularly.
- Economizers, fault detection and diagnostics (FDD), and acceptance testing are tied to California energy code requirements on many systems. Confirm which apply to your equipment.
- Trend logs turn controls from a black box into evidence. Seven days of data often explains a complaint that months of guessing did not.
Why controls deserve their own maintenance
Mechanical equipment gets the attention: compressors, fans, boilers. But the controls decide when each of them turns on, how long it runs, and what it responds to. A rooftop unit that runs all night because a schedule was overridden three years ago costs money every night without ever failing. A zone that is always too warm because its sensor sits on a sunny wall generates complaints that no amount of refrigerant work will fix.
This guide is organized as a walk through a building's control layers, from the simplest to the most sophisticated. Most buildings gain the most from the first three layers, and most of the remaining value depends on those being right.
Layer one: schedules and occupancy
The schedule says when a system is in occupied mode and when it may relax. Walk the building with the actual operating hours in hand and compare. Retailers change hours seasonally; offices shift to hybrid work; a school adds an evening program; a medical suite starts Saturday appointments. Each change should flow to the schedule, and often does not.
- Confirm the clock. Controllers drift after power interruptions, and daylight saving changes are not always handled automatically.
- Look for permanent overrides. A thermostat held on a manual hold, or a BAS point commanded to on, defeats every other strategy.
- Match zone schedules to real use. Storage areas, back-of-house rooms, and unused suites rarely need full occupied conditions.
- Plan start and stop. Optimal start (calculating when to begin conditioning so the space is comfortable at opening) beats a fixed early start for energy, if its inputs are healthy.
- Holiday and special-event handling. Decide who is responsible for changes and how those changes get recorded.
Layer two: setpoints, setbacks, and deadbands
Setpoints are simple but political. A cooling setpoint that is only a degree lower can add meaningful runtime across a year, and a narrow gap between heating and cooling setpoints can cause the system to fight itself, heating one hour and cooling the next. Energy code includes thermostat requirements such as setback capability and minimum deadband for many commercial systems. Check the current edition for your project rather than relying on memory.
| Item | Healthy practice | Common problem |
|---|---|---|
| Occupied cooling and heating | Reasonable separation between the two so they do not overlap | Setpoints within a degree or two, causing simultaneous heating and cooling |
| Unoccupied setback | Relaxed setpoints that still protect the building and equipment | No setback, or setback defeated by a manual hold |
| Occupant adjustment | Limited range at the zone sensor | Unlimited adjustment, with each occupant moving setpoints to extremes |
| Humidity handling | Considered for spaces with moisture-sensitive contents | Ignored until mold or condensation complaints appear |
| Critical spaces | Dedicated setpoints and alarm limits | Treated like general office space |
Layer three: sensors you can trust
Every control decision is only as good as the measurement behind it. Sensors drift, get painted over, get moved, or lose calibration. Typical culprits include space temperature sensors in poor locations (near a door, in direct sun, behind a decorative object), supply and return air sensors with loose connections, outdoor air temperature and humidity sensors in direct sun, and CO2 sensors that have never been calibrated.
Verify sensors against a calibrated reference during PM visits. Record the reading on the controller and the reading from the reference instrument side by side. A sensor that is several degrees off will silently cause either wasted energy or a persistent comfort complaint. Replace it rather than adjusting around it.
Disconnect or sample the outdoor air sensor and compare it to a handheld reading in shade. If the controller thinks it is warmer outside than it is, the economizer may never open when it should.
Economizers, FDD, and acceptance testing
An economizer lets a cooling system use cool outdoor air instead of running compressors. In Southern California's mild climate it can carry many hours of cooling, but only when the outdoor sensor, high-limit logic, dampers, and actuator all work. California's Title 24 energy code requires economizers on many commercial cooling systems and fault detection and diagnostics on many packaged units with economizers. Requirements depend on equipment capacity, climate zone, and whether work is new construction or an alteration, so confirm what applies to your project.
Where FDD is required, it is intended to flag problems such as dampers stuck open or closed, sensor faults, and simultaneous heating and cooling. Its value depends on someone reading the alerts. An FDD flag nobody acts on is the same as no flag. Acceptance testing, performed by a qualified person at installation for many systems, documents that the controls function as designed; keep those records, because they matter again when you sell, refinance, or replace equipment. See our rooftop unit guide for economizer failure patterns and the Title 24 guide for permit context.
Building automation and trend logs
A building automation system (BAS) ties many pieces of equipment into one interface. Whether you have a full BAS or a handful of networked thermostats, the capability worth using first is the trend log: a recording of values over time. Trend logs convert a vague complaint into an observation.
What to trend for a comfort complaint
- Space temperature and setpoint in the affected zone, so you can see whether the system is losing the fight or never trying.
- Supply air temperature and fan status, to confirm the unit is actually delivering.
- Outdoor air temperature and economizer damper position, to see whether free cooling is working.
- Compressor or heating stage status, to see cycling patterns and runtimes.
What to trend for an energy review
- Equipment run status outside occupied hours.
- Simultaneous heating and cooling indicators.
- Fan speed or static pressure on variable-air-volume systems.
- Demand or power data when meters are available.
Pick a sampling interval short enough to see the behavior you are chasing, and plan how long the data will be stored. A week of five-minute data often tells you more than a month of hourly data when you are tracking short cycling. Integration with a service history makes this stronger: readings logged against each asset in the DaVinci Portal give you the manual measurements to compare with the BAS data.
Keeping controls honest over time
- Assign an owner for schedule and setpoint changes. Unowned controls drift.
- Keep a current list of all controllers, their locations, and their passwords or access methods stored securely.
- Back up the BAS programming and database after any change. Lost programming turns a minor failure into a major one.
- Review override and alarm reports monthly, not annually.
- Re-verify sensors and economizers at least seasonally, ideally during PM visits.
- After tenant improvements or equipment replacement, re-check that new equipment is integrated, scheduled, and trended.
- Document what was done. A change log, even a simple one, pays for itself the next time someone asks why a setting is what it is.
These habits fit naturally into a preventive maintenance program, where controls checks are a line item rather than an afterthought. If your building has a mix of old and new equipment, our building controls and automation service page describes how we approach integration.
Frequently asked questions
What is the first thing to check if utility use seems too high?
Schedules and overrides. Look at when equipment actually runs outside occupied hours and whether any zone is held in a manual mode. This often explains a surprising share of usage without any equipment replacement.
Do I need a full building automation system?
Not necessarily. Smaller buildings often do well with networked programmable thermostats that have scheduling, remote access, and basic alarms. A BAS makes more sense as the number of systems, zones, and special spaces increases.
What is FDD and does my building need it?
Fault detection and diagnostics is logic that flags problems such as stuck dampers or failed sensors. California's energy code requires it on many newer packaged units with economizers. Whether it applies to your equipment depends on capacity, type, and installation date, so confirm during project planning.
How long should I log trend data?
For a specific complaint, one to two weeks is often enough if the interval is short. For energy reviews, a full season gives a more reliable picture. Ensure the system retains the data long enough to analyze it.
Can controls fix a failing system?
Controls can reduce stress and waste, but they cannot repair worn mechanical parts. Use trend data to see where the problem actually lives, then repair the cause.
Have a site this applies to?
Davinci Mechanical is the commercial and union division of Scottish Tom's Heating & Air. Send us the equipment list or the problem and we'll tell you what we'd check first.