There are two ways to write an HVAC maintenance schedule and only one of them survives contact with a real year. The common way is a calendar: January these units, February those. It reads well, it is easy to produce, and it is wrong from the first time a visit moves, because nothing in it knows why any date was chosen. The other way holds the interval on the unit and lets the dates fall out, which means a moved visit corrects everything after it without anybody editing a document.
Why the calendar version goes stale
A calendar schedule stores conclusions rather than reasons. When a February visit happens in April, every subsequent date for that unit is now wrong and only a person can fix them. Multiply by a building's worth of units and the schedule is either maintained by hand continuously or is quietly false, and it is almost always the second, because nobody has the time for the first.
The per-unit version, and what it needs
Each unit carries what it is, its interval or intervals, the date of its last completed visit and what a visit consists of. The due dates are derived, so completing a visit late moves the next one automatically. That is the whole mechanism, and a spreadsheet can express it as long as somebody recalculates, which is the step that gets skipped in a busy month.
More than one interval on one unit is normal
A rooftop unit might be on a quarterly filter and belt check and an annual coil clean and refrigerant inspection. That is two intervals on one asset, not two assets, and any schedule that allows only one per unit forces the second into a note where it will eventually be missed. Check this before committing to any tool, because it is extremely common and often unsupported.
Questions people ask about hvac maintenance schedule
Should filters be on the same schedule as services?
Same schedule, different interval, and driven by the environment rather than the calendar where you can measure it. A dusty intake changes filters far more often than a clean one, and a single site-wide interval will be wrong for both.
What about units under a contractor's agreement?
Keep them on your schedule too. The agreement covers who attends; it does not tell you whether they came, what they found, or when the next one is due.
How do we start if we do not have a unit list?
Walk the building and write one, including the roof, the plant rooms and anything serving a tenant area. The walk is where the missing units are found, and it usually finds some.