A tenant books after-hours air conditioning from 6 p.m. to 9 p.m. The question that decides whether they’re happy: at 6 p.m., do they walk into a comfortable office — or a stuffy one that’s still slowly catching up? The gap between “the system switches on” and “the space is actually comfortable” is warm-up time, and how an after-hours program handles it shapes tenant satisfaction, billing fairness and energy use all at once. This is where the courtesy start comes in.
Here’s why HVAC can’t deliver comfort instantly, what determines how much lead time a space needs, and how to set up an after-hours program that gets tenants comfort on arrival without wasting energy or starting disputes.
Why HVAC can’t deliver comfort the instant it’s switched on
When a building sits empty, its systems relax — setpoints drift into a wider “setback” range overnight and the indoor temperature wanders away from the comfortable target. Switching the system back on doesn’t fix that immediately. The equipment has to bring the space from wherever it’s drifted to back to setpoint, and that takes time. Controls engineers call this recovery time: the minutes (sometimes more) it takes to move a space from its unoccupied state to occupied comfort.
Central plant adds to it. A building running chillers or boilers needs those to spin up before they can do useful work, and the conditioned air then has to circulate through the zone. None of that is instant — which is the whole reason after-hours HVAC needs lead time built in.
What determines warm-up time
There’s no single number, because recovery time swings with several factors:
Outdoor temperature and season. This is the biggest variable. A mild evening might need 20–30 minutes; a brutally hot afternoon or freezing morning can need an hour or more to pull a space back to setpoint.
How far the space has drifted. The wider the gap between the current temperature and the target, the longer the recovery. A space sitting two degrees off setpoint is quick; ten degrees off is not.
Building thermal mass and insulation. A heavy, well-insulated building holds its conditioned state longer (good) but can also be slower to shift once it has drifted. A leaky, lightweight building drifts faster in both directions.
System type. Packaged rooftop units behave differently from central chiller-and-boiler plant; variable-air-volume and fan-powered reheat systems each have their own response. Some can heat a zone quickly; others are gradual by design.
Zone size. Conditioning one small suite is faster than bringing a whole floor up to temperature.
As a rough rule of thumb, many buildings land somewhere around 30 to 60 minutes of warm-up for a typical after-hours start, with the extremes of the year pushing longer. The practical takeaway: lead time is real, variable, and worth designing for rather than ignoring.
The courtesy start
A courtesy start (also called an early start or pre-conditioning) is the answer to the 6 p.m. problem. Instead of switching the system on at the booked time, the building starts it a calculated amount of time before, so the space is at comfort when the tenant’s booking actually begins. It’s the after-hours equivalent of the “optimal start” logic that building management systems use every morning to have a building comfortable by the time staff arrive.
Without a courtesy start, a tenant who books 6 to 9 spends the first half-hour in a space that’s still recovering — and reasonably feels they paid for three hours but only got two and a half of comfort. With one, the booked window is comfortable end to end.
The natural follow-up is who pays for the warm-up. The usual and fairest answer: the lead time is a courtesy — not billed — and the tenant is charged for the window they actually booked. Whatever you decide, it should be stated plainly in the lease so there’s no argument later, which our guide to including after-hours HVAC charges in your commercial lease covers. From the tenant’s side, knowing the booked window is comfortable end to end is part of what we explain in a tenant’s guide to after-hours HVAC charges.
Why there’s a minimum charge
Warm-up time also explains the minimum charge that nearly every after-hours program applies. If bringing a space to temperature and spinning up the plant takes 30 to 60 minutes of real energy and equipment effort, a 15-minute booking can’t possibly cover that cost. So programs set a minimum charged time — commonly one or two hours — that applies even to short bookings. It isn’t padding; it reflects the genuine floor cost of starting the system at all. Which billing model you wrap around that minimum is a separate decision, compared in our guide to after-hours HVAC billing methods.
Why notice was traditionally required — and how automation removes it
The old reason after-hours HVAC needed 24 or 48 hours’ notice was partly administrative and partly physical. Staff needed time to manually program the start and end into the controls, and the system needed enough lead time to pre-condition the space.
Modern systems collapse both. A booking platform connected to the building’s controls can apply optimal-start logic automatically — calculating the right courtesy-start offset from the outdoor temperature and the space’s known recovery time, then starting the plant just early enough to hit comfort at the booked time. The tenant simply books the hours they want; the system handles the warm-up invisibly. That’s what lets on-demand programs drop the notice requirement that manual programs depend on.
The balance: comfort without waste
There’s a real tension to manage. Start the system too early and you condition an empty space for no reason — wasted energy and cost, and on a building subject to performance standards, wasted energy that counts against your cap. Start too late and the tenant is uncomfortable and unhappy. Good optimal-start logic threads this needle: start as late as possible while still reaching setpoint exactly when the booking begins, and adapt that offset to the weather rather than using a fixed guess. (It’s worth knowing that warming a space as fast as possible isn’t always the lowest-energy approach, especially as buildings electrify and peak demand charges come into play — another reason to let smart logic, not a blanket rule, set the offset.)
For the wider picture of how after-hours HVAC works, start with our guide to what after-hours HVAC is.
Frequently Asked Questions
How long does it take for after-hours HVAC to warm up a space? Typically around 30 to 60 minutes, though it varies widely with outdoor temperature, how far the space has drifted from setpoint, the building’s thermal mass, the system type and the zone size. Extreme weather can push it longer.
What is a courtesy start for after-hours HVAC? A courtesy start (or early start) is when the building begins conditioning the space a calculated amount of time before the tenant’s booked start, so the space is comfortable at the beginning of the booking rather than still recovering. The lead time is usually provided as a courtesy and not billed.
Do tenants pay for HVAC warm-up time? Usually not. The fair and common approach is to bill the tenant for the window they booked and treat the pre-conditioning lead time as a courtesy. The arrangement should be stated clearly in the lease.
Why is there a minimum charge for after-hours HVAC? Because bringing a space to temperature and starting the plant takes real energy and effort that a very short booking can’t cover. A one- or two-hour minimum reflects that floor cost and discourages bookings too short to be worth starting the system for.
Do I still have to give advance notice for after-hours HVAC? With older manual programs, yes — staff needed time to program the system and pre-condition the space. Modern booking platforms connected to the building’s controls calculate the start offset automatically, which removes the notice requirement.
7NOX applies a courtesy start automatically so tenants arrive to a comfortable space, programs the building’s controls without staff intervention, and bills only the booked window — no notice required. See how it works.