Night setback and optimal start

Most commercial buildings are occupied for fewer than half the hours in a week. During the rest, there is little reason to hold full comfort conditions or bring in full ventilation air. Night setback (lowering heating setpoints) and setup (raising cooling setpoints) during unoccupied periods reduce the energy needed to condition the building. Optimal start and stop control then decides, day by day, exactly when to begin and end conditioning so that comfort is reached when people arrive and maintained until they leave.

These are among the oldest energy measures in building automation, and they are required by energy codes such as the NECB and ASHRAE 90.1. Yet trend data from existing buildings regularly shows them disabled, overridden or tuned so poorly that the savings disappear. This guide explains how they work and how to set them up properly.

How setback saves energy

Heat loss through the envelope and by infiltration is roughly proportional to the temperature difference between inside and outside. Lowering the indoor temperature overnight reduces that difference, so the building loses less heat while unoccupied. The building does have to be reheated in the morning, but the energy needed to recover is less than the energy saved by allowing it to cool, provided the setback period is long enough.

The larger savings in many commercial buildings come not from the setpoint change itself but from what goes with it:

• Air handlers shut off instead of running continuously.
• Outdoor air dampers close, eliminating the ventilation heating load.
• Exhaust fans stop.
• Pumps and central plant can shut down or run at minimum.

In a Canadian winter, eliminating overnight ventilation heating is often worth more than the envelope savings.

Typical setpoints

Setback and setup values are a balance between savings, recovery time and risks such as freezing or condensation. Typical starting points for offices and similar buildings:

These are typical ranges, not requirements. Occupied setpoints should follow ASHRAE Standard 55 comfort criteria and the owner's requirements. Deeper setbacks save more but lengthen recovery and raise the risk of condensation on cold surfaces or freezing in perimeter zones. Buildings with heavy masonry or concrete construction recover more slowly; lightweight buildings recover quickly.

Optimal start

A fixed start time, such as starting all air handlers at 5:00 a.m., is set for the coldest morning of the year. On most days it starts far too early. Optimal start calculates how long the building needs to recover based on current conditions and starts equipment just in time.

Typical inputs:

• Current space temperature and the occupied setpoint.
• Outdoor temperature.
• A learned recovery rate from previous days, often in degrees per hour, adjusted for outdoor conditions.

Most modern controllers and BAS platforms include adaptive algorithms that update the recovery rate automatically. ASHRAE Guideline 36 defines a warm-up and cool-down sequence with an optimal start calculation, which is a good reference for specifications.

Morning warm-up without ventilation

During warm-up before occupancy, outdoor air dampers should stay closed (except where codes or the owner require pre-occupancy purge), so the heating plant only has to warm recirculated air. Ventilation starts at the occupied time. Some buildings perform a pre-occupancy purge to flush contaminants; this should be timed and sized deliberately rather than happening by default.

Optimal stop

Optimal stop shuts down heating or cooling before the end of occupancy when the building will coast within an acceptable range until people leave. Savings are smaller than optimal start, and the occupied ventilation requirement still applies until the end of occupancy, so many designers limit optimal stop to the heating and cooling functions while keeping ventilation running.

Cold climate considerations

Deep winter in Edmonton, Calgary or Winnipeg changes the picture:

1. Freeze protection: perimeter zones with hydronic radiation, sprinkler piping in exterior walls, and coils near outdoor air dampers can freeze if setback is too deep or heating is fully disabled. Keep heating pumps running in cold weather and maintain a low limit.
2. Recovery capacity: if the heating plant is barely sized for the design load, it may not be able to recover from a deep setback on a cold morning. The optimal start algorithm will start very early, eating into savings, or fail to reach setpoint.
3. Heat pumps: air-source heat pumps lose capacity at low temperatures, and aggressive morning recovery can trigger electric backup heat and high demand charges. Shallower setback or staged recovery is often better.
4. Condensation and humidity: buildings with humidification should limit setback depth to avoid condensation on windows and in walls as surfaces cool.

Common problems

These issues show up repeatedly in retro-commissioning:

• Schedules that no longer match actual occupancy, or holiday schedules never entered.
• Zones permanently overridden to occupied mode from a past complaint.
• Optimal start disabled after a single cold morning complaint and never restored.
• Unoccupied setpoints set so close to occupied setpoints that nothing is saved.
• Air handlers that stop but outdoor dampers that do not close fully, leading to frozen coils.
• Night setback working on the BAS but local thermostats with their own schedules fighting it.
• Tenant after-hours requests that switch on the whole floor or building instead of one zone.

Verifying setback and optimal start

1. Review the BAS schedules, holiday calendars and override status for every system.
2. Trend space temperatures, supply fan status, outdoor air damper position and heating plant output for at least two weeks across a range of weather.
3. Confirm that fans stop and dampers close during unoccupied periods.
4. Check that start times vary with outdoor temperature and that spaces reach setpoint near occupancy time, not hours before.
5. Look for zones that never set back.
6. Interview the operators about complaints and overrides.

Our guide to trend logging and using BAS data explains how to set up these trends. For buildings where schedules do not fit actual use, occupancy-based HVAC control can complement or replace fixed schedules.

Frequently asked questions

Does setback save energy with heat pumps?

Usually, but with care. Shallower setbacks and gradual recovery avoid running backup resistance heat or exceeding electrical demand limits on cold mornings.

What about buildings occupied around the clock?

Hospitals, data centres and some residential buildings have little or no unoccupied time. Setback may still apply to specific zones such as clinics, offices and meeting areas.

Is setback required by code?

The NECB and ASHRAE 90.1 require automatic controls capable of setback and shutdown for most systems, with some exemptions. Check the adopted code edition for details.

Our control system verification work checks that these functions actually operate as specified.

Key takeaways

• Setback, setup and system shutdown during unoccupied hours are low-cost, high-value measures.
• Closing outdoor air dampers overnight is often the largest saving in cold climates.
• Optimal start replaces fixed start times with a daily calculation based on conditions.
• Freeze protection, recovery capacity and heat pump backup heat limit setback depth on the Prairies.
• Overrides and outdated schedules are the most common reasons the savings vanish.