Late August creates a peculiar operating day in commercial buildings. The morning begins cool enough that perimeter offices call for heat. By lunch, the west side is fighting solar gain. Interior conference rooms remain warm from people, lighting, and equipment, while a tenant near the lobby asks why cold air is still coming from the ceiling.
In the mechanical room, the picture can be even stranger. A boiler is maintaining hot water for reheat. Rooftop compressors are running. Several terminal boxes are nearly closed, yet their heating valves are open. The building automation system shows temperatures close to setpoint, so nothing appears to be in alarm. The property is technically comfortable, but it is paying to cool air and heat it again.
Simultaneous heating and cooling is not always evidence of a failed system. Commercial HVAC systems sometimes use reheat deliberately to control humidity, provide ventilation, or stabilize zones with different loads. The problem begins when heating and cooling overlap longer than the building actually needs, when controls fight each other, or when airflow and zoning problems force the system to use energy as a substitute for proper distribution.
This issue becomes especially visible during the last weeks of August in White Plains. Offices, restaurants, medical suites, retail spaces, and mixed-use buildings may move between cool mornings, humid afternoons, changing occupancy, and extended operating schedules within one day. Equipment that looked stable during continuous July heat can begin cycling, hunting, and reheating as outdoor conditions moderate.
Property managers reviewing broader operating costs can begin with our guide to improving HVAC efficiency without replacing the system. This article focuses on one specific diagnostic question: why does a commercial building call for heating and cooling at the same time, and how can that overlap be reduced without creating humidity or tenant-comfort problems?
Simultaneous heating and cooling should be understood before it is eliminated. Some reheat is intentional. Waste begins when controls, airflow, schedules, or zoning make the building oppose its own cooling effort.
The condition can occur at several levels of a commercial HVAC system.
Not every example has the same cause. A large office building may legitimately need cooling in its interior and heat at the glass perimeter on a cold morning. A medical or high-occupancy space may require dehumidified air that is reheated to avoid overcooling. A restaurant can have kitchen and dining zones with opposing loads.
The diagnostic question is whether the overlap is controlled, limited, and connected to a real building need. If heat remains active because a valve leaks, a sensor is inaccurate, or the supply-air temperature is unnecessarily low, the building is wasting energy. If a terminal unit reheats dry air to maintain humidity and ventilation targets, shutting it off blindly can create a different problem.
During a sustained heat wave, most commercial zones move in the same direction. Outdoor temperature is high, roofs and walls are warm, and cooling dominates. Shoulder-season weather is less orderly.
A White Plains office may begin the day with a cool exterior envelope and an interior that retained heat overnight. The east side warms first. The south and west sides follow later. Conference rooms gain heat abruptly when meetings begin. Lobby conditions change each time exterior doors open. By evening, restaurants and fitness tenants may be reaching peak occupancy as office floors empty.
HVAC systems respond according to their sensors, schedules, setpoints, and zone design. When those pieces are not coordinated, the building can spend the morning heating spaces that will need cooling two hours later. It can then overcool supply air to satisfy an interior zone and reheat it for perimeter rooms.
Late August also brings humid days when outdoor temperature is moderate. The building may need moisture removal without much sensible cooling. A system that cools air deeply enough to condense moisture may need controlled reheat before delivering that air to occupied rooms. That is a legitimate use of simultaneous cooling and heating—provided it is measured and managed rather than left to run continuously.
Commercial thermostats and zone controllers normally use a deadband between heating and cooling setpoints. For example, a zone may heat below one temperature and cool above a higher temperature, leaving a neutral range between them.
Problems develop when the deadband is too narrow or when tenant overrides move the setpoints toward each other. One occupant raises the heating setpoint. Another lowers the cooling setpoint. A local thermostat allows both adjustments, while the building automation system continues following a central schedule.
An inaccurate sensor makes the problem worse. If the heating sensor reads low while the cooling control uses a different sensor that reads high, the zone can appear to need both modes. Poorly located sensors near exterior walls, return grilles, supply diffusers, appliances, or direct sun can create similar conflicts.
The zone temperature remains near setpoint, but the heating valve and cooling damper repeatedly open and close. The system appears stable only because the two outputs are cancelling each other.
Check the actual heating and cooling setpoints, deadband, sensor calibration, override limits, occupancy mode, and whether more than one control device is acting on the same zone.
Central air systems often deliver relatively cool air so the warmest zones receive enough cooling. Terminal boxes then reduce airflow or add reheat for zones with lighter loads.
This strategy can be appropriate during peak summer conditions. In late August, however, the building may continue producing the same cold supply air even though most zones no longer need it. Interior rooms remain satisfied, perimeter boxes nearly close, and reheat valves open to prevent overcooling.
A supply-air temperature reset can reduce this conflict by allowing warmer supply air when cooling demand is low. The correct reset depends on humidity, ventilation, equipment type, terminal-box position, and the needs of the warmest zone. Raising supply temperature too aggressively can cause remote or high-load rooms to lose control.
Before changing the reset, facilities teams should review which zones are driving the air handler. One conference room with a bad sensor or weak airflow can force the entire system to produce colder air. Correcting the local problem may reduce building-wide reheat more effectively than changing every setpoint.
Variable-air-volume systems regulate zone cooling by changing airflow. When a room needs less cooling, its terminal box moves toward a lower airflow setpoint. Many boxes also contain hot-water or electric reheat for heating and temperature control.
If the minimum airflow is set higher than the room needs, cold air continues entering the zone even after cooling demand falls. The reheat coil then warms that air to prevent the room from becoming too cold. The building is not reheating because the room truly needs heat; it is reheating because the box cannot reduce airflow further.
Minimum airflow may be elevated for ventilation, pressure, diffuser performance, or equipment reasons. It may also be the result of a commissioning value copied from an old drawing, an actuator that never reaches its command, or a flow sensor that is out of calibration.
Reducing the minimum without understanding ventilation can create under-ventilated spaces. The correct process is to verify design intent, actual occupancy, terminal-box airflow, diffuser performance, outside-air delivery, and room pressure before adjusting the setpoint.
A central fan that maintains more duct static pressure than the system needs can push excessive air through terminal boxes and branches. Dampers spend much of the day nearly closed, creating noise and unstable control. Small changes in damper position produce large changes in airflow, and reheat becomes the easiest way to keep zones from overcooling.
Static-pressure reset can lower fan pressure when most terminal boxes are partly closed, then raise it when distant zones need more airflow. The control sequence should use reliable box-position or pressure feedback and preserve adequate flow to the critical zone.
High pressure may also be masking duct problems. A remote branch with poor fittings, leakage, or an undersized section can require excessive system pressure to receive its design airflow. Lowering the setpoint without correcting that restriction makes the distant room uncomfortable.
Our guide to commercial HVAC ductwork design and airflow efficiency explains how branch layout, fitting losses, return-air capacity, leakage, and balancing shape the pressure the fan must produce.
Interior commercial zones gain heat from occupants, lighting, computers, kitchen equipment, and neighboring spaces. They may need cooling even on a cool day. Perimeter zones respond more strongly to windows, outdoor temperature, wind, and sun.
When interior and perimeter rooms share one thermostat or one uncontrolled duct branch, the system cannot respond accurately to both. A sensor in the interior keeps cooling active while the perimeter becomes cold. The perimeter then relies on reheat, baseboard heat, or portable heaters.
Renovations often create this problem. An open office becomes private rooms. A conference room is added along the glass. A reception area is divided from the main floor. Supply diffusers are moved, but thermostats and return paths remain in their original locations.
Zoning corrections may include sensor relocation, averaging sensors, separate terminal control, revised branch ductwork, improved return air, or dedicated equipment for a space with unusual load. The right solution depends on how the building is occupied now—not how the floor was laid out twenty years ago.
Many commercial buildings use hot water for perimeter heat, terminal reheat, domestic water production, or other loads. As nights become cooler, operators may enable the heating plant for the season even while daytime cooling remains active.
The boiler may need to be available, but its enable logic should reflect actual heating demand. A fixed calendar date can start the plant weeks before the building consistently needs heat. A low outdoor-air threshold without time delay can enable boilers during one cool morning and leave them active all day.
Warm-weather shutdown, outdoor-air lockout, demand-based enable, and scheduled plant rotation can reduce unnecessary operation. These controls must be coordinated with freeze protection, domestic-water requirements, process loads, and spaces that genuinely need morning heat.
A leaking control valve can create heat even when the boiler sequence is correct. Hot-water coils, baseboard loops, and reheat valves should close tightly when commanded. Valve position shown on a control screen should be compared with pipe temperature or measured flow.
Economizers use suitable outdoor air for cooling when conditions allow. In late summer, cool morning air can reduce compressor use. Poorly functioning economizers can also create unstable operation.
A damper may open and close repeatedly near its changeover point. Outdoor-air and return-air sensors may be inaccurate. The control may evaluate temperature while ignoring high outdoor moisture. Relief air may not track the intake, changing building pressure. Mechanical cooling may remain active because mixed-air temperature never reaches the intended target.
The result can be a system that introduces cool outdoor air, runs compressors, and activates reheat within the same hour.
Economizer testing should include physical damper movement, actuator linkage, sensor calibration, mixed-air conditions, minimum position, relief operation, and the transition between free cooling and mechanical cooling. A dashboard command is not enough.
Commercial comfort problems often develop through reasonable local decisions. A tenant lowers a thermostat before a crowded meeting. Another closes a diffuser because the desk below feels cold. A portable heater is placed under a thermostat. A studio manager props open a door to improve circulation.
Each action changes how the zone behaves. A heater near the sensor can keep cooling active. A closed diffuser can raise duct pressure and shift air to another branch. An open door can create a return-air path that disappears when the room is occupied normally.
Tenant overrides should have limits and expiration times. Facilities staff need visibility into repeated adjustments, especially when one zone routinely drives central equipment. Communication should focus on the building response rather than blaming occupants. People create workarounds because they are uncomfortable.
Our article on the hidden HVAC conditions behind tenant comfort explains why complaint patterns often reveal zoning, airflow, and control problems that are not obvious from the equipment room.
Not all simultaneous heating and cooling should be removed. Commercial systems may require reheat to control humidity while maintaining a comfortable discharge temperature.
During humid weather, air is cooled below its dew point so moisture condenses on the coil. If that air is delivered directly to a lightly loaded room, the space may become too cold. Reheat raises the supply temperature without returning the removed moisture.
Healthcare, fitness, hospitality, high-occupancy, process, and ventilation-heavy spaces may use this strategy deliberately. Dedicated outdoor-air systems also commonly dehumidify ventilation air and then temper it before delivery.
The efficiency question is whether reheat is minimized and whether the energy source is appropriate. Heat recovery, hot-gas reheat, condenser heat, hydronic recovery, and variable-capacity equipment may reduce the need for separate boiler or electric reheat.
Removing reheat simply because heating and cooling appear together can cause high humidity, condensation, odor, and comfort complaints. The control sequence should preserve the latent performance the building needs while reducing unnecessary sensible overcooling.
Some VRF and heat-pump systems can transfer heat from zones that need cooling to zones that need heating. In that case, simultaneous calls can be an efficiency opportunity rather than pure waste.
The system still needs correct branch-controller operation, refrigerant distribution, sensor calibration, indoor-unit airflow, and controls. If cooling and heating demands are unbalanced, the outdoor unit may still reject or absorb additional heat. Defrost, minimum operating limits, and piping design affect performance.
A building should not assume that every simultaneous mode is being recovered efficiently. Trend data, manufacturer diagnostics, zone behavior, and electrical use can show whether the system is exchanging heat effectively or running opposing modes because of control errors.
The most useful investigation follows the building through a complete late-summer day. A ten-minute visit cannot show how morning heating becomes afternoon cooling.
The analysis should separate local faults from central strategy. One failed terminal box should not trigger a building-wide supply-air change. A central sequence that overcools every zone should not be addressed by adjusting dozens of local thermostats.
Establish a reasonable neutral range between heating and cooling. Limit tenant adjustments and make overrides temporary. Verify that all control layers use compatible values.
Allow supply air to warm when cooling demand drops, provided humidity, ventilation, and critical zones remain controlled. Use representative zone demand rather than outdoor temperature alone.
Reduce fan pressure when terminal boxes are mostly closed. Confirm that the distant or critical zone still receives required airflow.
Verify terminal-box calibration and minimum airflow. Coordinate ventilation requirements with the zone sequence so reheat does not compensate for excessive cold airflow.
Mechanical leakage and inaccurate feedback can defeat sophisticated controls. Field verification often finds simple causes behind long-running overlap.
Perimeter, interior, conference, kitchen, retail, and event spaces may need different sensors, schedules, airflow, or equipment.
Evaluate whether condenser heat, hot-gas reheat, hydronic recovery, or heat-recovery VRF can serve legitimate simultaneous loads more efficiently.
The White Plains HVAC service area includes office buildings, restaurants, medical spaces, apartments, retail properties, and mixed-use developments with overlapping schedules.
Properties in the White Plains 10601 service area may have several rooftop units, central air handlers, tenant-controlled systems, kitchen exhaust, elevator lobbies, and interior spaces that retain heat after perimeter zones cool down. Those differences make blanket seasonal changeover dates unreliable.
Yukos Mechanical’s White Plains brewery HVAC installation involved exposed ductwork, ventilation, equipment planning, and air distribution for a hospitality environment with changing occupancy and internal loads. It was not a simultaneous-heating-and-cooling correction, but it illustrates why one commercial space can move through very different thermal conditions within the same day.
Late-summer control work should be scheduled around actual operations. Restaurants need pressure and ventilation during service. Offices need quiet adjustments that do not disrupt tenants. Medical and wellness spaces may have tighter environmental limits. A control sequence is successful only when it works during the building’s real occupied hours.
Commercial buildings can require heating and cooling at the same time. The energy problem is not the existence of two calls; it is uncontrolled overlap that persists because the building is masking sensor, airflow, zoning, scheduling, or mechanical faults.
The last week of August provides a useful diagnostic window. Morning and afternoon loads move in opposite directions, boilers begin entering standby service, economizers become more active, and humid days reveal whether reheat is serving moisture control or simply correcting overcooling.
For White Plains property managers, the most productive approach is to trend the building, verify field conditions, and correct the sequence in layers. Start with sensors, valves, dampers, schedules, airflow, and zone design. Preserve intentional humidity control. Reduce the hours when central cooling and local heat are working against each other.
Yukos Mechanical helps commercial property teams evaluate rooftop units, VAV systems, controls, duct static pressure, airflow, zoning, economizers, reheat, and boiler enable sequences. Request a commercial HVAC controls assessment to identify where simultaneous heating and cooling is protecting the building—and where it is quietly wasting energy.
Review sensors, VAV airflow, reheat, boiler enable, economizers, deadbands, and control sequences before late-summer overlap becomes a full-season operating cost.
Request a Controls AssessmentThe building may have different loads in perimeter and interior zones, intentional humidity reheat, overlapping setpoints, excessive minimum airflow, high duct pressure, leaking heating valves, inaccurate sensors, or controls that enable heating and cooling too close together.
No. Reheat may be necessary for humidity control, ventilation-air tempering, or zones with genuinely different loads. It becomes wasteful when the overlap is caused by control errors, poor airflow, leaking components, or schedules that do not match occupancy.
Yes. A reasonable neutral range between heating and cooling can reduce rapid mode changes and opposing calls. The deadband should be coordinated across local thermostats, terminal controllers, and the building automation system.
A VAV box may deliver more cold air than the zone needs because its minimum airflow is too high, its flow sensor is inaccurate, or duct pressure is excessive. The reheat coil then warms that air to prevent overcooling.
Measure or trend zone temperatures, setpoints, damper positions, reheat-valve positions, terminal airflow, supply-air temperature, duct static pressure, outdoor conditions, boiler and chiller status, compressor stages, schedules, and tenant overrides.
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