How Rooftop HVAC Systems Handle Heating and Cooling Simultaneously

Walk into a modern high-rise office building on a crisp spring morning, and you might encounter a peculiar phenomenon. The south-facing offices, bathed in direct sunlight through floor-to-ceiling glass, are sweltering. The occupants are cracking windows or calling facilities to ask for air conditioning. Meanwhile, on the north side of the same floor, employees are shivering in the shade, reaching for space heaters and asking for the heat to be turned up.

This scenario, known as “diverse loading,” is one of the most complex challenges in commercial climate control. How do you keep everyone comfortable when half the building needs cooling and the other half needs heating—all at the exact same time?

For standard residential systems, this is impossible; your house is either in “cool” mode or “heat” mode. But for commercial properties utilizing advanced rooftop HVAC systems, simultaneous heating and cooling is not just possible—it is an expectation.

In this comprehensive guide, we will pull back the curtain on the engineering marvels that allow commercial rooftop units (RTUs) to perform this balancing act. From Variable Air Volume (VAV) reheat systems to the cutting-edge efficiency of Variable Refrigerant Flow (VRF) technology, we will explore how modern systems manage simultaneous demands to ensure total building comfort.

The Challenge of Diverse Thermal Loads

Before understanding the how, we must understand the why. Why would a building ever need heating and cooling at the same time?

It often comes down to the physics of the building envelope and internal heat gains.

• Solar Gain: The sun is a massive heat source. As it moves across the sky, it aggressively heats the east side in the morning, the south side at midday, and the west side in the afternoon. The shaded north side, however, remains cold, constantly losing heat to the outside air.
• Internal Loads: People, computers, servers, and lighting generate significant heat. A crowded conference room in the center of the building (the “core”) might need cooling even when it is 40°F outside, simply to offset the body heat of 20 people.
• Perimeter vs. Core: The perimeter of a building is at the mercy of the weather. The core is insulated by the perimeter rooms. This creates a permanent disconnect in thermal needs.

If your HVAC system cannot handle these opposing forces simultaneously, you are left with a building that is always uncomfortable for someone. Solving this requires sophisticated system architecture.

Strategy 1: VAV Systems with Reheat

The most common method for achieving simultaneous heating and cooling in North American commercial buildings—especially those using large rooftop units—is the Variable Air Volume (VAV) system with terminal reheat.

The Mechanics of “Cooling First”

In this setup, the main rooftop unit is essentially a “cooling factory.” It runs in cooling mode, supplying a steady stream of cool air (typically around 55°F) down the main trunk duct. This cool air is available to every zone in the building.

The VAV Box

Above the ceiling in each zone sits a VAV box. This box contains a motorized damper and a heating element (either an electric coil or a hot water coil).

• Cooling Mode: If a zone (like the south-facing office) needs cooling, the damper in its VAV box opens wide, letting the 55°F air flood the room. The heating element stays off.
• Heating Mode: If a zone (like the north-facing office) needs heat, the VAV box damper throttles down to a minimum airflow setting. Then, the heating element activates. It warms up the 55°F air from the rooftop unit before it enters the room.

The Result: Simultaneous Operation

The rooftop unit is technically only cooling. However, from the occupant’s perspective, the system is heating and cooling simultaneously. The south office gets cold air directly from the roof, while the north office gets warm air “manufactured” locally at the VAV box.

While effective and widely used, this method has critics regarding energy efficiency. You are essentially cooling air down on the roof only to pay to heat it back up at the zone level. However, modern codes and controls have made this process much more efficient, and it remains a staple of our Commercial HVAC Services.

Strategy 2: Variable Refrigerant Flow (VRF) Systems

If VAV with reheat is the traditional workhorse, Variable Refrigerant Flow (VRF) is the high-tech sports car. Originating in Asia and Europe and rapidly gaining popularity in the US, VRF systems offer true simultaneous heating and cooling through heat recovery.

Moving Heat, Not Creating It

Unlike a traditional RTU that pushes air through ducts, a VRF system pushes refrigerant through small pipes to individual indoor units (cassettes) in each room. The “magic” happens in a device called the Branch Controller or Heat Recovery Box.

In a heat recovery VRF system, the outdoor unit can circulate refrigerant to different indoor units in different states.

• The Cooling Zone: The indoor unit in the hot conference room acts as an evaporator. Liquid refrigerant absorbs heat from the room air, turning into a gas.
• The Heating Zone: The indoor unit in the cold office needs heat. In a standard system, the heat absorbed from the conference room would be pumped outside and wasted. In a VRF system, that hot gas is redirected directly to the cold office.

The Efficiency of Heat Recovery

This is the ultimate form of energy recycling. The system takes heat from the people in the conference room and moves it to the shivering employee in the north office. The outdoor compressor works significantly less because it is just managing the net difference between the heating and cooling loads.

This approach offers incredible energy savings and precise comfort control, though it requires specialized installation and maintenance expertise found in advanced Commercial HVAC Repair Services.

Strategy 3: The Dual-Duct System

Though less common in new construction due to the high cost of materials, dual-duct systems are a robust way to handle simultaneous loads, often found in hospitals or older institutional buildings.

Two Highways for Air

As the name implies, the rooftop unit supplies two completely separate airstreams through two separate duct networks:

1. The Cold Deck: One duct carries cold air (around 55°F).
2. The Hot Deck: A parallel duct carries hot air (around 90°F+).

The Mixing Box

Every zone has a mixing box where these two ducts meet. The thermostat controls dampers that blend the hot and cold air streams to achieve the exact desired temperature for that specific room.

• Pros: Exceptional control and responsiveness. No water pipes or electric coils are needed above the ceiling.
• Cons: Extremely expensive to install (double the ductwork). It can be energy inefficient if not controlled properly, as you are running heating and cooling plants simultaneously at full capacity.

Strategy 4: Water-Source Heat Pumps (WSHP)

For very large complexes or multi-story buildings, a water-source heat pump loop is a highly efficient way to manage diverse loads. This system often utilizes a cooling tower and a boiler on the roof, rather than a traditional packaged air conditioner.

The Water Loop

A loop of water circulates throughout the entire building. Individual heat pump units are installed in the ceiling of every zone. These units are connected to the water loop.

• Extraction: A unit in cooling mode pulls heat out of the room air and dumps it into the water loop (warming the water).
• Absorption: A unit in heating mode pulls heat out of the water loop (cooling the water) and puts it into the room.

Balancing the Loop

If the building is perfectly balanced (equal heating and cooling needs), the units just trade heat through the water loop. The south side warms the water, and the north side cools it back down. The central boiler and cooling tower don’t even need to run.

If the loop gets too hot (summer), the cooling tower on the roof turns on to reject the excess heat. If the loop gets too cold (winter), the boiler fires up to add heat. This system is brilliant for simultaneous operation because it inherently shares energy between zones.

The Role of the Economizer in Simultaneous Operations

In commercial rooftop systems, the economizer plays a crucial role in reducing the cost of simultaneous heating and cooling.

As mentioned in the VAV reheat strategy, the system often cools air down just to heat it up again. This sounds wasteful. However, if the outdoor air is 55°F, the rooftop unit can open its economizer dampers and use that free outside air as the “cold deck” supply.

The compressors stay off. The unit simply filters the outside air and sends it down the duct.

• Cooling Zones: Receive free cooling from outside.
• Heating Zones: Still use their local reheat coils, but the “cost” of the cold air was zero (just fan energy).

Properly maintained economizers are essential for this efficiency. If an economizer linkage is broken or a sensor drifts, you lose this free energy source. Our Commercial HVAC Services include rigorous economizer testing to ensure this “free cooling” mode is fully operational.

Control Systems: The Brains Behind the Operation

Hardware alone cannot manage the complexity of simultaneous heating and cooling. It requires a sophisticated Building Automation System (BAS) or modern networked controls.

Logic and Arbitration

The control system must make constant decisions:

• Supply Air Temperature Reset: If 90% of the VAV boxes are closing because they are too cold, the system can tell the rooftop unit to raise the supply air temperature from 55°F to 60°F. This reduces the need for reheat and saves energy.
• Static Pressure Reset: If dampers are closing, pressure builds up. The BAS tells the fan VFD to slow down, saving electrical energy.
• Morning Warm-Up: Before employees arrive, the system might go into a “heating only” mode to bring the whole building up to temperature, ignoring the usual simultaneous logic until occupancy sensors detect people.

Zone Voting

In simpler VVT (Variable Volume and Temperature) systems, which simulate zoning on smaller equipment, the system uses a “voting” logic. It polls all the thermostats. If 3 zones call for heat and 1 calls for cooling, the system switches to heat mode. The cooling zone just has to wait (or use a bypass). This is not true simultaneous operation, but “changeover” operation. For true simultaneous needs, VAV or VRF is required.

Maintenance Challenges in Simultaneous Systems

Systems that heat and cool simultaneously are inherently more complex than standard “on/off” units. They have more moving parts, more sensors, and more failure points.

The Reheat Coil Failure

In VAV systems, a common issue is the failure of the reheat valve or electric coil. If the coil fails open, it might heat the air continuously, fighting the cooling system. The occupant feels hot, so the VAV damper opens wider to let in more cold air. The heating coil just heats that air up too. This “fighting” wastes enormous amounts of energy and can go undetected for months without proper monitoring.

Sensor Calibration

Simultaneous systems rely on accurate data. If a discharge air temperature sensor is reading 5°F off, the entire logic of the system crumbles. The economizer might open at the wrong time, or the boiler might fire when not needed. Regular sensor calibration is a non-negotiable part of preventative maintenance.

Water Loop Quality

For WSHP systems, the quality of the water in the loop is critical. Scale, rust, or biological growth can insulate the heat exchangers, preventing the efficient transfer of energy between the loop and the zones. Water treatment is vital.

Design Considerations for Building Owners

If you are developing a property or planning a major retrofit, choosing the right simultaneous system depends on your specific constraints.

• First Cost vs. Lifecycle Cost: VRF systems are expensive to install due to the specialized copper piping and labor, but they offer the lowest monthly energy bills. VAV systems are standard and easier to service, but may have higher operational costs in climates with extreme swings.
• Space Constraints: Do you have room for large ductwork (VAV)? Or do you need the small footprint of refrigerant pipes (VRF)?
• Future Flexibility: VAV systems are relatively easy to modify if walls are moved. Moving VRF piping can be more labor-intensive and requires recovering refrigerant.

Conclusion

The ability to heat and cool a building simultaneously is a triumph of modern HVAC engineering. It transforms a glass-walled office building from a thermal nightmare into a comfortable, productive workspace. Whether through the air-blending mechanics of VAV, the heat-shifting wizardry of VRF, or the thermal sharing of a water loop, these systems ensure that the sunny side and the shady side can coexist in harmony.

However, complexity demands care. These are not “install and ignore” appliances. They are dynamic systems that require intelligent oversight and skilled maintenance. A VAV box with a stuck damper or a VRF system with a slow leak can quickly turn efficiency into expense.

To ensure your sophisticated rooftop system is performing its balancing act correctly, you need a partner who understands the science behind the machinery. Whether you need to optimize your current VAV controls, repair a heat recovery system, or explore a retrofit, trust the experts. Visit our Commercial HVAC Repair Services page to schedule a diagnostic or consultation today. Ensure your building is comfortable for everyone, no matter where they sit or what the weather is doing outside.