HVAC Systems & Peak Demand

What Is a Rooftop Unit (RTU)?

A rooftop unit (RTU) is a self-contained, packaged HVAC system mounted on the roof of a building. Unlike split systems (where the compressor is outside and the air handler is inside), an RTU contains every component of the heating and cooling system in a single weatherproof cabinet installed directly on the roof.

RTUs are the dominant commercial HVAC solution for buildings in the range of 1,000–50,000 square feet: big-box retail stores, grocery chains, restaurants, office suites, banks, and similar single- or multi-story commercial structures. A national retail chain with 600 locations might have 10,000 or more individual RTUs in its portfolio — each one a controllable electrical load that can either work with or against the building's energy cost profile.

Typical commercial RTUs range from 3 tons to 25 tons of cooling capacity, drawing 3–30 kW of electrical power when the compressor is running. Larger buildings may have dozens of RTUs serving different zones.

Key RTU Components

Understanding RTU components helps explain why demand spikes occur and why coordinating their operation is the key to demand management:

• 1
  Compressor

  The largest electrical load in the RTU, typically drawing 70–85% of the unit's total power consumption. The compressor pressurizes refrigerant to drive the refrigeration cycle. On startup, the compressor draws a brief inrush current spike — often 3–6 times the normal running current — before settling to steady state. When many compressors start simultaneously, these inrush spikes compound.

• 2
  Condenser Fan

  Draws outdoor air across the condenser coil to reject heat from the refrigerant. Runs whenever the compressor is operating. Adds 5–15% to total unit power draw.

• 3
  Supply Fan

  Circulates conditioned air through the building's duct system. In many commercial applications, the supply fan runs continuously (even when the compressor cycles off) for ventilation purposes. Draws 10–20% of total unit power.

• 4
  Economizer (if equipped)

  When outdoor air is cool and dry enough, an economizer allows the RTU to use outside air for "free cooling" without running the compressor. Economizer operation dramatically reduces energy consumption during shoulder seasons.

• 5
  Controls and Thermostat Interface

  The RTU's control board receives a call for cooling or heating from a thermostat or Building Automation System (BAS) and manages the sequencing of compressor, fans, and heating elements. This control interface is where demand management software integrates to coordinate unit operation.

HVAC's Share of Commercial Building Energy

According to US Department of Energy data, HVAC systems account for approximately 40% of total commercial building energy consumption — the single largest category, exceeding lighting, plug loads, water heating, and all other systems combined in most building types.

In buildings with high cooling loads — retail stores with frequent door openings and customer traffic, restaurants with kitchen exhaust requirements, or server rooms requiring 24/7 cooling — HVAC's share can exceed 60% of total energy consumption.

This concentration of electrical load in HVAC systems means that the behavior of HVAC equipment — when it starts, when it stops, how it cycles — is the primary driver of the demand spikes that appear on commercial electricity bills.

The Energy Stampede Problem

Imagine a retail store with 20 rooftop units serving different zones of the building. During the overnight setback period, all thermostats have allowed temperatures to drift 4–6 degrees above the daytime setpoint. At 7:00 AM, staff arrive and the BAS switches from setback to occupied mode, restoring setpoints to their normal daytime values.

Every thermostat in the building simultaneously sees a temperature well above its setpoint. Every RTU control board receives a simultaneous call for cooling. Within seconds, all 20 compressors start up in parallel.

Each compressor draws its startup inrush current — perhaps 150 amps at 240V, or about 36 kW each — for a few seconds before settling to its running draw of approximately 15 kW. For a brief window of 15–20 minutes, every unit is running hard, trying to pull building temperatures back to setpoint. Total building HVAC demand might peak at 300–400 kW during this window, compared to a normal operating demand of 150–200 kW.

This is the energy stampede — and it is completely predictable. It happens every morning during hot weather, every time the building returns from setback, and every time temperatures recover after a power interruption. The stampede is not an anomaly; it is the normal consequence of letting thermostats operate independently when all zones share similar conditions.

Traditional Solutions and Their Limits

The energy industry has long recognized the HVAC demand problem. Traditional approaches to solving it include:

• Variable Frequency Drives (VFDs):

  VFDs reduce the inrush current on motor startups and allow variable-speed operation. They are effective at reducing energy consumption and demand on individual units. However, they require hardware installation on each unit, cost $2,000–$8,000 per RTU, and require ongoing maintenance. For a facility with 30 units, the capital investment can exceed $150,000 — with no benefit from units not equipped with VFDs continuing to cause coincident peaks.

• High-Efficiency Equipment Replacement:

  Replacing aging RTUs with high-efficiency units reduces energy consumption significantly. But it involves enormous capital expenditure — often $8,000– $20,000 per unit — and provides no immediate demand management benefit until the replacement cycle is complete. High-efficiency units still cause coincident peaks if they start simultaneously.

• Manual Demand Limiting:

  Some older BAS systems include simple demand limiting features that shed all HVAC loads when a demand threshold is exceeded. The problem with this approach is that it is reactive — the spike has already occurred — and shedding all units simultaneously creates a different kind of demand event when they all restart. It also frequently impacts occupant comfort.

Each traditional approach addresses part of the problem, but none solves it comprehensively at the scale and cost-effectiveness that modern commercial portfolios require.

The Intelligent Queue Approach

The most effective solution to the energy stampede is conceptually simple: instead of letting all units respond to thermostat calls simultaneously, coordinate their startup timing so that demand is distributed across time rather than concentrated into a single interval.

This is the principle behind DemandQ's patented queuing technology. When multiple RTUs receive simultaneous calls for cooling, the system does not start all of them at once. Instead, it evaluates each unit's need — based on zone temperature, rate of temperature change, occupancy conditions, and time remaining until the measurement interval closes — and sequences startups so that total building demand stays within a controlled envelope.

Units that are furthest from setpoint start first. Units that are closer to acceptable temperature are delayed by minutes, not hours. By the time the later units start, some of the earlier units may have already reached setpoint and cycled off — keeping the aggregate demand curve flat rather than spiked.

Critically, this approach does not prevent equipment from running — it controls when it runs. The total thermal work performed is identical; only the timing is different. Occupants experience the same temperature conditions, because the algorithm ensures that every zone's temperature remains within the acceptable range at all times.