When to use: Simulate multi-zone HVAC control over a 24-hour period. Set heating/cooling setpoints and occupancy per zone, then watch the system cycle on and off as outdoor temperatures change and internal gains accumulate. Track runtime and energy usage per zone.
This simulator models multi-zone HVAC thermostat control over a 24-hour period, tracking zone temperatures against heating and cooling setpoints, simulating HVAC cycling, and recording runtime per zone as outdoor temperatures and internal gains change. Engineers and commissioning agents use it to visualize deadband behavior, evaluate setpoint scheduling, and understand zone thermal drift characteristics.
Zone control systems maintain each zone within a heating-cooling deadband: if zone temperature drops below the heating setpoint minus the deadband (typically 1.5°F), heating activates; if it rises above the cooling setpoint plus the deadband, cooling activates. The deadband prevents simultaneous heating and cooling and reduces short-cycling.
Zone temperature drift is driven by thermal exchange with the outdoor environment (proportional to the outdoor-indoor temperature difference and the building envelope conductance) and by internal gains from occupants, lighting, and equipment. In this simulation, internal gains are simplified as a constant BTU/hr rate per zone, with the kitchen receiving higher internal gains from cooking appliances.
VAV zone control in real buildings uses a VAV terminal box to modulate supply airflow between minimum and maximum CFM, with reheat at minimum flow if heating is needed. The zone thermostat sends a control signal to the VAV controller, which modulates the damper. This simulation uses simplified on/off logic to illustrate the fundamental heating/cooling sequence.
ASHRAE Standard 90.1 Section 6.4.3 requires setback temperature control for all HVAC systems — unoccupied setpoints are typically 55°F heating and 85°F cooling for commercial buildings. ASHRAE Guideline 36 (High-Performance Sequences of Operation for HVAC Systems) provides detailed control sequences for VAV systems including zone minimum flows, reheat limits, supply air temperature reset, and morning warm-up.
ASHRAE Standard 55 defines the thermal comfort zone in terms of operative temperature and humidity that occupants find acceptable. Zone control setpoints should target conditions within the ASHRAE 55 comfort polygon. ASHRAE/ICC Standard 100 and IECC Section C403 also set requirements for setback controls and occupancy-based temperature controls.
Deadband width is a key energy design parameter. A wider deadband (3–5°F) reduces HVAC cycling and energy use but allows greater temperature swing in the zone. A narrow deadband (1–1.5°F) maintains tighter temperature control but increases cycling frequency and runtime. Most thermostat manufacturers recommend 1–2°F deadband for comfort applications.
Morning warm-up and cool-down periods must be accounted for in zone control design. After a nighttime setback, a zone may need 1–3 hours to reach occupancy setpoint — the BAS pre-occupancy period must start early enough based on outdoor temperature and building thermal mass. Optimal start algorithms calculate the required lead time dynamically based on measured zone temperature and outdoor conditions.
Click Run Simulation to start the 24-hour day simulation. Adjust the speed multiplier to run through the day faster. Watch each zone card show its current temperature, HVAC state (heating/cooling/idle), and accumulated runtime. Toggle occupancy to see how it affects internal gains and zone temperature drift.
Adjust heating and cooling setpoints per zone using the + and − controls. A wider gap between heating and cooling setpoints creates a larger deadband, reducing cycling. Observe how the kitchen zone runs warmer due to internal cooking gains. The history chart plots all zone temperatures over time, revealing the characteristic sawtooth pattern of thermostat-controlled cycling.
The deadband is the temperature range between the heating setpoint and cooling setpoint within which the HVAC system is idle. If heating SP = 70°F and cooling SP = 76°F, the deadband is 6°F. This prevents the system from simultaneously heating and cooling, which wastes energy and wears out equipment. ASHRAE 90.1 requires a minimum 5°F deadband for all commercial thermostats.
VAV systems vary supply air flow rate to each zone to match thermal load, rather than varying temperature (constant volume). At full cooling load, the VAV box opens fully; at partial load, it throttles to minimum flow. At minimum flow, if the zone needs heating, a reheat coil in the terminal box heats the air. VAV systems are more energy-efficient than constant volume systems because fan energy decreases with the cube of flow reduction.
Occupied zones generate internal heat from people, computers, lighting, and equipment. A person at sedentary activity produces approximately 250 BTU/hr of sensible heat and 200 BTU/hr of latent heat. In an office with 10 occupants and 20 computers (200W each), the total internal gain is approximately 9,000 BTU/hr sensible, which drives the zone toward the cooling setpoint even in mild weather.
Setback reduces heating setpoints (to 55–60°F) and increases cooling setpoints (to 80–85°F) during unoccupied periods to save energy. ASHRAE 90.1 requires all commercial thermostats to have setback capability and 7-day programmable scheduling. For buildings with high thermal mass, setback is most effective — the mass retains heat through the night, reducing morning warm-up energy. For lightweight construction, shorter setback periods may be more economical.
Optimal start calculates the pre-occupancy heating or cooling lead time needed to reach setpoint by occupancy start time, based on current zone temperature, outdoor temperature, and empirical thermal response of the building. Instead of a fixed 2-hour warm-up regardless of conditions, optimal start might activate heating 30 minutes before occupancy on a mild morning and 3 hours before on an extremely cold morning, saving energy on mild days.
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