Pulse heating is a control method designed to deliver more stable and accurate temperature regulation in greenhouses that rely on on/off heating devices, such as steam valves or unit heaters.
In a basic on/off control system, the heating stage simply turns on when the temperature drops below the target setpoint and turns off once the temperature rises above it. While this approach appears straightforward, it often leads to significant issues due to residual heat. In systems involving steam pipes, heat exchangers, or other high-thermal-mass components, a substantial amount of heat remains trapped and continues to radiate into the space even after the valve or heater turns off. There is also a delay between when the heat stage turns on and when the temperature sensor begins to register the rise in temperature. Heat must radiate into the greenhouse zone, circulate through the air, and eventually reach the sensor location. By the time the sensor detects the temperature increase and the controller shuts the heating stage off, additional heat is still moving through the system. This causes the temperature to overshoot the target, followed by a long wait until it drops low enough to trigger heating again, resulting in large, undesirable temperature swings.
Pulse heating, also known as heat cycling, addresses this by pulsing the heating stage on and off in regular, repeating cycles instead of allowing it to run continuously until overshoot occurs. This delivers heat in small, controlled bursts, creating a smoother average heat input and dramatically reducing temperature fluctuations.
The system operates using a few key settings in the configuration screen:
- Heat Cycle Length: The total duration of one complete on and off period. In theory, shortening the cycle length can improve temperature stability, but doing so increases how frequently the heater switches on and off, which can place additional mechanical and electrical stress on the heater.
- Minimum time on for heat cycle: The shortest period the heating stage is permitted to remain active in any cycle (e.g., 30–60 seconds). This is especially important in gas fired heaters where it takes time for the ignitition cycle to complete. Make sure the minimum on time is long enough for the heat source to start generating heat.
- Minimum time on for heat cycle: The shortest period the stage must remain off (e.g., 30 seconds), which protects equipment from excessive rapid cycling and allows time for the system to respond to the added heat.
Once the configuration has been set up, you also need to check the settings page. If you have one, heat stage, you set the start percentage at 1% and the end percentage at 100%.
If you have multiple heating stages, you can overlap the stages so that the second stage starts activating even before the first stage is fully on.When using two or more stages of heating, the start time of each stage will be staggered so that all heat stages are not activating at the same time.
The Compass continuously calculates a heating demand percentage (0–100%) based on how far the current temperature deviates below the target. It then proportions the ON time within each fixed cycle accordingly:
- At 1% demand, the stage activates only for the minimum ON time (e.g., 60 seconds), then remains off for the remainder of the cycle. This gives the greenhouse a small amount of heat to offset minor heat loss.
- At 50% demand, it stays on for half the cycle and off for the other half. Typically the heating percentage will fluctuate up and down to maintain equilibrium.
- At 99% demand, the stage the stage is mostly on, deactivating only for the minimum OFF time (e.g., 30 seconds).
- At 100% demand, the stage remains on continuously throughout the entire cycle.
The pulse heating transforms a basic on/off system into something much closer to proportional control. By rapidly averaging heat delivery across short, repeating cycles, it minimizes overshoot and undershoot, resulting in finer, more consistent climate control.