Q&A about SAT advanced operation

[sergeantd83]

SAT is an advanced heating control system designed to intelligently manage boilers through full system awareness. Unlike opentherm thermostat projects that simply act as a master, SAT truly becomes the master, optimizing boiler performance, minimizing cycling, and maximizing efficiency by adapting to the unique characteristics of each heating system.

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How other OpenTherm thermostat projects operate

Typically, they calculate the Control Setpoint based on a weather compensation algorithm or a PID controller.
They then pass this value to the boiler to maintain the room temperature near the setpoint.
This works: the user is happy because the room stays warm, the thermostat is happy because it can control the boiler, and the boiler is happy because it can heat the home.

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Q&A

Q. Is this really the purpose of the thermostat — being the master, or just “acting” like one? Is this approach robust or naive?

A.
In reality, the thermostat is only acting as the master. It has no visibility into the actual heating system behavior. It simply calculates a Control Setpoint without considering key boiler parameters like Boiler flow water temperature, Relative modulation, or Max relative modulation.
As a result, many times the boiler can’t handle low setpoints properly and starts cycling, leading to increased wear and a reduced burner lifetime.

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Q. Why is SAT different and truly the master of the heating system?

A.
SAT controls the boiler by keeping the setpoint temperature in a smarter way. It continuously monitors the Room temperature, Outside temperature, Boiler flow water temperature, Relative modulation, and Max relative modulation to refine its calculations while respecting the heating system's limitations.

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Q. Why should a thermostat respect the heating system’s limitations?

A.
Every heating system is different:

• Different boiler brands, sizes, and minimum capacities.
• Different types of heating emitters (underfloor, radiators, or mixed systems).
• Different pipework and circulation pumps.

Because every home behaves differently, SAT defines a crucial threshold called the minimum setpoint — the lowest boiler flow water temperature that the boiler can maintain while running continuously without cycling at 0% modulation (lowest capacity).

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Q. Why is the minimum setpoint so important?

A.
When SAT calculates a Control Setpoint above the minimum setpoint:

• It sends the Control Setpoint unprocessed.
• It allows the boiler to modulate freely (sending a 100% Max Modulation value).

When the Control Setpoint is below the minimum setpoint, SAT activates Low-Load Control to prevent boiler cycling:

• Sets the Control Setpoint to the minimum setpoint (ensuring stable low modulation).
• Lowers Max Modulation to 0%, letting the boiler run as smoothly as possible.
• Calculates the duty cycle percentage based on the calculated Control Setpoint and the Boiler flow temperature.
• Translates the duty cycle into ON/OFF intervals of 20 minutes.

Through Low-Load Control, SAT effectively "shrinks" a 24kW system down to behave like a 5kW system.

Benefits of this operation:

• The boiler runs longer without interruption.
• Higher ΔT (temperature difference) across the system.
• Reduced cycling.
• Lower return water temperature = Higher efficiency.
• Less wear on the burner and plumbing.

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Summary

SAT is not just a thermostat acting like a master — it is the master.
By continuously adapting to the heating system’s capabilities, SAT ensures maximum efficiency, reliability, and longevity of your heating system.