Q&A about SAT advanced operation

[sergeantd83]

Let’s start with the operation on most OpenTherm projects. They calculate the Control Setpoint based on a weather compensation algorithm or a PID controller. Then pass this value to the boiler and the room temperature stays near the setpoint. That’s fine no issue with that. The user is happy because the room temperature is near the setpoint, the thermostat is happy because is able to control the boiler and the boiler is happy because is able to heat the home.

Q. Is this really the purpose of the thermostat? Being the master or just “acting” as a master? Is this approach robust or a naive approach?

A. The real answer is that the thermostat is “acting” as master since has no idea what is happening in the heating system. The thermostat calculates a Control Setpoint that doesn’t account any of the boiler parameters ( Boiler flow water temperature, Relative modulation, Max relative modulation ) and heating system capabilities. Most of the times the boiler can’t cope with such low Control Setpoint values and is cycling. Which tends to reduce the life time of the burner due to wear and tear.

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

A. SAT controls the boiler and keeping the temperature on setpoint in a completely different way. It monitors every moment the Room temperature, Outside temperature, Boiler flow water temperature, Relative modulation and Max relative modulation to refine its calculations while respecting the heating system limitations.

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

A. Every heating system is unique. Different boiler brands, sizes and minimum capacities. Different heating emitters ( Underfloor or Radiators or Mixed systems ), pipes, circulation pumps. That means every home has a different behavior. That led us to define a threshold. We call it minimum setpoint.

Minimum setpoint is the lowest boiler flow water temperature that the boiler can keep while running continuously without cycling at 0% modulation ( Lowest capacity ).

Q. Why minimum setpoint is so crucial for the heating system?

A. When SAT calculates a Control Setpoint that is higher than the minimum setpoint, boiler works in a normal cycle mode. This means:

a. Sends the CS value unprocessed

b. Lets the boiler modulate itself by sending the Max modulation value of 100%.

When SAT calculates a Control Setpoint that is lower than the minimum setpoint inevitably has to control the boiler in duty cycles ( ON/OFF cycles ) otherwise the boiler will start cycling. This function is the Low-Load control. In order to achieve such a control SAT works like this:

a. Sets the Control Setpoint to the Minimum setpoint ( The lowest Boiler flow water temperature that the boiler is able to handle without overshooting at 0% relative modulation ).

b. Lowers the max relative modulation to 0% ( The boiler works as smooth as it can).

c. Calculates the duty cycle percentage based on the calculated Control Setpoint and the Boiler flow water temperature.

d. Translates the duty cycle percentage to 20 minutes ON/OFF intervals.

Low-Load control helps the boiler to work as much as it can at its minimum capacity. It is suppressing the system from a, let’s say, 24kW system to a 5 kW system. The benefits of this operation are:

a. The boiler runs longer.

b. ΔT of the system is higher.

c. Cycling is reduced.

d. Lower return water temperature which leads to higher efficiency.

e. Less burner and pipes wear.