So I think SAT autodiscovered your board. Correct?
Thatâs correct!
Installed 4.0.0 for use with my EMS-ESP connected Buderus boiler. So far it looks like SAT handles low load situations much better than Buderus themselves. Itâs only 16 hours in, but current metrics look promising. I will follow up in a couple days with more data.
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Still figuring out if I can use SAT⌠I have underfloor heating and Honeywell Evohome valves for the bedrooms. I cannot directly control those valves (can only control the target temperature). Is it now correct to assume that I cannot leverage SAT to control the temperature in the bedrooms?
Sat does not control the temperature. It tries to control your gas boiler or heat pump as efficiently as possible based on the heating setpoints you set with your thermostat.
You will need a gateway though inbetween your Honeywell and the gas boiler.
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I would like to share some more âunder the hoodâ informations about SAT.
Letâs 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. Thatâs 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.
Is this really the purpose of the thermostat? Being the master or just âactingâ as a master? Is this approach robust or a naive approach?
The real answer is that the thermostat is âactingâ as master since has no idea what is happening in the heating system. The thermostat calculates a Control Setpoint that doesnât 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 canât 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.
Why SAT is different and actually IS the master of the heating system?
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.
Why a thermostat should respect the heating system limitations?
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 ).
Why minimum setpoint is so crucial for the heating system?
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, letâs say, 24kW system to a 5 kW system. The benefits of this operation are:
a. The boiler runs longer.
b. ÎT 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.
Is SAT more efficient compared to other thermostats?
Our research showed that SAT controlled boilers are using 30% less gas than traditional ON/OFF thermostats.
I hope I made some things more clear about SAT operation.
Thanks for this great explanation! Can you maybe also indicate how we can measure how much better SAT is then the normal thermostat? Should I then think off lower gas per degree day or more around cycling of the heater?