Ask An Engineer: PID Control and the Triac
Brad Pretty, 2 years ago   |    11   |    6 min read   |    11052

30%. You can save up to 30% of your electricity bill by using Mysa to control your electric heaters. If you’re familiar at all with smart thermostats, you know that the big promise is that they can save you money through automation and control. Only heating when you need to heat is a big part of maximizing your savings, but it doesn’t account for such a big jump. So what does?

Modernizing how a thermostat works, for a start.

You’d be surprised how antiquated thermostat technology really is. While smart thermostats have done a lot to bring innovation to the low voltage category, the vast majority of line or high voltage thermostats are still built around technology that’s severely out of date. That’s why we’ve worked to build brand new hardware from the ground up. Mysa’s really smart – WiFi compatible, integrates with smart homes, app access – but at its very core, it’s a sophisticated piece of energy regulating hardware that uses innovative electrical technology and components engineered to maximize accuracy and safety.

Yes, this blog’s talkin’ bout hardware.

Circuits. Heat Sinks. Sensors. Wires. Resistors. Chips. Single pole. Double pole. That kind of stuff.
The electrical and mechanical design of Mysa is a huge part of why you save money and energy, but one we rarely talk about. So let’s forget about the cool things like voice control and the Mysa app for just a second and instead talk about the equally cool PID cycles. Y’know, the things that keep your heater in check and your room exactly how you like it.

Traditionally, there’s been 3 different ways that a thermostat makes sure the temperature remains at its setpoint: ON/OFF, Proportional Control, and PID.


The Wooden Torch of Thermostat Technology

ON/OFF means that the heater turns up to full power, reaches its temperature, then shuts off. When it senses that the room has dipped below the setpoint, it repeats this process. Using full power and being reliant on temperature actually falling below the set point means 2 things for the average user: It uses more energy than necessary and it fluctuates from the setpoint by as much as a few degrees at a time. With no feedback control, this means that it will often rocket past the setpoint, giving you an uncomfortable heating experience. You likely won’t find ON/OFF settings on anything but the most basic thermostats, but cheaping out on your thermostat will cost you exponentially more in the long run due to the severe shortcomings of ancient technology.


ON/OFF Thermostat graph showing temperature fluctuation and energy loss


The Stone Hammer of Thermostat Technology

It got a little better, though. Eventually Proportional control was developed. Instead of firing up to full power and then switching off, it works with a little bit more nuance. Essentially, it readjusts the amount of energy outputted as it approaches the setpoint (it’s not limited to only using max power). This makes it a lot more reliable, and ensures that there’s less variance when it comes to temperature fluctuations. However, it’s still limited in its scope. It’s not perfect, and the goal of precision actually works in a counter-intuitive way when it comes to energy use. While the heater isn’t always on max, it’s constantly working to make adjustments with what are often inaccurate sensors.  This means that the heater is on far more often than it should be. So while it’s more precise, it still needlessly wastes energy. But hey, at least it’s on the right track.

Proportional control showing less variance and fluctuation than ON/OFF control


The Flying Car of Thermostat Technology

Mysa comes fully loaded with the best temperature sensor tech around, and uses advanced algorithms to more accurately reach your setpoint.

PID control is a souped-up version of proportional control. It stands for Proportional – Integral – Derivative, all of which are factors that contribute to an algorithm that determines the rate of energy use. Mysa augments a traditional Proportional control with Integral control – this adds another variable and greatly adds to our thermostat’s precision. Although it is often grouped alongside the other two variables in common terminology, Mysa does not actually incorporate Derivative calculations into its heating algorithm – there’s no real application for it.

Mysa uses much more precise sensors and technology in order to more closely automate the reading of temperatures, to estimate the rate of change, and to minimize the discrepancy while heating. By monitoring multiple factors, it maximizes the efficiency of the heater and helps conserve energy. Another integral part is our custom designed heatsink and triac, which work in tandem to keep things in check. Our triac design allows for current to pass quickly through the thermostat, regulated with our onboard software, and sent back to the heater, all while our custom heatsink design ensures that the triac doesn’t overheat from the high voltage passing through it.

PI(D) Control of Temperature with Mysa

It’s Our Duty to Save Energy

Duty cycles are one of the more advanced hardware features that you can actually see working in the app. When the software on the thermostat reads the current input and calculates the data from all the sensors, it has to judge the length of time it sends outputs to the heater. This check happens every 15 seconds, meaning that 4 times a minute, your Mysa is precisely calculating your comfort level.  These slivers of time are when your thermostat goes through a duty cycle and they show up on your app as a flame icon under each thermostat or zone. The flame animation acts as a gauge for the duty cycles: a full flame means that your heater is at 100%, and anything less than full is measured in 10% increments.

After going through the duty cycle numerous times, a more advanced application of this technology will eventually allow Mysa to “learn” a room – understand the basic physics of reaching a setpoint in a given space. This will allow it to give scheduling recommendations with pinpoint accuracy.


We’re in the Future, Man.

Buying efficient hardware that actually works will pay for itself in a short amount of time. If you buy cheap, simple hardware, you’ll end up paying much much more over the life of the device. It’s like you took out a loan and are paying a ridiculous interest rate just for being inefficient. It’s a much better idea to go with something that works, that will pay for itself in no time, and is built with cutting-edge hardware. With this in mind, PID control is a major element in Mysa’s goal of conserving energy and ultimately, saving you money. Outfitting your home with Mysa may cost more upfront, but it will quickly pay for itself through tangible energy savings every month.

There’s no way Mysa can reach its full potential without it. What’s more striking is that we’re only going to unlock more of its power, refine it further, and give you more control over your home heat. It’s amazing what we can do with software, but it’s also helpful that we’ll be able to chart energy use and precisely heat your room. Every little bit counts, and together we can use technology to help us create a better future. We’ve already started.




  1. Tom Reeser

    I have line voltage that provides voltage for another thermostat in another room. I find that your thermostat will not work in this condition. Are your working on one that would provide this technology ? If so, when would it be produced?

    1. Rebecca Collins

      Hi Tom,

      If you mean that you have two heaters on the same circuit, and they are daisy-chained, then Mysa would actually work! If that is not exactly what you mean though, just let us know with a bit more information. Thank you so much!

  2. Jerald L. Reisman

    Great technology!! Anxious to watch them at work this winter!

  3. Steve Parus

    So is the duty cycle varied in 10 % increments? But at what frequency ? Is it the 60 Hz line frequency? Is its output turned on for 10%, or 20%, etc of each 1/60th A/C power line duration?

    Or when 15 seconds is mentioned, does it mean its output is turned on for that percentage of a 15 second duration? Or does it mean the duty cycle is recalculated every 15 seconds?

    1. Brad Pretty

      It means that the duty cycle is adjusted every 15 seconds and output to a percentage of that 15 seconds. So 10% output means 1.5 seconds of activity in order to maintain the setpoint.

      – Brad

      1. Patrice Hebert

        We’re not hearing the thermostat click every 15 seconds, there’s no mechanical relay here, right?

  4. Tom Murphy

    nice tutorial


  5. Zach O

    So the reason I can hear my heater turn on and off every few seconds is that its maintaining the room temp and not trying to catch up?

    1. Brad Pretty

      Exactly. In the long run, it uses way less energy.

  6. Steve Humphrey

    We have our first Mysa thermostat connected to (2) forced air wall heaters. It works great. The temperature in the room is so much more stable. We are moving to electric hydronic base board heaters to reduce the noise. We have installed our first baseboard heater in another room that currently has a bi-metal thermostat. Does the PID algorithm and learning apply to electric baseboard heaters with the same results?

    1. Brad Pretty

      They should work fine, but give our support team a shout and they’ll be able to confirm and help.

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