Hysteresis vs PID

28. November 2017

There are two commonly used temperature regulators. Classic hysteresis where the actual temperature is allowed to fluctuate around the setpoint within certain limits. Or the advanced PID – the so called learning algorithm – which adjusts the heating / cooling performance based on past differences between the actual temperature and the setpoint. PID gradually adjusts its parameters in order to find the best fit for the response time of the heating / cooling system.

  • With hysteresis it is easy to understand what the heating / cooling system is doing – if the setpoint is not reached, the heating / cooling system keeps working. But it results in temperature fluctuations.
  • PID is not transparent – the setpoint may not be reached and still, the heating / cooling device may be off at times. Why is that? Because the PID algorithm has calculated that it shall decrease the performance in order to avoid upcoming temperature fluctuation.

Hysteresis is more popular. PID is being used by experts who like to fine tune its numerous parameters in order to achieve the perfect result.

Which one shall you choose? Well, honestly, it doesn’t matter much. If you keep the hysteresis tolerance low or the pulse width frequency in PID short (see gallery), you end up with similar results. The key point is to use 24VDC thermoelectric valves or solid state relays and control them with this TapHome device with open collector non-relay outputs. In case you do use relay outputs, it may be worthwhile to keep them at default settings so that their lifetime is not shortened.