Thermostat Tuning

Introduction

Your average thermostat rarely, if ever, gives you fine tuning options (except maybe [hysteresis](http://diy-zoning.blogspot.com/2007/07/rite-temp-gpmg8085c-informal-review.html hysteresis)). DZ thermostat is based on a PID controller with windup protection.

PID controller tuning is not as simple as it may seem ( google it up if you doubt). Whereas DZ PID controller can be tuned up according to classical PID tuning tutorials, there are some concepts understandable to non-geeks.

PID Controller Configuration XML Tag

Looks like this (see Configuration Guide for details):

<bean id="pid_controller-6500000055FF1A26" class="net.sf.dz3.controller.pid.SimplePidController">
  <!-- Setpoint (all configuration values are in SI Units only) -->
  <constructor-arg index="0" value="20.5"/>
  <!-- P -->
  <constructor-arg index="1" value="1"/>
  <!-- I -->
  <constructor-arg index="2" value="0.0000002"/>
  <!-- D, careful with it -->
  <constructor-arg index="3" value="0"/>
  <!-- Saturation limit, 3 is a reasonable default -->
  <constructor-arg index="4" value="3"/>
</bean>

Heating vs Cooling

This one is simple.

• For cooling, both P and I are positive.
• For heating, both P and I are negative.

Hysteresis, a.k.a. Temperature Swing

• Decrease P to make the swing wider. This is good for your unit (less start/stops = longer unit ilfe, higher runtime = higher efficiency), but bad for comfort.
• Increase P to make the swing narrower. Bad for your unit, good for your comfort. Remember, though, the price is high - your unit life, and, even more importantly, your electric bill (due to less efficient operation (see HVAC short cycling).

Size Does Matter

• For small rooms, consider decreasing P. Yes, the swing will be wider, but you'll save some money on the bill.
• For big rooms, consider increasing P if your HVAC unit runs too long.

Fighting "Stale" Air

If your HVAC unit is stopped for extended periods of time (this happens mostly in mid-season, when ambient temperature is close to your setpoint), the air in your house starts feeling "stale". To avoid this, increase I. Be careful with this value - make sure you have saturation limit set to something other than zero, and PID signal graphs enabled so you can diagnose a problem should you accidentally remove a few extra zeros from the I value.

Magnitude of change

• Sane values of P seem to lie between 0.25 and 10
• Start with default value for I (above) and multiply or divide it by 10 to get the next candidate for consideration. Did I mention be careful?