Wood Heating
System Design
Control Logic
Software Design
Electrical Schematic
Simple system w/ storage
Domestic Hot Water
Heat Storage
Solar Hot Water
System Components
User Guide
Programming Guide
Failsafe Design
Sample Application
LM35 Sensor Assembly
Pinout Info
Poor Man's VS Circ
Plastic Pipe Collector
Forum Solar-TodayWood-TodayBurn Planner
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Timer Rules

Timer rules set a variable to a value (in seconds) when the specified condition is met. The target variable will automatically count down each cycle. The template is as folllows:

Set Element1 for Element2 seconds when Element3 (is / becomes) (true / false)

Element1 must be a variable. It is given the value of Element2 when the specified conditions are met. Element1 should not be used as the target of any other rule. Variables that are the target of timer rules are decremented every cycle of the control process until they reach zero.

Becomes vs. Is

In timer rules, the rule can be triggered when a specified variable IS true (or false), or when it BECOMES true (or false). This creates two very different behaviors.


If the rule uses IS as the criteria, then the timer will be reset to the specified value every cycle that the chosen element has the specified value. This is useful if you want to time from the end of a condition. For instance, you might want to keep track of whether the wood boiler could still produce heat. Simply measuring output temperature isn't adequate, since it will fluctuate over time. A reasonable rule might be to consider that the wood boiler is out of fuel if its outlet temperature stays below 160 degrees for 30 minutes. This requires a set of rules, one of which is a timer rule. The first rule we need is a rule to determine whether the wood boiler is hot. We'll create a differential rule:

Set WoodBoilerHot if WoodBoilerOutlet is at least ZeroDegrees more than WoodBoilerMinimum with a deadband of CoupleDegrees

This sets a variable (WoodBoilerHot) that tells us whether the wood boiler is hot or not. Now we can set a timer based on the wood boiler outlet being hot:

Set WoodHeatTimer for BurnoutTime seconds when WoodBoilerHot is true

As soon as the wood boiler outlet temperature exceeds WoodBoilerMinimum, the first rule sets WoodBoilerHot to true. It will stay true as long as the wood boiler outlet is hot enough, although it may not be true for a few minutes now and then if a slug of cold water is introduced from an inactive zone.

As soon as WoodBoilerHot is true, WoodHeatTimer gets set to the value of BurnoutTime - we'll say 1800 seconds. It will keep getting set to 1800 seconds every cycle as long as WoodBoilerHot remains true. As soon as WoodBoilerHot is no longer true, WoodHeatTimer will start to count down, reaching zero if WoodBoilerHot remains false for 1800 consecutive seconds.


The other behavior for timer rules is BECOMES. In this case, the timer is set when the specified element makes a transition to the specified value. It then counts down, only resetting if the element makes another transition to the specified value. Consider an example where you want to do something different during the time that the boiler is first coming up to temperature. A thermocouple in the flue could detect a rise in flue temperature and trigger a timer:

Set FlueHot if FlueTemperature is at least SeveralDegrees more than RoomTemperature with a deadband of CoupleDegrees

This tells us that the flue is enough above room temperature to indicate that a fire has been started (or is in progress).

Set FireStartTimer for StartUpTime seconds when FlueHot becomes true

The FireStartTimer will be set as soon as the flue temp climbs above the threshold. It will then count down regardless of whether the temp stays above the threshold or drops below again. The only way it would be reset is if the temp drops below the threshold and then rises again.

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