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Rules to Change Variable Values

While it may be tempting to assume that the control system is done at this point, there are still rules to change the top and main floor setpoints based on whether heat is coming from oil or wood..

There are two options to resolve this:

  1. Add another condition to each zone valve rule, requiring that backup disable be true before the NFCS will open the zone valve. This will work, and will result in the zones being managed by the thermostats when the wood boiler is out.

  2. Add some logic to automatically select different setpoint values depending on whether heat is coming from wood or oil.

While either approach will work, the second choice is preferable, in part because we may elect to implement time-of-day setback at some point and this will lay the groundwork for that.

Variables

Well need variables to hold the temperatures that we want to use for the comfortable and cool setpoints. We'll call them 'Comfy' and 'Cool' and give them values of 72 and 65.

We want our new setback rules to be based on whether or not the wood boiler is hot. At present, we don't have a variable that keeps track of that condition. We set 'Backup Disable' based on wood boiler temperature, but using that to determine our setback isn't a good idea - we might elect to disable the backup heat source for other reasons. For instance, once we've added heat storage we'll likely want to disable the backup boiler whenever there's heat available in storage.

The best approach is to create a variable to keep track of whether the wood boiler is hot - 'Wood Boiler Hot' would be a reasonable name, and we'll give it an initial value of zero..

We'll create these variables using the Data Elements tab.

Rules

Next, we'll want to create a rule that sets the value of 'Wood Boiler Hot'. That will be the exact criteria that we're now using to set 'Backup Disable'. The easiest thing to do is merely change the existing rule. Select 'Wood Boiler Hot' instead of 'Backup Disable' for the first field and update the rule.

Now there's no rule that sets 'Backup Disable', but we'll take care of that in a minute.

First, we'll create rules to set our top floor and main floor setpoints to 'Comfy' if the wood boiler is hot. These will be logical rules, and they'll look like this:

Set Top Floor Setpoint to Comfy if Wood Boiler Hot is true

Set Main Floor Setpoint to Comfy if Wood Boiler Hot is true

That takes care of the situation where the wood boiler is hot. We need another pair of rules to set the setpoints to 'Cool' if the wood boiler is not hot:


Set Top Floor Setpoint to Cool if Wood Boiler Hot is not true

Set Main Floor Setpoint to Cool if Wood Boiler Hot is not true



Finally, we need to replace the rule that set the 'Backup Disable' output. That can also be a logical rule based on the 'Wood Boiler Hot' variable:

Set Backup Disable to TRUE if Wood Boiler Hot is true





These rules will manage the heating system as desired, maintaining the living space at a higher temperature when the wood boiler is operating. We could have more elaborate versions of these rules - different temperatures for each floor, for instance.

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