System With Parallel Wood Boiler
In this step a wood boiler has been plumbed in parallel with the oil boiler. The wood boiler is added to the left side of the diagram. The rest of the system is the same as it was before.
The parallel plumbing allows either boiler to provide heat. The two circulators have built-in check valves to prevent reverse flow through either boiler.
The wood boiler has a 'dump zone' and inlet protection valve to provide safe and effective operation. Neither of these devices require any external control.
The wood boiler has a built-in controller that turns on the wood circulator when the wood boiler outlet temperature is above 160 degrees. The built-in controller also shuts down airflow to the firebox when the outlet temperature exceeds 180 degrees.
Desired System Behavior
The intent is to use the wood boiler as the primary heat source, and keep the oil boiler as a backup heat source for times when no one is home for long enough that the wood boiler goes out. At this point, it's good practice to spell out the control behavior that's desired as explicitly as possible. This list of desired behaviors becomes the basis for designing the details of the control system.
Control System Issues
There are two primary issues with controlling this system. First of all, there is nothing to prevent the oil boiler from coming on. When any zone calls for heat, the oil boiler will come on even if the wood boiler is running. Since the wood boiler is providing lots of heat, the oil boiler won't have to run as long, but it will still run as needed to maintain its own temperature at 180 degrees. There needs to be a way to disable the oil boiler when the wood boiler is able to provide heat.
Second, there's no provision for keeping the living space at a warmer temperature when heat is coming from the wood boiler. The thermostats don't have any way to operate at a different setpoint depending on the heat source.
Thermostats and Temperature Sensors
Most heating systems use thermostats. They are relatively simple devices that are easy to use. However, they do have one significant limitation - they can only tell you 'warm enough' or 'not warm enough'. Some thermostats do have a second stage output that will provide a second contact closure if the temperature drops more than a certain amount below the setpoint. This is a little better, but still not as useful as knowing the actual temperature. Temperature sensors allow control decisions to be made based on actual temperature.
To enable this system to control the living space temperature based on the heat source, temperature sensors will be added on the top floor and the main floor so that the NFCS can monitor temperatures directly.
The existing thermostats will be retained, but they will be set to a much lower temperature and used only to control the backup oil boiler.