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Heating System Design Summary

At this point, there is a plumbing design for the heating system as well as an electrical design for the control system. There is a high-level description of how the system should behave, although that definition is filled with vague terms such as 'comfortable'. Here's a discussion of the desired system behaviors with a brief description of how each could be accomplished.

  1. The system should use heat from the wood boiler if heat is available.

    This happens automatically. The wood boiler's built-in controller will turn on the wood boiler circulator whenever the wood boiler is hot.

  2. The oil boiler should not run if the wood boiler is capable of providing heat.

    The 'backup disable' relay will accomplish this. However, there will need to be some way for the NFCS to know that the wood boiler is capable of providing heat. A temperature sensor will be added to measure the temperature of water at the wood boiler outlet. One or more rules will be needed in the NFCS to accomplish this behavior.

  3. The system should manage the zone valves to keep the top floor and main floor temperatures at a comfortable temperature.

    The 'force' relays for each zone will allow the NFCS to control heat delivery. The top and main floor temperature sensors will provide the information necessary to manage temperatures. One or more rules will be needed in the NFCS to accomplish this behavior.

  4. The system should maintain the domestic hot water temperature according to the aquastat in the DHW tank.

    As with the top and main floors, the 'force' relay will allow the NFCS to manage the DHW. The 'DHW demand' relay provides a signal that allows the NFCS to sense when the DHW aquastat is calling for heat. One or more rules will be needed in the NFCS to accomplish this behavior.

  5. If the wood boiler is not hot, the system should automatically switch to provide heat from the oil boiler.

    This can be accomplished by turning off the 'demand disable' relay. This is really the flip side of requirement #2.

  6. If heat is coming from the oil boiler, the living space temperature should be maintained at a lower level.

    This will require rules in the NFCS. All of the information needed for these rules is available from the various temperature sensors.

  7. If the NFCS is not connected or is not running, heat should be provided by the oil boiler

    This is provided by the failsafe control box. No rules or NFCS functions are required.

Control System Block Diagram



NFCS Inputs and Outputs

These are the inputs and outputs from the NFCS as determined by the current system design:

Temperature Sensor Inputs

  1. Top Floor temperature

  2. Main Floor temperature

  3. Wood Boiler Outlet temperature

Discrete Inputs

  1. Domestic Hot Water heat demand (from DHW aquastat)

Discrete Outputs

  1. Top Floor Force Relay Coil

  2. Main Floor Force Relay Coil

  3. Hot Water Force Relay Coil

  4. Backup Disable Relay Coil

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