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
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Failsafe Operation

While not absolutely necessary, it would be very desirable to make sure that our oil backup system works whether or not the NFCS is running. This is most easily done by adding a relay box to the system, separate from the NFCS.


The thermostats should provide a demand signal to the oil boiler even if the NFCS is not present or not operating. The NFCS also needs to be able to operate each zone valve. To accomplish this, a relay will be added to each thermostat circuit. This will be referred to as a 'force' relay, as its purpose is to allow the NFCS to force a zone valve to open even if the thermostat isn't calling for heat. Since the thermostats will be set to a lower temperature and used only for control of the oil backup heat system, the force relays will be the normal method of providing heat to the heating zones.

This relay is installed so that when it is unpowered, the thermostat is connected to the zone valve motor just as before. When power is applied to the relay, the contacts supply power directly to the zone valve motor. The relay coil is powered by a discrete output of the NFCS. The same approach will be taken with the Main Floor and Hot Water zones.

Demand Signal

There must also be a way to prevent the demand signal from reaching the oil boiler when the wood boiler is able to provide heat. This signal is created by the zone valve limit switches.

To disable this signal, another relay will be added. When unpowered, it will allow the demand signal to go to the oil boiler. When powered, that signal will be interrupted. This 'backup disable' relay will controlled by the NFCS.

The left side shows the original demand signal wiring: any zone valve limit switch closing connects the oil boiler demand contacts, telling the oil boiler that there is a demand for heat.

The right side shows the new configuration. The 'backup disable' relay breaks the demand circuit path, preventing the oil boiler from seeing a demand signal.

All these relays are installed in a separate enclosure that contains only relays. This is called the 'FailSafe Controller'. It allows for normal operation of the oil backup heat source in the event that the NFCS is not present or is not operating.

Putting it all together, here's a schematic of the complete FailSafe Control Box. Items in blue are controlled by the NFCS.

Note that there is one additional relay labeled 'DHW Demand'. This relay is powered by the hot water tank's aquastat. Its contacts are connected to an NFCS discrete input so that the NFCS can tell when the hot water tank is calling for heat.

The FailSafe box will be mounted next the NFCS. It will have connectors for thermostats, zone valves, discrete inputs and outputs from the NFCS, and a connector that will go to the oil boiler demand contacts.

The following photograph shows a partially wired FailSafe box intended for a more complex application. The small Molex connector is 24Vac power. All other I/O is through standard RJ-45 and RJ-11 connectors.

Some relays have 12Vdc coils and are powered by the NFCS. Others have 24Vac coils and are powered by thermostats, aquastats, and other remote devices. The relays are in sockets mounted on DIN rails for easy service. The relays in this particular application have manual test capability and visual indicators to show the relay state.

FailSafe Control Box

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