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Navien NFB-C compact commercial fire tube boilers

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Intuitive 7" touch screen control for easy set up and diagnostics.

Built-in hardware for boiler pump, 3 heating zones or indirect tank control.

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Another example is found in Figure 2 showing a relay that powers a blower.

has the voltage supplied from the circuit board run through field installed dry contacts for both the low voltage (for thermostats and such) and for high voltage contacts (for pumps and motors and such).

Figure 3 shows how I wired the system. On this job there was an existing fan centre relay (FCR) that energizes the system pumps and provides the 24 volts for the thermostat circuit. The relay in the fan centre has a normally closed contact and a normally open contact. The normally open contact is the dry contact that I used for the 120v pumps. So, one side of the contact is fed 120v, and when it closes it travels through the terminals to energize the pumps. That contact closes when the “G” terminal is energized.

I left the pump circuit wired through the FCR and added a single-pole single-throw (SPST) 24v relay for the low voltage (thermostat) call for heat circuit. I used terminals 2 and 4 on the relay as the dry contacts the manufacturer requires to energize the call for heat.

The FCR becomes the dry contacts to energize the system pump, and the new SPST relay I installed becomes the dry contacts for the low voltage call for heat circuit.

IS THERE SUCH A THING AS “WET CONTACTS?”

There is such a thing as wet contacts, but they aren’t normally referred to as being wet. Wet just means that the contacts indicated on that equipment have power. Wet contacts are typically powered by the boiler (or other unit) to energize field installed controls and equipment such as thermostats, motors and pumps.. Wet contacts can be hot all the time, or energized by the unit when required. Most high limit controls, most fan limit controls and most pressure switches have control voltage running to them and then through them. Thermostats are wet contacts. They get their power from the circuit board and it directly influences the activity of the circuit, from the R terminal on the board, up to the R terminal on the thermostat, to the W terminal on the thermostat, and back to the board to call for heat. Wet contacts are powered contacts that directly power the equipment.

Not All Manufacturers Wire The Same

The manufacturer for the boiler on this particular job uses wet contacts, but they don’t want us to use their low voltage and high voltage wet contacts to directly energize the thermostat circuit or the relay circuit for the system pumps. They fear the additional field wiring will be too taxing on their circuit board. The manufacturer

As you can see by Figure 3, I made sure the thermostat circuit energized both the “G” terminal on the FCR as well as the coil on the SPST relay I mounted on the wall. The dry contacts on the SPST relay complete the circuit in the boiler to call for heat and the dry contacts in the FCR energize the system pump. The dry contacts, in essence, become the thermostat for the boiler.

Other Alternatives

Remember, wet contacts mean the equipment supplies the power. A possible application for wet contacts in a boiler might be a set of terminals to run a circulator pump for the DHW storage tank. The boiler control will energize that terminal when the aquastat calls for heat. If there are two terminals in that circuit, one will be the hot, and the other may be the neutral for the pump. Either way, the manufacturer runs the low voltage load (thermostats, aquastats), and/or the high voltage load (pumps, motors) right from the boiler. That is what wet contacts typically do. Alternatively, some manufacturers use dry contacts in their terminal strips. Again, the manufacturer does not provide power to dry contacts, you have to provide the power to the terminals for your load. An example is when there is a low voltage signal for space heat, the circuit board on a boiler might close a set of dry contacts on the terminal strip that will be rated for 3 amps at 120v. So, you apply 120v to one terminal and you can wire the other terminal to the pump. Be sure the neutral side of the pump is wired to the boiler as well. When the call for heat is terminated, the circuit board will open the contacts shutting down the pump.

When you find yourself navigating a boiler wiring project, you need to read the instructions carefully to identify which terminals are dry and which terminals are wet, and how the manufacturer wants them wired.