How to navigate wiring a boiler

ELECTROMECHANICAL CONTACTOR DE-ENERGIZED

On a recent service call I was replacing an instantaneous water heater with a combi-boiler. The boiler has low voltage terminals to engage the call for heat, and high voltage terminals to engage system pumps. But the manufacturer requires that the installer use field-installed “dry contacts” to engage those terminals when energizing the heating cycle. Dry contacts have a power source going through them that is independent of the control circuit. It’s like a contactor or relay. You have control voltage that controls when the contactor or relay coil is energized, and you have the dry contacts that close because of the action of the coil.

When the coil closes the dry contacts a load voltage runs through those dry contacts to energize a motor, valve, compressor, or other components. Again,

Electromechanical Contactor Energized

the control voltage that energizes the coil on the relay is independent or separate of the load voltage that actually energizes the equipment.

For example, a contactor in an A/C condenser has a 24v coil and a set of dry contacts. When the coil is energized from the furnace or air handler (as indicated by the control signal in Figure 1) it closes the contacts, which are wired independent of the 24v that energized the coil. Now one of the screw terminals on the dry contacts will have the line voltage (as indicated in Figure 1, right), which in this case is 120v. When the coil closes the dry contacts, the voltage is carried through to the other terminal, which is the load terminal shown in Figure 1), and energizes the compressor. It’s a switch that turns on equipment, but the switch is controlled by a different voltage source.