Articles » Relays - Simple and Inexpensive Control
Relays are in essence really a lot like a light switch that is turned on and off by another device. The use of relays in heating controls can allow us to introduce logic into how and when certain parts of the system run. Relays designed specifically for the heating industry tend to be rather expensive, but less expensive relays can be purchased at electical/electronics supply stores.
The coil acts as a switch which can turn the current on or off to a given device (or switch between two different devices). The relay has a ‘common’ (C) terminal. When the coil is not powered, the C terminal is connected to the ‘Normally Closed’ (NC) terminal. When power is applied to the coil, the C terminal is then connected to the ‘Normally Open’ (NO) terminal.
One application would be to have the circulator on your wood boiler charge the coil on the relay which would disconnect the power (or thermostat control) to your fossil fuel boiler to ensure that it does not run. You can also use inexpensive relays in place of expensive heating ones by having a thermostat charge the relay and turn a zone valve or pump on and off.
With enough relays in your system you can actually create a good deal of logic. Relays have different current and amp ratings for the coil and for the switch. You must always make sure that you choose a relay that is rated for your specific application. For instance, in the example above of using a thermostat to turn on or off a zone pump, the coil would need to be rated for 24vac (assuming you use a 24vac thermostat) and the other portion of the relay would need to be rated for 110vac and have enough amp rating to handle your pump. (I like to use 12amp relays for this application to account for the amp spike that occurs when the pump is first engaged.)
The RIB RIBU1C SPDT Relay
This relay is very versital as it can use either 24vac or 120vac on the coil and independantly use either one on the NO and NC side of the relay. Since these are the most common currents used in heating systems, this device would work well. It is prewired and mounts to the whole of a standard junction box. It is rated for 10amps and includes a light indicator to know when it is charged or not.