I've been looking for "isolation relays" to install an Ecobee 3 smart thermostat on my 2-wire T/T furnace (replacing a Honeywell round thermostat). I know I want to use the 24VAC transformer/relay method (option 2) detailed on the Ecobee site, but I can't seem to find any details on the isolation relay required. Looking at the pictures, the relay is a SPST relay (see image), but it has two sets of contacts, one for NC and one for NO setups. I'd like to know what the proper default is for this situation.
My instincts say NO (I assume closing the loop tells the furnace to turn on), but I won't have a chance to check the setup (2 hours away) for a month. Would a SPST-NO relay like this be appropriate, or am I going down the wrong path?