Isolation relay for Ecobee smart thermostat: NC or NO

relaythermostat-c-wire

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.
SPST isolation relay

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?

Best Answer

You're correct, you'll use the normally open (NO) terminal. When the thermostat calls for heat, it will energize the relay coil. The energized coil will close the contact in the relay between common (C, COM, etc.) and normally open (NO).

When the coil is not energized, the contact connects the common (C) terminal to the normally closed (NC) terminal. When the coil is energized, the contact connects the common (C) terminal to the normally open (NO) terminal.

NOTES:

  • When looking at schematics/wiring diagrams, all the components will be shown in the "normal state" ("shelf state"). That is to say, they'll be shown as if they were laying on a storage shelf, with no power sources connected to them.

The relay you linked to should work, however, it's probably a bit overkill. If you're only switching the thermostat terminals from a boiler, you're probably talking about milliamps @ 24 VAC. The relay in the link can switch 10 amps @ 240 VAC.

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