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?

  • did you end up configuring it? i'm struggling with a similar setup right now – Radu Bogdan Oct 12 '17 at 22:10
  • It was pretty straightforward, wiring just like the ecobee instructions. The only hard part was selecting the right components, and only because there are so many choices. In the end, most of the options work fine though. The linked relay works great, and I found a 120V -> 24V transformer for ~$15. Running 25 ft. of 3-wire thermostat cable and installing the transformer/relay took about an hour total, was up and running that same afternoon. – Dustin Wheeler Oct 16 '17 at 15:52

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.


  • 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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It's best to confine yourself to Code legal components, at least on the 120V side. Any 120V wiring must be contained inside a Class I wiring method, i.e. inside junction boxes.

For instance many excellent transformers suitable for powering an Ecobee are made built into a junction box cover, or mount to a 1/2" knockout hole. When installed in the obvious way, the result is the 120V wiring stays inside the junction box, and the 24V wiring is outside.

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