This article applies to all ecobee thermostats except SMART and EMS.
ecobee thermostats require a C (common) wire to power on. If you only have two wires at your current thermostat, there are a few installation options available, depending on your application.
Note: You will not be able to install the Power Extender Kit (PEK) with heat-only 2-wire systems. The PEK can only be used on furnace or heat pump systems that do not have a C wire — this is because the PEK requires R (power), G (fan), and Y (compressor) wires to operate, and these 2-wire heat-only systems do not have these G and Y wires.
Firstly, take a look at your heating systems control board to see its wiring connections
Warning! This step requires you to work with wiring at your HVAC Control Board. If you are not comfortable working with your HVAC wiring, contact ecobee customer support (1-877-932-6233) or hire a professional installer.
To better understand your choices and what options will be applicable, you'll need to look at the two wires that come from the thermostat and where they connect to the furnace/boiler system—this is most important!
If your furnace/boiler connections have something similar like these (R & W or TH/TH), then installation option 4 (FAST-STAT COMMON MAKER WITH R/W contacts) is the best choice:
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| Furnace/boiler control board | Gas Valve |
If your furnace/boiler connections look like the ones below (T/T), then installation options 3 (EXTERNAL TRANSFORMER WITH ISOLATION RELAY) or 5 (FAST-STAT Common Maker WITH T/T contacts) are best:
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| Oil Primary Control | Aquastat |
OPTION 1. RUN A NEW WIRE
Option 1 involves running a third wire to use as your C (common) from your furnace or boiler to your thermostat. This is applicable for non-T/T furnace or boiler connections. The connections that are needed at the furnace or boiler for this option are R, C, and W.
Below is an example of the wiring.
Note: The R wire must go into RC instead of RH.
If you find your system does not have a C (common) terminal, you will need to identify where the common can be connected to. Every furnace and boiler that has a 24VAC transformer will have a common terminal—just trace the 2 low-voltage wires and see where they connect to. If there is an existing connection point, then tap the C wire coming from the thermostat at this point. Sometimes cutting/stripping the common wire for splicing the thermostat's C wire may also be required.
Always make certain you are connecting to the (secondary) low voltage side and NOT the (primary) line voltage side of the transformer—otherwise immediate damage will result! We recommend consulting with a licensed contractor if you're not comfortable re-wiring with high voltage equipment.
OPTION 2. EXTERNAL TRANSFORMER
The plug-in transformer will power on the thermostat from a nearby plug outlet near the thermostat's location. This will require the two wires from the plug-in transformer to be fed to the thermostat location so that the ecobee can power on.
Your two existing wires would go to Rh and W1 and the two wires from the transformer would go to Rc and C. Below is a diagram illustrated how the ecobee would be wired in this scenario (ecobee3 backplate shown). Note: The R wire from the external transformer must go into RC instead of RH.
OPTION 3. EXTERNAL TRANSFORMER WITH ISOLATION RELAY
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| 24VAC plug-in transformer | Isolation Relay |
Below is a diagram illustrated how the ecobee would be wired in this scenario (ecobee3 backplate shown). Note: The R wire from the external transformer must go into RC instead of RH.
OPTION 4. FAST-STAT COMMON MAKER WITH R/W contacts
Option 4 entails using your existing two thermostat wires and installing a FAST-STAT Common Maker for furnace/boilers with R & W connections. You will not be required to run a new wire from your thermostat to your furnace/boiler.
Below is a diagram illustrating how the ecobee would be wired with the common maker in this scenario.
Note: The R wire must go into RC instead of RH.
A C (common) wire must be identified at the furnace/boiler system for the FAST-STAT to connect to. If you find your system does not have a C (common) terminal, you will need to identify where the C wire can be connected to. Every furnace/boiler that has a 24VAC transformer will have a common terminal—just trace the 2 low-voltage wires and see where they connect to. If there is an existing connection point, then tap the FAST-STAT C wire at this point. Sometimes cutting/stripping the common wire for splicing the FAST-STAT C wire may also be required.
Always make certain you are connecting to the (secondary) low voltage side and NOT the (primary) line voltage side of the transformer—otherwise immediate damage will result! We recommend consulting with a licensed contractor if you're not comfortable re-wiring with high voltage equipment.
OPTION 5. FAST-STAT Common Maker with External Transformer
Option 5 entails using your existing two thermostat wires and installing a FAST-STAT Common Maker for furnace/boilers with T/T connections. You will not be required to run a new wire from your thermostat to your furnace/boiler.
Below is a diagram illustrating how the ecobee would be wired with the Common Maker and plug-in transformer in this scenario.
Note: The R wire must go into RC instead of RH.
This option is best for when a C (common) wire can't be identified at the furnace/boiler system for the FAST-STAT to connect to, or if you have T/T connections at your equipment.
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