Heat only 2-wire boiler/furnace installations

The ecobee3 requires a C(Common) wire to receive a constant 24VAC. If you have only 2 thermostat wires, there are a few options to complete your installation depending on your application.

1) Run a new thermostat cable with a minimum 3 conductors (RH, C, W1)

2) Run a new thermostat cable with a minimum 3 conductors (RH, C, W1), add an external 24VAC transformer and Isolation relay for systems with T/T contacts

3) Use a Fast-Stat Common-Maker with the existing system 24VAC transformer and thermostat cable for TH/TH or R & W furnace/boiler connections

4) Use a Fast-Stat Common-Maker with the existing system 24VAC transformer and thermostat cable for T/T furnace/boiler connections.

 

To better understand your choices and what options will be applicable you'll need to look at the two wires that come from the t-stat and where they connect to the furnace/boiler system....this is most important.

 

If your furnace/boiler connections have something similar like these (TH/TH or R & W) then option 3 is the best choice.

2-wire_heat_only_1.PNGterminals_R_W.PNG

 

 

If your furnace/boiler connections look like these ones circled then option 2 or 4 are best suited.

2-wire_heat_only_t-t_3.PNG2-wire_heat_only_t-t_4.PNG

 

 

Option 1)

Standard 3-wire application for non T/T furnace/boiler connections. Connections that are needed at the furnace/boiler for this are: R, C and W. Here is the applicable wiring configuration.

ecobee3 - 1-stg heat only.jpg

If you find your system does not have an C(Common) terminal you will need to identify where the common can be connected to. Every furnace/boiler that has a 24VAC transformer will have a Common. Just trace the 2 low-voltage wires and see were they connect to. If there is an existing connection point then tap the ecobee3 common wire at this point. Sometimes cutting/stripping the common wire for splicing the ecobee3 common 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 transformer otherwise immediate damage will result.

transformer3.PNG

 

 

Option 2)

A standard 120/24VAC 20VA step-down transformer and an Isolation relay are needed and can be purchased from an HVAC Wholesaler. These come in different configurations/sizes but will do the same thing.

Figure 1. Transformer

transformer.PNG 

Figure 2. Isolation relay

Isolation relay.JPG

 

And here is how it would be wired with the ecobee3 using these components.

E3 with Isolation relay.jpg

 

 Option 3)

Use your existing 2 thermostat wires and Fast-Stat Common-Maker Fast-Stat (website: http://www.fast-stat.net/) for furnace/boilers systems with R & W connections.  You will not be required to run a new thermostat cable from t-stat location to furnace/boiler location.

ecobee3_FastStat_R_W.PNG

common-maker.PNG

A C-wire must be identified at the furnace/boiler system for the Common-Maker to connect to. If you find your system does not have an C(Common) terminal you will need to identify where the common can be connected to. Every furnace/boiler that has a 24VAC transformer will have a Common. Just trace the 2 low-voltage wires and see were they connect to. If there is an existing connection point then tap the Common-Maker C-wire at this point. Sometimes cutting/stripping the common wire for splicing the Common-Maker 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 transformer otherwise immediate damage will result.

transformer3.PNG

 

Option 4)

Use your existing 2 thermostat wires and Fast-Stat Common-Maker (Fast-Stat website: http://www.fast-stat.net/)  for furnace/boilers systems with T & T connections.  You will not be required to run a new thermostat cable from t-stat location to furnace/boiler location.

ecobee3_FastStat_T_T.PNG 

common-maker.PNG

A C-wire must be identified at the furnace/boiler system for the Common-Maker to connect to. If you find your system does not have an C(Common) terminal you will need to identify where the common can be connected to. Every furnace/boiler that has a 24VAC transformer will have a Common. Just trace the 2 low-voltage wires and see were they connect to. If there is an existing connection point then tap the Common-Maker C-wire at this point. Sometimes cutting/stripping the common wire for splicing the Common-Maker 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 transformer otherwise immediate damage will result.

transformer3.PNG