ecobee

Heat only 2-wire boiler/furnace installations

This article applies to the ecobee3, ecobee3 lite, and ecobee4

 

The ecobee requires a C (common) wire to receive a constant 24VAC and remain powered on. If you only have two thermostat wires at your current thermostat, 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 thermostat 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.

terminals_R_W.PNG     2-wire_heat_only_1.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_4.PNG  2-wire_heat_only_t-t_3.PNG

 

 

Option 1 entails running a third wire to use as your C from your furnace/boiler to your thermostat. This is applicable for non-T&T furnace/boiler connections. The connections that are needed at the furnace/boiler for this option are: R, C and W.

Below is an example of the wiring.

* When installing the ecobee3 lite or ecobee4, the R wire will go to RC instead of RH.

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 common wire coming from the thermostat at this point. Sometimes cutting/stripping the common wire for splicing the thermostat's 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! We recommend consulting with a licensed contractor if you're not comfortable re-wiring with high voltage equipment.

 

transformer3.PNG

Option 2 entails running a third wire, as well as installing an external 24VAC transformer with an isolation relay. This is applicable for a furnace/boiler with T/T contacts.

A standard 120/24VAC 20VA step-down transformer as well as an isolation relay 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

 

Below is a diagram illustrated how the ecobee would be wired in this scenario.

* When installing the ecobee3 lite or ecobee4, the R wire will go to RC instead of RH.

 

E3 with Isolation relay.jpg

common-maker.PNG

 

Option 3 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 illustrated how the ecobee would be wired in this scenario.

* When installing the ecobee3 lite or ecobee4, the R wire will go to RC instead of RH.

ecobee3_FastStat_R_W.PNG

A C-wire must be identified at the furnace/boiler system for the Fast-Stat 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 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 transformer—otherwise immediate damage will result! We recommend consulting with a licensed contractor if you're not comfortable re-wiring with high voltage equipment.

 

transformer3.PNG

common-maker.PNG

 

Option 4 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 illustrated how the ecobee would be wired in this scenario.

* When installing the ecobee3 lite or ecobee4, the R wire will go to RC instead of RH.

 

ecobee3_FastStat_T_T.PNG

 

A C-wire must be identified at the furnace/boiler system for the Fast-Stat 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 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 transformer—otherwise immediate damage will result! We recommend consulting with a licensed contractor if you're not comfortable re-wiring with high voltage equipment.

 

transformer3.PNG

 

 

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