ST-2E to NCM-300 - Extra wires

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Joined: Wed Apr 19, 2023 7:38 pm

ST-2E to NCM-300 - Extra wires

Post by ajcast5201 »

Hi everyone,

After reading other experiences regarding the upgrade from the ST-2E to the NCM-300, I decided to give it a shot myself and save some change. I've received the new controller and am accounting for each wire on the current setup, as I understand it to be a 1:1 swap.

On the thermostat side, I've had no issues matching wires. The same can be set for the damper motors. However, when looking at the transformer and system areas, there is what looks like a jumper Transformer 1 to System R. There are also two wires in Transformer 2. Additionally, there are two wires in System Y. I've attached pictures.

Full transparency - I had someone install two Nest thermostats, and they were able to work by adding a jumper to the O/B ports on the right side of the ST-2E (I have to switch the wire when changing from heat to cool or vice versa).

Thanks in advance. Looking forward to learning more about thermostat wiring.
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Joined: Sat Jul 08, 2023 2:14 am

Re: ST-2E to NCM-300 - Extra wires

Post by delam0 »

The first thing to note is the jumper wire you mentioned from Transformer 1 to System R. This jumper wire ensures that the system receives the necessary power from the transformer. So, when you swap to the NCM-300, you should also connect the R terminals of the new transformer and system using a jumper wire.

Regarding the two wires in Transformer 2, it's important to identify their purpose. These wires are likely connected to different components in your HVAC system.
Similarly, the two wires in System Y need to be identified to understand their role in your system. Y wires are typically associated with cooling systems, so you may have a dual-stage cooling system or some other configuration that requires two wires. Make sure to note down their connections and replicate them accordingly when installing the NCM-300.

Since you mentioned using Nest thermostats previously, it's worth noting that the O/B ports on the right side of the ST-2E are used for heat pump systems. The jumper wire you added allowed you to switch the wire when switching between heating and cooling modes. When installing the NCM-300, you should consult its documentation or contact the manufacturer to determine the appropriate wiring for your heat pump system.
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