I changed my old 'round Honewell with mercure' thermostat 2 wires with a Nest 3rd generation. I plugged the old 2 wires 24AC on Rh and W1 of NEST. But, there a problem :
The first time the system runs all good, as described with the following step below:
- In usual mode, the thermostat Honewell closes is own contact up to reach the temperature, but the relay of the furnace operate in safety mode when the furnace is overloaded : It means, when the furnace runs to much longer it needs to cool down and the relay of the furnace seems to bypass the closed contact of the Honewell thermostat.
- It means, with this old technology as solid as a rock we don't see the well job done realized by the relay of the furnace.
Here the problem when overload appears :
- The NEST is directely powered by the 2 wires from the relay as 24AC. But when the furnace is overloaded, his own relay bypass the NEST and 'cut the power' then appears the error like 'no power e74'.
- My understanding is when the furnace is overload, I have this message.
- I disconnected/reconnected the NEST from the socle and the message was always there. Few time later, I do the same and all seems go back to the normal : I suspect the furnace relay go back to the initial mode as relay works in normal mode with secure mode in background in case...
How to prevent the futur :
- The first case when the e74 appears should be : I do nothing, let the message on the screen up to the furnace relay go back to the normal mode, and it means delivery the 24AC from his own relay. At this step I don't know if the NEST will work properly after, even if it works yet on his battery. Do someone can enlight me of the NEST behavior?
- The second case not should be : if after a overload furnace we need to act manually on the NEST to make disappear the e74 or reset the NEST, it will be a big problem as like holiday periods. In this case, the NEST is an awful device. At this step what is the procedure for the NEST runs normally after this e74?
- The third case : to prevent the error e74 when the furnace is overloaded and runs on his own security mode, we should use a separate power. At this step, what kind of wiring we should do on the NEST? I have only choice to use an external power like transformateur 110/12 to bypass the power from the relay.
If someone can enlight me on this issue, he will make my day and my christmas !
Best regards,