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Nest 3rd generation thermostat with error e74 error

Community Member

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,


Community Member

The next tomorrow seems really instructive about the error e74  and this is my understanding about what's happened :

  • As explained, I used the 2 wires 24AC from the old thermostat 'round mercure' and I connected them to Nest 3Gen Rh And W1 . In fact the power is coming from the furnace relay with this only 2 wires.
  • Here's the logic. The furnace turns with 2 interruptors : (R) is the relay of the furnace, and (T) is the thermostat.
  • When the target of the temperature is perfect, (R) is close and delivery the power with the 2 wires, and (T) is open because the temperature is reached, then the furnace is doesn't running. 
  • When the temperature goes below the target, (R) is close and delivery the power with 2 wires, and (T) is close, then the furnace runs.
  • When (R) is close and (T) is close, the furnace runs BUT if the furnace is overloaded before to reach the target temperature THEN (R) is open to bypass the (T) which is closed. In this context, the Nest lost the power and runs with is own battery. This is the safe mode of the combination furnace/relay.

What I saw this morning :

  • The Nest bottom indicates 'The power cord that connects your equipment to the thermostat may be unplugged' (its a translation language/english) with 2 tools symbols. I didn't touch the thermostat at all.
  • I lauched the Nest Apps Android and the first screen was the symbol of the house with the message OFFLINE.
  • I touched the screen and the message was 'POWER SUPPLY OF ERROR' with E74: No power to the Rh wire.
  • I done absolutly nothing because I suspected the (R) was opened as security mode and it's means not powering the Nest, and I decided to wait maximum one hour to see if all will goes back to normal.
  • The time between (R) close, then (R) open, then (R) close was around 30 minutes and this time is done by the relay.

Now, I see that as a 'power outage' and when we have a power outage in our area nothing works in the house. But I'm concerned by the following questions :

  • We are living in area where the winter is very cold and sometime the electric networks falls down. If the power outage is above the capacity of the Nest battery, what's happening when the power goes on? Does it's existing a default value in this kind of situation? I believe so, but I'm looking for the answers.
  • At this moment, I'm suspecting that I will have a 'power outage' every day. My understanding is the battery will be used and will be replaced more often. Or it's be better to power the Nest by others methods as external 24AC C-Wire or USB side?
  • What is the new cabling on the socle if I use 24AC C-wire? Does Someone have an answer?
  • What is the new cabling on the socle if I use USB? Does Someone have an answer?

I hope this message will help some people who have this kind of problems,