Intermittent E74: No Power to Rh wire for 3rd gen learning Nest thermostat (Heater only, not AC)

[JBussi]

We are getting an intermittent E74 error during heating. The heater will come on for 5, 10 or 15 mins, but doesn't reach the programmed temperature, then turns off and the E74 code is generated. It will then come on again after a little while, but stops again with the same error. It will go through this cycle until it eventually reaches the programmed temperature. During the summer, when using the AC, we didn't have any issues. The issues didn't start until fall when we switched over to heating. Based on other threads for this issue I've looked at the wiring and I didn't see anything that jumped out at me there as being an issue. I also looked at the drain pan for the furnace and didn't see any standing water and also looked at the furnace drain pipes and didn't see any clogging. Just wondering if anyone else had a similar issue and found the fix. Thanks!

[zoeuvre]

Hi JBussi, 

Uh-oh! I'm sorry to hear that your thermostat is not reaching the programmed temperature, then turns off and the E74 code is generated. Check on the following: 

1. Aside from drip pan or drain tubes, check also the condition of the HVAC fuse. Occasionally, fuses burn out due to one or more reasons. If your HVAC fuse burns out, the system might cut off the power to the thermostat and cause an E74 power error. 
   • Turn off the main power to the HVAC system at the breaker box. Note: The HVAC system can have multiple breakers, so make sure you turn them all off.
   • Check the furnace and fan and make sure there are no lights that might indicate the system still has power. Note: If there are still lights on the furnace or fan, the system might still have power, and you need to turn off the main power to their home at the breaker.
   • Search for the HVAC fuse. It’s usually on the furnace control board. Note: If you need help to find your control board, check your owner’s manual. Fuses can be different for each type of system. You might find a small U-shaped fuse with a number 3 label and is pink or purple colored. You might also find a round fuse.
   • Remove and inspect the fuse for damage.
   • If the fuse seems burnt or has discoloration, consider a fuse replacement. To learn more about fuse replacement, consult the user guide for the HVAC system or contact a Nest Pro.
   • Make sure to reattach any panels on the furnace. Note: The furnace might not work if the customer doesn't reattach the panels.
2. Check if you have an energy savings box. Some utility companies install boxes that cut off the power during high-demand (peak) times. These energy savings boxes can cause the error. 
   • Search for an energy savings box near the electric meter (possibly near their air conditioner).
   • Search for a small box with 2 lights on it (one light is red, the other is usually green). This box allows the utility company to cycle the customer’s air conditioner off and on during certain peak times of the day.
3. Gather the power readouts from the thermostat. 
   
   • Go to Settings > Technical Info > Power and list down the reading for Battery, VOC, VIN and lIN.
4. Make sure the thermostat wires are properly stripped and seated in the thermostat base.
5. If there’s a common wire connected in the C terminal, remove it.
6. Move the power wire to the unused power terminal. If the power wire is in RH, then move it to RC. If it's in RC, then move it to RH. 
7. If the previous thermostat is available, ask the customer to install it. 

It is worth noting that you turn off the power to your HVAC system when you are about to touch some wires on your thermostat base. If you are not comfortable checking on the following, consider contacting a pro.

Let us know how it goes.

Best,

Zoe