Intermittent E74 error code

[GLConboy]

Intermittent E74 code (no power to Rh) on a heat-only thermostat ('downstairs').  I frequently get an E74 error on a heat-only thermostat that has 2 wires. They are connected to W1 and Rh.  The issue then resolves spontaneously.  However, it feels like there is heat going to forced hot water baseboards.

The upstairs thermostat (same model) is connected to heat and AC.  It has no issues.

At a loss.

[zoeuvre]

Hi GLConboy, 

Uh-oh! I'm sorry to hear that your thermostat is showing E74 code. Check on the following: 

1. Make sure your breakers are on.
   • Go to the breaker box and check if the breakers flipped.
     • If yes, reset the breakers.
     • If not, proceed to the next step.
2. Check 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 you doesn't reattach the panels.
3. 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 your air conditioner off and on during certain peak times of the day.
4. Check your air filter.
   • Your HVAC system needs to have a good airflow to work properly. A clog in the air filter can limit how much air flows over the cooling coils. If the clog persists, the cooling coils can freeze and cause the customer’s system to shut off.
   • Find your HVAC system’s air filter. The air filter is usually behind a grate in the hallway.
     1. Check your walls and ceiling.
     2. If you can’t find the air filter, search for it inside the furnace near the fan.
   • If the air filter seems dirty or there seems to be a clog, replace the filter with a new one. If the issue is because of frozen cooling coils, you need to wait a while for the HVAC system to thaw before they can use it again.
5. Gather the power readouts from the thermostat. 
   
   • Go to Settings > Technical Info > Power and list down the reading for Battery, VOC, VIN and lIN.
6. Make sure the thermostat wires are properly stripped and seated in the thermostat base.
7. If there’s a common wire connected in the C terminal, remove it.
8. Move the RH power wire to the unused RC power terminal.
9. If the previous thermostat is available, kindly 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

[GLConboy]

Done all of that. And the thing is it spontaneously works without my doing anything. The Nest upstairs is also connected to the furnace but the A/C also. However, the A/C blower also turns on when the baseboard heat comes on

[Markjosephp]

Hey there,

I'm jumping in to see how I can help.

Thanks for looking into this, Zoe.

@GLConboy, it's great that everything seems to be working spontaneously, but I understand the concern about the A/C blower turning on when the baseboard heat comes on.

To troubleshoot this, can you gather the power readings in the Technical Info settings? This would help us determine the amount of power it receives from your system. Also, if you have the C wire and have it disconnected, then move the power wire to the other power terminal. Can you confirm the message code that you received?

I'll look forward to your response.

Warm regards,
Mark

[GLConboy]

E74

wiring reporr

no power to Rh wire detected

nest.com/e74

battery 3.703v

Voc 0.01v

Vin 0.01v

lin 8 ma (0)

[Jenelyn_O]

Hi there GLConboy,

Thanks for providing your thermostat's power readout. It looks like you have the Nest Learning Thermostat, which has two power terminals (RC and RH). To identify if it has something to do with your wire or with your thermostat terminal, please move your power wire from RH to RC terminal. After that, check your thermostat's power reading again to compare. 

For your safety, ensure to turn the breaker off before touching the wires. 

I appreciate your input, Zoe and Mark.

Regards,

Jenelyn

[GLConboy]

What do you mean by the other power terminal? There is a white wire also but it has always been disconnected. How do I attach a pic?