If your Nest thermostat gives you an “E74” error code, it is notifying you that no power to the Rh wire is detected. When that happens, your first reaction might be to look for the HVAC technician’s telephone number. But is that necessary? Can’t you troubleshoot and fix the issue yourself?
Usually, you can troubleshoot an E74 issue yourself and in many cases, you can also rectify the problem without calling a technician. Although the reasons why there is no power to the Rh wire can vary from a loose connection to blocked drain pipes, the Nest troubleshooting procedures make it easy to find and fix the issue.
To understand the E74-troubleshooting procedures better, we’ll first have a brief look at an Rh wire as such and then discuss various troubleshooting steps. In most cases, you will probably be able to rectify the issue yourself. However, if the problem persists after you’ve gone through all the recommended steps, you’ll have to call a qualified HVAC technician to check your system.
More about the Rh wire
“Rh wire” is the abbreviation for “Red heating wire.” With electrical wiring, a red wire indicates a wire carrying current – normally 120 or 240-volts and, importantly, you can be shocked when touching it. However, your thermostat’s red wire only carries a 24-volt current and can’t shock you, and doesn’t have to be red. But some thermostat manufacturers like to use red wires for all wires carrying current, regardless of whether it is 240-volt or 24-volt.
In your HVAC system, the Rh wire is the connecting wire between the thermostat and the HVAC system’s heating system. It should be connected to the thermostat’s Rh terminal. If the system is working correctly, the wire delivers electric power from the transformer 24/7.
If the wire is not connected or broken, or the thermostat cannot detect the Rh wire, no power can be sent to the heating system.
Basic troubleshooting if you don’t have power to the Rh wire (Error code E74)
As there might be various possibilities why- power is not sent to the Rh wire, you’ll probably have to troubleshoot quite a few aspects before finding the problem. For your convenience, we’ve organized the troubleshooting steps in a logical order, starting with the “easier” steps.
By following these steps you don’t have to do “complicated” troubleshooting procedures while the problem might have been easily solved in one of the first steps.
To get an overview of the recommended steps, have a look at the list of the steps we’ve compiled. If you understand the logic of our steps you can get the detail by reading on.
| Action | Reason | |
| Step 1 | Check for loose wires. | Power can’t flow if connections are not secure. |
| Step 2 | Reconnect all loose wires. | Loose wires other than the Rh wire can also cause that power doesn’t go to the Rh wire. |
| Step 3 | Check your HVAC system’s fuse. | A blown fuse stops the flow of power to HVAC components, including the Rh wire. |
| Step 4 | Unclog the drain pipe if necessary and check the HVAC’s float switch. | A clogged pipe can cause the water level to rise and disconnect the float switch which then cut the flow of power. |
| Step 5 | Step 5: Check for a faulty relay in an outside unit (condensing unit). | A faulty relay can cause the fuse to blow and this stops the flow of power. |
Step 1: Check whether the Rh wire is in place and connected and rectify if needed
As the E74 error code means that there is no power to the Rh wire, it is logical, as your first step, to check whether all the wires, including the Rh wire, are in place and securely connected. The chances are good that the loss of power is because of a loose connection.
Find loose connections with Nest’s “Tech Info Diagram” feature
If you quickly want to verify whether there is a connection issue with the Rh and other wires you can use the excellent “Tech Info Diagram” feature of Nest thermostats. To use the Diagram, go to the main error page where you’ll find the option to open the feature. The diagram highlights all loose connections in red. If the Diagram shows lose connections, go to Step 2.
If no loose connections are showing on the Diagram, you might want to skip Step 2 to save some time, but it is recommended that you still execute the step to enable you to manually test all the connections on your thermostat.
Step 2: Reconnect loose thermostat connections
Before you remove your thermostat’s face from its base, turn off your HVAC system’s electricity completely. Although the thermostat’s wire only carries a 24-volt current, it is always good safety practice to turn off the power to the whole system before opening your thermostat.
Remove the thermostat’s display from the base on the wall and inspect all connections and wires. If some wires are not connected, re-insert them into the thermostat’s terminals. To re-insert a wire properly, press down the connector button and push the wire as far as it will go.
You know the wire has been properly installed if the connector button remains in its position when you let it go. You can test the newly inserted and all the other wires whether the wire is securely connected by trying to lightly pull it from the terminal. If you can pull a wire out, re-insert it securely.
When you’re satisfied that all the connections are securely in place, fit the display back to the base.
Then, power on the HVAC system and if you don’t get the E74 error code anymore, you’ve fixed the problem. If the problem persists, you have to troubleshoot further.
Step 3: Check your HVAC system’s fuse
After you’ve determined that there’s no problem with your thermostat’s wiring and that all the connections, including the Rh wire, are in place and secured, the HVAC’s main fuse is the next aspect to check.
Generally, the fuse is positioned to the right of the HVAC control unit. You have to take the fuse out to determine whether it is still in working condition. To get the fuse out, turn off the power to your HVAC system again, open the fuse’s housing and remove the fuse.
The fuse has a transparent “window” with a u-shaped wire visible inside. You know the fuse is blown if this u-shaped wire is broken. Then you have to replace the fuse with the same type and model as the one that has blown. Remember, the color of a fuse indicates the current rating. So, ensure that you replace the fuse with a fuse with the same color.
Turn on the power again and if the error message is not shown anymore the issue has been resolved. However, if the fuse blows again there is a great possibility that your HVAC has other underlying issues that cause the fuse to blow. It is recommended that you then contact a licensed technician to rule out and fix any HVAC issues.
If the existing fuse’s u-shaped wire has still been intact when you’ve removed it from the housing, clean the contact points on the fuse and re-insert the fuse back into the fuse’s housing. If the error message is not shown anymore, dirty fuse contacts have been the problem.
If you’ve checked all the connections and are sure that the fuse is in working condition, but there is still no power to the Rh wire, keep on following the troubleshooting steps to look for possible faults, including issues that could have caused the blown fuse and have thus contributed to the loss of power to the Rh wire.
Step 4: Unclog the drain pipe if necessary and check the HVAC’s float switch
Sometimes your HVAC drain pipe gets clogged and a clogged pipe may cause the HVAC’s float switch to disconnect. A disconnected float switch stops the power to your devices which will result in an E74 error message.
Fortunately, you do not need professional assistance to unclog and clean a drain pipe. To do it yourself, look for the outside valve and suck all the material causing the clogging out with a vacuum. This may take some time, and you also may require a strong tool to insert into the pipe to loosen hard material before you start vacuuming.
When the drain pipe is unclogged, check that the float switch is still connected. If it is disconnected, move the float to the bottom manually. Wait for a few minutes and try to restart your thermostat to see whether power is going to the Rh wire.
Step 5: Check for a faulty relay in an outside unit (condensing unit)
The loss of power can also be contributed to your air conditioner’s main unit outside your house. After years of use, it can happen that the relay simply goes bad. A faulty relay may cause the HVAC’s fuse to blow and that can again will stop power to go to the Rh wire.
If this is the problem, it is one of the rare instances where you’ll most probably need an electrician or HVAC professional to repair it for you.
Things to remember about troubleshooting a Nest thermostat
Relatively easy to troubleshoot a Nest thermostat
The Nest thermostat is one of the easiest thermostats to troubleshoot. It is relatively easy because the errors listed are very precise. With many other thermostats, you don’t get an indication of what the problem might be. The thermostat or HVAC components just start to malfunction or stop working.
The Nest thermostats have a well-designed and sophisticated error-code system. Thus, if you receive the E74 error message you know it is the Rh wire that is not getting power from the thermostat. You immediately know where to start with the troubleshooting.
To rectify a Nest problem usually a DIY job
In most cases, you will be able to rectify the problem yourself. However, if none of the DIY methods works, it is recommended that you call in professional help. In other words, if you are troubleshooting an E74 error and you’ve followed all our recommended steps but the problem still persists, you have to call a qualified HVAC technician or electrician.
Always be safe and put off the HVAC system before working on it
And also remember that although the thermostat wires only carry a low current of 24-volt, it is always good practice to put off the whole system before your start working on the wires.
Especially make sure you turn off the thermostat and the HVAC system before changing the fuse, looking for loose connections, or cleaning the drain pipes.
Wait before re-starting the system
After you’ve repaired a fault found during the troubleshooting, it is always good practice to wait for a few minutes before turning the system on again.
If needed, recharge the Nest Thermostat’s internal battery
Sometimes it is necessary to recharge the Nest thermostat manually after an E47 error has been rectified. Without a charged battery the thermostat can’t work and thus, no power can be sent to the Rh wire.
If no power has been going to the Rh wire for more than 12 hours because the thermostat has not been receiving power for more than 12 hours for any reason, its internal battery might have drained.
Generally, as soon as the power is restored, the thermostat will first charge the battery before turning ON. Your Nest Thermostat will show you with a blinking light that it is charging.
If you still receive an error message after a few hours, you might have to charge the Nest thermostat’s internal battery manually through a USB connection. When the thermostat is working again the power to the Rh wire may be restored.
Conclusion
When no power is coming from the thermostat to the Rh wire, you can troubleshoot yourself and in most instances, you’ll be able to rectify the problem yourself.
Fortunately, Nest thermostats are equipped with a sophisticated error-code feature which makes it easy to detect problems. If you follow the steps we recommend for an E74 you’ll troubleshoot logically and effectively.
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