Nest thermostats are great, convenient devices that let you control the temperature in your home remotely. Unfortunately, they can occasionally experience hardware issues.
One common issue is no power to the red wire on your Nest thermostat. This can occur for various reasons, but fortunately it’s an easy fix without needing professional assistance.
Loose Wire Connections
Your Nest thermostat may display an E74 error code indicating there is no power to the Rh wire, which indicates it has not been connected correctly. In order to restore functionality, you’ll need to reconnect the Rh wire so your Nest can detect it once more.
The Rh wire is a red wire that provides 24-hour electric power to your furnace or heat pump. It’s located inside your HVAC system and attaches to either the thermostat’s R terminal, depending on whether you have dual transformer setup or not.
Before installing a Nest thermostat, ensure all wires are clearly labeled and connected to each other. Doing this will prevent any confusion when connecting the thermostat to your HVAC system.
If you’re having trouble connecting your wires, inspect the thermostat for loose connections by taking the display away from the wall and making sure each wire is securely fastened in place. Typically, there will be two wires for Rh and Rc or one single wire (usually with a jumper wire connecting its terminals).
It’s also wise to inspect your control unit fuse. If it has blown, this could be the cause of the no power to rh wire error on your Nest.
Another potential cause of no power to the Rh wire is an installation error by your HVAC installer. They could have accidentally peeled away wires and exposed the Rh wire while attaching them to the thermostat.
As wires settle over time, they may come into contact with each other, leading your thermostat to display an E74 error. At that point, your thermostat won’t be able to charge its battery or provide backup power.
Loose wiring can also cause your thermostat’s fuse to blow, necessitating replacement. If this occurs repeatedly, contact a professional and have them repair the issue for you.
Clogged AC Drain Pipes
If your Nest thermostat is showing an E74 error indicating it does not have power to the rh wire, there are several steps you can take to resolve the problem. These include inspecting the connection of the rh wire, clearing out the drain pipe, and inspecting the HVAC float switch.
The red wire connecting your thermostat to your heating and cooling system is known as the thermostat wire. In case of dual transformer setups, this wire may be unlabeled red with no “H” attached; on single furnaces it may have both colors attached.
Your thermostat typically connects to your AC with a jumper wire that joins the red wire on the thermostat to the red wire on your furnace. This makes it simple for you to monitor heating and cooling conditions as well as adjust thermostat settings remotely.
It’s wise to inspect the rh wire connection regularly in order to guarantee its condition. This is especially crucial if you live in an area prone to high humidity levels, such as a bathroom or kitchen.
To locate the rh wire connection on your thermostat, remove the back cover and look for a small box with a red LED. This is your condensate overflow switch which should be located near or within your HVAC’s air handler or near its outlet; its purpose is to prevent water infiltration into your home.
If the rh wire is clogged, use a wet/dry shop vac to clear it out. This will only take three minutes but will make an immense difference in how well your AC runs.
Once the blockage has been cleared away, use a diluted vinegar or bleach solution to disinfect the pipe and break down any dirt and debris that has built up inside. This will help break down any hardened spots on the inside.
If you don’t feel confident clearing the blockage yourself, call a professional for assistance. They will clean out your AC’s drain line and inspect the condenser pump associated with your HVAC system as well as identify what caused the E74 error code.
Low Thermostat Battery
If your Nest thermostat is giving an E74 error message that indicates it can’t get power to the Rh wire, there could be several causes of this. It could be something as simple as a loose wire connection or something more serious like an issue with your HVAC pipe or control unit.
First and foremost, inspect for loose wire connections. Do this by taking apart the display and reconnecting its Rh wire, then visually checking to make sure everything connects securely.
Another way to determine if there’s an issue is by opening the thermostat and inspecting its wiring inside. Use either a flashlight or magnifying glass to locate any loose wires.
Unfortunately, sometimes installers make a mistake and accidentally pull or strip the wires during installation. It could also occur if wires start to touch each other over time and cause the fuse to blow or the entire thermostat to shut off.
When this occurs, you’ll need to replace the fuses and relays that power your furnace. Doing so will resolve issues with no power to the Rh wire as well as an E74 error message on your thermostat.
If the fuses on your air handler have been replaced and yet the E74 error still occurs, it might be time for an HVAC professional to examine the float switch. If it is broken, condensation won’t drain properly from pipes which could cut off power to your thermostat.
Low thermostat batteries are another common cause of the E74 error message. You can try manually charging your Nest thermostat using a USB cable, or replace the AA alkaline or 3-volt lithium batteries with new ones.
To charge your thermostat, pull off the display and plug it into a USB port. A blinking red light on the front will indicate that it’s charging, though it may take up to an hour for full battery recharge.
Any loose connections or clogged AC drain pipes can cause the E74 error message. To resolve the problem, take out the float switch and clean any AC drain pipes before restarting your thermostat.
HVAC Control Unit
When your Nest thermostat displays the E74 error “No Power to Rh Wire,” it means there is an issue with the Rh wire connecting your furnace to the thermostat. This wire supplies 24-hour volt power from the transformer to your thermostat; if not enough power is supplied, your Nest thermostat will display this message and turn off either your air conditioner or furnace.”
No power to the Rh wire can be caused by several reasons, such as a blown fuse, broken connection or clogged float switch. If any of these issues exist in your setup, they need to be addressed prior to trying to resolve the nest no power to rh wire error.
First, verify that your thermostat’s RC and Rh wires are connected correctly. If they’re not, you can try reversing the power flow between them using a jumper wire; this should correct the problem and restore power to your thermostat.
A potential cause of no power to the Rh wire could be a malfunctioning contactor relay in your AC system. Over time, this relay generates sparks and could eventually blow out all your fuses. For safety’s sake, call an experienced HVAC technician to replace this part of your system; touching it could result in serious electrical shocks or even death.
To replace a relay on an AC unit, first open up your AC unit and disconnect any wires connecting to it. Afterward, disconnect the relay from its contactor and take out.
You may need to use a 24V common wire transformer. This inexpensive piece of equipment can provide the needed voltage for your thermostat to restore it to proper working order.
Another possible reason why your thermostat’s Rh wire may be dead is that the wire has accidentally made contact with another one in your thermostat. This typically occurs due to incorrect installation; if these wires were stripped incorrectly, they could now be in direct contact with each other and resulting in the loss of power to the Rh wire.
If your Rh wire still doesn’t have power, it could be due to a damaged transformer or wire damage from corrosion, nails, or pests such as mice and rats.
