Ways to Fix Nest "No Power to Rh Wire" (Do This!)

With the advancement of smart technology and software, smart homes are becoming extremely popular. Whether you want to go all out with a smart home or simply include one or two pieces of smart technology, a Nest thermostat is a great place to start. While they offer many advantages over other thermostats, Nest thermostats still have their fair share of problems. One of these problems is when you suddenly have no power to the Rh wire.

If your Nest thermostat is displaying the E74 no power to Rh wire error code, there’s a problem with the Rh wire connecting the furnace to the thermostat. This problem will result in power loss to the furnace and is often caused by a blown fuse, a faulty transformer, or a flooded float switch.

The E74 error code is one of the most common that Nest thermostats encounter. They can be caused by many different things, several of which you’re able to fix yourself. In this article, we’ll take a deeper look at the Rh wire and what often causes it to fail. Let’s get started!

Combined air conditioning and heating units come with thermostats. These contain two sections, cooling and heating. Combined thermostats have Rc and Rh wiring for these. The “R” indicates a live electrical wire. Rc wires are responsible for the cooling system.

On the other hand, Rh wires (“red heating” wires) are connected to the heating system. They are responsible for 24-hour volt power from your transformer. Rh wires connect directly to your thermostat’s Rh terminal. The Rh wire between your thermostat and furnace allows for constant operation of your thermostat without the need for batteries.

You will first realize this error in one of two ways. You might wake up in a pool of sweat because your AC mysteriously shut off. Or you’ll spot the E74 error code on the Nest display screen. It will read, “No power to Rh wire detected.”

What happens when there is no power to the Rh wire? Your air conditioning will stop working. Luckily, the most common cause of this is an easy fix. Chances are your Rh wire has a loose Rh terminal connection. But if removing the display to reconnect the wire doesn’t work, there may be another issue.

Another common cause of no power to Rh wire is clogged AC drainage pipes. Sometimes condensation can’t properly drain from the lines. When this happens, the air handler float switch cuts power to the thermostat. This is to prevent any overflow and water damage.

To find out if this is your issue, go to the float switch by the air handler. Check if the switch is working by rattling the floating mechanism at the top. It should make a clicking sound if it’s working properly. No clicking sound? The switch is probably disconnected due to a clogged drainage line.

If that is the case, clean your drainage pipes with a wet Shop-Vac. First, locate your drain line and suck out excess water and any lodged objects. Then return to your condensate switch. Manually move the float back to the bottom. Wait a few minutes, your thermostat should be up and running again.

Because the Rc and the Rh wires are often jumpered together inside the thermostat, there’s a chance that the contactor relay on your air conditioner has gone bad. If your wires are jumpered together, a problem with the air conditioner can manifest itself as a problem with the Rh furnace wire.

Attempting to replace an AC contactor relay is extremely dangerous and can result in electrical shock. The contactor relay often retains power even after the air conditioner is disconnected or has no power running to it. Touching the contactor, even with power completely shut off to the unit, can carry enough voltage to seriously injure or kill you.

If you suspect the contactor relay is the cause for your no power to Rh wire issue, you should contact an HVAC technician and allow them to perform the repair.

Another reason that you don’t have power to your Rh wire for your Nest thermostat is that the Rh wire might be making contact with another wire in your thermostat. When a bare Rh wire makes contact with another bare thermostat wire, it will result in a blown fuse and a loss of power to the Rh wire.

The most common reason for an Rh wire contacting another wire is due to installer error. It’s possible when your HVAC installer was running the thermostat wire, that he or she accidentally stripped the wires without realizing it. If the wires were stripped in the right places, the Rh wire could now be exposed and eventually make contact with another exposed wire.

It’s also possible that your installer stripped the wires down too far while they were attaching the wires to the thermostat. Wires settle ever so slightly over time, and the Rh wire could eventually make contact with another wire inside the thermostat. This will also blow a fuse on the furnace, which you’ll have to replace.

If you repeatedly replace a blown fuse but every time you start your furnace it repeatedly blows the same fuse, an Rh wire touching another wire is likely the problem.