2 Ways To Fix Your Nest Smart Thermostat When No Power Is Reaching The RH Wire (Error Code E74)

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The Nest smart thermostat from Google is one of the best in the business, no questions asked.

From its elegant form to its deep and seamless functionality, it’s an amazing addition to a smart home network, one that will keep you comfortable and save you a few bucks on your energy bills in the process.

However, no matter how good your thermostat is, you’re bound to encounter the dreaded E74 error code eventually.

2 Ways To Fix Your Nest Smart Thermostat When No Power Is Reaching The RH Wire (Error Code E74)

E74 - No Power To RH Wire

The E74 error code plaguing Nest users all over the world means that the RH wire is being starved of power, which is nice to know I suppose, but for most people (me included) that means very little.

You might as well tell me that the neutron capacitor has under-juiced the hyper-space time-paradox transducers.

So, I thought it would be a neat idea to actually tell you what you can do to try and solve this mysterious error, rather than just sit by, blankly rolling the words “RH wire” over your brain like a nickel over knuckles.

1. Overloading Furnace - A Power Cut

An E74 error code means that something is interfering with the flow of power, preventing it from reaching the RH wire. 

What is this famous RH wire, you ask? It’s the primary power input for your HVAC system, so needless to say, it’s essential that it’s fed a sufficient wattage.

Next, we need to investigate what’s causing the power cut. Your first port of call should be to simply check that the RH wire is firmly in place. If it has come loose, there’s a good chance that’s the issue.

Should you find the RH wire is secure, it’s time to check the condensate network in your HVAC system.

When the summer months come around and the weather gets warm, it can kick your HVAC facilities into overdrive, and as it struggles to keep temperatures in your home down, a significant amount of condensation develops in the system.

If there’s some kind of blockage in the drainage pipes, the moisture can build up, overflow, and cause unnecessary water damage to your home.

Most systems will have an integrated sensor that monitors the build-up of condensation, and once it gets to a certain point, it triggers the condensate switch, halting power on its way to the RH wire, and preventing floods from occurring.

To get everything back in working order, you’ll need to clear the blockage in the drainage pipes with a wet/dry vac and give the condensate pump a look over too. Once the lines are cleared, it’s a good idea to run some white vinegar through them to break down any residual debris.

Once completed, pay your HVAC unit a visit, and you should hear it whirring away as normal. In the unfortunate event that your Nest thermostat is still running the E74 code, you’ll need to move on to the HVAC control unit fuse.

Sometimes, when there are sudden fluctuations in power, this fuse will blow, so it’s possible that when your HVAC system shut itself down due to an overload of condensation, the fuse went along with it.

To check if your fuse has blown…

  • Make sure your HVAC system and thermostat are turned off.
  • Locate and remove the fuse.
  • Look into the transparent casing of the fuse to see if the white U-shaped wire is intact. If the wire is broken, your fuse has blown. Replace it, and your HVAC system should immediately fire back up.

2. Call in a Professional Technician

If you’ve tried all the fixes above, it could be that there’s a problem with your HVAC contactor relay.

These parts of your HVAC system can fail for a number of reasons, but mostly on older units, so if your HVAC system is an absolute fossil, you can pretty much guarantee that this is the problem.

The bad news is that it’s really not a good idea to try and replace the contactor relay yourself, so pick up that phone and call in some professional help.

There are numerous types of relay, so even picking out the right one might pose a conundrum to the average homeowner, but an expert will have no problems identifying the parts you need.

You may also benefit from contacting Nest support directly, as they might have some novel ideas on what the problem could be.

Summing Up - The Bottom Line

Well, that’s that. The reason your Google Nest Thermostat is flashing you the “No power to the RH wire” error is that something is interrupting the voltage to the power wire of your HVAC system.

Hopefully, it’s just down to a loose RH wire, but it’s more likely to do with a clogged drainage pipe or pump, which can in turn spark a blown fuse.

If you still can’t get your HVAC system up and running after replacing the fuse, it’s time to call it quits and call in some outside help. Best of luck!