Thermostat will not power on with C wire, but it has voltage


I recently purchased a Honeywell wifi thermostat and tried to hook it up as a replacement for an existing battery powered unit. I have a typical forced air HVAC setup with gas furnace (no heat pump), and I have replaced several thermostats in the past at other houses on very similar systems and never had any issues.

In this case, when I removed the original battery powered unit, the C wire was tucked behind the wall, so I manually connected it back up to the wall plate and inserted the new thermostat, but it would not power on. Then I tried putting the battery powered unit back in (removing its batteries, the manual says this will work), and still that one would not power up. I got out my volt meter and measured 20 volts across R and C, and 27 volts across R and G. I put the old thermostat back in with the batteries and that unit is still working fine.

Any idea why I am not able to power either thermostat when using the C wire, but I can measure voltage across...

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I bought a RTH8580WF1007 wi-fi thermostat but it will not power up!
my thermostat had a white, green, yellow and wire, the red wire was connected to the thermostat at two different screws. I connected the white green, yellow and red wire. I only connected the red wire to the screw labeled red. The thermostat does not power on! I do not have a email I'd and password ***** and can not figure out how to set my account.
JA: Just to clarify, do you think this is a larger HVAC problem, or something specific to the thermostat?
Customer: Thermostat only my old one work as it should. I think it is a wiring issue
JA: Do you plan on doing the work yourself?
Customer: Yes I installed the thermostat there are only a fee low voltage wire. The single red wirethatt was connected to two screws on the old thermostat has me puzzeled
JA: Anything else we should know to help you best?
Customer: That's all I can think of the other wires mated to points on the...

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THERMOSTAT WIRE CONNECTIONS - CONTENTS: Guide to Thermostats for Heating and Air Conditioning Systems. Wall mounting, leveling, and wiring requirements vary depending on the type of room thermostat and what it needs to control. How to select the proper type of replacement thermostat for air condtiioning or heating systems. POST a QUESTION or READ FAQs about heating, air conditioning, and heat pump thermostat installation and wiring REFERENCES

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

Room thermostat installation & wiring guide: this article series explains the basics of wiring connections at the thermostat for heating, heat pump, or air conditioning systems.

We provide Honeywell, White Rodgers & other thermostat wiring diagrams and explanation showing how to wire a room thermostat, including just what connections to make and how wires and connectors are color coded to...

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Installation guide...

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FAQs. Technical support is also available by email or by phone: 1.877.932.6233 (North America) 1.647.428.2220 (International) Compatible systems ecobee3 works with most centralized residential heating and cooling systems. Heating: up to 2 stages Cooling: up to 2 stages...

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Items included in box ecobee3 thermostat with Large trim plate back plate and trim plate Screws and drywall plugs Remote Sensor and stand Information booklets Power Extender Kit Double-sided adhesives (optional) wire labels Installation Guide Quick Start Guide...

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Before making the jump to a smart thermostat, become familiar with the c wire. The most common problem people have when making the jump to a wifi thermostat is unknowingly lacking what’s known as the c (common) wire. Here’s my neighbors not so pleasant experience:

The great powers of Tony Fadell’s Nest Learning Thermostat seemed so magical. As soon as I convinced my wife it was worth the investment, I went straight to Amazon Prime. After unboxing the glorious puck thermostat and I proceeded to use my awesome handyman skills to read the directions (skills are not needed, Nest makes it super simple). When it started having trouble connecting it to my wifi, I realized my common wire does not have enough voltage to power the Nest! – My Neighbor

Lesson learned: Most modern thermostats require what is known as a c wire. They really are pretty smart thermostats, so extra power is needed for maintaining a the screen, wireless connection and powering it’s small processor.

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i have this old honeywell thermostat that has mercury inside. and it also has a blue c wire connected to it.

i tried to install a new wifi thermostat, the screen was totally blank. i talked to the honeywell tech people.

the person told me that my old thermostat requires low voltage, i forgot, maybe less than 15 v. therefore, even though i have a c wire. it is not big enough to power the new wifi thermostat.

he asked me to look at the furnace, i don't see any circuit board, i only see this. the blue wire is behind the other wires.

my questions are.

1. do i really have a low voltage c wire? and it is too weak to power the new wifi thermostat?

2. is there any way for me to make the c wire increasing to 24 v? or is there anyway to get myself a damn c wire?

3. if i ask a hvac tech person to install it, do i need to pay for any extra...

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They get home and dust off the tool box. Never reading any thermostat installation directions, they proceed to change the thermostat. Some are successful at changing the thermostat and some are not. Those that are most likely read some thermostat installation directions or had someone read the thermostats installation directions for them. The ones that are not successful end up calling a professional in to install a new thermostat.

Successful Thermostat Installation Advice Article (opens in a new window)

Thermostat Installation Advice - How to Wire a Thermostat

The bottom line advice to most people is to call an HVAC professional if something is wrong. Whether it is with your thermostat or HVAC system. The problem may not be with the thermostat. And you may exasperate the problem which will cost more in the long run. Additionally, If you have a multi-zone system, a high-efficiency heat pump or even just a heat pump, a regular split-system AC and a boiler for...

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More likely the multimeter is measuring RMS.

I took 2 minutes and found this AC to AC 12V 400mA and a cheaper 500mA at Jameco. This 18V with 2200mA is also from them matches the 16 to 24 Volt suggestion with current to spare, but it is likely to waste some watts of power.

Note that Jameco stocks are very fluid.

Those links will die within a few days or months - but the actual stock will be replaced with similar items almost as quickly.

If you really want to use your 28 Volt...

Yes you could use a dropping resistor but you would also need to have a shunt in parallel with the thermostat to provide a reliable current draw.

Your thermostat will very likely have times where it powers down and draws almost zero current because it is done for now and waiting before it does anything else.

At those times your thermostat will see 28VAC again because the dropping resistor will not be dropping anything.

You need a Shunt resistor that draws at...

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This guide answers these questions. How to install a thermostat? What tools do I need to install a thermostat? What do each thermostat wire? What is a C wire? Do I need a C wire? Do I have a hidden C wire? What are C wire alternatives? How not to fry your thermostat wires or transformer while Installing your thermostat? How to avoid short circuiting your thermostat?

Thermostat replacement could be very easy if your old thermostat was set up right. But if your old thermostat cables and the system is complicated it could be a real pain. If you are not careful you might fry your new smart thermostat.

Here is a step by step instructions how to replace your old thermostat with your new smart thermostat. Please pay attention, read the whole thing once before you start your replacement.

Before you Install your smart wifi thermostat

Make sure you check our “What kind of Heating and Cooling system do I have?” article first. You have to know the type of...

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If you’re considering buying a new Wi-Fi thermostat, you might be overwhelmed by all the different options. Chances are your research has left you with more questions than answers. How do you know what kind of system you have? Do you know if a thermostat is compatible with your system? Will you be able to install it yourself? One of the most important factors to consider is whether your system has the common, or c-wire. If you aren’t an engineer or familiar with HVAC systems and how they work, wiring can be intimidating. Before your purchase, understand what a c-wire is and how it works with your thermostat.

What is a C-Wire?

A c-wire, is a low-voltage wire that runs from the heating and cooling equipment to your thermostat. It provides constant 24-volt power. Typically, this wire is connected to the “C” terminal on the system and the “C” terminal on the thermostat base. However, some manufacturers use different labels like “B” and “X”. So it is important to...

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On this page we have listed some popular questions with answers that our customer’s have asked us in their emails. I hope this will help answer some of your questions about heating and air conditioning. More questions and answer will be coming soon. If you have a question please email us anytime: We would love to help you out and have your business! Thank you so very much for visiting our website! One of my favorite quotes by David Jeremiah: “Nothing is more valuable than helping another person succeed!” We would love to help you troubleshoot and repair your furnace or air conditioner problem! Steve & Barbara Arnold

Popular HVAC Problems with Answers:

1. Customer problem: I am going through an ignitor almost every year. I think it is because my furnace to cycle off and on too much. What could be the cause for my furnace cycling too much?

Answer: Furnace short cycling is very hard on the furnace and hard on your energy bill. Furnace...

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Install the replacement wall plate.

Use the new wall plate as a template to mark where the holes you need to drill for the screws will be. Use a level if necessary. Then, drill the holes, and screw the replacement wall plate into its new position on the wall.

/b/ba/Replace a Thermostat Step 7.360p.mp4

If your new thermostat has a mercury tube (that is to say, if your new thermostat is rather old school), your device needs to be completely level or it won't render accurate readings. Using a level is very important in this scenario and is not just for aesthetic reasons. Make sure you're drilling holes that match the size of your screws. A 3/16" drill bit is fairly standard. Your thermostat...
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I'm befuddled and have spent far too much time on this already so I'm hoping someone here can help. We recently bought a home in a very rural area so it's difficult to get vendors/technicians to head out here.

I want to replace an older (12 years?) battery-operated thermostat with a wifi thermostat (Honeywell's 7 day programmable wifi thermostat, model YRTH6580WF1007 purchased from Costco). The house has both heating (propane gas powered furnace) and AC.

The existing thermostat had Y, W, R, G wires connected and THREE extra wires (brown, orange and blue). There was a jumper cable from R to Rc.

I climbed up into our attic crawl space to get to the furnace. The former owner/builder put the Bryant furnace up there on its side under the eaves so it's a pain to get to. There is clearly a blue wire connected to the C wire on the circuit. See photo:

Note that I also saw a second bundle of wires on the circuit just above this set of wires that don't seem...

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When installing any thermostat, the first important thing that you should take care is the c wire. You need to know that c wire can offer power to the thermostat. To the old models, they were often dialing, and they didn’t need to consume a huge power. Meanwhile, the modern thermostats with Wifi or backlit display require a stable supply. Therefore, the thermostat c wire (also called common wire) allows the constant flow of 24 VAC power to the thermostat.

In a technical way, the power runs from the R (red) wire, but not progressively (not on its own). So, in order to make it progressive, you need to have a common wire to fulfill the circuit. Of course, when the circuit is accomplished, 24V energy will constantly flow. In case that you are going to buy a wifi thermostat, you can install it by yourself. If you used to change a light switch or container, you could perform it – expecting your system has already a C wire.

In case that your system is equipped with a c-wire,...

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General Information | Installation Questions | Troubleshooting

General Information

Do programmable thermostats really save energy?
What does a thermostat do?
How does a programmable thermostat work?
Can a mechanical or manual be replaced with a programmable thermostat?
Can I use a Filtrete (TM) Thermostat from 3M with my system?
How do I know if I have a Heat Pump?
What is a Heat Pump?
What is Emergency heat for on a heat pump?
How do I know if I can use a Filtrete (TM) Thermostat from 3M with my heat pump?
Will a Filtrete (TM) Thermostat from 3M control a two-speed blower?
Can I use this in my trailer or motor home for heating and cooling?
Can I use a Filtrete (TM) Thermostat from 3M with a two-stage gas, oil, propane or a two stage electric furnace system (W1 and W2)?
Will a Filtrete (TM)...

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I'm in an old home with only 2-wires as well, but I risked trying to see if Nest would work, and it has been performing spectacular.

My only issue is with the Vera Nest plugin not yet fully supporting UI7, so I have to settle for only having automatic scene control of the Home/Away function. That supprisingly has been working out well though, the moment I get out of bed the Nest is switched to Home mode via a Vera scene, and heats the downstairs up to a comfortable temperature already before I even make it downstairs (Home set to 69F). When I go to bed it goes into Away mode and keeps temp above 50F.

Have it running for little over a month now, and already see savings on my utility bills. Am on weird ever changing schedules, so programming a regular thermostat would never have worked for me, and it would happen quite often where I had forgotten to turn the thermostat down. Having Z-Wave sensors now do that fully automatic is easy to get used to, and at this rate it...

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In a New York Times article published yesterday, writer Nick Bilton discusses a software glitch that caused his Nest Learning Thermostats to stop working.

As Bilton describes it, "Although I had set the thermostat to 70 degrees overnight, my wife and I were woken by a crying baby at 4 a.m. The thermometer in his room read 64 degrees, and the Nest was off." Additional customers noted similar problems through Nest's online forum.

The Google-owned company claims to have addressed the issue by pushing out a firmware update to "all Nest Thermostats that were affected," but Bilton (and likely others) had already sworn off their Wi-Fi thermostats by then.

Software bugs certainly aren't new to the Internet of Things. Every gizmo that's "smart" and "connected" can be exposed to non-hardware-related hiccups and hacks.

The Ring Video Doorbell recently updated its own software to account for a flaw that made it relatively easy for hackers to access proprietary Wi-Fi...

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The promise of building a smart home lies in how easy the devices are to connect: you can link your smart thermostat to a smart lighting set-up—all in the flip of a switch. As we talk about often on GearBrain, building a smart home can be simple, as long as you understand the details along the way. But one of those details is a little known element called a "C-Wire," or as it's known in the HVAC (heating, ventilation and air conditioning) industry, the "common wire."

In learning about smart Wi-Fi thermostats and how they work, we came across this little wire and could not believe how important it was to running a smart home. The C-Wire enables the continuous flow of 24 VAC power to your smart thermostat.

Many of today's new smart Wi-Fi thermostats require you to have this little wire to run their connected devices. But not many homes have a C-Wire, causing a lot of frustration among buyers. Why? Not many manufacturers of smart Wi-Fi thermostats inform you of the need...

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Last updated: 2/10/2016

Back in the olden days, thermostats were simple on/off devices that didn’t need their own continuous power supply. Modern thermostats with Wi-Fi and backlit display, by contrast, need a steady supply of juice.

The C wire, or “common wire” enables the continuous flow of 24 VAC power to the thermostat.

Technically speaking, power flows from the R (red) wire, but not continuously (not on its own, anyway). To make it continuous requires a common wire to complete the circuit. When the circuit is complete, 24V energy will flow continuously.

If you’re considering purchasing a smart thermostat, you’re probably thinking of doing the installation yourself. After all, if you’re able to change a light switch or receptacle, you’re skilled enough to install a smart thermostat – assuming your system already has a C wire.

If your system has a C-wire, it might be in use or just tucked away behind your current thermostat.

If your...

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When you read through the reviews I have on my website, you will notice that lots of manufacturers state their thermostats need a common wire in order to work properly. However, just because you don’t have the blue wire mentioned above, that doesn’t mean you can’t use various models.

My research has taken me on quite a journey in this respect, and so far I have come across three options that will help you overcome the common wire issue so many people suffer.

Unused C Wire Terminal

So you’ve decided to install your new thermostat and discover that you have an unused wire but you do need a C wire connection. In this really short video you will learn how to connect an unused wire so it can be used as your C wire, which in a lot of cases has solved the problem of a thermostat not working.

Substituting a G Wire for a C Wire

As you will notice from the information above, your ‘G’ wire connects to the fan (as industry standard) and is green in color....

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Power to the unit is the first thing to look at.

Check power off of the transformer (24VAC)

Check power in at the thermostat.

If you have power to the unit, then there should be power input to the transformer. If you have power input to the transformer but no power output from the transformer, replace the transformer. If you have power output from the transformer then check for power at the thermostat, if you have power at the thermostat, then replace thermostat, if you have no power at the thermostat check wiring from transformer to the thermostat.

If all you did was ductwork and no work on the electrical system then it is likely a bad luck situation for you. You touch it, you own it. Most likely you will need to get it repaired even though it is not your fault.

If you can get a technician in there and have him troubleshoot the problem and report to the HO that there was no way you caused the issue you could get away with it.


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The C-Wire, or “common wire” enables the continuous flow of 24 VAC power to the thermostat. It’s a wire that (if you have one) runs all the way from your furnace to your thermostat.

Questions about the C-wire are by far the biggest drivers of traffic to this site, so I’ve written this article to help you understand what the C-wire is, help you find your system’s C-wire, and help you decide what thermostat to buy based on what wires you have and/or are willing to install.

This bundle green, blue, yellow, white, and red color wires is known as “5 conductor” wire and it is a common type of thermostat wiring.

To find out if your system has a C-wire

The simplest way is to pop your thermostat off the wall and look at the wires behind it and where they are hooked up. Remember, the colors don’t really mean anything. Think of the colors as a suggestion: the C-wire is usually the blue wire, but that’s not a standard.

Best case scenario: you find five wires...

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In this article, I am going to explain the function and wiring of the most common home climate control thermostats. This information is designed to help you understand the function of the thermostat to assist you when installing a new one, or replacing or up-grading an old one. We will focus mainly on the basics of home heating / cooling thermostats, and first of all, I will explain the function of them.

The thermostat is the control device that provides a simple user interface with the internal workings of your homes climate control system. By the use of an adjustable set-point, the job of the thermostat is to turn on either the heating or cooling system to maintain the desired room temperature in the home, and to turn off the system when the desired temperature is achieved.

The most basic of systems (such as an older ‘heat only’ forced air / gas furnace with a standing pilot light) only need two wires for control. They connect to a two-wire thermostat (generally a...

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