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122

More than you've ever wanted to know about the C wire: Lets start by explaining what the C wire is, and why it's needed. Ye olde thermostat In the olden days thermostats were simple switch devices, that used Mercury Switches to complete the circuit and turn on the heat/AC. Mercury switches were commonly used in bimetal thermostats. The weight of the ...


14

Wiring Diagrams Locate the schematic In most cases, a wiring diagram can be located inside the HVAC unit. Typically, it will be on the inside of the access cover. Find the transformer on the schematic Once you've located the diagram, you'll want to look for this symbol. Or the actual text that says "Transformer", which may be labeling a crudely drawn ...


12

Use an Add-a-Wire Device Assuming your thermostat and furnace are compatible and you're just short on conductors between them, you could use one of these add-a-wire devices. This device won't work for C wires. However, it does allow one to combine two of the other relay wires down to 1, freeing up a dedicated wire for C. For example, in a standard 4-wire (...


8

If you're building a new building and having new HVAC installed, I would simply put 1/2" conduit in the wall and be done. That way, you let the installer run the wire they need. I don't know that I would do this without at least consulting an HVAC installer to see where they would put the unit and the air return (typically the thermostat is as near the ...


7

If your furnace has a C terminal then you can run a new wire from the C terminal to your thermostat. You can connect as many wires to the C terminal as you'd like as long as you do it in parallel (ie: all wires must terminate on the terminal itself, do not "daisy chain" them). In the case when your furnace does not have a C terminal, you can utilize an ...


5

I also have a thermostat that requires a C wire hookup for wifi access. The spec calls for a 24 volt DC and I presently have it wired to share a 24 volt termination on my AC air handler. What I discovered was that this termination in the air handler does not provide a constant output of 24 volts. Instead, it cycles as called upon by the air handler and ...


5

Judging by this manual, you have one of these: You described it as terminal 4 but the diagram just shows two terminals labelled "T", but that is fine: I found a manual for an EnviraCom device which shows terminals 2 and 3 are 24vac power: This means we have the right connections, and according to the R7184 manual: EnviraCOM™ Current Available: 150 mA So ...


5

Install equipment in real junction boxes I've done a lot of wiring and I cannot imagine how you would get a ground wire anywhere near a 2x4. Your wires (including ground) are doing one of two things: traveling inside a conduit (pipe) into a junction box, or travel inside a cable and the whole cable enters a junction box, so ground is inside the junction ...


4

It's a bit high but whether it will be a problem depends on your thermostat and how it tolerates the high voltage. Also consider that the furnaces controller board is also receiving this voltage. Before you start anything else, check to see if your thermostat can be calibrated and complete this step first. To rule out the higher voltage as the cause of ...


4

Yes you can terminate any number of connections on the C terminal. All of the connections will be in parallel. This is very common.


4

Um, I don't know where you got the term "power stealing". What you describe as that, is exactly the normal way to do that thing, and the right way... unless you are aware of some capacity issue on the transformer (as is sometimes seen in transformer-relay combos). As far as the R and C terminals, any HVAC transformer has two pins on the 24V side... They ...


3

If there's an easy way to get from the basement to the main ceiling, then you could just run an entirely new cable for your thermostat. There's nothing that says you have to follow the existing path. Just abandon the old cable. You could even take advantage of this to move the thermostat to a better location, if you were so inclined.


3

It appears according to the manual for the Mastertrol Automatic Balancing System (MABS) EZ Zone (EZ-2 and EZ-4) Control Panels (PDF), that M1 is used for C. Click for larger view This does not appear to be the manual for your device, so this information may not be correct. The manual for the MABS 2 Zone Panel (PDF), also shows M1 being used for C. ...


3

Normal voltage range for 24 volt HVAC systems is 20 to 30 volts. Normally if you are in that range to the upper/lower bound of accuracy of your measuring equipment you're fine. An induction transformer will read higher when lightly loaded. All of them I've ever worked with over the years do this, even modern switched mode supplies have a lower bound under ...


3

The obvious answer is to run a new thermostat cable with five wires instead of trying to use jumpers etc that in the end just end up causing more troubles, like destroying a circuit board. In most cases if can be as simple as attaching a new one where the old one goes up the wall from the basement if it's open. If it's a finished basement then a little more ...


3

Forget the wires. Use a wireless thermostat, placing the receiver in the basement at your furnace and the transmitter wherever you want it. The Venstar Add-A-Wireless Thermostat T1119RF might do the trick. If you musts have the wifi thermostat, put it in the basement, and add a remote sensor to the mix.


3

The C-wire substitution requires you to use the G-wire. When using the G-wire as a common you have to change it to C at the thermostat, and then to a common in the bottom panel of the unit. In your situation, your specific unit uses an older style control board made by White Rogers. This early revision of the board did not have a terminal bus on the board ...


3

You'll have to connect the blue wire in the thermostat cable at both ends. Start by connecting the blue wire in the thermostat cable to the Comm terminal in the furnace. Then connect the blue wire in the thermostat cable to the C terminal on the thermostat.


3

Okay... After reviewing your new photos, I think I have it figured out. This is what your system looks like You'll have to excuse the glitter pens, it's the only thing I could find on my wife's desk. It's pretty simple actually. When a thermostat calls for heat, the valve for that zone opens, and the boiler is also signaled via the auxiliary switch in ...


3

Note: the solution here is the querent's -- I'm simply writing it up in a CW answer to get this off our plate as the querent has not returned to migrate it from his question-edits to an answer himself. SOLVED (01/04/15): I decided that the T terminal was exactly what I needed so gave it a try and everything is working perfectly. I also pulled in some 18-5 ...


3

You'll start by wiring the furnace and A/C as follows: Red from the furnace connects to Red from the thermostat cable and Red from the ADD-A-WIRE™ device. White from the furnace connects to White from the thermostat cable. Green from the furnace connects to Green from the ADD-A-WIRE™ device. Blue From the furnace connects to White from the A/C condensing ...


3

"C" is not always labelled as such. If you can determine the two wires on the 24V side of the transformer (load side), then find which of the two wires on that side is running to "R" on the thermostat. The wire NOT running to "R" is always "C".


3

Technically, there should only be one wire connected to the screw terminal. However, it's common practice in the HVAC industry to connect more. If you want to do it the proper way, you should use a twist-on wire connector (or other connector), and a length of scrap wire to make a pigtail. Disconnect the wires that are currently attached to the terminal. ...


3

According to this document, the internal wiring of that relay looks like this. It sounds like it's connected something like this. When the thermostat calls for heat, the T terminals are connected through the thermostat. This causes the relay to energize, which closes the contact between terminals 5 and 6 (It also closes a contact between 3 and 4). When ...


3

I had the same problem with the same model switching relay and following the encouraging description given in a comment on Tester101's answer, made the same modification to expose a connection which provides the 'C' terminal. This is shown in the photo below. This is a view of the rear of the modified back of the circuit board (not normally visible without ...


3

You simply have to connect the new wire you ran between the "C" terminal on your thermostat and the "C" terminal on the furnace board. You can test with your multimeter, you'll get 24VAC between R and C terminals. Judging from your photo you have an air conditioner (it will be connected to Y and C), and there is already something else on C -- perhaps a ...


3

Since you have a R841C (with an integral transformer) You have your terminals on the relay confused. Connect RH to the HOT terminal from the relay and W1 to to the NEUTRAL terminal from the relay. C on the Nest is left unconnected, and you do not need a separate transformer here. If this was an R841D (standalone relay) You have your terminals on the Nest ...


3

Typical "thermostat wire" is a NEC/UL type CL2 cable, just like your speaker wire says it is, so as a replacement for an 18/2 thermostat cable, this will work just fine.


3

With only two wires between the thermostat and the boiler, you're not going to be able to power the thermostat, and call for heat. You'll need at least three wires for that. The C terminal on the controller is likely the COM terminal, though I can't say for sure. According to the schematic, pin 2 of the PCB connector is C. But I'm not sure where it's ...


3

According to the wiring diagram for your system (i.e. Figure 2 in the linked PDF) -- the yellow wire on the 24VAC side is the C wire (it's the wire that goes to the compressor but not to the 'stat in the diagram). Just incorporate the green wire from the thermostat cable into the nut with the yellow and green wires in it on the 24VAC side, and you'll be set!...


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