Yesterday I tried to replace my old bell-style thermostat with a new digital one. It turns out that the new one requires a 24vac line that the old bell-style model didn’t need. I put the multitester to the other wires that were not previously connected, but I got that two of them were 15vac and one was 19vac. Back in my days studying electronics at tech school, I recall that when doing TTL logic that required 5vdc, 4.2 volts was considered close enough and that it was actually somewhere around 3.5 volts that the logic gates would start tripping erratically. So, what I’m wondering is whether the 19vac wire is supposed to be my 24vac wire, and if 19 volts is enough to power the unit.
You need to trace the wires.
Go to your AC unit and open it up. You’ll see a schematic inside, and you should see where the control wires from the thermostat connect. Find the 24AC transformer, and see if it’s connected to any of the thermostat wires. Sometimes older thermostats don’t use a ground, and the wire is disconnected. I wouldn’t just hook up any old wires and expect it to work. You must know which are your power supply lines…
Looking at the AC unit, the panels don’t seem to want to come off, and I’m not eager to press the issue until I know more, but the diagram on the front showed an array of screw posts, and one of them was the C-line I’m looking for. In theory, if I got to that and I saw which color wire was connected to that, that would be my thermostat power voltage? And I should read 24vac between that and the R-line?
In theory, C is common from the transformer, and R is hot, so yes, those two should be your 24vac.
Unless your unit is a Lennox,
R= 24V Hot red wire
W= Heat white wire
G= Fan green wire
Y= A/C yellow wire
C= Common side of transformer. (often this the blue wire)
Many times 5 wires weren’t pulled to the stat because years ago houses only had heat and 2 wires were sufficient. Typically, now 4 are needed, and often 5 (for the common)
Not all stats require a common connection—and for many it is optional. However, some require it to operate. Look at your literature.
On your transformer there will be leads for the incoming power (typ 115V) and 2 leads for the secondary. They are often yellow and blue, or red and blue or other combination. Trace them out to find the common.
That will be in the furnace, not the A/C unit.
Yes. Typically R is hot, and C is common; 24V between them.
No. Many times you won’t have a good path to common at the stat so while 24V is likely present in the R wire/terminal, the other wires aren’t providing you a good path to ground/common.
It is likely that there is no wire presently at your stat location that goes back to common, for obvious reasons. (your old stat didn’t need it)
Were 4 wires hooked up? If so, they were Power, Fan, Heat and Compressor (A/C)
Were there extra wires present at the stat location in the jacket? If so, hook one of them up to your C terminal and then go downstairs and hook that same wire to your C terminal in your furnace.
You can do it. We can help.
In case I wasn’t clear before…
If you don’t have a good path to common you will get some funky readings from your meter at the stat location. Don’t trust what your meter tells you unless you know you have a clear path to common.
Typically, 19V would be a problem. You should be reading better than 20V and really more than 22V. Generally, you’ll read between 22V and 27V, with 24/25V being most common.
And if you have a camera phone and want to PM me I’ll walk you through it if needed…
It turns out that the secret to opening the panel is to slide it upward first. Beyond that I can see the terminals mentioned, and I can also see all the unconnected wires from that cluster at the thermostat. I took a picture, but typically it came out extremely blurry. Suffice it to say that I can clearly see the terminals and all the wires that lead to the thermostat, and another set that lead away somewhere else, I presume to the heater or something. It looks like I could strip the black wire, connect it to the terminal labeled Com 24U, then hook the other end of the black wire to the C post of the thermostat mounting plate. That end is a bit of a problem because that clutch of wires all bound in a common sheath have only so much reach – it won’t stand to be pulled more than a centimeter out of its hole even with pliers, and the terminals I need to screw them down to are too far apart.
So, it looks like I’ll have to do a delicate splicing maneuver, with little room to work with. I don’t have any of this gauge of solid wire lying around, but I have stranded wire of about the same gauge that I could salvage from an old power supply. Stranded wire is not so good for a solid Western Union splice, but I could solder it in place. Or should I be trying harder to pull the wires out of the wall? Would they really have left this little excess room?
You’re doing good so far. It looks like you have extra conductors available to you for a common. I assume that the panel you’re referring to is the furnace and not the outside A/C unit.
The other wires leading somewhere else are probably the wires leading to the A/C unit outside. (Once again, you should be indoors at the furnace, not outside with the A/C unit)
Yes, it is a stupid thing to do, but there is no shortage of idiots. A thinking, considerate professional realizes that maybe 5 years from now there may actually a need for those extra conductors and leaves excess wire and tucks it back into the wall. The idiot snips it off close.
You can try a little harder with a pair of pliers. (lineman’s or stout long nose) Don’t tug on them. Grab them and apply steady pressure and see if there is any give. Don’t go crazy pulling! You can simply break the wires. They’re not delicate, but they’re also not going to stand up to you putting your shoulder into it. They may have stapled the wires to the studs before drywall. (another stupid thing to do, as leaving the wires looses in the wall gives the opportunity to use the old wire as a lead when wanting to pull new wire)
If there is no give at all, you’re either doing the ‘delicate splice’ or your pulling new wire into that wall with a fish tape. (something that may be tricky if you haven’t done it before and don’t have a fish tape) One caution: I’ve applied pressure on the jacket and had the wire snap back into the wall. It can be a pain to fish it out of the wall sometimes. Be easy loosing the tension with the pliers.
Lastly, the outer jacket can be pealed back if needed. It’s just plastic. It’s just an outer jacket with 5 conductors inside. (or 6, 7, 8 or even 10) Hopefully you won’t need to pull new wire.
But you’re clearly on the right track.
Important
Do this with the switch to the furnace off. 24V won’t hurt you, and most often feels like an unpleasant tingle.
However…if you accidentally touch the 24V Hot and 24V Common together you’ll create a direct short and toast your transformer.
I haven’t had a common ground to short out yet so far, but I have noticed that while unscrewing the red line I’ll get little sparks even though I switched all the AC/Heater power off at the fuse box. My best guess was that somewhere in the system a capacitor was still holding a charge. But there is an additional switch at the indoor unit, kind of like a light switch. That does seem to stop the unit from running. It would be nice if that were sufficient, since the journey to the fusebox is a harrowing one.
It is that switch you need, no other. Turn it off.
That spark is because the unit is on, and that is evidence that 24V is present, for sure.
There are no capacitors in the low voltage circuits.
You were likely switching off only the A/C unit.
The 24V comes from your furnace which is always on a separate breaker.
Alright. The unit is plugged in and apparently working. Thanks everybody for the advice. In particular, the fact that the one switch was really all I needed to turn off saved me from trips back-and-forth through the jungle that surrounds my fuse box, which was a tremendous source of hassle.
I solved the wire problem by using a butt connector. It seemed like an ideal solution. Crimp, crimp, and Bob’s your uncle, right? One of these days I should get so lucky. My mortal strength proved a poor substitute for the Crimp-o-tron 9000 the manufacturer seems to have assumed I’d have access to. I actually broke the crappy needle-nose pliers that came with my PC repair toolkit.