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Old 10-16-2012, 07:48 PM   #16
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New thermostat with no 'C' wire


mentalcase, watch out with the RTCOA radio that comes with the 3m Filtrete thermostat. I have a 3m-50, and within 3 months, the radio module died. I had nothing but problems with the RTCOA radio, so after it died, I got the Our Home Spaces radio and never have had any problems like I did with the RTCOA.

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Old 10-16-2012, 08:34 PM   #17
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New thermostat with no 'C' wire


Did you look at the venstar link I had posted? http://www.venstar.com/Thermostats/ACC0410/
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Old 10-16-2012, 08:43 PM   #18
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New thermostat with no 'C' wire


Why go to all of that work & expense, when they already have the wire there. Even if they did not have the extra wire, it would be no problem for them to pull up a new wire set to allow them to attach to C.

hvactech126, you need to learn how to make work easier, not harder.
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Old 10-16-2012, 08:57 PM   #19
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New thermostat with no 'C' wire


Greg, WHAT? There are 4 wires present and the module and Diode allow for use of the 4 wires to operate as 5.
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Old 10-16-2012, 09:04 PM   #20
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New thermostat with no 'C' wire


Again, why buy a extra item, that is not needed. Read post #10, the OP can do the fix without paying more for something that is not needed.
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Old 10-16-2012, 09:22 PM   #21
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New thermostat with no 'C' wire


Greg, that is IF he decides to run new wire and if he CAN run wire. While it looks like the wire is not stapled he does not know for sure.
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The wire looks to to be free for the most part the only issue might be fishing through the fire block, but I figure that was already notched out for this wire to be run.
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Old 10-16-2012, 09:30 PM   #22
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Even if it is stapled, there should be no problem pulling the new wire, if you would read that they have a direct route from the thermostat to the basement.
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Old 10-16-2012, 09:50 PM   #23
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calm down GREG.... Don't make mountains out of mole hills....
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Old 10-16-2012, 09:54 PM   #24
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hvactech126, who is getting upset. Looks like you are the only one getting all riled up, after someone pointed out that your effort is too much work & money.
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Old 10-16-2012, 09:59 PM   #25
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New thermostat with no 'C' wire


Deep breath guys.
Deeeeep breath.
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Old 10-19-2012, 10:26 PM   #26
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New thermostat with no 'C' wire


I have a similar problem. I have a separate heater and A/C, and only five wires running up to the thermostat. One is Rh and one is Rc.

To free up a line for common, I'm considering jumpering Rh and Rc in the basement, and then running up a single R line. Then I'd wire a common to the A/C and send that up the now-free line.

I haven't opened up the controller on the furnace, but the A/C is basically a bunch of separate components attached by wire nuts. So, I should just be able to wire the common into the nut shared by everything on the transformer pole that isn't connected to Rc. As far as I can tell on the A/C neither pole on the 24V side is grounded.

As far as I can tell jumpering Rh and Rc in this way shouldn't create any safety issues - I imagine that more than a few homes are configured this way by accident (jumper left in place on thermostat for a 5-wire setup). If the common poles were grounded on both units there might be an issue if they weren't in phase from what I can find online.

Can anybody think of something that might go wrong with such a setup? If so then I'd probably just break down and run another cable.
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Old 10-19-2012, 11:13 PM   #27
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New thermostat with no 'C' wire


Separate heater and A/C?

Do you mean a standard split system. or you have a heating unit that doesn't even run its blower when your using the A/C.

Posting pics often helps.
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Old 10-19-2012, 11:23 PM   #28
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New thermostat with no 'C' wire


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Originally Posted by beenthere View Post
Separate heater and A/C?

Do you mean a standard split system. or you have a heating unit that doesn't even run its blower when your using the A/C.

Posting pics often helps.
Heater is a summer/winter boiler, and the A/C is an A/C. They're in separate rooms actually. So, they're as separate as they get. The house is fairly old - the wiring for the heat is quite ancient, but I doubt there are any problems with it.

I've been doing more reading - it seems like jumpering two separate transformers is probably unwise. In theory it sounds like it should work as long as at least one of the 24V systems is not grounded. If both units are grounded on the common side and the reds are jumpered, then the transformers will be in parallel and likely short.

It also looks like if I don't jumper them then I can't use the common on the A/C either, since the thermostat expects it to be from the heat (though if they actually were connected by ground that wouldn't be a problem). My heater doesn't really have an accessible common (the transformer on that is on a circuit board and without taking the whole thing apart I doubt I could get to the other pole of the transformer and it has no common terminal).

I'm still interested in opinions, but the more I think about it the more likely it is that I'll probably just get a 24V AC adapter, run one more wire up to the thermostat, and jumper the other side of it to Rh.

A photo of the A/C would be amusing. The schematic on the panel is for a furnace, and inside it is basically just a bunch of anchored components attached by wires and nuts. That does make it easy to access the common on that, however.
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Old 10-20-2012, 12:13 AM   #29
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Then can't do it the way you said. Each unit has its own transformer.

Just run a new set of wires from teh A/c and use its Common.
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Old 10-27-2012, 10:02 PM   #30
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Then can't do it the way you said. Each unit has its own transformer.

Just run a new set of wires from teh A/c and use its Common.
That won't actually work. Most thermostats expect common to be paired with R - not Rc (though that doesn't matter if they're jumpered).

I was thinking about it and realized that no matter what I did I'd effectively have a transformer wired in parallel (a standalone one, or one from the heater/AC). There is a risk that a line is grounded in any of those cases.

I checked with a voltmeter and found 0V AC across the two R lines, which means there shouldn't be much current across them. I measured AC amps and got about 15uA, which is almost nothing. So, I went ahead and joined the lines freeing up a 5th wire for common, and ran that from the AC. Since the R's were now paired the common could come from either transformer.

All worked fine.

However, I do agree that this is something that carries a bit of risk with it, and anybody without a decent understanding of electricity should not attempt this, and anyone attempting it should check for voltage differences across any lines they intend to cross, and measure the leakage current. If somehow in the future both 24V circuits get grounded there is the potential to burn out a transformer, which is easily a $100 repair bill depending on the system and whether you call in help.

Oh, and in the process I learned that my AC wiring wasn't that great to begin with - somebody crimped a spade connector of the wrong size and it was hanging by just a few strands of wire and came loose.

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