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OK... so in trying to install a thermostat (and testing to see if the old thermostat was bad) I inadvertently jumped the R and C... (I actually jumped the R and G terminals but the G terminal was connected to the C terminal back at the HVAC). So I shorted it out, no? How bad might the damage be?

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Why don't you hook it up correctly and see if it works? And then worry about what might be broken if it doesn't work. – Steven Jun 29 '13 at 19:29
Thanks - well, I did and it does not work! At least I think I did. – Ken Feds Jun 29 '13 at 19:47

If you short the R wire to the C wire, you've effectively created a very low resistance closed circuit across the secondary of the 24V transformer in the HVAC system. This causes the current in the circuit to be high, which may quickly overheat the wires. In a typical household circuit, the circuit would be protected by a circuit breaker (or fuse) which is designed to open (trip) to prevent damage to the wires in this situation. Since the thermostat is on the secondary side of the transformer, the circuit breaker for the HVAC system may react too slowly, or not at all.

Ohm's law says that current (I) in a system, is equal to voltage (V) divided by resistance (R). If we assume it's a 24V transformer, and that there's 50' of 18 AWG wire between the transformer and the thermostat. So voltage is 24 volts. We'll say resistance is about 0.6 Ohms; since in this example we have 100' of total wire in the circuit (50' R, 50' C), and 18 AWG wire is about 6 Ohms per 1000'.

V = 24
R = 0.6
I = V/R = 24/0.6 = 40 amperes.

Since 18 AWG wire is only rated to carry 14 amperes @90°C, the 40 amperes running through the wire could be enough to damage it.

Wire resistance and ampacity values from WikiPedia
These values are an estimate, and intended for example purposes only.

The first thing you should check, is to make sure the circuit breaker for the HVAC system is not tripped.

If the breaker is not tripped, it's possible that the coil wires in the transformer or the thermostat wires are damaged. You'll want to start by doing a continuity test on the R and C wires, to insure they are intact and not damaged.

You'll also want to do a continuity test on the secondary of the transformer, to insure it's not damaged. To do this, you'll have to completely disconnect the transformer from the wiring, which means you'll also have to turn off the circuit breaker (and make sure power is off with a non-contact voltage tester).

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Very good answer. 40 amps on the secondary is about 8 amps on the primary (40 x 24/120 = 8) neglecting the transformer resistance, not enough to trip a breaker. It is less than that since there is resistance of the transformer secondary (i.e., the 0.6 in your calculation would have been a lot higher). P.S., G should be the fan and shouldn't be connected to C, to R I could understand if the fan were always on. – Richard Raustad Aug 29 '13 at 4:20

I encountered a similar issue, except that the root cause may have been the 3M radio stat (wifi enabled) thermostat ran out of batteries... which it uses instead of the input voltage (R) to control the thermostat. The manual says never let it run out of batteries (which seems impossible... bad design). In tracing the issue, I found a light on my furnace/air handler that indicated a fuse had blown, found the blown fuse and replaced it. I put everything back together, and no AC. So I thought maybe my fancy wifi thermostat was bad, and reconnected the original. It powered up (using R) but when I set it to cool, the AC didn't turn on. So to eliminate a short in the wiring, I did a continuity test (resistance) between the common (C)/black and any of the others (Y, G, W) except R and they all appear as a shorted to C. R is open. So it seemed to power on the thermostat fine, but then when it went to supply power to the Condenser Relay on the outside unit, the old thermostat just poof smoked and it appeared a relay on the thermostat PCB had met its maker. So it appears that when the R is switched to the Y and G to turn on the AC and fan, the shorts don't make things happy. Next, I went to the furnace/air handler, and opened it, found the wiring on that end, disconnected from the screw terminal bank and tested. I no longer saw the shorts from C to Y/G/W, on the wire (whew, the short is not in the walls). I suspect that the transformer is bad. In fact everyone is saying 24V (and there's some confusion if that's supposed to be AC or DC on the internet). So I measured my transformer secondary voltage, it said 32VAC on my multimeter. Then turned off power, and checked the continuity of the secondary side (Red/Blue) and they appeared as shorts to one another. Then tested the primary side (White/Black) and they appeared as shorts to one another. Luckily primary to secondary of the transformer was not appearing as a short.

So now the question... In the final answer above, they said we'd want to do a continuity test on the secondary of the transformer. Can anyone describe what the correct process to diagnose the transformer is? I'm fairly sure this is the part that got blown up by my wifi thermostat running out of batteries... but want to be sure before running around town buying parts and swapping them out.

As an update, I think I was the culprit after all. I'd been doing some framing work in my attic, I traced the issue to a short between the Y in the furnace going outside to my condenser unit's relay. I troubleshot by removing the spade connectors from the relay outside and found I still had a short. So I tested if other colors in the same bundle had the short and they were fine, so I moved over to another color pair in the bundle and that worked. So I'm guessing I hit the 1-in-a-million shot with my nail gun and put a nail between the hot and common wire out to the outside relay. The batteries running out seems to have been coincidence.

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