Thermostat short circuit

A Common Problems when installing smart thermostat

You touched R and C wires while trying to install Smart Thermostat and now it’s not working?

What could be damaged if I shorted the R wire to the C wire at my thermostat?

R and C wires touched, and sparked?

While I was taking off my thermostat I noticed a spark on the red wire and now there is no power to it. I cycled the circuit breakers and still nothing?

Replaced an old thermostat. Wires touched creating a spark?

I installed Smart wifi thermostat and nothing seems to be working now. The system literally seems “dead.”

So how do I fix my smart thermostat?

I didn’t flip the breaker off before taking off the thermostat, it’s not working. What’s wrong with it.

These questions came up often on HVAC forums on the internet often. There are lots people answering the question I think the following answer is most complete one.

What happens if you forget to flip off the breaker?

If you forget to flip off the breaker when replacing your thermostat.

1-You might have fried the Hot(R) or Common(C) wires.The wires have to be replaced, get some 18/7 thermostat wires and replace them.

2-You might as well blown the fuse on the control panel. The fuse is just an auto fuse, it’s either 3amp and purple color, or 5 amp and ember color. Just get a new fuse and replace it if it’s blown.

3-The Worst case scenario is frying something major. Transformer or the control board going to be replaced because the fuse failed bad or melted into the control board.

If you fried the transformer or any other major parts do not attempt to fix it yourself.

Replacing your transformer is a more dangerous project since it has a much higher voltage connected to it, I advise you to contact a professional about the issue. If you want to do it your self, you can check out the videos on youtube, but beware, your fire insurance might not cover any mistakes you make during this DIY project. You have been warned.

What happened when you touched R and C wires?

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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. You Touched R and C wires. 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 the 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 https://en.wikipedia.org/wiki/American_wire_gauge#Tables_of_AWG_wire_sizes These values are an estimate and intended for example purposes only.

What to check if you touch R and C wires?

Thermostat Troubleshooting

The first thing you should check is to make sure the circuit breaker for the HVAC system is not tripped since you touched R and C wires.

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 ensure they are intact and not damaged.

You’ll also want to do a continuity test on the secondary of the transformer, to ensure 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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How to Fix R and C wire short circuit?

If the problem is the circuit breaker. Turn the breaker on and your problem is solved. If you fired a wire or your transformer you have to replace the damaged wires or transformer. Replacing the wires is fairly easy, you just have to buy an 18/7 thermostat wire and rerun the wires from the control panel to your thermostat.

Replacing your transformer is a more dangerous project since it has a much higher voltage connected to it, I advise you to contact a professional about the issue. If you want to do it your self, you can check out the videos on youtube, but beware, your fire insurance might not cover any mistakes you make during this DIY project. You have been warned.

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