I have a Lennox system (G27M series). The transformer has a 3amp breaker. It trips time when the R wire and W wire are connected (heat is on).
To find out where the circuit is short, I attempted to measure the resistance between R wire and W wire, and found the reading is zero ohm (thermostat was taken off and furnace was unplugged). Is that normal? Is it a sign that the control board is broken?