I found a furnace that had the common on the transformer grounded to the cabinet?and the common of the contractor for ac there also ?

I would isolate from ground, the relays and contactors need the 24 volts with no connection to ground.

Crazy ? I disconnected it and connected the common to the ac and disconnected additional common from transformer that was connected to ground also and it wouldn't run on heat. I know what your saying as for the common and leg of power . That's why I'm confused.

Your Heat, Compressor, and Blower Contactors/Relays have 1 side of the transformer connected, they get the other side of the 24 volts from the thermostat. You will need to use a unused wire, or use an additional wire.

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Originally Posted by Stratmando

Your Heat, Compressor, and Blower Contactors/Relays have 1 side of the transformer connected, they get the other side of the 24 volts from the thermostat. You will need to use a unused wire, or use an additional wire.

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Yes. But I don't know why the grounded the common.

Yes. But I don't know why the grounded the common.

They may have cheated? Not having the extra wire and were lazy, they used the ground, it would work, they may even have done it in the old days.
It is potentially dangerous as a problem could cause your low voltage and thermostat wiring to be Line Voltage Levels.
Find where the relay/contactor for heat connects to ground, and connect to your new wire.