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I just recently installed an Ecobee3 lite and am having an issue with it in the heating mode.
My system consists of a Goodman furnace (GMNT120-5D), Goodman Condensing unit (M/N buried under snow at this time), and General Aire Humidifier.
My existing t-stat was a White Rodgers battery powered non programmable model. A 5 conductor t-stat cable was run, without connecting the C wire. The control board on the furnace only has the R, W, G terminals. A separate 24V transformer is mounted above the board.
I connected the wires at the new Ecobee t-stat per the instructions (using the C wire, not the PEK) and had this part verified as correct by Ecobee support after sending them a picture.
At the furnace I left all of the other wire terminations alone and then connected the C wire to the 24V transformer.
For the first couple of heating cycles everything is fine and operates as it should. Then the system will call for heat, turn the hot surface ignitor on, open the gas valve, burners light and then immediately shut off. It will keep doing this even if I cycle power off and back on.
For now I have disconnected the C wire and reinstalled my old thermostat until I can get this figured out and I am hoping that some one here can help me.
I have sent pictures to Ecobee but it doesn't seem to help.