No the two H's have to have 6V between them. Its a 6V heater, either 6V DC or 6V rms AC.Its not a good diagram. I suspect A is signal ground, you measure the EMF developed at B relative to A (the op-amp circuit gives a high input impedance.I'd try to find a better datasheet for this kind of sensor. The mV is probably meant to represent a voltmeterMight this be the same sensor: http://www.parallax.com/Portals/0/Downloads/docs/prod/sens/CO2SensorDatasheetMG811.pdf - that implies you need a very high input impedance CMOS opamp to measure the signal (input impedance 10^11 ohms or larger).
Yes that's the same sensor. also, I do not have the money to buy the pre-made board as it costs $50. I don't know what to do :[