Deaerators are generally controlled at a low pressure. You put in enough steam to heat the feedwater (mix of return and make-up) so it reaches the saturation point at the design pressure. You then vent the top to get rid of the non condensable gasses that come out of the water, with the most concern being oxygen. So, if you don't get rid of the O2, you could pit the tubes in the boiler and get a failure, but the conductivity would be little effected.
From memory, the stripping stream is 1/4 of 1% of the heating steam. The amount of steam vented from the deaerator should be just enough to form a slight plume.
Conductivity is only slightly effected by the deaerator.
