This window has three 9' x 6' plate glass panes separated by about a one inch space. Between the outer pane and the middle pane there is a lot of moisture and the outer pane fogs up frequently. There is a breather tube (about 1/2 inch) drilled into the wood spacer between each set of panes (two tubes) but the copper tube is capped off inside the wall. Should I open this tube up and would that draw off the moisture? There is no moisture between the first and second panes.

So is this factory made thermopane or not? The moisture is condensation due to warm, moist air touching colder glass. Are you running a humidifier?

These windows were put in individually 32 years ago. There is a matching set on the other side of the same fireplace in the same wall. Each plate glass pane weighs about 350 lbs. We never had a problem with moisture until we had the windows cleaned in December. It took four men from the glass company to get the panes out and put them back. The right hand set is still without moisture. The company came back and plugged up the breather tubes in case that is where the moisture is getting in. But it hasn't helped at all; may be even worse. Since then I've done some research but the advice is conflicting. I know about breather tubes compensating for pressure differences at different altitudes. And I read that those tubes can be useful for drawing off moisture that gets into the window from a break in the seal. I'm looking for another opinion as to whether this theory is valid.

My theory is the left panel was not properly reinstalled. The outside panel is allowing too much cold air inside which then condenses on the outer surface of your inner most glass. Tell the glass company they screwed up and you are doing a "call back." They will know what you mean.