hvac
I have recently had an energy audit for my home and discovered a large amount of air leakage. As a result I have learned about Stack Effect, and how it impacts a home's ability to retain warm or cool air, and as a result it's energy requirements.
I wonder what the impact of an open top floor window would have on Stack Effect. Would a single window change the flow of air in the home enough to cause or exacerbate stack effect, given a home that is appropriately insulated and air sealed?
Rain a day after pointing work will help, not harm, the cure. Freezing weather would harm the cure.
High efficiency furnaces output low-temperature flue gas - more heat in your house, less up the chimney = higher efficiency.
Flue gas is always moisture-laden, and starting at a low temperature and then dumping into a cold masonry chimney means the moisture will condense out inside the chimney, rather than exiting as a gas and condensing outside the chimney. Depending on the gas quality, this condensate may be somewhat acidic, thus corrosive (attacks mortar, steel etc.)
It would be worth checking your furnace manual for chimney recommendations - many of the very high efficiency units actually want to be vented into PVC that's sloped to let the moisture drain out (or at least has a drain at the bottom, if vertical.) The temperature (on those furnaces) is so low that PVC is perfectly adequate for the temperature, and it has good chemical resistance. A stainless steel liner would be another possible option - a masonry liner might well not. In any case, what you use should comply with what the new furnace wants, which may not be what the old furnace had (or needed.)
I did this, almost exactly as you have drawn (in my mothers home when I was a teenager). You will need to support the back end if you cannot support the bottom "lip" of the AC from sliding forward. In other words, the AC will have torquing-force.
The reason I used a shelf for my AC was because I couldn't support the top. But since you are putting the AC at the top of the window, then you need to support the bottom from pushing both down and inward (or set it on a shelf). You will need to build/cut a groove (not just a front lip). A front lip is bad for two reasons: 1. A front-lip-only would mean that you would be able to lift the back of the AC up (when lift it, the bottom lip will slide out of the back of the window). 2. If you plan to unscrew the lip to remove the AC, it may slip forward suddenly (when the last screw comes out).
You need to consider taking the AC out of the window (and putting it in). So leave a little slop/space/room in the groove to lean the AC forward and lift it out. Normally it's not a problem, because you can lift most windows... but you will be at the top of the window. If the bottom lip is 1 inch wide, you will probably need to cut the groove 1 1/4" wide.
Also, I would add one or two more studs under the "ledge", so that it can't fall.
Best Answer
I think you're asking where air would come from in a modern, well-sealed home, in order to replace that which escapes through an open window. The answer is simply that there's no such thing as a "sealed" home. Even brand new, ultra-efficient homes have countless tiny leaks. Door seals. HVAC penetrations. Recessed light fixtures. Gaps in vapor barriers. They happen.
Older homes were so leaky that it was almost as though a window was open at all times. Modern homes reduce that leakage dramatically, but you're still probably talking the equivalent of a 3-4" pipe through the wall. By opening a window, the convection and escape create low pressure that dramatically increases the airflow through those leaks, probably on the order of ten times as much.
There's also the natural mixing that occurs, which has probably as much effect or more as the overall leakage volume.