It's at least a gallon of humidity - seems excessive to me...
wait for it ~ ~ ~ ~ wait for it ~ ~ ~
A house that old probably isn't as air tight as a construction one.
Bingo~!!
A 250 year old house is more holy than the Catholic pontiff
My walls are made from oak beams, draw bored together.
In the original part my first floor joists, seen from the cellar, are round Oak logs with axe hewn squared ends only at the joints. In the the new addition the massive oak beams are hand squared with broad axes and then refined with Adzes. The upper floors of the building are made from water sawn spruce framing. It's an old building.
The outer walls all around the building have no sheathing. The clapboards were nailed right onto the frame and mortar and rubble was poured in the void for insulation. Makes for a hell of an interesting time when trying to find routes for plumbing, electrical and HVAC.
Suffice it to say that humidity in the air inside my house won't stay in the house very long.
However, to your original point about a gallon or three being a lot of water to inject into a house's atmosphere in so short a period.
That' a very complex matter because of all the variables.
How much water is in the air anyway:
If you precipitated all the water from all the world's atmosphere all at once you'd have maybe 2 inches of water on the ground every where (in a magical universe where the water just sat on the ground). That's a lot of water.
But the water is not evenly distributed in the atmosphere. High altitude air has much less water than sea level air. So most the water is pretty close to the ground. Even then, some areas are more humid than others. And of course water seeks a normative state so it migrates from wet areas to dry ones Much like salt in solution will seek to become evenly distributed in a given liquid volume. But unlike the salinity in a beaker, water in the atmosphere has all sorts of forces operating on it to concentrate it more in some areas than others.
And of course there's the fact that in winter, my house hold air could use a gallon or two of water because it's so dry. In the summer the AC will pull the water out of the air and condense it and dump it.
So all in all a few gallons of water liberated in any hour long boil won't make a meaningful difference in my house, or any house.
But that's not the whole story. Take that 2" of water that you'd get if you extracted all the water from the air at once.
Now think about spilling a few gallons of water across all the first floor of an average home.
That's not a lot of water in any one place. It'd be spread thin to cover all the first floor space with an even coat. Not thin at the molecular level, but so thin that you'd be hard pressed to be able to sop much up with a napkin.
It would not come close to that 2" mark, that's for sure. Hell I'd say it wouldn't even make an appreciable difference.
So all in all a few gallons of water is not a big deal, I don't think.
