What happens when you burn hydrogen?
Imagine a sealed, rigid container holding 2 parts hydrogen and 1 part oxygen. A spark in the center will set off the burn, and the result will be water vapor.
My speculation is that the flame will move outward from the point of the spark, and that the H and O will burn along the path of the flame. Most of the H and O will be consumed but there will be residual atoms that weren't close enough (not tightly packed) to combine. That will be a very low-pressure area.
The flame itself will bring heat, which will cause the water molecules and remaining H & O to move more quickly: this creates a high-pressure wave. As the combustion consumes the remaining available H & O, the overall pressure in the container will decrease.
So my guess is that if you start with the gases at 1 ATM, there will be a brief over-pressure up to say 1.3 ATM that will last for a very short time, followed by a quick dropoff of pressure down to say .1 ATM.
As the contents cool, the vapor will fall to the bottom of the container and the temperature of the remaining gas in the container will drop to ambient.
Will the entire process be exothermic? Or will the condensation effectively remove the heat energy from the container?
These are the things I think about late at night or early in the morning.