Shower pump to existing gravity fed system

[Russt12]

I'm planning out a shower pump for my parents house.

I'm putting a twin pump by the hot water tank.

I see the S flange is useful to draw water from several inches down in the tank.

But since the tank vent is now under negative pressure, why does air not get drawn into the tank?

 

[cross thread]

The cylinder is under pressure from the cold water storage tank above it so it’s not under negative pressure