Heat pump water heater location ?

[Ludington]

HTP has stainless steel tanks and lifetime warranty, Rheem Marathon is composite and lifetime warranty. Rheem 4 year warranty extension kit adds a second anode rod. Most water heaters with a longer warranty are the same tank with a larger or additional anode rod. Depending on my needs I would either by a standard tank and check/change the anode regularly, or get a tankless if I have gas, currently I only have electricity so I would probably do a lifetime tank with a thermal mixing valve for capacity.

I was looking at the marathon information, they come with a vacume pressure relief valve, (so the tank doesn't collapse from negative pressure) how the heck do you get negative pressure in a hot water heater that's essentially preasurized all the time ?