My idea is to bury a ~250 gallon water tank in the backyard. The city domestic water would be plumbed to it and then the house would always draw water from it ensuring the water is always fresh in the tank.

City water pressure is 85 PSI, nominal. I'm guessing the tank would see intermittent higher pressures.

Is this doable?

Can a fiberglass tank work at those pressures? A metal tank would bring up more issues I don't like, including cost.

check around the web for well water holding tanks,some with bladders some without.250 is a lot but they have 120 gal or so size's.they would be plumed for the pipes you would be using.
check around for used ones and you could put a couple 120's in a line to get what you need.
a well water holding tank would be above ground and easy to maintain.

Think that with water pressure tanks that around 120 gallons is going to be about as large as you can find easily. You can use more than one tank to gain volume but the drawdown on a 120 gallon tank is roughly a third of the tank size so you would only get 30 to 40 gallons out of one before you lost pressure. A non pressurized tank would give you the full volume of the tank but would require a pump of some sort.

Consider running the tank as a non-pressurized system rather than pressurized. You'll probably need a float switch to the valve (like a toilet tank) that lets water into the tank so it doesn't overflow. Then a pump to get it into the house. Using the domestic supply as a direct line into the house won't work if the domestic system goes down. It will be without pressure so won't "push" the water into your home.

You'll also need some system in place like a double check valve to ensure you don't siphon back into the domestic system should it lose pressure. Otherwise there goes all your water. Additionally, you'll face fines if you hook it up the way you're looking at: static systems like that can't be hooked to a domestic system or they risk contamination from stagnant water flowing back into their system. Hence the double check valve to eliminate backflow.

"""The city domestic water would be plumbed to it and then the house would always draw water from it ensuring the water is always fresh in the tank. """


We have had several incidents where city water became unsafe to drink, usually relating to sewer pipes or drainage issues.

Would it make more sense to have it hooked up to city water for filling and draining, but not actively hooked up all the time?

my first thought was that if it's pressurized that the water would flow back into the city lines, but the check valves mentioned would stop that. I do like the idea however, it seems a good way to store water out of the way and unseen.