fire sprinkler pressure tank(s)

I designed and installed a fire sprinkler system in the house I am building. The fire system has three CPVC legs. Each leg ends at a toilet so every time we flush it puts fresh water in the system. I had planned on using pressure tanks to supply the system since I'm on a well. However, I have recently found out that the volume of a pressure tank is not the working (drawdown volume). I need about 210 gallons of drawdown volume. Is this reasonably possible? I wanted to avoid a storage tank with an auxillary pump. I want the system to work at least a little while without electricity. Plus with all of the drawdowns for the toilet flushing I don't think an auxillary pump will work out for me.

The way I arrived at 210 gallons: The sprinkler heads use 14 gpm each. Code requires that I have enough water to run two sprinkler heads for 10 minutes. The well pump is 7 gpm. 280 gallons minus 70 gallons is 210. I know the well pump doesn't turn on right away but this will get me close.

The fire sprinkler system is not required in my area. But my neighbor's house burned down several years ago and I have been on several fire departments over the years. If one goes in it has to meet code.

Also, thanks for this forum. I also installed the dwv and Rehau pex systems with the help of this forum.

heads

At what pressure do the heads deliver that gph? Your storage tank's pressure and volume will decrease as water is drawn from it. The draw down from a storage tank is directly related to its initial air pressure charge. If you precharge it to a level just below the pump's turn on pressure, it will be "empty" shortly after the pump starts if it cannot deliver the demand from the sprinkler head, which yours cannot. As your precharge pressure is reduced, more water can be stored in the tank, but it will not be released until the pressure in the system degrades due to an excessive draw, and at the same time the system pressure will also be reduced to the level of the residual air charge in the tank at that point, so the sprinkler heads efficiency will also suffer. In order to have 210 gallons at 14 gpm and at any acceptable pressure, your storage tank may have to be in the neighborhood of 1,000 gallons.

You mentioned that "code" requires 2 heads @ 14 GPM. I expect the same code requres a sprinkler system to be designed and installed by a certified fire system contractor.

Out here , folks are opting for an auto-start gasonline driven pump taking suction from the swimming pool. This is for outside protection. The inside sprinklers are usually on city water. Your well situation requires a better design to be effective.