Can someone tell me the correct formula for calculating the pressure tank volume using in domestic water application not for HVAC. I tried the most common one but it's generating too much big capacity tank even for less flow rates values and Its results are not logic at all. Thanks in advance.

You can use a pump with an integrated speed drive and keep your system under constant pressure. It will save space and have the optimum energy consumption plus premium features. Have a look for Grundfos E solutions like CR-e or for small domestic applications CM-E

To avoid switching ON/OFF surge/water hammer use a VFD

There is a very good article in the January issue of Plumbing Engineer magazine by Mr. Chris Edondson about how ASHRAE 90.1-2010 will affect domestic water pressure boosters. You may want to read it before you make any decisions on VFD's, tank sizes, sensor placement, etc.

Check Hydro MPC data booklet from Grundfos , there a lot of tools will support to select the right pressure tank

Thanks all for your answers.

Input value /output value
min relative pressure from pressure switch with calibration p2 (bar) 2
max relative pressure from pressure switch with calibration p1(bar)4
required flow ql/min 50litre
pump power p9kw) 0.75 kw
coefficient K k(min) 0.2267
usefull internal reserve 11.338
nominal volume of the tank V1(I) 28.3
refer back to varem tanks italy