After the tanks are drawn down, they will present a much bigger load on the pump as it tries to keep up with the water use and try to fill those big tanks at the same time.
Suppose, we have a 40/60 pressure switch. Suppose the pump puts out 4 gpm. Suppose the tank capacity is exhausted, and let's neglect the reserve between the air pressure precharge and the cut-on for this part:
- If the pressure tank is 20 gallons we get 4 gpm available for immediate continuous use.
- If the pressure tank is 172 gallons we get 4 gpm available for immediate continuous use.
Now suppose a different non-standard scenario. We have one 86 gallon tank (tank A) with a 36 psi precharge in parallel with a multi-dome construction tank (tank B) with a precharge of 28. Now when the pump goes on we could still draw some water down to 36 psi from tank A and B both. Below that, we could draw water down to 28 psi supplementing the 4 gpm from the pump.
What would be the effect of the 28 psi precharge on tank B? I think the diaphragm stretch gets limited by the dome, so it does not contribute beyond a certain pressure. However tank A is contributing fully.
Alternate water treatment scenario: Suppose there is an iron filter that needs 10 gpm for 8 minutes every 2 days at 2 am to backwash properly, but we have the 172 gallon tank with a 38 psi air precharge. We put a Fleck 5810 XTR2 valve on the iron filter. That offers flexibility. Instead of a single 8 minute backwash, we program in three 4-minute backwashes with 5 minutes between backwashes. The of the first 2 backwashes will be dropped to 4 gpm, so the media is not properly expanded. That could happen any time during the first backwash. The following backwash(s) will have enough volume stored to give a minimum of 4+4=8 minutes of backwash.
Most softeners use less than 4 gpm for their backwash, so a softener would not be a problem anyway.