In a chlorination system the pump gets the attention and the contact tank does the work. Chlorine needs time in contact with the water to disinfect, and the tank is what provides it. Undersize it and the system doses correctly, smells correct, and disinfects incompletely, with nothing at all to indicate the shortfall.
Why contact time is the variable
Disinfection depends on chlorine concentration multiplied by the time it is in contact with the water, so a given result can be reached with a higher dose over a shorter time or a lower dose over a longer one. At home the sensible balance is a modest dose with real retention, because a high dose is unpleasant to handle, harder on the carbon stage that follows and more likely to produce taste complaints. The tank is what buys the time, and everything else in the system is arranged around it.
Size on peak flow, not on daily use
This is where sizing goes wrong. A tank that gives ample retention at average draw may give very little when a shower, a washing machine and an outside tap run together, and that is precisely when the most water is passing through. Work from the plausible simultaneous draw of the house. On a well, also consider the pump's delivery rate, since the flow through the tank is set by the pump rather than by anyone's choice, and a large pump pushes water through a small tank quickly.
Short circuiting, and the tank's internals
A tank only provides retention if the water actually stays in it. Poorly arranged inlet and outlet connections let water take a short path from one to the other, so the nominal volume overstates the real contact time considerably. Tanks intended for this duty use inlet and outlet arrangements, and sometimes internal baffles, that force the flow to travel the length of the vessel. A repurposed pressure tank without those features will give less contact than its size suggests.
Proving it works
The check is a chlorine residual measured downstream of the contact tank and upstream of the carbon filter. A measurable residual there shows the water's demand was satisfied with enough chlorine left over to disinfect through the retention period. No residual means the dose is too low, the solution is stale, the injector is blocked or the demand has risen. The EPA notes private wells are not federally regulated under the Safe Drinking Water Act, so this monitoring is the owner's. Our well chlorination guide covers the whole system.
Questions people ask about chlorine contact tank
Why is a contact tank needed at all?
Disinfection depends on chlorine being in contact with the water for long enough. Without retention it reaches the taps before it has acted.
How is the tank sized?
From the household's peak simultaneous draw and the pump's delivery rate, not from daily or average use.
What is short circuiting?
Water taking a short path from inlet to outlet, so real contact time is far less than the tank's volume suggests. Baffles and connection placement prevent it.
How do I check it is working?
Test for a chlorine residual downstream of the tank and before the carbon filter. A measurable residual confirms demand was met.