How to Size a Pressure Tank, and Why the Cheap One Costs More

The smallest tank that fits is the most expensive component you can buy on a well system, because you pay for it in pump life rather than at the counter.

Pressure tanks are bought on price more than almost any plumbing component, and it is easy to see why. They all look like the same blue cylinder. One is $200 cheaper. The plumber says either will work, which is true.

The part that does not get said is that the cheaper one will have your pump starting two or three times as often for the next fifteen years, and the pump costs $1,600 to $3,800 to replace.

The short version. You are not buying a water container. You are buying pump starts, or rather the absence of them. That is the whole function.

Drawdown, which is not the number on the label

A tank marked 20 gallons does not give you 20 gallons of usable water. Part of the volume is the air charge that makes the whole thing work.

The number that matters is drawdown: the water you actually get out between the pump switching off at cut-out pressure and switching back on at cut-in. On a typical setup that is somewhere around a third of the nominal volume, and it varies with your switch settings.

So a 20 gallon tank might give you around 7 gallons of drawdown. Run a shower at 2 gallons a minute and the pump starts every three or four minutes. Halve the tank and it starts every ninety seconds.

Over fifteen years that difference is measured in tens of thousands of pump starts.

Why starts are the thing that matters

A submersible motor is not wearing out while it runs. Running is what it is built to do. The damage happens at the moment of starting, when the motor pulls a heavy inrush of current and heats up.

A pump that starts a handful of times an hour will see out its expected eight to fifteen years. A pump that starts every ninety seconds all day is being asked for years of starts in months.

That is why we size tanks toward the generous end. It is the cheapest life insurance available for the most expensive component on the system.

How the sizing is actually done

Three inputs, none of which is the size of the old tank.

• The pump’s flow rate. A pump delivering more gallons a minute fills the tank faster, which changes the run time a given tank buys you.
• What the household draws. Four bathrooms, a washing machine and irrigation is a different system than a two person house. Irrigation in particular is a long steady draw that punishes a small tank.
• The switch settings. The gap between cut-in and cut-out pressure directly sets drawdown. A wider gap gives more usable water per start, within what the rest of the plumbing is happy with.

The target is simple: the pump should run for a decent stretch when it runs, and it should not start again five seconds later. Everything else is arithmetic.

The step everybody skips

A new tank arrives pre-charged from the factory to a generic pressure. That figure is almost never right for your system.

The air charge has to be set against your actual switch cut-in pressure, with the system drained so you are measuring air and not water. Get it wrong and a brand new correctly sized tank will short cycle exactly like the failed one it replaced, and the homeowner concludes the new tank was faulty.

It takes a few minutes and it is the difference between a tank that works and a tank that is just installed. There is more on what happens when it is wrong in short cycling and what it does to a pump.

Bladder tanks and the old galvanized ones

Plenty of Shore houses still have a galvanized tank with no bladder in it, air and water sitting in direct contact. Air gets absorbed into the water over time, so the tank waterlogs on a regular cycle and somebody has to drain it to restore the charge.

If yours needs draining periodically, that is what you have, and a modern bladder tank ends the routine permanently. It is the most common upgrade we fit on older systems.

What to do at the same time

The switch, the gauge and the tank tee are the same age as the tank. Replacing them during the same visit costs very little extra and saves paying for a second visit when one of them fails next year. A tank replacement runs $600 to $1,500 with those included.

If the pump is also near the end of its life, the two jobs together are cheaper than the two jobs apart. See well pump installation and replacement and the full price list.

The service page is pressure tank installation and replacement.

What size pressure tank do I need?

It depends on what your household draws and on the pump’s flow rate, not on the size of the tank that is there now. The goal is long pump runs and few starts, because starts are what wear a pump out.

What is drawdown?

The amount of water you actually get out of the tank between the pump switching off and switching back on. It is a fraction of the tank’s total volume, which is why a tank labeled 20 gallons does not give you 20 gallons.

Is a bigger pressure tank always better?

Up to a point, and bigger is almost always kinder to the pump than smaller. The limits are space and cost, and we will tell you where the sensible stopping point is for your house.

Do I need to set the air pressure on a new tank?

Yes, and it is the step most often skipped. A tank is pre-charged at the factory to a generic figure that is almost never right for your switch settings. Setting it correctly at commissioning is what makes the tank work.

What does a pressure tank cost?

A replacement runs $600 to $1,500 installed, and it is worth doing the switch, gauge and tee at the same time since they are the same age.

Written by the team at Shoresteady Plumbing, Temperanceville, Virginia. Master plumbing licensed in Virginia and Maryland. Prices shown are typical ranges for the Eastern Shore, not a quote. Your written estimate is the only number that binds either of us.

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