The pressure tank is the most misunderstood component of a private well system. Most homeowners don't know what size their tank is, what the pre-charge pressure should be, or that an undersized tank can cut their pump's life expectancy from 10–15 years down to months. This guide covers the actual engineering — the drawdown formula, pre-charge rules, and the 1-minute runtime rule that every Central Florida well owner should understand.
What a Pressure Tank Actually Does
A submersible well pump is a high-amperage electric motor that doesn't like short, frequent starts. Every startup draws 4–7 times the motor's running current and generates significant heat. Without a pressure tank, the pump would cycle on every time any fixture opened — thousands of cycles per day, and a burned-out motor in weeks.
The pressure tank's job is to hold a usable volume of water at pressure, so small water demands (flushing a toilet, washing hands) draw from the tank rather than starting the pump. The pump only starts when tank pressure drops below the cut-in pressure, and runs until it reaches the cut-out pressure. The water delivered between those two pressures is called the drawdown.
The Drawdown Formula
The engineering equation used industry-wide:
Drawdown = [P1V / (P2+14.7)] − [P1V / (P3+14.7)]
- P1 = pre-charge pressure (absolute)
- P2 = cut-in pressure (absolute)
- P3 = cut-out pressure (absolute)
- V = total tank volume in gallons
- Add 14.7 psi to any gauge reading to get absolute pressure
Worked Example: 85-Gallon Tank
At a 30/50 switch (30 psi cut-in, 50 psi cut-out): Drawdown ≈ 25.1 gallons (acceptance factor ~0.295). That means the 85-gallon tank delivers 25 gallons of water between pump starts.
At a 40/60 switch (40 psi cut-in, 60 psi cut-out): Drawdown ≈ 21.9 gallons (acceptance factor ~0.258). Higher pressure feels better at fixtures, but you get 3 fewer gallons per cycle.
At a 20/40 switch: Highest acceptance factor but lower delivery pressure. Rarely used in modern residential installations.
The Pre-Charge Rule
Pre-charge pressure (the air pressure in the empty tank, measured at the Schrader valve on top) must be 2 psi below the cut-in pressure:
- 30/50 switch: pre-charge 28 psi
- 40/60 switch: pre-charge 38 psi
- 50/70 switch: pre-charge 48 psi
Why this matters: if pre-charge is too low, the bladder collapses too soon and effective drawdown drops. If pre-charge is too high (above cut-in), the tank never fills properly. Check pre-charge annually with a standard tire gauge — the tank must be drained of water first for an accurate reading.
The 1-Minute Runtime Rule
Industry standard for pump protection: minimum 1 minute of pump runtime per cycle, 2+ minutes ideal, and at least 1 minute off between cycles. This gives the motor heat time to dissipate into the surrounding water.
Why 1 minute? A submersible motor below the water table is cooled by the water flowing past it. Startup heat takes about 60 seconds to fully transfer away. Runs shorter than that concentrate heat in the motor windings; repeated short cycling causes cumulative thermal damage that ends the motor well before its mechanical lifespan.
At typical household flow rates (8–12 gpm), you need roughly 8–24 gallons of drawdown to meet the 1-minute rule. That's why an 85-gallon tank with 25-gallon drawdown is a reasonable baseline for most 3–4 bathroom homes.
Bladder Tanks vs. Air-Over-Water
Older systems used air-over-water tanks — no bladder, just air pressurizing water directly. Over time, air dissolves into the water, the tank waterlogs (fills with water, loses pressure reserve), and the pump short-cycles. These required annual air recharging and were prone to failure.
Modern bladder tanks use a butyl or polypropylene bladder to separate air from water. The air charge stays on its side; water fills the other. Waterlogging is eliminated under normal operation. Bladder failure (after 10–20+ years) is the main long-term concern.
Signs of a Waterlogged Tank
- Rapid pump cycling — on for 15–30 seconds, off for 15–30 seconds, repeat
- Pressure gauge spikes instead of rises smoothly
- Tank feels unusually heavy (water-filled rather than air-filled)
- No air escapes when you press the Schrader valve (should release some air)
- Reduced or no drawdown — pump starts the moment any fixture opens
If the tank is air-over-water style and original, it can sometimes be recharged. If it's a bladder tank showing these signs, the bladder has failed and the tank needs replacement.
What Short-Cycling Does to Your Pump
Submersible pump motor startup draws 4–7× the running current. Each startup:
- Heats the motor windings
- Stresses the starting capacitor
- Wears the motor bearings
- Cycles the pressure switch contacts
A pump designed for 10–15 year service that cycles 4× per minute all day can see motor failure in months rather than years. The economics: a 25-gallon-drawdown tank upgrade costs $300–$600. A pump replacement costs $1,500–$4,500. The math works every time.
Sizing for Central Florida Homes
General guidance based on household size and pump GPM:
| Home Size | Peak Demand | Recommended Tank |
|---|---|---|
| 1–2 bath | 6–10 gpm | 36–46 gallon nominal |
| 2–3 bath | 8–12 gpm | 62–86 gallon nominal |
| 3–4 bath | 10–16 gpm | 86–120 gallon nominal |
| 4+ bath with irrigation | 15–25 gpm | 119+ gallon nominal or multiple tanks |
"Nominal" is the volume listed by the manufacturer. Actual drawdown depends on pressure switch settings and pre-charge (see the formula above).
Common Mistakes
- Assuming tank size from label volume. An 85-gallon tank at 40/60 delivers only 22 gallons of usable water.
- Skipping the pre-charge check. New tanks ship with pre-charge set for a 30/50 switch; if you run 40/60, you must increase pre-charge to 38 psi.
- Adding a "bigger tank" without checking switch settings. Doubling tank volume doesn't help if short-cycling is caused by switch hysteresis.
- Ignoring short-cycling for months. Every short cycle accelerates pump death.
Get Help Sizing Your System
Quality Filters And Pumps handles the full stack — well drilling, pump installation, pressure tank sizing, and ongoing service. Free site assessments across Marion, Alachua, Citrus, Lake, and Orange Counties. Chase Norris is a Florida licensed water well contractor with 15+ years of experience in Central Florida karst.
Call (352) 268-9048 or contact us online.