Fiberglass Well Pressure Tank: What to Check Before Buying

A fiberglass well pressure tank is worth considering when steel-shell corrosion or handling weight is a concern. But fiberglass describes the reinforced shell—not the tank’s usable water capacity, internal separator or control requirements. Choose the construction for the installation environment, then size the tank for the pump and pressure settings.

What “fiberglass” actually means

These are composite pressure vessels, not simply hollow fiberglass containers. Pentair describes WellMate vessels as a polyethylene liner wrapped with continuous-strand fiberglass and epoxy resin. Flexcon’s Flex-Lite uses a polypropylene shell reinforced with fiberglass and epoxy, with a butyl-rubber diaphragm and polypropylene lower water chamber. The water-contact materials and the pressure-bearing reinforcement have different jobs. (WellMate construction, Flex-Lite specifications)

In a precharged well tank, incoming water compresses the trapped air. When the pump stops, that compressed air pushes water back into the plumbing. The tank therefore supplies water between pump starts; it does not produce pressure independently of the pump. A properly sized tank reduces frequent, short pump cycles. (Flexcon operating instructions)

Check that the product is actually a well-system pressure tank. WellMate also offers retention tanks and traditional air/water tanks. Similar-looking vessels do not necessarily have the same internal equipment or purpose. (WellMate product categories)

Fiberglass versus steel: the useful distinction

The principal material advantage is avoiding rust in a steel pressure shell. Weight can also make access and handling easier: Flexcon states that Flex-Lite is approximately 35% lighter than a comparably sized steel tank. That is a manufacturer-specific comparison, not a universal figure for every composite tank. (Flex-Lite product information)

A rustproof shell does not make the complete installation maintenance-free. There are still an air valve, water connection, internal separator and connected piping to consider. Flexcon calls for annual professional inspection for leakage or corrosion at the tank and its connections, and warns that aggressive water conditions can affect performance and lifespan. (Composite-tank maintenance instructions)

Do not assume that fiberglass automatically means longer life or better pressure. Compare the exact models’ drawdown, ratings, service parts and warranty terms rather than treating shell material as a performance rating.

Size by drawdown, not the gallon label

Drawdown is the water delivered between pump cut-out and the next cut-in. Total tank volume includes the air space and is much larger than usable drawdown.

For example, Flexcon lists its 50-gallon FL17 at:

Switch settings, cut-in/cut-out	Listed drawdown
30/50 psi	17.0 gallons
40/60 psi	14.7 gallons

These figures assume precharge set 2 psi below cut-in. Raising the operating pressure band reduces usable drawdown in this example, even though the physical tank is unchanged. (Flex-Lite sizing chart)

For a conventional on/off system, a preliminary sizing relationship is:

Required drawdown = pump delivery in gallons per minute × required run time in minutes.

If the pump delivers 10 gpm and its required minimum run time is one minute, the preliminary requirement is 10 gallons of drawdown—not a 10-gallon tank. Flexcon’s manual uses at least one gallon of stored water per gpm of pump capacity; check the pump/motor manufacturer’s actual runtime requirement before final selection. (Flexcon sizing guidance)

Use delivery at the system’s working head, not an unrelated maximum-flow figure. Our well-tank drawdown guide covers the sizing calculation. For a variable-speed drive or regulating-valve system, use the controller’s tank requirements instead of applying conventional sizing unchanged; see constant-pressure system choices.

Check precharge and repairability by model

Do not copy an air-pressure setting from another tank. Flexcon specifies 2 psi below pressure-switch cut-in for its composite well tanks. Pentair’s general WellMate FAQ specifies 4 psi below cut-in. Confirm the instructions applicable to your exact model and controls before adjusting it. Both manufacturers require pump power off and the tank drained for checking precharge. Flexcon also warns against adjusting precharge on a damaged tank or one showing diminished integrity. (Flexcon precharge instructions, WellMate precharge guidance)

Repairability is another separate question. Pentair says the WellMate air cell can be replaced, with instructions supplied in the replacement kit. Confirm the matching part and service procedure before buying; do not assume every composite tank has a replaceable bladder. Never disassemble a pressurized vessel. (WellMate air-cell service guidance)

Before approving a replacement

Ask the supplier or installer to confirm:

• Duty and potable-water suitability: the exact model’s approved application and certification.
• Capacity: drawdown at your pressure settings, matched to pump delivery and runtime.
• Ratings and protection: allowable pressure and temperature, plus the specified relief valve.
• Fit: connection type, dimensions, base support and service access.
• Exposure: freeze protection and current written instructions for the intended location, especially burial.

For perspective, Flexcon’s composite manual requires installation by a qualified professional, vertical mounting, a fully supported level base, no freezing exposure and a properly sized relief valve set no higher than 100 psi—even though the tank’s maximum working pressure is 125 psi. (Flexcon installation limits)

For burial, get confirmation for the exact model: the composite manual gives underground instructions specifically for the BG series, while the Flex-Lite sales sheet also advertises underground suitability. Do not treat that sales description as a complete installation procedure. (Composite-tank burial instructions, Flex-Lite sales sheet)

If the purchase is prompted by rapid cycling, first establish whether the existing system has lost precharge, effective drawdown or another control function. A fiberglass replacement is a material choice, not a diagnosis; start with water pressure tank problems.