Boshart Tank Pressure Drops to Zero? Pro Troubleshooting

The root cause of this failure mode is a fundamental misunderstanding of the physics governing a captive air pressure tank. The air pre-charge in the bladder acts as a pneumatic spring, providing the force to push water out into the plumbing system. The system is engineered so that this air ‘spring’ is slightly ‘softer’ than the pump’s activation pressure. Specifically, the pre-charge must be 2 PSI below the pressure switch’s cut-in setting. When the pre-charge is incorrectly set above the cut-in pressure (e.g., a 42 PSI pre-charge on a 40/60 PSI switch), the bladder can expel the entire volume of water from the tank, with the internal pressure dropping from 42 PSI directly to 0 PSI as the last of the water is ejected. The system pressure only reaches the 40 PSI cut-in threshold after the lines are empty, forcing the pump to start against zero back-pressure, a highly damaging condition.

This ‘zero-pressure start’ inflicts severe stress on the pump’s electrical components. A submersible motor starting under no-load conditions experiences a massive inrush of current, far exceeding its normal startup amperage. This instantaneous demand generates significant heat in the motor windings, which can degrade the winding’s enamel insulation over time, leading to turn-to-turn shorts and eventual motor burnout. The pressure switch contacts also suffer. Instead of breaking a live load under stable pressure, the switch closes on a system with no back-pressure, causing a more violent electrical arc across the contacts. This repetitive, high-amperage arcing leads to pitting and carbon buildup, eventually causing the switch to fail prematurely.

Mechanically, the consequences are just as severe. When the pump activates and instantly sends high-pressure water into an empty plumbing system, it creates a powerful hydraulic shock, commonly known as water hammer. This shockwave travels at the speed of sound through the water, slamming into the pump’s internal check valve, the impellers, and the thrust bearings. For Boshart’s multi-stage pumps, this can cause fatigue cracks in the Noryl or stainless steel impellers and puts immense axial load on the thrust bearings, accelerating wear. Furthermore, this violent jarring action can compromise the motor’s mechanical shaft seals. Over time, these repeated shock events can allow trace amounts of water to bypass the seals and enter the motor housing, contaminating the dielectric oil fill and leading to catastrophic failure of the entire pump and motor assembly.

• Identify Pressure Switch Settings: Before any adjustments, you must know your target. Turn off the two-pole breaker supplying power to the pump. Remove the plastic or metal cover from the pressure switch (typically a grey box near the tank) and look for a label or stamping inside that indicates the cut-in and cut-out pressures, such as ’40-60′ or ’30-50′. The lower number is your critical cut-in pressure.
• Isolate and Completely Drain the Tank: With the power still off, close the main ball valve located on the pipe after the pressure tank. Then, open the lowest faucet in the house (like a basement utility sink or outdoor spigot) and let it run until all water and pressure are completely gone from the tank. The tank must be at 0 PSI water pressure to get an accurate air reading.
• Measure the Existing Air Pre-charge: Locate the air valve (Schrader valve) on the top of your Boshart PT/PTD tank, protected by a plastic cap. Use a reliable, low-pressure tire gauge to check the air pressure. This reading is your current pre-charge. It is highly likely you will find it is higher than the cut-in pressure you identified earlier.
• Adjust Pre-charge to Specification: The correct pre-charge is exactly 2 PSI below your pump’s cut-in pressure. For a 40/60 switch, set the air to 38 PSI. For a 30/50 switch, set it to 28 PSI. Use a bicycle pump or a small air compressor to add air, or press the pin in the valve to release air until you hit the precise target.
• Monitor Pump Cycle and Amperage Draw: After restoring power, observe the pump’s behavior. A properly charged tank should allow for a minimum pump run time of 60 seconds for most residential pumps. If you own a clamp-on ammeter, safely clamp it around one of the hot wires going to the pump (at the pressure switch or control box) and verify the running amps are at or below the Full Load Amps (FLA) listed on the motor nameplate.
• Inspect Thermal Overload Protection: If your system has an external control box, locate the thermal overload reset button (often a red button). If this button has been tripping, the hard starting caused by the incorrect pre-charge is a very likely cause. Continued tripping after the fix indicates a more serious issue with the motor or wiring.
• Confirm Elimination of Water Hammer: Once the system is recharged and operating, listen for the absence of a loud ‘bang’ or ‘thud’ in the pipes when the pump kicks on. The correct air charge provides a cushion that should eliminate the severe hydraulic shock.

For a standard service call where the issue is confirmed to be an incorrect air pre-charge, a homeowner can expect to pay between $150 and $325 USD. This price typically includes the service call fee, one to two hours of a licensed technician’s labor, and the use of professional diagnostic instruments. This assumes the pressure tank itself is functional and no parts are required. The repair itself is quick, but the cost reflects the technician’s expertise in correctly diagnosing the problem without creating new ones.

If the diagnostics reveal a failed bladder, the scope and cost of the job increase substantially. The cost of a new Boshart PT series pressure tank can range from $250 for a small PT14 to over $700 for a large PT86, plus 2-3 hours of labor for draining, removal, installation, and disposal of the old unit. A complete tank replacement will generally fall in the $700 to $1,400 range, depending on tank size and site accessibility. Should the megohmmeter test indicate a failing motor requiring the pump to be pulled, the cost escalates significantly. A full pump replacement job, including the use of a pump hoist rig, a two-person crew for several hours, and the cost of a new pump, motor, wire splice, and check valve, will typically range from $2,000 to $5,000+.