Pump Losing Prime with Low Filter Pressure and Bubbles at the Returns
An 18,000-gallon plaster pool built in 2004. A 1 HP pump on a pad about 3 ft above pool water level pulls through a single skimmer on a 45-ft run of 1½-inch PVC; there is no main-drain line to the pad and no suction-side check valve. The owner reports the pump loses prime overnight and must be re-primed each morning. When it does run, fine bubbles discharge from the returns. The filter gauge reads 8 psi where the owner says it used to read 14 psi; the needle drops to zero immediately when the pump shuts off. The pump basket O-ring is flattened and hard, the lid has a hairline crack near its edge, and the skimmer weir is missing. Pool water level is at the middle of the skimmer throat. No water is visible on the pad and the pump seal shows no drip. There is no freeze protection on the equipment.
1. Diagnose the low filter pressure and the bubbles at the returns.
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Low pressure plus bubbles at the returns plus overnight loss of prime is the classic signature of air entering on the suction side. Air drawn into the pump reduces the mass flow the impeller can move, so the filter sees less flow and the gauge reads low; the same air is what you see discharging from the returns. A return-side restriction such as a dirty filter does the opposite - pressure rises, flow falls. Do not confuse the gauge dropping to zero at shutdown with a gauge fault: a filter pressure gauge should return to zero when the pump stops, and a needle that stayed at 14 psi with the pump off would be the defective condition. Note also that the pad sits 3 ft above water level, so the pump is lifting water and any small opening on the suction side will admit air rather than weep water - the absence of a visible leak at the pad is expected and not evidence against a suction leak.
2. Rank the likely causes and describe how you would confirm the leak.
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Rank by accessibility and by fit with the evidence. Most likely: the hardened, flattened basket O-ring and the cracked lid, both at the pump - the crack opens under vacuum. Next, the missing weir: without it the skimmer does not maintain throat level, and with the pump lifting 3 ft a vortex can form and pull air even at a normal water level. Then the underground suction line or its fittings, which is the expensive possibility and should be tested last. Field confirmation: with the pump running, trickle water from a hose over the lid, O-ring, and any visible suction fittings; if the bubbles stop while water is flowing, you have located the entry. Additionally, shut the pump down, plug the suction port at the pump inlet with an expandable test plug, and pressure-test the line back from the skimmer; a falling gauge or escaping air confirms a buried-line breach. Recommend the O-ring, lid, and weir be replaced regardless, since all three are consumables.
3. Why does the missing weir matter more than it looks on a pad-mounted-above-water system?
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The weir is a one-way dam: it floats and keeps the water in the skimmer throat at a level above the pool surface, so the pump draws a thin sheet of surface water rather than pulling air. Remove it, and the throat level drops as the pump draws; because this pump is lifting water 3 ft, the suction is already working harder, and a vortex can form at the throat and entrain air into the line. That air reaches the impeller, breaks prime, and shows up as the bubbles at the returns and the low filter gauge. The weir also keeps floating debris in the basket instead of letting it drift back into the pool when the pump stops. It is a low-cost part whose absence produces symptoms that get misdiagnosed as a buried-line leak, so replace it and re-check before recommending any excavation.