A pump that has lost prime is the quietest failure on the equipment pad. The motor runs, the timer says the system is on, and from the yard nothing looks wrong. Inside the strainer pot there is a pocket of air, or nothing but air, and the pool has stopped circulating. Nothing is being filtered, a salt cell or inline feeder that needs flow has stopped dosing, and the pump is heating its own shaft seal. Left until the next weekly visit, that can mean a green pool and a seal replacement at the same address.
The complaint covers two different faults. A pump that will not prime at all is usually starved of water, or is drawing in so much air at the pump itself that it can never clear the suction line. A pump that primes and then loses it later usually has a smaller air leak, a water level that drops during the day, or a suction line that drains back while it is off. The checks overlap, and the order you work through them in is what saves the time.
This covers what priming involves, how to do it properly within the time the manufacturers allow, the order a technician diagnoses the problem in, the safety points for opening a pump that may have run dry, and what a lost prime costs a route.
A pool pump is a centrifugal pump. The impeller throws water outward into the volute, and that movement creates suction at the inlet and pressure at the outlet. Spin the same impeller in air and it moves almost nothing, because air is far too light to carry the energy. Priming means filling the pump and the suction pipe with water so the impeller has something to push.
Pentair’s IntelliFlo manual puts it plainly: “Pumps do not hold prime, the pool piping system has that task.” The pump keeps its prime only while the piping in front of it stays full of water and airtight. If air leaks in faster than the pump can push it out, or the water source runs dry, any pump will lose prime. That is why most of the diagnosis below happens on the suction side.
Dry running matters because of the shaft seal, which is cooled and lubricated by the water passing through the pump. Hayward says a pump running without water overheats the seal until it melts, and that running dry for long enough can damage the PVC and other plumbing as well. Jandy’s Stealth manual says running dry for any length of time can cause severe damage. No manufacturer publishes a safe dry-running time. The times they do give are priming windows, and those assume the pot was filled first.
A pump that failed to prime can be scalding inside
Pentair warns against opening the strainer pot of a pump that has failed to prime or has run without water in the pot, because it can hold vapour pressure and scalding water. Turn the power off at the breaker, make sure the suction and discharge valves are open, wait until the pot is cool to the touch, and then open it with care. Pentair also advises letting a hot motor cool for 20 minutes before working on it.
Most failed priming attempts fail because a step was skipped rather than because the pump is faulty. The sequence below is common to the Pentair, Hayward and Jandy manuals. Where a detail differs between models, follow the manual for the pump in front of you.
Power off, and isolate a pump below the waterline
Switch the pump off at the controller and at the breaker. If it sits below the water level, close the suction and return valves first, as Pentair and Jandy both instruct, or the pool will drain out through the open pot.
Open the valves
Hayward’s instruction is that all suction and discharge valves must be open at start-up. Pentair’s troubleshooting also asks you to confirm the suction valve is working as well as open, since a handle can turn without moving the gate behind it.
Fill the strainer pot to the inlet
Fill the pot to the level of the suction inlet, as Pentair and Hayward specify. A bucket from the pool is quicker than a hose. Refill it before every new attempt, because the first fill often drains away into the suction line.
Clean and seat the lid O-ring, then hand-tighten
Wipe the O-ring, the lid and the pot’s sealing surface, and clear grit, leaves or hair from the seat; Jandy notes that debris there lets air in and makes the pump hard to prime. Lubricate as the manufacturer specifies and tighten the lid by hand, with no tools.
Open the filter’s air relief and stand clear
Water cannot climb the suction line unless the air ahead of it can escape, and the filter is where that air collects. Open the manual air relief valve, and stand clear of the filter as the pump starts, as Pentair instructs.
Start it, and close the air relief on a steady stream
Prime a two-speed pump on high speed, and let a variable-speed pump run its priming cycle. The pot should fill and the bubbles under the lid should stop. Close the air relief once it gives a steady stream of water with no sputtering.
If it has not primed in time, stop
If it has not caught prime inside the time its manual allows, turn it off, let the pot cool, and work through the diagnostic order below before you refill it and try again.
| Pump and manual | Priming time | Worth knowing |
|---|---|---|
| Hayward Super II | Allow a reasonable time; five minutes is acceptable | Depends on the suction lift and the length of the suction pipe |
| Pentair SuperFlo, two-speed | Prime on high speed; no longer than 8 minutes | Keep the water halfway up the skimmer opening |
| Pentair IntelliFlo VS+SVRS | Maximum priming time 1 to 30 minutes, default 11, then an error | Safety vacuum release is off while it primes |
| Jandy Stealth | Depends on the elevation and length of the suction pipe | If it will not prime, check for a suction leak |
Hayward Super II
Pentair SuperFlo, two-speed
Pentair IntelliFlo VS+SVRS
Jandy Stealth
From each manufacturer’s installation guide for the model named. Other models and later revisions differ, so check the manual for the pump on the pad.
Variable-speed pumps manage priming themselves. Pentair’s IntelliFlo VS+SVRS checks for enough water flowing in the basket and keeps trying at its maximum speed for up to its maximum priming time before reporting an error, and with too little water in the basket it can raise a dry priming alarm within seconds. A primed-sensitivity setting changes how much flow the pump must see, and the manual warns that setting it too high can make the pump decide it has primed when it has not. A priming alarm protects the seal but leaves the pool standing still, so treat it as a reason to find the cause. Lengthening the priming time to silence it does nothing for a worn lid O-ring.
A pump above the water has to lift water up the suction pipe, and the manuals tie priming time to that height and to the length of the run. Pentair’s IntelliFlo VS+SVRS manual says not to install the pump more than 8 feet above the water level or 3 feet below it, and Jandy recommends mounting its Stealth pumps close to the water, within about 1 foot of the water level. The plumbing matters as well. Jandy warns that high points such as an inverted U trap air, and Pentair asks for a straight run in front of the inlet of at least five times the pipe diameter, with no 90° elbow directly into the pump.
On check valves the manuals disagree. Jandy recommends one on a Stealth pump’s suction line, where it holds water in the pipe while the pump is off. Pentair says suction check valves must not be installed with the IntelliFlo VS+SVRS, and Jandy itself warns that some safety vacuum release systems are not compatible with check valves, so read the manual before adding one. A check valve stuck open lets the line drain back whenever the pump stops, and one jammed shut starves the pump.
Pumps below the water level, including most above-ground pool pumps, are fed by gravity, and Jandy notes that opening the filter’s pressure release valve is enough to prime one. When a pump in that position will not prime, lift is not the problem. Look for a closed valve, a blockage, or trapped air with nowhere to go.
Decide which problem you have before taking anything apart. A pump that will not prime at all is usually starved or wide open: water is not reaching it because the level is low, a valve is shut or a line is blocked, or air is entering at the pump faster than the impeller can clear it, typically through a lid that is not sealing or a missing drain plug. A pump that primes and loses it later usually has something slower, such as a small leak that grows a bubble under the lid, a level that dips below the skimmer in the afternoon, a sticking weir, a basket that loads up, or a line that drains back overnight.
| What you see | Likely cause | Check first |
|---|---|---|
| Motor runs, pot stays empty or nearly empty | Water is not reaching the pump | Water level, valves, a blocked skimmer or line, and whether the pot was filled |
| Pot fills, then a steady stream of fine bubbles | Air drawn in on the suction side | Lid O-ring and seat, then drain plug and union O-rings |
| Primes on the visit, found airlocked later | A slow suction leak, a dipping level, or the line draining back | Lid O-ring, the water level through the day, any check valve |
| Bubbles coming out of the returns | Air entering on the suction side | Suction joints, working outward from the pump |
| Pot full of water, weak flow, low pressure | Clogged impeller or a restricted suction line | Impeller, then the skimmer and drain lines |
| Water leaking between the motor and pump body | Failed mechanical shaft seal | Seal replacement, quoted as a repair |
Motor runs, pot stays empty or nearly empty
Pot fills, then a steady stream of fine bubbles
Primes on the visit, found airlocked later
Bubbles coming out of the returns
Pot full of water, weak flow, low pressure
Water leaking between the motor and pump body
Bubbles from the returns are listed in Hayward’s Super II troubleshooting as a sign of a suction air leak. The seal symptom is from Jandy’s Stealth troubleshooting.
Work from the cheapest check to the most expensive. The first few take a minute each and need nothing but your eyes, and there is no sense pulling a pump apart to find the water an inch below the skimmer.
The first two checks need no tools. The last two usually end in a repair you quote rather than one you do on the stop.
Pentair’s guidance is to keep the water halfway up the skimmer opening. Below that the skimmer gulps air, the pump loses prime, and if nobody notices it carries on running dry. Check the level with the pump running: a vortex in the skimmer or a weir slapping open and shut means it is taking air. Evaporation, splash-out, backwashing and a failed auto-fill all lower it, and a pool losing more water than evaporation explains needs leak detection. A vacuum hose pushed into the skimmer before it is full of water sends a hose-length of air to the pump, which is an easy way to cause the callback yourself.
A packed skimmer basket or pump basket starves the pump as surely as low water, and Pentair asks for the pump basket to be checked at least once a week. A weir stuck in the raised position stops water entering the skimmer, so the pump empties it and starts drawing air. Look into the skimmer throat for a toy, a bag or a wad of leaves, and check the pump basket for cracks, since Hayward notes that without the basket, debris clogs the impeller. Then check the valves, which get left in the wrong position after vacuuming, spa use, winterising or someone else’s repair. If the skimmer and main drain have separate valves, prime on each in turn. A pump that primes on one line and not the other has told you where the problem is.
If the pump has water and still will not hold prime, look for air entering at the pump itself. The lid O-ring comes first. A ring that is dirty, dry, flattened, stretched or nicked lets air in, and so does a cracked lid or grit on the seat. Jandy’s answer to bubbles in the pump basket is to confirm the water level, then clean the O-ring and its seat, refit it and hand-tighten the lid. Replace a ring that has taken a set rather than greasing it.
Next come the drain plugs. The plug on the strainer pot sits on the suction side, and one with a missing or perished O-ring draws air. Plugs removed for winter deserve a look in spring, since Pentair suggests storing them in the pump basket. Then check the suction-side union: Jandy’s procedure is to tighten it while the pump runs, and if the bubbles continue, to shut down, open both unions, clean and refit the O-rings and hand-tighten. Last is the threaded adapter into the pump inlet. Failed thread sealant there lets air in, and one service company’s guide calls that joint the most common air leak of all.
Use the lubricant the manual names
Pentair specifies a silicone-based lubricant for its lid O-rings, while Jandy’s Stealth manual asks only that the ring be cleaned and seated. Petroleum products, petroleum jelly included, attack some O-ring rubbers, EPDM among them, and make them swell and leak. Carry a lubricant that is safe for the rings on your route, use a thin film, and never rely on it to make a worn ring seal.
If the pump end is tight, the leak is further back, at any joint or valve between the pump and the pool that sits above the waterline. Include the valve stems: Pentair lists the glands on suction valves among the places to check. With the pump running, run a hose over each joint in turn and watch the lid, because when the water covers the leak, the bubbles in the basket stop. Shaving foam spread over a suspect joint will be drawn in, and close to a bad fitting you can often hear the air hissing in.
Hayward’s Super II troubleshooting splits the problem in one move: block the suction as close to the pump as possible and see whether it can pull a vacuum. If it cannot, with water in the pot, the fault is at the pump, and Hayward’s list runs from the suction-side fittings through the motor’s voltage and any obstruction in the pump to the shaft seal. If it can, the problem is upstream, either a blocked suction line or a leak in the piping. Freeze damage belongs here too. Pentair warns that water in the pump can freeze once the air drops below 35°F, and a hairline crack after a cold snap draws air. An underground leak is a job for leak repair, and repeated freeze cracks are an argument for closing pools properly.
A pot full of water with weak flow, and filter pressure well below its clean starting pressure, points at a blockage. Pentair lists a clogged impeller among the causes of a priming error as well as low flow. With the breaker off, Jandy’s first step is to remove the lid and basket and look inside the pump for debris. If it cannot be cleared from there the pump has to come apart, and on Pentair’s pumps the impeller’s anti-spin bolt has a left-hand thread. Debris that reached the impeller got past the basket somehow, so find out how. A blocked suction line looks the same to the pump, and a skimmer line that primes poorly while the drain line primes well is the clue. A check valve stuck open shows up as a pump found airlocked the morning after you primed it.
A failing shaft seal usually shows as water leaking between the motor and the pump body, but Jandy notes it can leak air as well, and Hayward’s list for a pump that will not pull a vacuum ends with replacing the seal. When fitting a new one, Pentair and Hayward both specify soapy water on the rubber parts, and Hayward warns that petroleum or silicone lubricants can make the seal leak. After a long dry run, check the pot, basket and nearby plumbing for heat damage as well.
Last come the problems built into the installation. A pump too big for its suction plumbing, or mounted too high above the water, cannot get the water it is asking for, and Jandy describes the result as cavitation that makes noise and shortens the pump’s life. On a variable-speed pump a lower speed may settle it. On an oversized single-speed pump the real fix is a different pump or different pipe, and that is a quote rather than a visit.
Turn the pump off at the breaker, and not only at the timer or automation, before opening the pot, pulling the motor or reaching for the impeller. On a variable-speed pump, never open the drive enclosure, because Pentair warns that its capacitor bank holds a 230-volt charge even with the power off. Treat a pot that has run without water as hot and possibly under pressure. Test the GFCI on the pump circuit routinely: Pentair’s instruction is that one which trips without the test button being pressed indicates current flowing to ground, and the pump should not be used until a qualified person has corrected it. Wiring belongs to a licensed electrician or a qualified pool professional.
Nobody in the pool while an SVRS pump primes
Pentair’s IntelliFlo VS+SVRS switches off its safety vacuum release protection while it primes, and the manual says nobody should be in the pool during that time. Clear the pool before priming. While you are working on the suction side, check every suction outlet cover: Pentair’s instruction for one that is loose, cracked or missing is to shut off the pump and keep the pool closed until a certified cover is fitted.
The causes at the top of the diagnostic order can be fixed on the visit if the truck carries the parts: topping up the water, clearing baskets and the weir, setting valves, fitting new lid, plug or union O-rings, and priming the pump properly. Anything that needs the pump opened or the plumbing cut is a repair to quote and have authorised, such as a shaft seal, a stubborn impeller, a cracked adapter, a check valve or a freeze-cracked fitting. New glued joints need time to cure before they hold suction, which on its own often means a return trip.
Replacement becomes the better answer when the wet end is heat-damaged from running dry, the pump body is cracked, or the seal keeps failing because the pump keeps losing prime for a reason the owner will not fix. Hayward’s warning that prolonged dry running can damage PVC is the reason to check the unions and pipe next to a melted pump before quoting for the pump alone.
Replacing the pump rather than repairing it again
What it gets you
What it costs
A lost prime is cheap to fix and expensive to miss. A pump that lost prime the day after your visit has not circulated the pool since. Nothing has been filtered, a salt cell or inline feeder that needs flow has not dosed, and the chlorine you added has been used up in still water. In warm weather that is how a clear pool turns cloudy and then green before the next visit. The recovery takes several trips, and if the pool was in your care, most of them are unpaid.
Water level is where responsibility gets awkward, because owners control the hose, the auto-fill and the backwash. Tell the owner in writing where the level needs to be and why, and make sure your service agreement says who keeps it there and how equipment repairs are authorised. A photo of the level at the skimmer, taken on the visit you raised it, is what settles the question later.
The on-the-stop fixes depend on what is in the truck. Lid O-rings for the pump models on your route, drain plugs and their O-rings, union O-rings, a compatible lubricant and a bucket take up almost no space, and they belong on the same weekly restock as your test reagents.
Before you leave a pump you have just primed
0 / 6The habit that heads off these calls is small: look at the pump lid and the skimmer on every visit. A bubble under the lid that is bigger than last week, or a level creeping toward the bottom of the skimmer mouth, is a lost prime that has not happened yet. Catching it then costs an O-ring and a sentence to the owner. Finding it after four days of dry running costs a seal, a recovery and some of the customer’s confidence.
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