RV water pump won't prime: The order to check things

What "won't prime" actually means

These are demand pumps. They start when a tap opens and stop when pressure builds, and the makers rate them to lift water from a tank a stated distance below. When one runs and produces nothing, the pump is almost always doing its job against a supply it cannot draw from.

SHURflo list the symptom this page is about as two separate entries in their troubleshooting matrix: "Pump Loses Prime" and "Pump Will Not Prime". Their documented causes across those rows include "Inlet Air Leak", "Kinked Inlet Hose", "Clogged Tank Outlet", "Clogged Tank Stainer", "Small I.D. Hose at Pump Inlet", "Collapsing/Non-Vacuum-Rated Hose", "Pump Too Far From Tank", "Dirt/Worn Check Valve", "Cracked Strainer", "Loose Pump Fittings" and "Low Voltage".

Read that list once more, because only one entry on it is the pump itself.

The order to check, and why it is this order

The sequence below is not arbitrary. It runs from the cheapest and most likely to the most expensive and least likely, and it follows what the makers themselves list first.

One: the valve position, before anything else

This is the one owners skip, and SHURflo document it explicitly. Their troubleshooting matrix names "Bad City Water/Tank Fill 3-way Valve" as a cause of low flow, and carries a separate symptom row headed "Tank Fills on City Water". A selector valve left in the wrong position, or a fill valve that no longer seats, breaks the prime path without anything being broken.

If your RV has a winterising kit, the same logic applies and it is the more common version. (For the wet-interior problem that follows a cold trip, see condensation inside an RV.) the valve that lets the pump draw from an antifreeze jug is a valve that, left open, makes the pump draw air. Check both before touching a tool.

Two: will water free flow from the tank

REMCO publish an actual number rather than a principle: when diagnosing a pump that does not prime, "Water should free flow from tank at 2 GPM minimum." That is a useful test because it separates the tank side from the pump side in a single step. Disconnect the pump inlet and see what the tank will deliver on its own. If it trickles, the restriction is upstream and no pump will fix it.

Three: the strainer

Every maker lists a clogged strainer, and SHURflo also list a cracked strainer, which is the version people miss because a crack lets air in rather than blocking debris. Their Aqua King checklist opens with it: "Is the strainer clogged with debris?"

On the same pass, look at the strainer housing for cracks and check that the bowl is seated. A cracked bowl passes air, and air is what stops a pump priming.

Four: the suction side, for air

This is the single most-cited cause across all four makers, and it is worth quoting each of them because the agreement is the point.

• SHURflo: "Is the inlet tubing/plumbing sucking in air at plumbing connections (vacuum leak)?" and, in the industrial series manual, "Inlet tubing/plumbing for severe vacuum leak."
• Flojet, on failure to prime: "Air leak in intake line."
• REMCO, on a pump that does not prime: "Leak on inlet side of pump. Check tightness of hose connections at pump, filter, and tank. Check valves at inlet side."
• Aquatec, on a pump that keeps running: "Air leak on inlet side of pump."

A suction leak does not weep water, which is why it is hard to find. It draws air in. Check every fitting from the tank to the pump, and remember the tank end, which is the one nobody looks at.

Five: the hose itself

The makers are specific about this and it is where a well-meant repair goes wrong. SHURflo require "a minimum of 1/2" [13mm] Inner Diameter plumbing" and state the consequence: "Smaller ID plumbing will cause cavitation, high back pressure, low flow and noise." Flojet specify 1/2" flexible hose, "preferably braided or reinforced".

Two further requirements matter and both are documented. The hose must be vacuum-rated, because the suction side is under vacuum and a hose that collapses under it stops the flow: SHURflo's own warning is that "Standard garden hose is not suitable as inlet tubing as it may collapse under vacuum, reducing water flow." And the inlet should be flexible, not rigid: "The pump ports and strainer should not be connected to plastic or rigid pipe, or the pump's normal motion will transmit through rigid plumbing causing noise, and possibly loosening or cracking components."

That last line is the mechanism by which a rigid install creates the air leak in step four, which is why the two belong together.

Six: how far the pump is from the tank

Each maker publishes a hard suction-lift ceiling, and they differ by model, so the figure that applies is the one in your own manual:

• SHURflo Lo-Flo 105: self-priming "up to 2.5 ft. vertical", and "can be located up to 2.5 feet above the water level"
• SHURflo Aqua King II: "Self-priming up to 6 feet" and "Mount the pump within 6 feet (Max) of the tank for best performance and pump life"
• Flojet Triplex: "self-priming up to 9 feet suction lift", with the installation step adding "mount within 6ft (1.8m) of the water tank"
• Aquatec RV Aquajet II: "PRIMING CAPABILITIES: 13 Feet"

A pump installed above its documented ceiling is operating outside the manual, and no amount of bleeding will fix it.

Seven: air trapped in the system, including the water heater

SHURflo name this directly, twice, and it is the step owners never think of: "Is there water in the tank, or has air collected in the hot water heater?" and "For air trapped in outlet side (water heater) or pump head."

The related maker instruction is the one that explains bleed procedure. Flojet's start-up step is to "Allow pump to prime with discharge line … open, to avoid airlock", and their triplex instructions say to open all faucets before switching the pump on. Aquatec put the underlying rule plainly: these pumps "prime only if all pressure is relieved from outlet port."

Practically: open a cold tap, and if the water heater was drained for winter, make sure it has refilled, because a heater full of air is a heater that will not pass water.

Eight: voltage at the pump, while it is running

Several makers list low voltage as a no-prime cause, and one gives a number. REMCO state that "With either a fully charged battery or a running generator, you should be getting at least 11 volts at the pump", and publish the operating window for their 3200 series as 10.8 to 14.5 volts operational, 12.0 to 13.6 optimal. Aquatec rate their pump's performance at approximately 11 VDC at the pump.

SHURflo's own prime checklist includes "Proper voltage with the pump operating (±10%)", though they publish no absolute floor. The test is the same either way: measure at the pump while it runs, not at the battery. A reading taken at rest tells you nothing about a motor under load.

Nine: the check valve

A stuck or debris-blocked check valve appears on three of the four makers' lists, and Flojet name both versions: "Defective pump check valve" and "Debris in check valves", with the diagnostic wording "check for 'jammed' check valves". SHURflo's list adds the condition most people do not expect: "Are the pump valves held open by debris or is the rubber swollen?"

A swollen valve is a valve that no longer seals, and it can happen without any debris at all.

Ten: the pump head, and the point to stop

Only now does the pump itself come into it. Documented causes at this end are a punctured diaphragm, a cracked housing, and loose drive assembly screws. Flojet list "Punctured pump diaphragm (pump leaks)" and "Crack in pump housing"; SHURflo add "Cracked Strainer" and "Loose Pump Screws".

Before you replace anything, note what the makers say about pressure: SHURflo state the inlet "must not exceed 30 psi maximum", Flojet Duplex II give the same 30 PSI ceiling, and Aquatec allow 60 PSI. REMCO's instruction is blunt: "Never subject the pump to pressures above 90 PSI maximum." If someone has been forcing water or air into the system to make it prime, the pump may now be the problem because of the attempt.

What the four makers list, side by side

The consistency is the useful part. These are the makers' own words for the causes of a pump that runs but does not prime:

One row deserves a note, because it is the pump equivalent of a mislabelled switch: REMCO list a "Diverter valve in 'fill' mode" among the causes of a pump that keeps running, with the remedy "Close and/or rotate any valve used for filling water tank". A valve set to fill the tank bypasses the pressure side entirely, and the pump will run indefinitely against it.

Two different faults that get called the same thing

A pump that cycles on and off with no tap open is a different failure from one that will not prime, and the makers split the causes. Flojet frame it as restricted or undersized discharge delivery. SHURflo and REMCO both put a real leak on the pressure side first: SHURflo instruct you to check "Output side (pressure) plumbing for leaks, and inspect for leaky valves or toilet", and REMCO's list adds a leak on the outlet side and low voltage.

A pump that will not shut off shares causes with both. SHURflo list air trapped in the outlet line or pump head, valve debris or swelling, and a pressure switch with a by-pass set higher than the shut-off. Aquatec add a specific warning about the switch itself: "Excessive adjustment of the pressure switch could cause low system pressure, and rapid ON/OFF cycling, reducing pump and motor life", and note that the warranty does not cover improper adjustment. Do not tune the switch by guesswork.

Before you put a tool on it

The makers are consistent about two things. Flojet's first caution is to "Disconnect power from the system before working on the unit to avoid personal injury, damage to the surrounding environment and/or damage to the unit." And the pump gets hot: SHURflo warn that "the surface temperature of the motor may exceed 250°F [120°C]", Aquatec that exterior surfaces "may exceed 150°F after only 15 minutes of operation", and REMCO's instruction is to "disconnect electrical power and wait 30 minutes before touching pump."

One more from Aquatec that matters when you are chasing a leak: "As long as there is inlet water pressure, the pump will not stop forward flow of water even if the motor is turned off." If you are working on the inlet side while connected to city water, the system needs a positive means of shutting the supply off, because the pump is not one.

What this costs

The parts in this list cost very little: a strainer bowl, a length of vacuum-rated half-inch hose, a few fittings, a fuse. A replacement pump is a larger number, and the point of the order above is that the expensive step is the last one, not the first.

The order to spend in: nothing until you have confirmed water free flows from the tank and the suction side holds vacuum. Then a hose and fittings if they are undersized or not vacuum-rated. Then a strainer if it is cracked. A pump only once the inlet has been proven, because a new pump on a restricted or leaking inlet behaves exactly like the old one.

Sources

• SHURflo pump troubleshooting guide, for the symptom matrix including "Pump Loses Prime" and "Pump Will Not Prime", the documented cause rows, and "Tank Fills on City Water" and the bad city water or tank fill three-way valve.
• SHURflo Aqua King II RV fresh water pump manual, 911-1026, for the six foot self-priming and mounting limit, the half inch minimum inlet diameter and the cavitation warning, the flexible-hose and rigid-pipe instruction, the will-not-prime or sputters checklist including air in the water heater, the will-not-shut-off checklist, the 250 F motor surface warning, and the dedicated circuit and fuse requirement.
• SHURflo Lo-Flo 105 series RV pump manual, 911-585, for the 2.5 foot vertical prime limit, the 5 amp fuse requirement, and the three-item will-not-prime list.
• SHURflo 2088 industrial series manual, 911-396, for the "severe vacuum leak" wording, the will-not-shut-off list, the 30 psi maximum inlet pressure, and the voltage tolerance check.
• SHURflo Pro Blaster II manual, 911-1035, for the six foot vertical prime and the instruction that standard garden hose is not suitable as inlet tubing because it collapses under vacuum.
• Flojet Triplex automatic water system pump manual, for the nine foot suction lift, the ten amp fuse and ten to fifteen amp switch requirements, the mount-within-six-feet instruction, the failure-to-prime and will-not-turn-off lists, the prime-with-discharge-line-open instruction, the 1/2 inch flexible hose requirement, and the disconnect-power and hot-surface cautions.
• Flojet Duplex II industrial series manual, for the conditional self-priming note about suction tube size and foot valve, the failure-to-prime list including jammed check valves, the 30 PSI maximum inlet pressure, and the electrical shock warning.
• REMCO Aquajet series manual, for the does-not-prime and keeps-running cause lists, the 2 GPM free-flow inlet requirement, the 11 volts at the pump figure, the 90 PSI maximum, the diverter valve in fill mode, the low voltage causes, and the burn hazard and 30 minute wait instruction.
• REMCO fuse and wiring sizing guideline, RSD-20-01, for the operational and optimal voltage windows and the low voltage protection range.
• Aquatec 550 series Aquajet RV data sheet, for the 13 foot priming capability, the 60 PSI maximum inlet pressure, the approximately 11 VDC test condition, the heat sink surface caution, and the positive shut-off requirement because the pump passes inlet pressure through.
• Aquatec IMI-104 installation manual, for the pressure switch over-adjustment warning, the cycling consequence, and the exclusion of improper adjustment from warranty.