Variable-speed pumps save the most power at their lowest speeds, and a lot of them are set up to run slow and quiet all day. That's good for the electric bill. It can also leave the heater showing a flow error and the salt cell making no chlorine, because both are built not to run unless enough water is moving through them. Nothing has broken. The pump just isn't moving enough water for the equipment downstream.

This guide puts the manufacturers' minimum flows side by side, shows the message each piece of equipment gives when the pump is too slow, and explains how to set speeds and schedules so they work together.

Safety first

Everything here is done at the pump's keypad, the automation panel and the outside of the equipment. Don't open a heater, pump drive or control panel. If you have a gas heater and smell gas, leave the area and call your gas utility from a safe place before anything else; Jandy's heater manual opens with the same instruction (JXi manual, p. 1).

Why a slow pump stops the heater and the salt cell

Heaters, heat pumps and salt cells each have a water pressure switch or flow switch. It closes only when enough water is moving, and until it does, the equipment won't run. On a Jandy JXi, for example, a pressure switch at the header inlet senses whether water is reaching the heater, and if not, the heater shows CHECK FLOW and stops (JXi manual, p. 7). Pentair says plainly that if an IntelliChlor cell doesn't receive adequate flow, no chlorine is produced (IntelliChlor guide, p. 2).

The switches are there to protect the equipment. Hayward warns that running an H-Series heater below its minimum flow can let it dry fire or let the water boil, tripping the high limits and possibly damaging the heat exchanger (Hayward H-Series guide, p. 61). Jandy says flow below the JXi's minimum may cause nuisance operation, damage the heater or turn it off (JXi manual, p. 24).

Pentair's own pump manual says that lower speeds are typically used for filtration and heating (IntelliFlo VS+SVRS manual, p. 6). That's true as far as it goes. "Lower" still has to be above the heater's minimum, and on many pads the lowest speed the pump can run isn't.

The minimums, side by side

Only figures printed in the manuals are listed here.

Equipment	Minimum flow	Maximum flow	What it shows when flow is too low
Jandy JXi gas heater (200, 260, 400)	30 gpm	100 gpm	CHECK FLOW (JXi manual, pp. 7, 24)
Pentair MasterTemp gas heater	20 gpm (175, 200) to 40 gpm (400)	120 gpm	ERR PS (MasterTemp manual, pp. 14, 48)
Hayward Universal H-Series gas heater	Set by model	125 gpm	LO among others (Hayward H-Series guide, p. 61)
Hayward HeatPro and Summit heat pumps	30 gpm, more at higher water temperatures		PS or FLo (Hayward heat pump guide, pp. 25-26, 33)
Pentair UltraTemp heat pump	30 gpm		LOW WATER FLOW, or HIGH REFRIG (UltraTemp guide, pp. 27-28)
Jandy VersaTemp heat pump	20 gpm (2000R, 2500R), 30 gpm (3000R)	70 gpm	FAULT- NO FLOW (VersaTemp manual, p. 6)
Pentair IntelliChlor salt cell	25 gpm	105 gpm	Red Flow light (IntelliChlor guide, pp. 5, 21)
Hayward AquaRite salt system	11 gpm for the flow switch to engage		No Flow (Hayward AquaRite guide, p. 27)

Pentair gives the IntelliChlor's design range as 25 gpm, plus or minus 5, up to 105 gpm, and says flows over 80 gpm need a bypass loop for best chlorine production (IntelliChlor guide, p. 5). Hayward's heat pump guide notes that a heat pump may need more than its minimum when heating water to higher temperatures, such as a spa (Hayward heat pump guide, pp. 25-26).

The manufacturers say so outright. On the JXi, Jandy lists a variable-speed pump set too low as a cause of CHECK FLOW, alongside a pump that isn't running, low pump pressure and a pressure switch fault (JXi manual, p. 46). Hayward's AquaRite guide says that if a variable-speed pump is running and No Flow appears, try increasing the pump speed (Hayward AquaRite guide, p. 27).

Speed isn't flow

A pump shows its speed in RPM, or as a percentage, but the equipment cares about gallons per minute, and the same speed moves different amounts of water on different pads. A dirty filter, full baskets, the plumbing and valve positions all change the flow at any given speed. So does switching to spa mode or running water features off the same pump. That's why one pool's heater runs fine at a speed that leaves the pool next door showing CHECK FLOW.

Some pumps can work in flow instead of speed. On a Pentair IntelliFlo3 VSF, each program can be set as a speed (a percentage of maximum) or a flow in gallons per minute (IntelliFlo3 VSF guide, p. 5), with minimum and maximum flow limits (p. 8). If the pump shows a Speed Limit warning while trying to reach a flow and its limit is already at 100%, Pentair says the pump may not be able to reach the flow you've asked for (p. 11), which is a sign of a restriction, not just a setting.

Setting it up so everything works

Find the minimum for each piece of equipment in the table above, or in its manual. The highest one that runs at the same time is the one that matters.
Give the pump a speed for heating and chlorinating that meets that minimum. Leave the quiet speeds for the hours when nothing else needs flow.
Line up the schedules. A salt cell only makes chlorine while water is moving through it, so the hours it needs have to fall in the pump's faster periods. The same goes for a heater.
Check it after cleaning the filter, and after it's been running a while. Flow drops as the filter loads, so a speed that's just enough on a clean filter may not be later.

How that's done depends on what's in charge:

A pump on its own schedule, with a timer for the heater. Hayward says that where an old time clock is kept to power other equipment such as a heater or booster pump, it should be set to power it only while the variable-speed pump is running at a flow rate suitable for that equipment, as set in the pump's timers (Hayward TriStar VS manual, p. 8).
Automation. Automation can give each circuit its own pump speed. On a Pentair IntelliCenter, when a pump is linked to several circuits with different speeds, it runs at the highest of them, and when there's a mix of RPM and GPM settings, the GPM settings take priority (IntelliCenter guide, p. 2). So giving the heater circuit a faster speed lifts the pump whenever the heater is on.

Signs the pump speed is the problem

The heater or salt system works on high speed, or in spa mode on some systems, and shows its flow message on the everyday low speed.
The flow message appears only at certain times of day, which match the pump's slow periods.
It started after a pump was replaced, a new variable-speed pump was programmed, or the schedule was changed to save power.
It gets worse as the filter gets dirtier and better after cleaning it.

If the message stays on at a speed that should be well above the minimum, with clean baskets and filter and the valves set to send water through the equipment, it's no longer a settings question. The next suspects are a restriction, a failed pressure or flow switch, or wiring, and those need testing. For the messages themselves, see:

Jandy heater and VersaTemp codes (CHECK FLOW, NO FLOW)
Pentair MasterTemp error codes (ERR PS)
Hayward heater and heat pump codes (LO, PS, FLo)
Pentair UltraTemp alarm messages (LOW WATER FLOW)
Salt cell not producing chlorine (No Flow, Flow light)

When to call

Call when a flow message stays on at a speed that should meet the minimum, when you're not sure what your pump, heater and salt system need or how your automation is set up, or after a pump replacement when nothing downstream has worked right since. We check the flow at the equipment, the pump's speeds and schedule, the filter and valves, and the pressure and flow switches, and tell you what has failed, or what needs changing, before anything is quoted.

If you don't know what's wrong, a diagnostic is $120, and it's credited toward your repair if you go ahead. Start from variable-speed pump repair, pool heater repair, heat pump repair or salt system repair, or answer a few questions in the Pool Problem Finder. Photos of the pump's display, the heater's rating plate and the salt system's label help.

Sources

Every manufacturer statement above links to this list. Page numbers are the printed page numbers in each manual.

Zodiac (Jandy), JXi Gas-Fired Pool and Spa Heater Installation and Operation Manual, H0574300 Rev M (2024).
Pentair, MasterTemp Pool and Spa Heater Installation and User's Guide, P/N 472592 Rev. Q (7/30/23).
Hayward, Universal H Series Heaters Troubleshooting Guide, TSG-UHS16b (2017).
Hayward, Hayward/Summit Heat Pump Troubleshooting Guide, TSG-HTPMPa (2018).
Pentair, UltraTemp Heat Pump Installation and User's Guide, P/N 474099 Rev. AA (5/4/23).
Zodiac (Jandy), VersaTemp JRT Series Heat Pump Installation and Operation Manual, H0744700 Rev H.
Hayward, AquaRite Troubleshooting Guide Residential, TSG-AQR159d (2017).
Pentair, IntelliChlor Salt Chlorine Generator Installation and User's Guide, P/N 520589 Rev. R (8/7/23).
Pentair, IntelliFlo VS+SVRS and IntelliPro VS+SVRS Variable Speed Pump Installation and User's Guide, P/N 356916 Rev. D (11/6/23).
Pentair, IntelliFlo3 VSF and IntelliPro3 VSF Variable Speed and Flow Pump User's Guide, P/N 356088 Rev. C (5/30/23).
Hayward, 2.70 THP VS Pump Family Owner's Manual (TriStar VS), 1011786 Rev B.
Pentair, IntelliCenter Pool Control System Troubleshooting Guide (02/2023).

Pentair, MasterTemp, UltraTemp, IntelliChlor, IntelliFlo and IntelliCenter are trademarks of Pentair. Hayward, HeatPro, Summit, Universal H-Series, AquaRite and TriStar are trademarks of Hayward Industries. Jandy, JXi and VersaTemp are trademarks of Zodiac Pool Systems. They are named here only to identify equipment.

Why a variable-speed pump set too low stops your heater and salt cell