You turn on the pool heater, but the display says LOW FLOW, WATER SW, FLO, or shows a pressure-switch message. The heater is telling you it cannot confirm enough water flow to run safely.
This does not always mean the heater is broken. The cause may be in another part of the pool system, such as the water level, pump, filter, valves, or pipes.
KALOS FAQ 5.02
What causes a LOW FLOW, WATER SW, FLO, or pressure-switch pool-heater error?
The heater isn't seeing enough water moving through it. A dirty filter, low water level, pump problem, valve position, or blocked flow path can cause it.
Table of Contents
Why water flow matters
A pool heater moves heat into the water. The pump must keep that water moving through the heater. If flow is too low, heat can build up where it should not.
That is why the heater has a pressure or flow safety. The message often means the safety is doing its job. A technician should check the real water flow before replacing or adjusting any switch.
Common causes of a flow message
1. A dirty filter or full basket
A dirty filter or a basket full of leaves can slow the water before it reaches the heater. Check whether normal filter or basket care is overdue.
If you already know how to service these parts, follow the maker's shutdown steps. If you are not sure, leave the cleaning or opening of equipment to a pool professional.
2. The pool water is too low
If the water drops too far below the skimmer opening, the pump may pull in air. You may see bubbles at the return jets or air under the clear pump lid.
Bring the water back to the level recommended for your pool. If the level drops again quickly, look for the cause instead of only adding more water.
3. The pump is not moving water well
The pump may be off, on the wrong schedule, running too slowly, losing prime, pulling in air, or having a repair issue.
Look and listen from a safe spot. Is the pump running? Does the basket under the clear lid look full of water? Does the return flow look normal?
If the pump sounds rough, leaks, will not prime, or keeps shutting off, stop the heater and call for service.
4. A valve or bypass is in the wrong spot
Pool and spa systems may have several valves. Some systems also have a bypass that sends water around the heater.
Take a photo of the valve positions and note whether anyone changed them. Do not force a valve or copy a setting from a photo online. Every pool is piped differently.
5. Something is blocking the water path
Debris, scale, a check-valve issue, or a problem inside the heater may slow the flow. These causes need testing. The error message alone cannot tell you which part is at fault.
6. A switch or wire needs testing
If water flow looks normal, a technician may test the pressure switch, wires, and heater controls. The goal is to prove the cause, not to assume the safety switch is bad.
Safe details to check before you call
Without opening equipment or changing settings you do not know, write down:
- The exact message and heater model
- Whether the pump is running and looks primed
- Whether water is returning to the pool normally
- Whether bubbles are coming from the return jets
- Whether the water level is normal
- When the filter and baskets were last cleaned
- The valve positions, shown in a photo
- Whether the message appears in pool mode, spa mode, or both
These details can help the technician find the cause faster.
Keep the heater safety in place
Never bypass, tape, jumper, or adjust a pressure switch or another heater safety. Do not open electrical or burner areas. Do not keep forcing the heater to run.
If you smell gas, hear banging, see soot, or find water touching electrical parts, turn the heater off with its normal control if you can do so safely. Move away and call for help.
Get your pool comfortable again
The fix may be simple pool care, a pump or valve repair, a blocked water path, or a heater control problem. A KALOS pool-heater technician can check the full water path and make sure the heater runs safely after the cause is fixed.
If your heater shows LOW FLOW, WATER SW, FLO, or a pressure-switch message, call or text KALOS Services at (352) 243-7099. Tell us the exact message and whether the pump seems to be moving water. We will help you choose the next step.
Getting ready for cooler weather? See when to run your pool heater.
