Pool Heater Water Flow Issues
When You first start servicing pool heaters the water flow circuit can feel a bit intimidating.
In this article I’m going to cover the pool circuit basics and common water flow issues that screw up our heaters.
How Pool heaters work
Pool heaters generate heat via compression refrigeration (heat pump) or gas fueled flame. This heat gets passed off into the water through the heat exchanger inside the pool heater. Heaters are equipped with a water pressure switch that opens on low pressure (I.e. pump off or flow restriction). However, just because the water has “pressure” doesn’t necessarily mean there is adequate flow to remove heat from the exchanger.
Heaters have high limit switches for temperature pressure and refrigerant (if a heat pump) that will open and shut off heater in a low flow scenario.
In a heat pump when flow is low the system will run high head pressure. In a gas heater the exchanger can begin to cavitate and make a loud banging noise.
In my experience 1/3 of pool heater service calls are actually a water flow issue. Knowing how to diagnose and fix a water flow issue is a must for pool heater techs
The pool pump circuit
The pump pulls in from the pool/spa drains and skimmers. Drains are on the bottom of pool, skimmers are at the surface. Multiple valves are used right before pump to isolate different sections of the intake if needed. Pool, spa or skimmer can each be individually shut off from the pumps intake.
Water enters the pump at the clear sight glass where a large screen trap is positioned to catch any chucks entering the pump. This screen trap can be easily cleaned by turning off pump and removing the clear sight lid.
Water exits pump into the pools filter. This is a large cartridge that has a filter inside. If clogged, Filter can be removed by turning off pump, unscrew lid and lift out the filter. When needed you can leave filter out and replace lid temporarily for testing purposes
After filter, water goes into the pool heater or solar heat. Usually here, there is a water bypass valve right at heaters intake, this would allow water flow to go around heater (if not in use) instead of through it.
Next the water passes through the heaters heat-exchanger and out.
Water then flows through a back flow safety before entering the high chemical chlorinator. This back flow safety’s is most commonly a one way spring loaded valve that closes when water flow stops. This protects heater from concentrated chemical flowing back into exchanger when pump is off.
Next in line are multiple vales directing water flow to the pool/spa jets and or waterfall/fountain fixture.
Pools with a spa can have two automatic actuators installed, one at the pumps pool/spa intake valves and one at the pool/spa jet return valves.
When in spa mode valve at intake closest of section that pulls water out of pool drains and skimmers, only allowing water to be pulled from the spa drain.
And the second auto actuator closest off jet supply to pool only feeding the spa jets. This is how the sap can be isolated and heated separately from the pool.
Common water flow issues and fixes
A dirty pool filter wins for most popular water flow issue on a heater service call.
This can be diagnosed by removing the filter and running pump again without filter in.
If the issue was in fact a clogged filter, you should have no water flow problems now.
Fix: Have owner get a filter replacement/cleaning.
Another common water flow issue is an open heater bypass valve. A bypass valve is put in place near intake, to stop water from flowing through the heater when itâ€™s not in use. When in bypass, the heaterâ€™s water flow switch (I.e. pressure switch) should open and heater will not run.
Bypass valve is shown as closed in picture below.
Air in the water circuit is another common flow issue.
If the pool lever is low the pump will began pulling air in through the skimmer. You will notice low water flow and lots of bubbles from the jets.
Turn valve from skimmers off. This will pull water just from drains at bottom of pool/spa. In picture below the valve circled can be turned to block water intake from skimmers.
Fix: Let owners know pool needs to be filled before skimmers are opened again.
Most water pressure and flow problems can be easily diagnosed and fixed once there’s a clear understanding of the pool circuit.
— Bert Testerman
Bryan Orr is a lifelong learner, proud technician and advocate for the HVAC/R Trade