Get Tech Tips
Subscribe to free tech tips.
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. If you would like to brush up on the basics of pool heaters before diving into troubleshooting tactics, you can check out our other two articles on pool heat pumps: Pool Heat Pumps, The Basics and Introduction to Pool Heat Pumps.
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,” that doesn't necessarily mean there is an 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 the heater in a low-flow scenario.
In a heat pump, when the flow is low, the system will run high head pressure. However, 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 the pool; skimmers are at the surface. Multiple valves are used right before the pump to isolate different sections of the intake if needed. The pool, spa, or skimmer can each be individually shut off from the pump's 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 the pump and removing the clear sight lid.
Water exits the pump into the pool's filter. This is a large cartridge that has a filter inside. If clogged, the filter can be removed by turning off the pump, unscrewing the lid, and lifting out the filter. When needed, you can leave the filter out and replace the lid temporarily for testing purposes.
After the filter, water goes into the pool heater or solar heat. Usually, there is a water bypass valve right at the heater's intake that would allow water flow to go around the heater (if not in use) instead of through it.
Next, the water passes through the heater's heat exchanger and out.
Water then flows through a backflow safety before entering the high chemical chlorinator. This backflow safety is most commonly a one-way spring-loaded valve that closes when water flow stops. This protects the heater from concentrated chemicals flowing back into the exchanger when the pump is off.
Next in line are multiple valves directing water flow to the pool or spa jets or the 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, the valve at intake closes off the 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 closes off the jet supply to the pool, only feeding the spa jets. That is how the spa can be isolated and heated separately from the pool.
Common water flow issues and fixes
A dirty pool filter wins for the most popular water flow issue on a heater service call.
This issue can be diagnosed by removing the filter and running the pump again without a filter in.
If the issue was, in fact, a clogged filter, you should have no water flow problems now.
Fix: have the 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 the intake to stop water from flowing through the heater when it's not in use. When in bypass, the heater's water flow switch (i.e., the pressure switch) should open, and the heater will not run.
The bypass valve is shown as closed in the picture below.
Air in the water circuit is another common flow issue.
If the pool level is low, the pump will begin pulling air in through the skimmer. You will notice low water flow and lots of bubbles from the jets.
Turn the valve from skimmers off. That will pull water just from drains at the bottom of the pool/spa. In the picture above, the valve circled can be turned to block water intake from skimmers.
Fix: let owners know that their pool needs to be filled before the 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.