Pool Heater Water Flow Issues

As you may already know, we work on pool heaters at Kalos as a way of opening ourselves up to some extra business during the winter. We find that it's a worthwhile line of work, and it may be something that other residential HVAC companies may want to consider doing. This tech tip by Bert is about pool heater troubleshooting, but we have two other articles on the basics if you'd like to start there: Pool Heat Pumps, The Basics and Introduction to Pool Heat Pumps.

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 HVAC School's other articles on pool heat pumps linked above.

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, about 1/3 of pool heater service calls are actually due to water flow issues. 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 at 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. You can easily clean this screen trap 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.

Spa mode

Pools with a spa can have two automatic actuators installed: one at the pump's 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 the contest for 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 after removing the filter.

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 the 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.

—Bert Testerman

P.S. – We also have some videos of Bert's Kalos meetings about pool heaters. You can click the links below to check those out.

Pool heaters (general)

Pool heat pumps

Gas pool heaters


12/21/18 at 07:25 AM

Good tips. Pool filters are often DE or Sand filters and not just removable cartridge filters. Most would agree that DE filters are the best while cartridge filters are the easiest to service. A DE or Sand filter has to be backwashed to a drain or to the yard and usually the media (DE or Sand) is then replaced. These filters have a Multiport Valve with several positions that should be carefully changed with the water flow off. There is typically a pressure gauge on these filters which will read higher when the filter is restricted or lower when other lines are restricted. However its necessary to know normal pressure for reference which vary. Often 10 psi is close to normal but it depends on the pool lines, pumps and filters. Around 10psi high is often a clogged filter. On the multiport valve, the “Recirculate” position bypasses the filter while “Waste” sends the pool water down the drain or into the yard. Normally you do not want to open one of these filters, especially a DE filter, unless you are experienced and have replacement parts including specialized O rings.

FILTER: Keep it here most of the time, except when backwashing, rinsing or wasting (draining).

RECIRCULATE: Use this if the filter is broken, at least you can circulate the water.

BACKWASH: Use this setting to reverse the flow in the filter and send water out of the waste line. Make sure appropriate drain valves are open to drain or yard. The filter media (DE or Sand) will be “washed” out and then must be replaced with new media.

RINSE: Use this setting for 20 seconds after backwashing to rinse tank.

CLOSED: Put here to close off flow from the pool, usually to work on the equipment. Do not operate pump with valve in closed position.

WASTE/ DRAIN: Another filter bypass setting, this setting sends the water out of the waste pipe (hose), instead of returning it to the pool. Used to lower pool water level or to vacuum to waste.

Lars Vonder
Lars Vonder
5/31/21 at 01:20 PM

Good tips. Need some advice though. I have a very simpel setup at the moment with a low flow (<2-3m3/h) pump with Sand filter attached to a 3kW Heater. No bypass or valves whatsoever. The pump has broken and I am replacing it with a 6.6 m3/h pump/sand filter. What problems might I be facing with this increased flow capacity? My main concern is the specifications of the Heater. All it says is that the recommended flow is 1.3 m3/h.


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