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.

Spa mode

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 it’s not in use. When in bypass, the heater’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

Related Tech Tips

Gas Law Conceps for HVAC/R
If you went to school and learned the “Gas Laws” early on and it seemed boring a LAME but then later things like mass flow rates and air flow conversions and compression ratios seemed HARD. Well… It's because HVAC is hard if you don't understand the concepts behind the gas laws, even if you never […]
Read more
Strip Control Wires Properly
Erich Vinson is a tech from Colorado and one of the most entertaining people I interact with online. He wrote this quick tech tip on stripping back the outer jacket properly on control wires and it happens to also be something I preach. Thanks Erich. In the first picture (above), you can see what happens when […]
Read more
The 10 Commandments of the HVAC/R Technician
One trait I've seen with good technicians is that they take their jobs VERY seriously, but they learn not to take themselves too seriously. A few months ago I had someone tell me online that I must think I'm the A/C “god” because I'm always telling everyone the “right” way to do things. This got […]
Read more

One response to “Pool Heater Water Flow Issues”

  1. 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.

Leave a Reply

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.


To continue you need to agree to our terms.

en English