Troubleshooting Marine Air Conditioning Water Flow Problems

Marine air conditioning is a major comfort factor for medium to large recreational boats, especially in warm climates. Systems from manufacturers like Dometic and CTM are built for marine environments, but like all equipment that relies on raw water cooling, they are susceptible to performance issues—most commonly caused by water flow interruptions.

Raw water flow problems are one of the top causes of marine A/C failure. If your system is shutting down unexpectedly, not cooling effectively, or triggering “HPF” (High-Pressure Fault) or “LO” (Low Flow) error codes, inadequate water flow could be to blame.

In this article, we’ll walk through how to troubleshoot and resolve marine A/C water flow problems to keep your Dometic or CTM system running smoothly.

Understanding How Raw Water Cooling Works

Before diving into troubleshooting, it helps to understand the basic principle. Both Dometic and CTM marine A/C systems rely on raw seawater pumped through a heat exchanger (condenser coil) to remove heat from the refrigerant cycle. After passing through the A/C unit, the heated water is expelled overboard via a discharge thru-hull.

If water flow is blocked or restricted at any point—whether due to clogged strainers, failed pumps, or fouled lines—the system can’t offload heat efficiently, causing pressure build-up and triggering automatic shutoffs to protect the unit.

Common Symptoms of Water Flow Problems

Water flow issues don’t always present the same way. Here are some of the signs that your marine air conditioning system may be suffering from poor raw water flow:

• A/C unit shuts off after a few minutes of runtime

• Error codes like “HPF,” “LO,” or “Hi P” appear on the digital control panel

• Weak or non-existent water discharge overboard

• Unusual sounds from the pump or unit

• Water pump is hot to the touch

If you’re noticing any of these problems, it’s time to inspect the raw water side of your A/C system.

Step-by-Step Troubleshooting Guide

1. Check the Overboard Discharge

Start with the easiest inspection point—your A/C water discharge port.

• Is water visibly flowing overboard while the system is running?

• Is the flow rate strong, or is it weak or pulsing?

A weak or absent flow is the clearest indicator of a water flow issue. If you don’t see water discharging, shut the system down and proceed with further inspection.

2. Inspect the Raw Water Strainer

The strainer is the first line of defense against debris and marine growth entering your cooling system.

• Shut off the seacock before opening the strainer

• Remove the basket and clean out any debris, grass, or barnacles

• Check for cracks or warping in the basket

• Inspect the strainer housing O-ring for damage and reseal it properly

A clogged or damaged strainer is one of the most common causes of reduced flow.

3. Test the Water Pump

The water pump is a vital component in any raw water A/C system.

• Turn on the system and listen for the pump activating

• If it doesn’t run, check the power supply (breaker, inline fuse, or relay)

• If the pump runs but no water flows, it may be airlocked or damaged

• Check the pump impeller for wear or blockage

• Inspect the pump housing for leaks or corrosion

CTM units often pair with self-priming pumps, while Dometic systems may use centrifugal pumps that require careful placement below the waterline to stay primed.

If the pump hums but doesn’t move water, try priming the system manually by adding water to the strainer or pump intake hose.

4. Verify Hose Routing and Integrity

Inspect all raw water hoses from the seacock to the pump, and from the pump to the unit and overboard discharge.

• Look for kinks, collapsed sections, or loose clamps

• Ensure no air pockets are trapped in uphill hose runs

• Check for biofouling inside hoses (algae, barnacles, etc.)

• Replace any soft, cracked, or overly aged hose

Poor hose routing can lead to cavitation or loss of suction at the pump.

5. Inspect the Thru-Hulls

Marine growth or debris can clog both the intake and discharge thru-hulls.

• With the boat out of the water, inspect intake grates and discharge ports

• Use a rod or soft brush to clear blockages

• Consider installing a scoop strainer to protect the intake

Make sure all thru-hulls are properly bonded to prevent galvanic corrosion, which can lead to internal degradation of metal fittings.

System-Specific Considerations

Dometic Systems

Dometic A/C systems may include Smart Touch or Passport I/O digital controls. Some common diagnostic tips include:

• Use the control panel to read fault codes like “HPF” (high pressure fault) or “LO” (low voltage/flow)

• Ensure all control cable connections are clean and secure

• Use the Dometic app (if compatible) for advanced diagnostics

Dometic systems are sensitive to pressure conditions; even minor restrictions can lead to shutdowns.

CTM Systems

CTM Marine Air Conditioners are designed with user-friendly troubleshooting features:

• Indicator lights or digital readouts provide fault codes

• Quiet, brushless water pumps may require occasional calibration or inspection

• CTM recommends using strainers with a mesh size that blocks smaller debris (40 mesh is typical)

Many CTM models support multi-zone configurations, so ensure the flow loop isn’t starved by another unit upstream.

Pro Tips for Preventing Future Flow Issues

• Flush the raw water system with fresh water at least once a month to remove salt buildup

• Treat raw water lines seasonally with barnacle buster or descaling solution

• Replace pump impellers and hoses every 1–2 years, depending on usage

• Keep spare impellers, strainers, and hose clamps onboard for quick fixes

• Inspect your system at the start and end of each boating season

Installing a flow switch or smart monitoring device can alert you early to flow interruptions and help prevent unit damage.

When to Call a Professional

If you've cleaned the strainer, primed the pump, checked hoses, and inspected thru-hulls but still face shutdowns, the problem may be deeper—such as a fouled condenser coil or an internal blockage. In this case, professional service is recommended to avoid damaging the compressor.

An A/C technician can:

• Pressure test the raw water loop

• Clean the condenser coil with acid solution

• Evaluate refrigerant pressures and system health

• Replace worn or corroded internal parts

Raw water flow issues are one of the most common causes of marine A/C failure, but they’re also some of the most preventable. By routinely inspecting strainers, pumps, hoses, and thru-hulls—and knowing the specific behaviors of Dometic and CTM systems—you can keep your air conditioning running efficiently and avoid the hassle of mid-season breakdowns.

For high-quality marine air conditioning components, including replacement strainers, pumps, and Dometic or CTM systems, turn to S & S Dockside Marine Supply. Whether you're troubleshooting a flow issue or planning a full upgrade, our expert team can help you keep your cool—literally—every time you leave the dock.