Marine A/C Cycling Too Often? Diagnose Raw Water Flow Issues
- Andrea Ryan
- Oct 27
- 4 min read
Nothing ruins a day on the water like an A/C system that just won’t stay consistent. One minute your cabin feels cool and comfortable — the next, your marine air conditioner cycles off, leaving you wondering what went wrong.
If your Dometic or CTM marine A/C system is cycling on and off too often, the culprit may not be electrical at all — it’s often a raw water flow problem. Understanding how your cooling water loop works (and how to keep it running smoothly) can save you from unexpected downtime and costly repairs.
Understanding the Role of Raw Water in Marine A/C Systems
Every marine A/C system relies on a raw water pump to draw seawater through a strainer and into the heat exchanger (or condenser). This seawater removes heat from the refrigerant, keeping the system efficient and preventing it from overheating.
When water flow is restricted — even slightly — the condenser can’t dissipate heat properly. As a result, the system overheats and the compressor shuts down prematurely, leading to rapid short cycling.
In short:
🚤 Good water flow = steady cooling.
🚫 Poor water flow = short cycling and potential compressor damage.
Common Causes of Raw Water Flow Problems
While marine A/C systems are durable, the raw water loop operates in a harsh environment. Salt, marine growth, and debris can all restrict flow. Here are the most common causes:
1. Clogged Raw Water Strainer
This is the number one reason for restricted flow. Over time, the strainer collects algae, grass, and debris that reduce water movement.
Fix: Check and clean your strainer weekly, especially if cruising in warm or shallow waters.
2. Blocked Intake Through-Hull Fitting
Barnacles or seaweed can clog the intake. If the A/C cycles off and you’ve already cleaned the strainer, the problem may be below the waterline.
Fix: Use a flexible wire or brush to clear the intake. In severe cases, haul-out cleaning may be needed.
3. Air Lock in the Pump
After maintenance or layup, air can become trapped in the pump housing. The impeller spins, but no water flows.
Fix: Prime the pump by loosening the outlet fitting until water begins to flow freely.
4. Failing Water Pump or Impeller
Pumps that run dry, overheat, or lose their seal can reduce output over time.
Fix: Check for leaks or excessive noise. Replace the impeller or the pump if water output remains weak.
5. Undersized or Worn Hoses
Old, kinked, or undersized hoses can choke off water flow, causing uneven cooling or frequent shutdowns.
Fix: Inspect all lines for soft spots, cracks, or restrictions. Replace as needed with proper marine-grade hose.
Symptoms That Point to Poor Water Flow
If your A/C system is cycling too often, look for these warning signs:
Reduced cooling efficiency even when the compressor runs.
Warm discharge water (it should be cool to the touch).
System tripping breakers after running for short periods.
Visible air bubbles or sputtering discharge flow.
These are all clues that your raw water system isn’t moving enough volume through the condenser.
How to Troubleshoot Water Flow Like a Pro
Start at the Discharge: Is there a steady stream of water exiting the hull? If not, check upstream.
Inspect the Strainer: Shut the seacock and open the strainer lid. Clean debris with a soft brush.
Check the Pump: Verify it’s running and not cavitating. Feel the housing — it should be cool.
Look for Kinks or Air Leaks: Any hissing or bubbles indicate suction-side issues.
Flush the Condenser: Over time, scale and salt build up inside the coil. Use a marine-safe descaling solution.
If you’ve checked these and the system still cycles, the issue may be electronic — such as a faulty temperature sensor or control board — but water flow should always be your first stop.
Preventing Future Water Flow Problems
Preventative maintenance is the key to reliability and comfort onboard. To avoid water flow problems:
Clean your strainer weekly during active use.
Flush the system with fresh water when leaving the boat for extended periods.
Inspect and service pumps every 6–12 months.
Use marine-safe antifouling for intake fittings in high-growth areas.
Run your system regularly to prevent buildup.
Brands like CTM and Dometic design their marine air conditioning systems to be robust, but consistent maintenance keeps them performing at peak efficiency season after season.
Why Short Cycling Matters
Short cycling doesn’t just affect comfort — it shortens your system’s lifespan. Frequent on/off cycles strain the compressor, increase amp draw, and reduce cooling consistency. Over time, this can mean expensive replacements and lost cruising time.
If your A/C is running erratically, fixing water flow issues early can prevent more serious system failures later.
Final Thoughts
Your marine A/C should make life on the water more enjoyable, not more frustrating. By keeping your raw water flow clean, unrestricted, and steady, you’ll extend the life of your Dometic or CTM system and enjoy reliable cooling wherever you cruise.
At S & S Dockside Marine Supply, we carry a full range of Dometic and CTM A/C systems, pumps, strainers, and accessories — everything you need to maintain steady comfort onboard. Whether you’re diagnosing an issue or planning an upgrade, our team is here to help you keep your cool this boating season.
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