Troubleshooting Marine Air Conditioning

Why isn't my boat's air conditioner working?

9 Tips To Get Your Marine A/C Working

1. Clean your boat's air filters - all of them

Dirty boat AC air filter
Does your boat's filter look like this?

  • It's important to learn how to clean boat air conditioner filters and coils.
  • Your boat's AC system probably has at least 2 air filters. Many people only check and clean one of them. The second filter is usually on the unit itself. If you have a split system with separate air handlers, you likely have at least one filter on those. You should remove and clean or replace all the air filters on your boat at least once a season. If you are living on your boat, we suggest checking and cleaning them 2 or 3 times a season.
  • Look for any blockages or debris on the the air vents or returns - which could be in the engine room, the cabin or near the unit itself

2. Check your boat's circulation pump (raw water / sea water pump) for water flow

When you turn on your boat's A/C water pump, water should immediately begin flowing out the discharge port from the chilled water system. If you don't see a steady, fast flow of air conditioning water, turn off your system and look for blockages:

  • Remove and clean your raw water strainer basket (i.e. Groco raw water strainer)
  • Remove any debris on your below water line thru hull strainer. Note that insects like these spaces when your boat is out of the water, so make sure you check and clean them before launching your boat
  • Remove hose clamps and inspect each section of hose for blockages or buildups. Remove those blockages or replace the hose
  • Watch this video to see one approach to cleaning out the circulation pump and hoses on your marineac system. There are some other options for marine ac line cleaner as well. Also check out this page about cleaning out your boat ac saltwater system

    Dometic Marine HPF code. High Pressure Fault marine ac. Marine Airrrr High Pressure Fault. Mermaid marine High Pressure Fault. Marine Strainer.
    A clogged strainer can break marine ac units. These often cause High Pressure Faults (HPF codes)


3. Check your airflow - especially if your evaporator coil is freezing over

A properly functioning evaporator coil is cold enough to quickly freeze the moisture in the air IF it doesn't have enough airflow. In other words, if your evaporator coil is freezing, it isn't that your system is cooling too much, but that there isn't enough air flowing over the coils to keep them from freezing and your air conditioning system will stop working. Most boats with ac use seawater and air to ensure that your system operates at the right temperatures.

  • Check and clean or replace all of your air filters. This is by far the most common problem with boat ac units we see in the field. Refer to the owner's manual for locations of all of the air filters on your marine ac unit
  • Look for kinks, broken connections or blockages in the ductwork. If you had work done recently on your boat, you may find ducts that were compressed or blocked that need to be fixed
  • Measure the size of your vents and returns. Compare the total square inches of each with the manufacturers requirements to ensure ample air flow. It is possible that your system doesn't have enough airflow for certain weather conditions (like hotter, more humid air) or water temperatures. In other words, your system may work fine until the air is too humid or the seawater is too warm - because you don't have enough air flow to compensate for the greater workload of your boat's AC system which will cause your marine air conditioner coils to freeze
  • Check for any clothing, towels, blanket or pillows that might be blocking the airflow out of your boat a c system. Even blocking one vent can cause some marine a c systems to throw a code and shut down

    My Dometic Marine AC is blowing warm air
    New units are compact and cool a lot of air!

4. Check your display for settings and error codes

  • Some boatac displays are very hard to read. You may have left your unit on the heat setting, humidity only or ventilation only mode. Or, it may default to settings that aren't what you expect. You may find it easier to reset the power and start over if you aren't getting cool air
  • Look for error codes and refer to your owner's manual

5. Check your power

  • Marine air conditioners draw more power when starting then when operating. Make sure that all of the correct circuit breakers are on, that all of your connections are free of corrosion and appropriately sized. Use a multi-meter to test the voltage and the current flowing when your unit is running
  • always recommends having dedicated circuits for marine air conditioning on your boat because they often draw much more than other systems on your boat.

6. Check your condensate pan and water pump

  • Many older marine AC units had condensation pans that could corrode over time and cause leaks. Some corrosion in the plumbing may even cause blockages in the flow of condensation out of your unit. Over time, this could stress your ac unit and cause performance issues. It could also cause water damage to your boat if it leaks on other systems
  • We recommend installing units that have stainless steel condensate pans and also suggest using a dedicated marine ac water pump for condensation - to keep stress down on your bilge. These smaller centrifugal pumps are designed to be much quieter than most bilge pumps
  • Check the filter on your condensate pan and the condition of the pan nipple

    Elite marine ac, fischer panda marine ac 50000
    Even factory-installed AC units have probems sometimes!

7. Check your refrigerant (i.e. Freon) level

  • If you boat ac system doesn't have enough refrigerant, it won't function properly. Generally, we have found nearly all systems with low refrigerant have leaks that are either difficult or impossible to fix and usually indicative of systemic issues (especially corrosion). Unless your air conditioning unit is brand new, it probably isn't worth fixing. Refrigerant is expensive and leaks aren't usually the only problem

8. Check your evaporator coil and condenser coil for debris - but, DON'T CLEAN THEM YOURSELF*

Mermaid marine ac
Damaged evaporator coils block airflow leading to freezing coils and shut down units

Frozen coils on marine air conditioner
Time to gently clean the coils.

  • Depending on the location of your unit and design, the coils may require disassembly or removal of interior cabinetry to access.
  • Coils are extremely fragile and require special tools to clean. If you try to clean your AC evaporator coils with a shop vac, you will almost definitely bend them. Bent coils stop airflow which leads to other issues. WATCH THIS VIDEO TO SEE WHAT HAPPENS WHEN YOU CLEAN COILS WITH A SHOP VAC.
  • Dust is a terrible heat conductor compared to the titanium or aluminum that your coils are made from
  • *If you don't want to or can't call a professional to clean your coils, follow these steps:
    • Follow your manufacturers recommendations for materials and techniques
    • Your manufacturer may sell the specialty tools needed to gently clean your coils
    • There are many different coil designs and one solution doesn't work for all of them. Please either call a professional or follow your boat ac manufacturer's recommendation carefully

9. Give a call

  • If you are having any trouble getting your boat air conditioner to work this season, please give us a ring. We are happy to help you choose the best boat air conditioning and provide the best boat ac services for your boat

Common Symptoms of Marine Air Conditioning Not Working

Troubleshooting my boat's air conditioner can be tough. Here are some examples of what we hear from customers about why their boat ac unit isn't working:

  • "Why is my boat's ac pump is not pumping water?"
  • "My marine ac compressor not turning on."
  • "Marine air high pressure switch turned off my boat ac."
  • "My marine hvac worked last year. But, I turned on my Dometic this year and it blows warm air"
  • "My Cruisair SMX marine ac unit is making loud noises when I turn it on"
  • "How do I fix my boat's air conditioning system?"
  • "My Dometic Cruisair marine ac compressor doesn't start"
  • "My Dometic Marine Air Systems AC coils are freezing up"
  • "My boat's Dometic Marine AC isn't circulating water or it pumps for a while and then stops working"
  • "How do I troubleshoot my marine air conditioner?"
  • "My Cruisair SMX12 boat ac doesn't work"
  • "Why won't my boat's ac turn on?"
  • "My Dometic marine air conditioner fan doesn't work"
  • "My Cruisair split system fan is running, but the compressor won't start"

If any of these sound familiar, please look at our top 9 suggestions below.

Also check out this video to learn how to test your boat's AC system on land BEFORE you launch.