Keeping your boat cool and your guests comfortable on the hottest of summer days is important, but knowing what size air conditioner is right for your boat is not always easy to determine.  There are a number of factors that effect how many air conditioners and what size unit(s) you will need to properly control the climate on your vessel.  There's even a little bit of math involved!  However, using the guidelines below, you should be able to pretty accurately identify the right air conditioning solution for your boat.

Type of Air Conditioner System

A good place to start with selecting an air conditioner for your boat is to first identify which type of air conditioner you'll need.  There are three main types of air conditioners used on boats: self contained units, split units and chiller systems. Each offers its own unique features and advantages.

A self-contained air conditioner is a unit where all of the components of the air conditioning system are mounted together on one chassis.  They use a special refrigerant, and will require some sort of connection to sea water for cooling.  A self-contained unit is usually stored somewhere in the living area out of sight. These units are best suited for smaller vessels under 40 feet in length.

A split or central air conditioning system has the components of an air conditioning system separated between two different units.  One unit is known as the condensing unit, and it contains the compresser, the condenser and the electrical system.  The condensing unit usually is placed in the engine room.  The other unit is known as the evaporating unit, and this piece contains the blower and the evaporating coils.  The evaporator is usually installed somewhere in the living area.  Split systems are usually best suited for vessels up to 80 feet in length.

Chiller systems are powerful systems with large capacities.  They work by a water loop that circulates between the chiller unit and air handlers.  There is no refrigerant used in chiller systems; instead the water is cooled and pumped through various cabins throughout the vessel.  Chillers are perfect for large boats and superyachts because they have a more flexible load management and reduced peak electrical requirements.

Do You Have the Power?

A very important consideration when installing a new air conditioner on your boat is understanding what your power needs will be.  While the unit may not draw much electricity when hooked up to shore power, the story will be very different when underway.  And especially when sailing or at anchor, the power load can become large.  It is important that you have properly sized your electrical sources on your boat to support your air conditioning system as well as all the other electrical devices you have on board.  Whether you use a generator or a battery bank and inverter system, you will want to properly calculate whether you are generating enough power to support your new air conditioning system.

Cooling AND Heating?

Some marine air conditioners feature heating as well as cooling.  Models like the Webasto FCF series offer reverse-cycle heating that pulls heat from the water in order to warm up the cabin.  This can be handy for extending your overall boating season, and is an important consideration when selecting an air conditioner for your boat.

Calculating Capacity

Here's where your math skills come in to play!  The first thing you'll want to do is identify each space that you are interested in having cooled by the air conditioner.  You'll want to classify each space according to its location on the boat - either below deck, mid-deck or above-deck.  The reason for this is that different areas of the boat require more effort from your air conditioner to be cooled, and so the location of these areas will factor into your calculations for how much output from your air conditioner is required.  For each area, calculate the square footage of the rooms, add them together, and then multiply the load factors listed below.  The load factors will depend on the climate in which your boat will mostly reside - moderate or tropical .  Use moderate load factors when the air temperature is less than 95°F and the water temperature is less than 85°F.  Select tropical when the air temperature is less than 105°F and the water temperature is less than 95°F.

  Below Deck Mid-Deck Above Deck
Climate Moderate Tropical Moderate Tropical Moderate Tropical
Load Factor 60 80 90 120 120 150

So, if you have 100 square feet of space below deck in moderate climate, you would multiply 100 X 60, for a result of 600.  This would be your needed capacity (BTU per hour).  Add all three sections up for your total required capacity.

How Many Air Conditioners Does it Take, and Where to Put Them?

Once you know your capacity, the next question you'll want to answer is how many air conditioning units are needed to do the job well.  This decision will largely be made based on the layout of your boat and whatever limitations there may be for ducting to circulate the air around your boat as well as room for plumbing components needed to supply and discharge water for the units.  You'll also want to consider how much flexibility you will want for different areas of the boat.  Will you want independent thermostat control for your bedroom from the rest of the living quarters?  As for ducting, to ensure good air flow it is recommended that there are no runs of ducting exceeding 15 feet in length.  If tight turns are required, that could restrict air flow as well.  You may be able to theorhetically use only one air conditioner to cool the living quarters on your boat, but space restrictions may require you to add a second unit for maximum comfort.

The location of the air conditioner(s) is also affected greatly by your space available.  Each unit should have sufficient access to a return-air path, and the discharge ducting should easily be routed to a high point in the cabin.  Many units, like the Webasto FCF series, offer a rotatable blower to allow for a variety of installation options.

The Right Pump for the Job

If this is a new air conditioner installation, you'll also have to install a number of seawater cooling components as well.  An inlet thru-hull, a sea water strainer, a seacock valve, an air conditioning pump, a discharge fitting and plenty of water hose will all be required for proper installation.  For a 5000 BTU unit, we recommend a 320 GPH air conditioning pump for proper cooling. For a 9000 BTU, 12000 BTU or 16000 BTU unit, a 580 GPH pump should be sufficient.  For units larger than 16000 BTU, you may need a pump with an excess of 1000 GPH power.

Ducting, Grilles & Other Components

The size of the ducting and grilles that you use will depend on the size of the air conditioner that you select.  Smaller units like 5000 BTU or 9000 BTU may only need 4-inch ducting, whereas a 12000 BTU or 16000 BTU unit may require 5-inch ducting.  The same can be said for supply and return grilles.  The space you have for these components will also be a factor in selecting the right equipment for your system.  For additional assistance selecting the right components for your air conditioning system, just reach out to one of the ABYC-certified experts at Marine Parts Source at (866)388-0390.

Hopefully, the information above can get you started in identifying the best solution for keeping your boat comfortable for all your guests and also for extending your boating season.  As always, we highly recommend that you consult with an experienced and ABYC-ceritfied marine mechanic if you do not feel confident in conducting any of the work mentioned in this article.   And, of course, feel free to give Marine Parts Source a call with any questions.