Specifying Guidelines

Now that we know what components are needed to complete a Marine Air system and have an idea of the type of system that is needed, we can proceed in selecting the items for each particular application.

• Step 1 - Required Capacity
  Divide your boat into three basic load areas:
  
  (1) Below Deck – cabins where the hull slopes inward toward the keel and there are minimal port lights and hatches.
  
  (2) Mid Deck – areas on the main deck with small or shaded windows.
  
  (3) Above Deck – areas with large glass surfaces and direct sunlight.
  
  Measure the length and width of each room to be conditioned to determine the square footage. It is assumed that your boat has average headroom of about 6-1/2 ft. (2m) and you have an average amount of furniture. If one end of the compartment is narrower than the other, take your measurement in the middle. Multiply the length by width to get the area of each. 
  
  Determine which load factors to use from the table below and multiply the area of each cabin by the load factor to determine the required air conditioner capacity.

Temperate: 95°F (35°C) air, 85°F (35°C) water, moderate humidity.
Tropical: 105°F (41°C) air, 95°F (35°C) water, high humidity.

•  Step 2 - Number of Units
  The number of air conditioners and their locations will be determined by the size and layout of your boat, and the space limitations for ducting and plumbing.
  
  The typical maximum ducting run is 15 ft (4.5m), but if there are many bends then the overall length must be reduced to ensure good airflow. Also consider which areas require independent thermostat control, and which cabins will be served by ducting or a secondary air handler (where the only temperature control is by reducing airflow with an adjustable grille or fan-speed control).
  
  See the figure below for some suggestions. You should check with your Marine Air dealer for specific guidance.

Comparison of typical central and self-contained systems

• Step 3 - Location
  As in step 2, the location of the units will be determined by your boat layout.  Check the appropriate specification sheet for unit sizes, and make sure that there is sufficient space to service the unit and remove if necessary.
  
  The self-contained unit or air handler must have an open return-air path, and be located such that the discharge ducting can be routed to a high point in the cabin. The return-air grille does not need to be directly in front of the unit, in fact, the system will be less noisy if there is an indirect path for the return air to follow. Most units have blowers that rotate which will allow routing the discharge duct in the most direct path to reduce restrictions.

•  Step 4 - Seawater Components
  It is normally recommended that you use one pump of adequate capacity for all of the air conditioning systems on board. The rule of thumb is to have 180 gallons per hour (3 gpm) of water per ton of air conditioning (one ton is 12,000 BTU/hr). If more than one Marine Air System shares a common pump, you will also need a pump relay and manifold.
  
  The table below shows recommended seawater flow rates and minimum inlet (through-hull) for a system of a given capacity. Your dealer can give you more guidance.

• Step 5 - Duct and Grille Sizing
  See the table below for recommended duct and grille sizing. Your Marine Air dealer can help with duct transition boxes and sizing of branch ducting, as well as with the large selection of grilles available, some of which include aluminum, plastic, and many species of wood.