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System Design

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'The simplest and most cost effective air conditioning solution available'

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How to Size Your A/C System

1) Required Capacity

Find the required capacity by dividing the vessel into three main load areas ;
Below Deck - Cabins where the hull slopes inward towards the keel with minimal port holes and hatches.
Mid Deck - Areas on main deck with small or shaded windows.
Above Deck - Areas with large glass surfaces and direct sunlight.

Multiply the length and width of each cabin to be treated to determine the area in square feet or metres. It is assumed the boat has an average headroom of about 6.5ft (2 mtrs)with an average amount of furniture. If one end of the cabin is narrower than the other take your measurement in the middle.

Using the table below, multiply the area of each cabin by the appropriate load factor number to find the required air conditioner capacity. For example, if your boat is in a temperate climate and you are measuring in square feet, you would multiply your total below-deck area by 60, your mid-deck area by 90, and your above-deck area by 120. (A temperate climate generally has 95°F (35°C) air and 85°F (29°C) water with moderate humidity; a tropical climate averages 105°F (41°C) air and 95°F (35°C) water with high humidity.)

Climate Below Deck Mid Deck Above Deck
Temperate 60 90 120
Tropical 80 120 150

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.

2) Number of Units

Taking into account the boat’s size and layout, determine the number of self-contained systems or air handlers needed.

Find out which cabins or areas will benefit best from a dedicated thermostat control, and which cabins can be served by ducting or a secondary air handler (where the only temperature control is an adjustable grille or fan-speed control).

3) Location

Taking into account the boat’s size and layout, determine the location of each self-contained system or air handler.

In addition to leaving enough room for plumbing and ducting, there must also be sufficient space in each installation location for servicing and/or removal of the unit.

A self-contained unit or air handler must have an open return-air path. However, 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. Never install the unit in the bilge or engine room or where vapors from these areas could reach the unit.

A self-contained unit or air handler must be located so the discharge ducting can be routed to a high point in the cabin. Rotate the blower to create the most direct path for routing the discharge duct. Poor airflow may result from a ducting run of over 15 ft. (4.5 m) or a ducting run with many bends. Plan for the shortest possible ducting run while limiting the number of bends.

4) Seawater Components

Use one pump of adequate capacity for all air conditioning systems on board. The basic rule is 180 gallons (681.4 liters) per hour (3 GPM/11.4 LPM) of water per ton of air conditioning (one ton = 12,000 BTU/hr). If more than one system shares a common pump, you will also need a pump relay and manifold.

The BTU/hr capacity in the table below shows recommended seawater flow rates and minimum inlet (through-hull) sizes.

System Capacity (BTU/hr) Seawater Flow Rate (gph/lph) Inlet Size
5-12,000 180/681 1/2 in.
16-24,000 360/1363 3/4 in.
30-48,000 720/2726 1 in.

5) Duct and Grille Sizing

Determine the proper duct diameter (Ø) and grille sizes for your air conditioning system. Use the table below to find the correct sizes, which are based on the system’s BTU/hr capacity.

Capacity (BTU/hr) Duct Return Air Grille
(sq. in./sq. cm)
Discharge Grille
(sq. in./sq cm)
5,000 4 in. 60/387 30/194
7,000 5 in. 80/516 45/290
10,000 6 in. 100/645 60/387
12,000 6 in. 130/839 70/452
16,000 7 in. 160/1032 80/516
18,000 7 in. 200/1290 100/645

Other A/C System Components

A complete air conditioning system requires controls, a seawater cooling system, an air-distribution system and electrical connections.

Controls: There are two types of controls, digital and electromechanical switch.

  • Digital: These keypad/displays are part of a microprocessor system with many advanced functions, including automatic fan-speed control, fault display, and a dehumidification program. Decorative bezels can be added to complement the vessel's interior decor.
  • Mechanical: These manual switches with two or three rotary knobs control the mode of operation, thermostat, and variable fan speed. Reverse-cycle models have automatic changeover between heating and cooling.

Seawater Cooling System

The seawater cooling system (shown in the illustration below) brings seawater into and through the system then discharges it overboard. It consists of an inlet through-hull fitting, seacock (water valve), strainer, pump, and overboard discharge fitting, all connected by hose or piping.

AC seawater plumbing

If multiple air conditioning units are served by a single seawater pump, then a pump relay and water manifold are required. A centrifugal seawater pump is recommended for efficient, quiet operation and long life. Centrifugal pumps are not self-priming and must be mounted below the water-line (install a self-priming pump for shallow-draft boats).

It is important that the seawater plumbing be self-draining, meaning that if the boat is lifted, all water in the piping will drain out. An air conditioning system plumbed this way will have no air locks which could disrupt the flow of seawater.

Air Distribution System

In cooling mode, warm cabin air is drawn into the self-contained unit or air handler through a return-air grille. It is then cooled and blown through flexible insulated duct and back into the cabin through a supply-air grille installed high in the cabin. The supply-air grille should be installed away from the return-air grille to ensure good circulation (see the illustration below).

Plenums, or transition boxes, can be installed in the duct to split the air flow into multiple ducts to serve one or more cabins.

AC duct plumbing

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