Air-Conditioning Systems - Part II


Chill Out, Part II

Last month we showed you how to determine your boat’s cooling needs as a first step in an air-conditioning installation or refit (see article here).Now that you know how much space you’re going to be cooling and the size of the unit you’ll need to cool it, it’s time to select the actual equipment.

A Typical seawater plumbing system for a marine a/c unit

Your boat’s layout will dictate where you put the air handlers. Check the unit manufacturer’s specification sheet for dimensions and make sure that wherever you put one, there will be sufficient room to service the unit and a plentiful supply of fresh air.

An air handler should be located so that it has an open return-air path and its discharge ducting can be routed to a high point in the cabin. Most units have blower outlets that rotate, which permits routing the discharge duct in the most direct path.

Don’t undercut the required capacity for the air-conditioning system. Use a raw-water pump of adequate capacity for each of the systems onboard. The rule of thumb is 250 gph of water per ton of air conditioning. (One ton of A/C equals 12,000 Btu/hour.) Table 1 shows recommended seawater flow rates and minimum through-hull intake sizes for Marine Air’s air-conditioning systems.


Table 1
System Capacity
Seawater Flow Rate
Inlet Size
5,000-12,000 250 ½"
16,000-24,000 500 ¾"
30,000-48,000 1,000 1"

Once return air is drawn into the handler, it’s cooled and then distributed via ducting into the appropriate space. It may first go through a plenum or “transition box” from which multiple ducts can lead to one or more areas. Use Table 2 to help you determine ducting sizes, plenums, and outlet grilles based on the Btus.


Table 2
Capacity (Btu/hr) Duct (in.) Return Air Grille (in.2) Discharge Grille (in.2)
5,000 4" 60 30
7,000 5" 80 45
10,000 6" 100 60
12,000 6" 130 70
16,000 7" 160 80
18,000 7" 200 100

Now let’s make sure you plumb the seawater system correctly and meet the electrical power needs, or nothing else will matter.

The seawater system consists of an intake through-hull fitting, seacock, strainer, raw-water pump, and overboard discharge through-hull, all connected by hose or piping. Choose a self-priming centrifugal seawater pump mounted below the waterline (that’s why you need a seacock on this end) for efficient operation. If a pump serves more than one air conditioning unit, you’ll need a pump relay and water manifold.

The plumbing must be self-draining so that if the boat is hauled, the system will drain, leaving no air locks to disrupt the water flow and reduce cooling efficiency. For shallow-draft boats where the pump cannot be mounted below the waterline, use a self-priming pump. The illustration on the previous page shows how your system should look.

Finally, you need enough A.C. power. Most air-conditioning units are available in three power ratings: 115V/60Hz, 230V/60Hz, and 230V/50Hz. Larger units may have three-phase compressors. Consult the unit’s spec sheet for the running current.

To select the right A.C. circuit breaker, multiply the compressor’s running amps by 2.5 then choose the next highest capacity breaker. If the pump is wired with the unit (not on a pump relay), add the pump amps before multiplying by 2.5. Wire size should be per ABYC requirements. If you’ll be running the system off a genset, make sure it can handle the compressor’s startup load; it’s higher than the pump rating. (See August “Yard” to learn more about the Dometic SmartStart system.)

Now you know the details of the system you need—get it installed and chill out.

This article originally appeared in the October 2011 issue of Power & Motoryacht magazine.

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