4. Air Ducting and Splitting:
Regardless of the btu size of the air conditioner, at least one vent must be within 4 to 5
feet of the unit. Insure though, that the airflow is not directed back towards the return air
grill. In the case of the 16,500 btu unit, the largest grill is required to be the closest grill.
Improper ducting accounts for the majority of marine air conditioning problems. Though
there are exceptions to the rule, the following rule of thumb applies to the number and
size of the supply air grills required for each btu size air conditioner:
5,200 btu unit: (2) four inch grills.
6,500 btu unit: (2) to (3) four inch grills.
9,000 btu unit: (3) to (4) four inch grills
12,500 btu unit: (Cool only) (3) or more four or six inch grills.
(With electric heat) Same as cool only.
(With reverse cycle heat) (1) six inch grill and (2) or more four inch
grills.
16,500 btu unit: (1) six inch grill and (2) or more four inch grills
24,000 btu unit: Minimum of three 6 inch vents/grills
When it comes to ducting, a general rule is that more grills are always better. Any time
more than one supply air grill is used, an air splitter is required. Air splitters come in a
variety of sizes and styles. Splitters may be mounted directly on the unit output air
collar, or placed in line a short distance from the unit where space restrictions preclude
mounting the splitter on the unit. Additional air splitters may be used at various intervals
along the duct routing to distribute air to multiple cabins. Wye’s are always preferable to
Tee’s. We do not use Tee’s for air splitters. Refer to FIGURE 1 for a typical ducting
scheme.
When surveying the boat for your air conditioner mounting spot, you must also decide
what size supply air grills will be used and where they will be mounted. The size of the
grill will dictate the size of the ducting. It is also important that you decide upon using
insulated or non-insulated air duct before beginning. Generally the ducting will be 4
inch, 5 inch or 6 inch diameter non-insulated duct. If you decide on insulated duct, add 2
inches to the outside diameter of the non-insulted duct. Typically power boats will use
non-insulated duct and sail boats insulated. Except for passing through a hot engine
room, the only benefit in using insulated duct is preventing condensation forming on the
exterior of the duct (sweating) which could cause water staining on fabrics. Because sail
boats generally sit lower in the water and have less glass than power boats, they tend to
produce more duct condensation than the power boat, however, space restraints may not
allow the total use of insulated duct and you may need to use a combination of the two
duct types. Some situations will require the use of non-insulated duct, but will require
insulating the duct after the installation with a insulation wrap. All duct runs should be
as short and straight as possible. Every 90 degree turn in a duct run reduces
performance by about 14%. Ducting should be tied to a permanent structure every foot
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