Mermaid Air Conditioner manual Air Ducting and Splitting

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