Marine Air Conditioning for 2 Cabins
One unit ducted fore and aft, or one unit per cabin.
Air Conditioning Zones and BTU for 2 Cabins
This is the thing most people have backwards. The sizer works from the volume of air that has to be cooled, and the volume does not change because you drew a bulkhead through it. A boat with two cabins needs the same total cooling as the same hull with one big saloon.
The decision that costs money. One unit ducted to both cabins is cheaper and quieter to buy, but the two ends of the boat never balance and the cabin nearest the box gets the noise. Two units cost more and solve both.
| On a typical 40 ft boat | Cooling |
|---|---|
| Total, subtropics | 16,000 BTU |
| Per zone, if you split it 2 ways | 8,000 BTU |
| Total in the tropics | 18,500 BTU |
| Total in temperate water | 13,000 BTU |
8,000 BTU per zone is 2.3 kW each — small enough that the per-zone hardware is cheap, which is exactly why the argument is about ducting and compressors rather than about capacity.
2-Cabin Marine Air Conditioning Installation Costs
Self-contained. A 14–16k unit under the saloon settee, ducted fore and aft.
Supplied and fitted on a boat of about 40 ft, ex GST, including the seawater side and the 240 V work.
The units on this site that suit this architecture at roughly 8,000 BTU a zone.
Ducting and Zone Control Limitations
Running long ducts from one box to reach every cabin is the cheap answer and it is usually the wrong one. Airflow falls off with duct length and with every bend, so the far cabin gets a trickle while the near one gets the noise. If a quote proposes to feed two cabins from a single self-contained unit, ask what the duct run to the furthest grille actually is, and ask what happens if that cabin does not cool. Agreeing that in advance is far cheaper than arguing about it in February.