Installing DC Air Conditioning on a Boat: The Planning Guide

A DC self-contained air conditioner is one of the more satisfying jobs on a boat because the unit itself is a single module: compressor, seawater condenser, evaporator, blower and electronics in one chassis, with the control panel on a cable. What takes the time is everything around it, and that is what this guide is for. Read it before you ask for a quotation and the installation drawing we send with it will make immediate sense; read it after and you will know what every line on the drawing is doing. It covers the Drake Marine DM-MAC DC range, 7,000–16,000 BTU at 12, 24 and 48 V, but the principles are the same for any seawater-cooled self-contained unit.

1. Where the unit goes

Under a bunk, in the base of a hanging locker or in a cockpit locker that backs onto the cabin: somewhere with a flat, dry, level base that can take the weight (18 kg for the 7,000 and 8,000 BTU units, 23 kg for the 12,000 BTU models, 31 kg for the 16,000 BTU), as low as practical so the seawater pump below it stays flooded, and as close as possible to both the cabin it serves and the house bank. Leave working room on the service side for the strainer, the filter and the hose connections; a unit you cannot reach is a unit that does not get its strainer cleaned. The footprints are 400 × 280 × 238 mm, 480 × 300 × 285 mm and 500 × 340 × 315 mm (W × H × D) for the three chassis sizes, and you need clearance above for the duct take-off and at the back for the seawater hoses and the drain.

Keep the unit out of any compartment where petrol, LPG or their vapours could be present, and out of the engine room unless the space is well ventilated and the drawing says so: the unit wants cool return air from the cabin, not engine-room heat, and the electrical equipment on board a boat with petrol or gas must be ignition-protected where vapours can collect.

2. The seawater circuit

The condenser is seawater-cooled, which is what makes a marine unit compact and efficient, and it is the part of the job that deserves the most care. The circuit is: a dedicated through-hull with a seacock, a raw-water strainer, a pump mounted below the waterline, a 16 mm (5/8″) hose to the unit, and a 16 mm hose from the unit to an overboard discharge above the waterline where you can see water flowing.

The through-hull should be on the centreline or low on the hull side where it stays submerged when the boat heels or rolls, and forward of any discharges. Use a proper scoop or flush intake fitted so it does not scoop air at speed. The strainer goes between the seacock and the pump, where you can open it; a clear bowl saves a lot of guessing. The pump is a continuous-duty seawater pump; the centrifugal type usually used is not self-priming, so it must sit below the waterline with the hose from the seacock rising steadily to it, no loops that can trap air. Keep the hose runs short, use marine-grade reinforced hose and double clips below the waterline, and bring the discharge out where the flow is visible from the deck or cockpit: the first check on any cooling problem is simply whether water is coming out.

Seawater temperature in Thailand is 28–31 °C most of the year. Warm water means the condenser has less to work with than it would in a cold climate, which is one reason we size units for the tropics rather than to a temperate rule of thumb, and it is why the strainer and the condenser need to stay clean: marine growth in a warm-water circuit arrives fast. Run the unit regularly, clean the strainer weekly in season, and flush the circuit at the end of the season.

3. Ducting and air

The unit blows cold air through a duct to a supply grille and draws warm air back through a return grille with the filter behind it. The duct size is 100–150 mm depending on the model, stated on the installation drawing for your unit, using insulated flexible marine ducting. Keep the run short and the bends gentle; every tight bend and every extra metre costs airflow (350, 450 and 600 m³/h for the three chassis sizes) and airflow is cooling. Place the supply grille high in the cabin so the cold air falls and mixes, and the return grille low or near the unit, with a free area at least equal to the duct area and preferably more; a starved return makes the unit noisy and inefficient. Never supply one cabin and return from another: the return air path has to be from the cabin being cooled.

One unit, one cabin or one zone is the rule. Splitting a duct to a second cabin can be made to work with short runs and a return path from each, but it is a compromise, and on a boat where two cabins need air conditioning two smaller units nearly always do a better job for similar money, with the second unit as a spare for the first.

4. Condensate

An air conditioner pulls water out of the air, a lot of it in the tropics, and the condensate pan under the evaporator has to drain. Gravity only: a continuous downhill run from the pan to a sump, a shower drain or a dedicated small sump pump, with no sags where water can sit and grow things. If the unit cannot be placed above a drain, a small condensate pump with a float is the answer; do not run the drain into the bilge and hope.

5. The DC circuit

This is a dedicated circuit from the house bank through a breaker or fuse at the battery end, with nothing else on it. The current is the whole point of choosing the voltage: 35 or 50 A at 12 V, 23 or 30 A at 24 V, 15 or 20 A at 48 V. The table gives the tinned marine cable size for a 3 % voltage drop over the round trip and the breaker or fuse to protect it, by one-way run length from battery to unit. It is a planning table; the installation drawing with your quotation gives the figures for your boat, and the installation must meet ABYC E-11 or ISO 13297.

Read the table and the voltage question answers itself. A 12,000 BTU unit at 12 V six metres from the bank wants 35 mm² cable and a 63 A breaker; the same unit at 48 V wants 10 mm² and a 30 A breaker. The cable size calculator handles other runs and the pump circuit. Use tinned marine cable, crimped and heat-shrunk terminals, and a breaker you can reach from the cabin so the unit can be isolated without going to the battery.

6. The house bank behind it

A DC air conditioner is only as good as the bank behind it. Plan the bank on the night’s amp-hours rather than on the amps on the label: a 7,000 BTU 12 V unit rated at 35 A uses roughly 170 Ah over an eight-hour night with the cabin pre-cooled and the thermostat cycling, and the bank should carry that as a shallow cycle, 50 % depth of discharge for a Fullriver AGM bank (Fullriver rate the range to 80 %, so the rest is reserve), 80 % for lithium. The run-time and battery calculator does this for each unit and adds the solar and charger time to put the night back; the guide to running air conditioning from batteries explains the reasoning. Charge fully and often with a temperature-compensated multi-stage charger; in the tropics that one setting protects the bank more than anything else you fit.

7. Commissioning

Open the seacock and check for leaks at every joint before power goes on. Switch on the pump circuit first and confirm a steady discharge overboard, then the unit. Set the thermostat a few degrees below cabin temperature and check the supply air is cold within a few minutes, the condensate is dripping to the drain, and the current draw on the battery monitor is close to the rated figure while the compressor runs. Note that figure and the overnight amp-hours on the first night; they are the two numbers that tell you the installation is right and the two to watch for the life of the unit.

Getting it done

Tell us the boat, the cabin and where she is kept and we specify the unit, the pump, strainer and through-hull, the ducting and grilles, the cable and the breaker, and the bank and charging to run it, with an installation drawing in the quotation: LINE @drakeco, WhatsApp +66 99 406 4680 ou e-mail. Yards in Phuket, Krabi and Trang are used to seawater-cooled self-contained units, and we are happy to work with yours or to point you to one. Free delivery in Trang, Krabi and Phuket; worldwide freight at cost, invoiced separately.