Air Conditioning



Air conditioning in a cruiser class boat is a very popular option. You not only get the convenience of air on board, but it is one option that tends to retain its resale value over the life of the boat. Even in northern climates, it is pretty common to see air conditioning on cruisers, especially if they are capable of heat as well. If you install it yourself, you can save up to half the cost of a factory-installed unit. And you can have the satisfaction that you did it yourself.

The typical marine air conditioner for a small boat consists of a self-contained unit using chilled-water. This is a bit different than the typical portable home window unit type air conditioner where a radiator performs the cooling function. It is interesting to note that chilled-water air conditioners are typically found in central air and commercial building systems.

While this is the most common type of air conditioner, it is not the only type available for marine use. At least one manufacturer makes a completely portable unit, which sits atop the v-berth hatch. While it does take awhile to set it up, the advantage is that you do not have to install anything. A popular RV supplier also makes a marine version of their popular RV system, but to me, a roof-mounted air conditioner just doesn't fit well with the aesthetics of most cruisers.

While it may be tempting to install one of those 4,000 BTU home window air conditioner units like those found for under $100 at the discount chain stores, the extreme "ugly-factor", difficulty in installing, and the possible durability and safety issues would prevent me from even considering them.

The typical small boat air conditioner - the kind I will be installing - is as follows:

Compressor Unit. Contains the major air conditioning components of the system, including the evaporator, compressor, blower fan, and control box. Some air conditioners also have the capability of producing heat, either by a "reverse cycle" or by a heat coil.

As boats become larger, manufacturers may install more than one unit. This creates air conditioning zones in effect, and is probably much easier than trying to install one large unit. Even though more than one unit is installed, a properly engineered single seawater circulation system can facilitate multiple units, supplied by a single thru hull, strainer, and water pump. For this project, my needs will be satisfied with a single air unit.

For the most part, air conditioners are not ignition rated - which means they cannot be installed in engine areas. Even if they were ignition rated, you would not want the air unit, supply, or return air ducts or plenums to route through engine compartments where exhaust fumes could be picked up. So keep the air conditioner out of the engine room.

Thru-Hull. This is literally a hole in the bottom of the boat, and serves to bring in cold water to the air conditioner. There are two types of thru-hulls. One type is a forward-facing clamshell, and is necessary when the intent is to run the air conditioner while underway. Since the air conditioner runs on 120VAC, this also means you must have a method of generating AC power via auxiliary generator or inverter. If you have no need to run AC while underway, you may be able to substitute a flush thru-hull, which should lessen drag.

I do not intend to run the air conditioner away from the dock, so I will try to use a flush thru-hull. One caveat in using a flush thru-hull is that after each boat trip, you might have to deal with an air-lock in the line. This can be alleviated by turning off the sea-cock before putting the boat in motion. Otherwise, you can buy a clamshell cover for the thru-hull.

Seawater Strainer. This component should be located below the waterline and be mounted close to the thru-hull. The function of the strainer is to protect both the water pump and air conditioner from becoming fouled with marine growth or other particulate matter.

Seawater Pump. Are typically centrifugal in design. They are typically 120VAC pumps, and are controlled by the air conditioner. Since the pump is often located in the engine room, it needs to be ignition proof rated. The plumbing from the thru-hull to the air conditioner should be as straight and short as possible, and ideally slightly uphill. This will also minimize any air air being trapped into the line.

Overboard Discharge. Completing the chilled-water system is the overboard discharge thru-hull. All of the air conditioners I am considering have a smaller diameter line from the pump through the air conditioner to the exit thru-hull, vs. the diameter from the inlet thru-hull to the pump. The units I am considering generally have an inlet thru-hull of 3/4in dia, while the exit thru/hull can be anywhere between 5/8 to 1/4 in.

Condensate Drain. It is not uncommon for up to 1/2 gallon of water per hour to accumulate in the condensate pan while the air conditioner is running. This must be drained somehow. It may be unsafe to dump this water directly into the engine room bilge, since the hose that allows this to happen could also allow gas fumes or engine exhaust to enter occupied areas. Either the forward bilge (if one exists), or shower sump is ideal for this purpose. One manufacturer makes a kit that will cause the water exiting the discharge thru-hull to "scavenge" the condensate. This might be an option if you cannot find a suitable method for collecting the condensate.

Control Unit. Regulates the air temperature, and if the air conditioner has a heating mechanism, controls both heating and cooling. Some units are simply a thermostat and set of knobs to control the fan and temperature, while more sophisticated units are digitally controlled, and can be programmed for different conditions.


A Note About Safety

This project is complex enough that a few minutes should be taken to througly review the US Coast Guard Regulation for Recreational Boats. It is known officially as 33 US Code for Regulations 183 (33CFR183), and can be found and downloaded on-line from several government web sites. The two areas of most concern with this project are electrical wiring and maintaining bulkhead integrity - in respect to ignition isolation. Before undertaking a project of this sort, it is imperative that these requirements be followed.


Air Conditioners

Before the start of any project, I spend a considerable amount of time doing research. The Internet is a good tool for this. Several manufacturers have web sites that allow you to compare air conditioner features. Three of the most popular air conditioners are Mermaid, Marine Air, and CruisAir.

Manufacturer's web sites:

    Mermaid Air Conditioners
    Marine Air Conditioners
    CruisAir Air Conditioners

As a general rule, when you purchase a marine air conditioner, it does not include all of the necessary components. As a do-it-yourselfer, you must typically gather all of the components for installation yourself. Several manufacturers do offer installation kits, and this might be a good short cut to obtaining all of the items. I am electing not to purchase an install kit because of my installation requirements, many of the items would not be used. However, it may be a good way to purchase all, if not most of the components as one kit.

Installation kits typically include a fresh-water circulating pump, thru-hulls, hose, ductwork, vents, and electrical components that are not included with the air conditioner unit itself.

Keep in mind that the installation components add a significant cost to the project, and can easily cost nearly as much as the price of the air conditioner unit.


Final Selection

As I was researching sources for the various units, I discovered that not all of them are "do-it-yourself" friendly. I had one sales representative tell me several times during a conversation that this is a job best left to professionals. I don't quite understand what means, but it resulted in my being more convinced than ever that this would be a project that I could do. I don't know if there is such a thing as a "Bachelor of Air" degree, but I might be wrong.

The extent of my current knowledge of air conditioners in general is that you turn them on, they cool you down. And, with marine units, they discharge water out the side of the boat as they are doing so. But I have tackled first-time projects before, and the technique is that you research, learn, then apply what you have learned. That being the case, the only way I could become a "professional" is by experience, and the best teacher of experience is simply do it.

I finally narrowed my selection to the following unit:

ManufacturerModelBTUCool AmpsHeat AmpsSize (W x D x H)Tech Data
MermaidM6-CEH-U6,5006.4A17.5A16.0 x 11.5 x 11.5Spec sheet

I chose this model after looking at quite a few different units. The general reasons for chosing this model were:

  1. Brand: Quite frankly, Mermaid impressed me. Mermaid intends their units to be installed by the do-it-yourselfer, and their target customer is the individual boat owner and marina installer rather than the boat manufacturer. They were very helpful in pre-sales information, answered all of my questions, suggested alternatives, and provided several great tips. They were also prompt - if asked by email, they often answered my questions by the next day. You can also talk to a technician via toll free number for technical support and installation questions while you are installing the unit. The installation is a bit on the technical side, so a source of help is always a good thing.

  2. Electric Heat option: I live in Michigan - so heat must be an option. I liked the idea of electric heat over reverse-cycle heat, because there are fewer components running during the heat cycle. In a reverse-cycle unit, the pump still runs, and there is a reverse-cycle switch to maintain. In the electric heat unit, only the blower and control circuitry are in operation.

  3. Availability: Mermaid units are available from vendors via, retail, mail order, or factory direct. In addition (and this is really important), you can purchase all of the various components, such as the seawater pump, ductwork, and other items directly from Mermaid. Since I am not going to use an installation kit, this is important to me.

  4. Cost: Whether it is due to the competition of the various retailers or other reason, I found you could get the Mermaid units from discount retailers at a significant price reduction. I found the unit I wanted on sale at Defender with a 20% discount - and during the sale, they included free shipping as well - which for the 60lb package is normally around $80 (Defender's Web Page).

  5. Form, Fit, Function: The unit I chose is physically the smallest of the units on my list, even though it had the highest BTU rating. And I like the idea of electric heat over the reverse cycle unit, being that I am in the northern climate.

  6. Warranty: The unit carries a 5-year warranty, which appears to be one of the best in the industry.

I could not see where I could go wrong with the unit I chose.


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