One, Two, Three on board AC; A marine air conditioning crash course.
The air conditioning inside your boat is most likely going to stop working while you’re living aboard. And it always happens on the hottest of days. 9 times out of 10 it’s going to be a relatively simple fix. But, just like anything, everyone is a newbie at some point. So, I’m going to try to give you a quick rundown on the easy stuff. That way you can knock out the head-slappers before having to make a call to the repair guy. Who, I can guarantee won’t be out in time to save you from having to make Sophie's choice over the ice in the beer cooler.
If your first visit to a marina was on a hot day, you probably noticed that almost every boat had a constant stream or two of water shooting out of a thru-hull on the side of the hull. Enough to grab your attention. The better-designed vessels out there channel this water in various ways to lessen the noise of splashing water, but the norm is to have it come out where it’s the easiest for the builder. These streams of water are the discharge from the air conditioning units on the inside of the boat. Unlike your home's air conditioner, marine air conditioners use water to cool the air. Air conditioners work via heat exchanging. At your house, this happens using the air outside of the home (it's that big noisy box with a fan in it on the side of your house). Differently, on board, fresh water on the outside of the boat is pumped through the inside of the hull to and through the AC unit to be then discharged overboard.
Using water from underneath the boat has its plusses and minuses. The big plus is that the water is much denser than air and can transfer heat more efficiently. This allows for a much smaller and efficient air conditioning unit. A "clean" AC unit on board can cool a boat relatively fast on the hottest day. The downside is that the water being pumped through the AC unit is full of dirt, tannins (think tea), and a whole rainbow of living critters. All of which try their damndest to clog the water flow through the system.
Part of the "basics" we need to cover is the path of the water through the AC system in the boat. Following the flow of water is the first troubleshooting trick. We're going to start from under the boat: The air conditioner system has a pick up on the bottom of the hull where it pulls in the water from underneath via a thru- hull, petcock & strainer (in that order, outside working in). Made of stainless steel, bronze, glass reinforced plastic, or titanium (longevity, strength, and desired equipment in that order). The thru-hull can best be described as a threaded pipe that has a turned out flange on one side. When the boat is out of the water, the thru-hull is slid through a matching sized hole in the hull to allow water in for various systems (engines, generators, raw water heads, AC systems, etc.). Using healthy amounts of sealant on the backside of the flange and any section of thread that will be in contact with the hull. The thru-hull is placed in the hole. A nut or triangular flanged nut is placed on the inside of the hull. The two are threaded together. Creating a compression sandwich. This leaves you with a pretty new hole in your boat. Not to worry, next comes the valve that closes the hole.
A bronze thru-hull made by Perko
Maintenance note: It is essential to have a service tech inspect your thru-hulls every time you haul the boat out of the water. Failure of a thru-hull means boat "go bye-bye." Don't freak though, they're pretty tough for the most part, and you'll get plenty of warning time before needing to replace them. It is recommended that they're replaced every ten years regardless of their condition. That's your call. But, you can bet that your insurance company will be checking to see that you did the right thing if ever it comes to that point. Keep those receipts, and your maintenance log up to date!
Leading inward; The combo has a valve incorporated in it (sometimes also referred to as the petcock). These valves close the thru-hull "hole" to allow for system maintenance and safety when the boat is left unattended for long periods. They sit threaded atop the thru-hull. They can be identified by a handle or knob coming off of them.
Thru-hull with petcock made by Groco . e.g. a hole in your boat with a valve on it to close off the in rush of water.
Maintenance Note: It's a good idea to work these valves at least once a month to help clear growth inside the valve. Also, they're best left closed if the boat is going to be left unattended and/or shut down for any length of time. These valves are the first choke point in the air conditioning system. Critters that like being in a place of high water flow with access to just a touch of sunlight love this spot. We get sponge-like critters growing all over ours. Working the valve open and closed a couple times breaks them free.
Sea Strainer. Connected to the petcock this catches debris before it enters the system…in theory.
Leading off from the valve, either by, direct connection or a connection hose will be the sea strainer. Sea strainers should be located on every thru-hull that collects water. The idea is pretty simple with several variants out there. Just like the name suggests it is a "strainer" of seawater. It's a stainless steel screen filter inside of a glass container that the water flows through. This keeps larger particles, debris and/or critters from entering into the system. Just like any filter they will clog over time and require servicing. In the Chesapeake Bay during the summer season, I remove and clean the screen once a month on our AC sea strainer. Sometimes more if growth is heavier than usual around the boat. Filter feeding critters attach themselves to the filter screen. This is an excellent environment for them. The high water flow of the system is carrying a ton of food and provides shelter from predators. I've found sponges, anemones, mussels and one time we even captured an eel in our AC strainer. As these guys gain purchase and grow on the filter screen, it starts to slow the water flow. This growth will eventually lead to complete blockage. If this happens, the AC unit will likely shut off. As most AC units have a safety mechanism to prevent overheating. However, following the flow of the system, the next component in our line of equipment usually doesn't come with this type of protection. If it runs dry, it can destroy itself. I'm speaking of the Air Conditioning pump:
March Pumps. A very common air conditioning pump found on boats to move sea water through the ac system. We have the one at the 9 o’clock position and its been good to us so far.
This pump is usually powered from your 120v panel and has its own breaker for that very reason. The pump is designed to move a high volume of water to the AC unit or units inside the boat. Some pumps can be quite powerful given that the AC unit(s) may be located high above the waterline or on the other end of the boat. The pump ensures constant delivery of fresh water to cool the heat exchanger of the AC unit itself. The pump's location has little to do with being fore to aft, or port to starboard within the boat. But, one essential aspect of the pump's location is that it's placed in a protected area within the bilge below the waterline.
Boating terms & a how to: "Waterline" is the level at which the resting water outside of the boat sits on the side of the hull. To determine this level on the inside of the boat, here's a trick. You'll need a 3 to 4-foot piece of clear tubing sized to match the output hose barb coming off of the sea strainer. Close the petcock and remove the existing hose that feeds the pump from the sea strainer. Attach and clamp the clear tube to the output barb on the sea strainer. Temporarily mount or have a helper hold the clear tube, routing it without kinks, to go straight up toward the ceiling. Then, you'll want to open the petcock and allow the clear tube to fill with water. Similar to a level glass, the height at which the water reaches inside the tube will be identical to the height of the water outside of the boat.
It's important that you plan to allow for adjustments in the waterline level based on how your boat moves through the water. A power boat that planes on the water will lift up while underway and will lower the waterline. Likewise, a sailboat under sail will heel (lean) to port or starboard, and this will change the relationship of the waterline to the AC pump. Why is this so important to your air conditioning? The vast majority of marine grade AC pumps are non-self priming pumps. Meaning they cannot generate suction to pull water into them. For the pump's impeller (little fan inside the pump head housing) to gain suction (or traction) in the water, it has to be already submerged in water before it starts turning. Mounting and plumbing the pump below the waterline will keep the pump's impeller submerged at all times. If the pump is mounted too close to the waterline, as the waterline fluctuates underway, the pump can lose suction, and water output can slow, or air pockets can build up in the system. If it's mounted above the waterline, it will fail to "push" water through the system. To clarify "push," let me explain how these pumps work. The majority of AC system pumps are centrifugal pumps in design. They "push" water by spinning it inside the pump head, creating centrifugal force with the spinning pump impeller. Giving the water only one way to escape the pressure; That's through the output nozzle. As the water gets pushed out of the output, the siphoning effect pulls more water into the impeller. If there is no water to spin/push, nothing happens. These pumps also use the water inside the pump head to lubricate the impeller and bearing. If the pump runs dry for an extended period, the bearing will heat up and melt, and you get to buy a new pump head or worse a pump. If your pump develops a clog and it will, it's going to let you know by how loud it gets. These pumps don't like anything in them. They will rattle if there's anything, including air, inside the pump head. If the pump starts making noise, shut it down, turn off your AC(s) and let it cool. Inspecting the pump impeller isn't as hard as you might think. Usually, all you'll need is a Phillips head screwdriver. Important! Close the petcock valve before doing this. Unscrew the 5 to 8 screws on the face of the pump head and slide it back out of the way. Be ready with an old beach towel or the like, some water is going to come out. Once the pump head cover is out of the way, this should uncover the impeller. The impeller is magnetic, and it will be held in place by the magnet attached to the motor side of the pump. Pull it out of the head to inspect and lube with a tiny amount of silicone based lube or vegetable oil if you're in a pinch. You'll need to replace it if it's missing any of the fins. Clean up any goop and debris inside the pump head. Put it back together in reverse order. Open the petcock, check for leaks, and fire it up. This usually fixes up pump noise. However, noise from the electric motor side of the pump could be a sign that the bearings in the motor are soon to go. In that case, purchase a new pump but wait until the original gives up the ghost before replacing it.
This is an explosion view of an Iwaki pump head. Note the red bearing located on the impeller. This will be the point of failure if the pump runs dry for an extended period. Iwaki makes a good product. Just make sure to get a unit that has a Japanese made motor. They seem to be a little better made.
Onward down the line! After being expelled from the pump, the water shoots down the hose to the AC unit's heat exchanger. This component looks like a coiled snake and will be attached to the AC unit. The heat exchanger is, on a basic level, a small tube inside of a larger one. The fresh cool water swirls around in the outer/larger tube that surrounds a small inner tube that has freon running through it. The two do not come in contact with one another but, they exchange heat through the metal surfaces that separate them. The now warm water then exits from the heat exchanger to another hose and makes its way to be discharged overboard via a connected thru-hull. (The freon is in a closed loop system within the AC unit itself, and only a certified technician should be messing with it.)
Marine AC Unit
The coiled copper tube is the heat exchanger.
