Category Archives: Air Conditioning Topics

Cleaning the A/C Raw Water Loop

Aboard Sanctuary, our normal summer and winter locations are mostly in brackish waters.  Local water temperatures rise into the mid-to-high 80s in the summer months.  Routine preventive maintenance is required to avoid the build-up of sea growth in the raw water circulation plumbing system.

The water that is circulated through heat pump plumbing systems is not biologically sterile.  It is drawn from the environmental water in which the boat is floating.  That water is referred to as environmental, or “raw” water.

All freon-based heat transfer systems work basically the same way.  In their cooling mode, heat is absorbed from the air in living spaces into the freon gas inside the sealed air conditioner system (R-12, R-22, R-134A).  The heat is transferred again from the internal freon gas through the metal walls of the unit’s heat exchanger into the circulating raw water.  The heat is discarded overboard, carried by the discharged raw water.  In their heating mode, heat is absorbed from the environmental “raw” water through the metal walls of the unit’s heat exchanger into the freon inside the sealed system.  That heat is transferred again within the unit into the air in the living spaces of the boat.

The maintenance issue in systems that circulate environmental “raw” water becomes the efficiency of heat transfer into and out of the circulating water.  The efficiency of the transfer of heat into and out of circulating raw water can be impaired by any accumulation of slime, algae and other biological “critters” attached to, and living on, the internal surfaces of raw water plumbing system components.  At its minimum, slime acts like an insulating blanket which impairs heat transfer.  In its worst case, it can build up to the point that it also occludes the lumen of the plumbing through which raw water circulates, acting like an insulating blanket AND decreasing water flow through the system.

To clean environmental biologicals from raw water plumbing fouled with sea growth, a couple of approaches can be tried, in order.  Raw water circulator pumps are high volume, low pressure pumps.  As a first step, if you have the capability to do so, close the intake thruhull, and swap a relatively higher pressure dockside fresh water hose to the raw water loop to the heat pump.  Do this just “downstream” of the circulator pump; i.e., at the discharge port of the pump.  The higher pressure of the dockside water may be sufficient to blow fouling material out of the raw water cooling loop system.  If this works, it’s less harsh than circulating chemical cleaning agents in the system.  If not, though, remove the dockside water hose and restore the circulator pump connection.   Re-open the cooling water thruhull, and prime the pump.  The next step is to place pool chlorine or bromine high test hypochlorite tablets (HTH) into the sea strainer basket.

Aboard Sanctuary, to control the growth of slime, algae and other sea life in the “raw” water loop of our two air conditioning heat pumps, we have used HTH pool “chlorine” tablets for several years.  The tablets are about the diameter of a Kennedy half-dollar, and about 5/16″ – 3/8″ thick.  We put as many of them as will fit into the sea strainer basket that supplies “raw” water to the air conditioners, and we then just run the units as normal.  These HTH pellets dissolve over 24 – 36 hours.  HTH works by releasing elemental oxygen into the raw water.  The elemental oxygen is a powerful oxidizing agent that kills and shrinks slime and other sea-life critters in the raw water loop. That allows the little corpses to slough off of internal plumbing surfaces and flow out of the system.  Physically clean surfaces allow better contact of raw water with the internal metal parts of the condenser/evaporator (cool/heat respectively) coils of the air conditioning unit, which increases heat transfer efficiency.

We perform this preventive maintenance procedure once or twice per season.

We do not use the HTH tablets in our genset or propulsion engine cooling circuits.  Doing so for the genset and the propulsion engine(s) would require them to be running to circulate the chemical.  With engine(s) not running, no raw water circulates and no cleaning would occur.  Also, the efficiency of heat transfer is not an equivalent issue with engines, and is not necessary, in engine cooling systems.

I hasten to add: “not necessary, in my opinion.”  There are significant design differences between air conditioning/heat pump raw water cooling systems and an engine’s raw water cooling system. In air conditioning systems, water flow is constant over a relatively small heat transfer surface area. Furthermore, in the first few minutes of operation, the temperature differential normalizes to a very steady state. Contact efficiency is very important to heat transfer. In engine cooling systems, flow is variable (with engine speeds) over a relatively high surface area. In engine systems, the amount of heat shed in the raw water cooling circuit varies widely, and the proportionate temperature differential is much higher. In engine cooling systems with high temperature differentials, the importance of contact efficiency is lessened. By and large, the capacity of engine cooling systems is significantly over-sized with respect to their nominal operating requirements. That large capacity accommodates extremes that may be encountered in severe operation.  Again, there is no analog to those severe operating conditions in air conditioning system design. So, the two systems are very different.

One common misconception is that calcium deposits will foul air conditioning and engine cooling systems. That is largely an “old wives tale,” often implied and promoted by product advertising. Yes, there are minerals in solution in sea water. Yes, those minerals can and do coat internal hose and metal lumen channels of raw water loops. But except under the most extreme situations, mineral deposits do not account for significant fouling of marine cooling systems. If they did, salt water would be a severe and ever-present disaster. Slime, algae, sea worms and barnacles – living sea creatures – are the problem. These critters grow quickly in warm sea water, foul plumbing lumens and impair heat transfer. It just so happens that de-mineralizing agents – RydLyme®, Lime-A-Way®, muriatic acid, hydrochloric acid – will also clean out slime, worms and dissolve the shells of barnacles and other sea critters. But, these agents are caustic in varying degrees to the internal metals of raw water-carrying loops, caustic to use safely, caustic to dispose of properly and safely, and generally “overkill” on a “fitness-for-purpose” scale. Free elemental oxygen, like that released by HTH, will kill biologicals without adversely affecting hoses, tubing, copper/nickel metals, etc. and they are generally safer for people to use and dispose of safely.

It is my opinion that HTH is the best first-use agent, and I suggest it as the primary, routine course of preventive maintenance. Reserve the caustic stuff for the less common, more intense and serious, corrective-action situations.


Diagnosing Air Conditioning System Performance

When we bought Sanctuary in 2004, there were two MarineAir heat pump units aboard.  They  were an aftermarket addition originally installed by the first owner of the boat. The salon unit is a 16K BTU unit that heats and cools the salon and aft cabin. The forward unit is a 5K BTU unit that heats and cools the vee berth and forward head.  The forward unit also has one single air vent in the salon.  In the spring of 2009, we replaced the then failed salon unit with a new Cruisair model STX16-HV, 16K BTU unit.  Sanctuary is fit with a single raw water inlet thruhull.  The inlet is plumbed to a Groco 3/4″ Sea Strainer, and then to a single Little Giant Model P510 circulator pump.  The pump head assembly is a low pressure, high volume, Model 3-MD-SC unit, which means 3/4″, Magnetically Driven, for use in Semi-Corrosive (salt water) applications.  The raw water plumbing loop divides at the output of the circulator pump.  The system divides into two loops from that point, one serving the salon 16K BTU unit and the other serving the forward 5K BTU unit.  The loops converge and re-join at a “tee” at the discharge thruhull.  On 100°F days on the Chesapeake Bay, we’ve generally only needed the 16K BTU unit to keep the boat comfortable.  If we have grandchildren or guests aboard, we also run the forward unit.

If the heating or cooling performance of the heat pump system seems to be struggling, there are several checks we perform, in sequence.

The first and most obvious checks are:

  • verify that the temperature control is calling for the compressor to run,
  • verify that the air handler filter is clean and allowing air to flow freely. If the air filter is dirty or missing, carefully clean the cooling coil fins. DO NOT poke at those coils with sharp tools.  Use a toothbrush or fingernail brush with nylon bristles to dislodge accumulated fuzz, and use compressed air from the back side of the coils to blow fuzz backwards out of the coils, if possible.  Sanctuary’s air filter collects dust, lint and fuzz at an astounding rate,
  • make sure the sea strainer is clear of debris, slime and/or barnacles.  Remove the strainer basket from the housing and reach into the basket to feel gently around for slime, algae and barnacles in the housing and water ports.  After doing this step, wash hands in bleach water.  Refer to our article on Mycobacterium Marinum, here:
  • Verify that the compressor is actually running, and
  • verify that the volume of circulating raw water is both “normal” and adequate.

In a scenario where the temperature control is calling for the compressor to start and run, but the compressor itself does not start and run, there are two different kinds of potential causes to rule-out.  Theses are:

      1. heat pump compressors have two safety switches (rarely, there can be three) that can prevent them from running.  The compressor will not run if either one is broken/defective.  One switch is a simple bi-metal switch that shuts the compressor off if it gets too hot in operation.  The other switch is located in the sealed plumbing system of the unit that contains the freon gas.  This is a pressure sensor that will stop the compressor if the gas pressure – sometimes called “head pressure” – gets too high.  (Some units also have a third sensor to detect “head pressure” that is too low.)  True hi-temperature and/or high “head” pressure are symptoms that suggest inadequate flow of circulating raw water.  A defective bi-metal hi-temperature switch is easy for a DIY user to replace.  Temperature switches are commonly available replacement parts and will cost in the range of $20 from an a/c repair shop or the manufacturer.  The pressure switch(es) can be diagnosed by a skilled DIY individual, but may require an a/c technician to replace.  That’s because replacing the pressure switch(s) does require breaking the sealed freon circuit.
      2. Heat Pump compressors are designed to start against the back-pressure of freon gas restricted by an expansion valve; that is, to start under load.  Compressor motors have two electrical windings, one that assists in starting the motor and one which serves to keep the motor running once it has spun up-to-speed.  A capacitor is used in the start winding to increase starting torque.  A disconnected or defective start capacitor will cause the compressor to fail to start.  The start capacitor is usually in the range of 30mfd to 50mfd.  These are universally available replacement parts and will cost in the range of $20 at any appliance store.

If you hire an air conditioning technician to service your boat systems, do not simply “replace the compressor” without asking about the above specific component failures first.  Ask about them – each one, and all.  I know several cruisers who have paid big bucks to have compressors replaced only to wind up with a very expensive new switch or start capacitor.

If the compressor does run, and if the air filter and sea strainer are both clear, check the quality and quantity of cooling water flow.

  1. Have an assistant watch the raw water discharge outlet, and sequentially clamp off each alternate raw water loop.  With a loop clamped off, the remaining loop should still provide a fairly normal volume of water flow.  If you do have significant decrease, you may have a kink or blockage in one of those lines.  Find and fix any mechanical kinks or blockages, until you have good water volume flowing on all loops.
  2. Use a digital thermometer to take the temperature of (“shoot”) the unit’s evaporator and condenser coils.  In cooling mode, the condenser coil contains raw water, and the discharge of the condenser should be only a degree or two Fahrenheit warmer than the incoming sea water.  (The actual difference will depend on the flow rate of raw water circulation, of course. The more raw water flowing, the less the temperature differential will be.) If the difference is greater than just a few degrees, suspect poor water circulation through that cooling loop, and the possibility of water plumbing fouled with sea growth.

The last assessment is to identify the possibility of low freon.  In cooling mode, the evaporator coil is where the cold air comes from. That coil should normally be in the range of 45°F – 55°F.  It is normal and desirable for that coil to have water dripping from it.  That water is called “condensate.”  The water will condense onto the cold coils from the warm, humid air passing over them.  This is the same process that causes frost to form in a refrigerator or freezer.  If the coil “freezes up” with actual ice deposits, that an indication of low freon.  Low freon, always caused by a freon gas leak, will require hiring an a/c service technician unless you have a unit containing R-134A gas, and have the skills, equipment and tools required to DIY.