Dometic – 5 Things You Need to Know Before Buying a Marine Air Conditioning System

Any questions…Please Ask.

Steve~

Let’s start with the basics…

Seawater flow thru your A/C system is primary…

Without water flow….You have NO cooling !

Service people on the docks (at least the better ones) can walk down the dock during the summer and tell which boats they are likely to be getting a call from next just by the flow of water (or lack of) going overboard on each boat….Lack of seawater flow can cause other problems besides loss of cooling…

• Lack of seawater flow causes high system operating pressures
• This, in turn increases the amperage draw from the A/C unit
• Possibly leading to: Burnt up wiring at switches or shore cord ends

Many of these symptoms can stem back to the cause of low water flow. If the flow is allowed to remain restricted,  compressor failure can also occur in some older systems.

Different than land based systems, Marine systems installed below decks have no fan to remove the heat from the refrigerant…They use water to carry away the heat for many reasons…

• Water is more efficient than air as a heat exchange medium
• Size of a unit with air cooling is much larger, thus making air cooling impractical on boats
• Air cooled units require a exit path for the heat so that the space they are in does not continue to heat up as they run, thus reducing their efficiency further

As you see from the picture above….It’s an example of a seawater strainer to a system that has seen little love, and this system of course did not cool.

Every boat owner should become familiar with their boats overboard flow rate when the system is clean, and become accustomed to glancing at it every chance they get…Once you have established what is normal for your boat, a simple glance may be enough to recognize a mounting problem, and thus head off a ruined trip due to the loss of A/C…

The first question a tech may/should ask when he gets a call Is…”How is the seawater flow ?”

More frequently than not…The owners answer is he thinks it ok, or Yes it’s flowing…Only for us to arrive to find not much more than a trickle….All this is ok for us as techs…but if you would like to save that expensive service call…Keeping your strainer from looking like the picture is a great start !

Seawater strainer maintenance is not hard to do…But of course every boat or installation is different. You should also know that the more the system/s run…The more frequently you will need to clean the strainer. During the winter months here in central Florida we find that due to cooler water (marine organisms seem to grow more slowly in cooler water) and obviously less usage you may not need to clean the strainer but once a month….Summer is a different story !

Depending on where your boat is moored, Tidal flow, Water Temp, and Running the system/s 24/7 can require cleaning the strainer as often as every week…I have even seen worst case situations where a boat may be moored in a pass with much tidal flow (and grass flats nearby) causing the grass to “Tumble” in the water down to where the thru hull is located, thus requiring the strainer basket to be emptied in a matter of hours…

Steve~

Not "Good" Ice

Though there can be many reasons for system icing…There are a few that the DIY boater can rule out before calling the service guy ( I can discuss other reasons in the forum but here are a few of the basic answers)

• Direct Expansion A/C (as opposed to Chilled Water) systems rely heavily on Air Flow across the evaporator coil in order to pick up the designed amount of heat the system was engineered to remove (BTU) and any loss of that designed air flow can cause the system to Ice.

Regardless of freon charge (explained later) the system must first and foremost have the correct amount of air flow…So air flow should be the first thing to check…Below are some reasons for reduced air flow…

• Dirty or clogged Return Air Filters…Some systems may have two filters installed…One at the Return grill, and a second that shipped with the unit on the evaporator fins ( folks tend to forget this one, or never knew it was there in the first place)
• Dirty or clogged evaporator coil/s due to not cleaning, or No/Poor return air filters installed.
• Fan Speed Adjusted Too Low (Sometimes done to attain a quiet blower)
• Undersized Return or Supply Ducting for the BTU System installed…This was the installers responsibility to size correctly, but all too often there is not sufficient space for where they want to run to size it properly…Thus a problem for the systems life, which is most always shortened dramatically from this practice…They should find another way, or reduce the size of the system to match the installation…Blowers can only move rated flow thru the proper sized duct.

Another reason which sometimes is overlooked…

• Very Cold Cabin…The colder the cabin…The less heat there is to pick up…Thus more Air Flow is needed to increase the amount of heat (that’s now not there) being picked up…A/C systems are called “High Temp” systems…they are designed to do exactly as the name implies…Air Condition…Not Refrigerate…That’s what Refrigerators & Freezers are designed to do…Your A/C system is not designed to cool much below 68 degrees F…Though it will try to do so if you ask it…
• Most of the newer Digital Cabin Controls have algorithms to “help” with this by shutting off the compressor for short periods until the desired room setpoint is attained, but they are not defrost cycles with coil heaters like you might find in a system designed to run at lower temps.

Last but Not least is…

• Low Refrigerant Charge causes Icing…You Ask Huh ???…Why would low freon cause it to ice ?…I thought the more freon the colder…Not So with refrigerants…I’ll try to explain briefly…

Pressure & Temperature Automatically Correspond with Refrigerants…The Lower the Pressure…The Lower the Temperature of the refrigerant ( and Vise Verse) Regardless of how much is in there…I’ll leave it at that for now but can explain further in another article, or in the forum if requested.

• The question here is…Why is the system low on freon ???

These systems are sealed, and should not require seasonal charging, like automotive systems that tend to leak because they have rubber hoses, o-rings & compressor shaft seals…These systems are all copper tubing, with soldered & mechanical flare joints.

• Did someone let out freon because the unit was running high pressure ? (usually an indication of  loss of seawater flow, or a dirty condenser)
• Has the system started leaking due to a poor connection (Flare or otherwise) or maybe electrolysis eating away at solder joints ?

If So…Then the leak needs to be located & repaired to save burning up the compressor…The compressor is the heart of the system, it’s expensive, and relies on the proper amount of refrigerant return to stay cool…It will run Very Hot without enough Freon return regardless of seawater flow…and even if the system (air handler or tubing) is icing…The compressor will run hot from that lack of freon.

Hope this helps answer a few questions, but if not…Or if it creates more…Drop me a line (preferably in the forum so that others can benefit as well)  Always glad to answer.

Steve~

Here’s what I can share at this point from the Cruisair/Marine Air perspective & what I have been told.

Self Contained R-22 units are no longer in production…sort of…

Since 417a is a drop in replacement for R-22 you can get the R-22 units (but they are shipping charged with R-417a)

We are told that the R-417a drop in replacement refrigerant for R-22 is 17% less efficient than the R-22 in the same unit (like running 60Hz equipment on 50Hz power)

So if you bought a new R-417a unit…You can evacuate the R-417a and re-charge that unit with R-22 to realize the same performance as a R-22 unit that was purchased before the production change…

You can also evacuate a older R-22 system & recharge with 417a…But the 17% performance drop still applies…

R-22 refrigerant should still be available for quite some time yet…But…The price will climb pretty quickly.

Edit:  5/1/2011…2020 is slated to be the end of production of R-22 in the US…None will be imported either…But at that point, if you still have an operational R-22 system, it can be evacuated and re-charged with R-417a.

The new R-410a cannot be used in a R-22/R417a system even with evacuation…It’s not compatible with the oil in the 22 system & the pressures will run too high in that system (metering devise is not set for it either)

My distributor has about 3 months worth of sales of units with R-22 available, but after they are gone he will only stock the new R-410a units….And is now only ordering Self Contained unit with the new R410a.

5/11/2011…R-22/R417 self contained units are no longer in stock.

Split or Remote systems are a whole different story…

One thing they are the same in is that none of the existing systems can run the new R-410a, but they can run the R-417a with the 17% drop in performance….but the problem is bigger with small split systems.

Typically the 16K & smaller split systems have always used piston type compressors as opposed to the rotary type that have been used in self contained units for many years now…

The reason for this is that the rotary type compressor does not lend itself well to long copper line runs…
Rotary crankcases don’t hold enough oil for the migration of oil with the refrigerant throughout the longer line runs in split systems…
It’s my understanding that the rotary can run out of oil before it has a chance to return with the refrigerant, and thus can ruin itself in a split system from lack of lubrication.

The compressor manufacturers are being forced to phase out production of the lower efficiency piston compressors by the EPA or whomever…

5/1/2011…We still have no info on when this might happen but as of this date…Replacement R-22 split system units with the older piston compressors are still in production…But only ship with R-417a which can be evacuated after installation & replaced with R-22 to maintain efficiency.

Scroll compressors would be a great alternative, but as of yet…The compressor manufacturers have not developed Scroll compressors in the smaller sizes that are required for our smaller Marine split systems (the larger than 16k marine units are using scrolls now)
I guess they figure the Marine market is too small to develop new small scrolls for, and that the rotary’s are fine for the home window unit market (which is what the the rotary’s were developed for)

I know that Dometic Cruisair/Marine Air has been testing small splits using rotary’s for quite some time now…So far they aren’t saying much…

Until then…They are only offering the same piston type smaller split systems and shipping with R-417a (the drop in replacement for R-22) to meet the new regulations….The Larger splits (over 16K) will be available as either R-417a (mainly for replacements of the older R-22 units) or the new R-410a units, but to utilize R-410a condensing unit you must also change the air handler…No need for new copper tubing runs.

Dometic is now offering their new “Emerald Series” split/remote systems with rotary compressors (16K & below) Also with  TXV’s (thermostatic expansion valve) metering devises at both the condensing unit & air handler (2 TXV’s)…Larger than 16K “Emerald Series” systems are Scroll compressors.

Steve~

I started this site last year to “put my feelers out”  about what owners and captains may want to learn and share about marine  a/c.

The support has been more than I ever expected…

So… My New Year resolution to you is expand the site we created…Because of your feedback I am going to begin adding to my store to allow you to order most of the common Marine A/C system items that I found you’ve needed,  but until the store is fully operational…As always I’m here for you to contact…

Some important information…Sadly…Due to EPA regulations there will be no more R-22 systems/units produced in 2010…

• If you need to replace a R-22 condensing unit in an existing system…You will need to purchase a R-417A unit, evacuate your R-22 (which you would do anyway) and re-charge the system with R-417A (drop in replacement for R-22)…No need to change the air handler.
• For new installations (split system or self contained) the systems have been re-designed to operate with environmentally friendly R-410A which operates at much higher pressures that the existing R-22 or R-417A systems…These R-410A units can Not be paired with the older air handlers.

Your local A/C guy will know what to do & have the equipment to make the change.

Your comments & feedback are always welcome & appreciated…I need things to help me write…So ask away…It does stir the thoughts…

Clean Sea Strainers to All, and to All a Good Year…Think of the A/C/Heat/ Refer Guy…He Does You…

Steve~

Hi Mark…Don’t know if you are still watching this thread, or if you have seen my thread over in the “Introduce Yourself” forum but here’s my take…

It seems that when the system was flushed, that it was flushed just well enough to break loose some stuff…Now it is floating around in the hoses and randomly blocking flow where it now lodges from time to time…
Once the pump shuts off the water drains back, and with it the loose stuff moves…Only to lodge again maybe somewhere else when the pump starts again…

If the water flow does not look right to you (and you obviously have been watching it since new) then it is not right, and will continue to get worse…Changing the pump will not fix a thing unless the pump had a problem…It can only pump water thru unrestricted passages, and that water it can pump will re-direct to the un-restricted passages…Like said…Path of least resistance.

From descriptions here & in the thread I started…It seems the 459 has what I would call a odd or improper A/C plumbing/pump setup…

This happens sometimes with boat builders…Generally speaking…In my experience the only way to get them to make a change in a poor setup is for enough customers to have a problem and complain…No amount of prodding from us A/C guys ever seems to help in effecting that change…It’s a shame too because we are the ones dealing with this stuff everyday…They deal with building boats everyday…

So what is the fix ?…Well marine growth will always be a problem but there are a few things that can be done to help…

Over the years the A/C systems that boat builders install have gravitated to the Self Contained all in one units as opposed to the Split/Remote systems that most utilised for years…They don’t need guys with refrigerant certification to install or buy these All in One units…They are easier to install, and freon leaks in connections of copper tubing line runs are no longer something they need to worry about warrantying….All they have to do now is run power, water hoses from the pump, and duct…

The problem with this is the water runs are now much further than when the “Central” type systems were installed all in one location close to the pump…Back then we had very little growth problems (just scale buildup in condensers that was easy to remove) and flow was much easier to plan & maintain…

What I have found since this change is that the further, and or higher you pump seawater…The more problems you will have with growth, and it has also created the problems I describe below with using larger pumps…

Directing a pumps flow to units of varying distance & height for even flow to each…Is a challenge to say the least !
Generally their answer is to install an oversized pump in an attempt to brute force flow…

No I have not seen a 459 but it has been described as having two units on the bridge and 2 down below…I don’t care how they manifold this setup…It still going to be very hard to get the proper flow to each unit, and the least little restriction can and will upset any kind of balance they have designed into that system…The Pump has to overcome the height (Head) it needs to pump to the units on the bridge so it likely has been over sized just to do that with enough volume for those units…The remaining water can then supply the lower units…

Over sized pumps are never really the best answer over time (though it may be cheaper for them) because a over sized pump attempting to pump thru say four 5/8″ condensers (in this case) is (even when the system is clean) likely not pumping it’s rated flow which will cause it’s impeller to cavitate…This causes cavitation burns to the bronze impeller & pump housing…Thus causing premature pump head failure.

Again having not seen the install…My answer would be to install two pumps…One for the upper units, and one for the lower units…
They both could then be sized for the job presented, the systems & pumps would be much happier with less overall maintenance.

Steve~

Hi Steve, love your web site – lots of great info. ! Our company does a lot of yacht maintenance / mechanical preventive maintenance type stuff here in Wilmington, N.C. – my question is, do you have an opinion concerning back flushing the sea water lines on marine air conditioning systems ? That is, do you think there is any benefit to occasionally (2 or 3 times per season), removing the water discharge hose and connecting a dockside water hose to the unit and pumping water backwards through the system. I have talked with some individuals who believe that doing so helps to remove/reduce the amount of marine growth in the coils and hoses, eliminating the need for or reducing the frequency of acid cleaning. Any advice or opinions you can offer would be greatly appreciated

Bill

Let me say first that I’m not against holding plate systems but…Some of the points I’m trying to make in the article are…

Holding plates are a much different animal…In order to get the performance/temp desired, and holding time at that temp…Many factors come into play such as insulation, size of box vs size of plate, door seals, top or bottom loading, plate solution temp etc…

A holding plate systems main design purpose is to keep a given box at a desired temp for a period of time, without running the refrigeration unit to keep it there for that period of time…

Basically…The temp of the box while running the unit has little to do with how a holding plate system was intended to be used…and given two plates of the same size (area used in the box) a holding plate will not offer anywhere near the BTU of heat absorption that a standard plate will offer…

• Running…Plate for Plate, a holding plate needs to be much larger to do that same job of heat absorption.
• A typical 134a standard plate system will run that plate surface at -10 F.
• A 134a holding plate system can run the tubing inside the plate…and solution (liquid) at that same -10 F….But the solution is a limiting factor in some ways…
• 134a system plate liquid is typically mixed to change states from a liquid to a solid somewhere between 0 F. to 26 F. Depending on if the plate was meant for a freezer or refrigerator box…
• It is also a larger mass to change the temp of, so running it down to -10 as you might imagine takes more time & energy…and Likely never getting the surface of the plate quite down to -10 F.

If a holding plate/box is not designed/sized as such (To hold a desired box temp when off) it can be much less efficient than a standard plate system that can recover the box temp much faster and may provide for less overall run time in a 24 hr period.

As you see…It’s hard to explain, but installing just any holding plate into a box because it will fit is almost never the answer to the most efficient or ideal box.

Holding plates can be over sized too, and thus undesirable if they keep the box too cold during that hold over period…Freezing lettuce etc…

What I’m saying is…A Hold over plate can and will continue cooling long after the t-stat has cycled the unit off if it has been run long enough to freeze the solution in the plate…and If it is enough BTU to compensate for the amount of heat leaking into the box…
If Not…Then you will need to run it longer to attain the desired temp in that box…
The thing is it will warm very rapidly back up to the point that the solution begins to thaw…and This is where the holding period is (during the thaw, or change of states of the solution) and the box temp during that period is related to all the factors I discussed.

If that holding period box temp is not what is desired…Then a holding plate has not gained you a thing…It has hindered things somewhat.

Am I making sense ?

The OP responded that I had…So I felt it might help here also…

Steve~

The original poster had a March magnetic drive pump that would not pump water, and after the usual back flush & priming sugguestions he could still not get it to pump water…Just before I read the thread he had found that either something had gotten past the strainer (or he didn't have one) and had lodged between the impeller magnet and the pump housing stopping the impeller from turning, but not the motor from running…

Then the questions & discussion centered around how their systems reacted to loss of water flow and how & when their pump ran…Some of it was info that was a bit misleading for others depending on what type of system & control they have…So I posted these two posts below to try to help out…It's good info so why not share it here on my own site too ?

• My first post…

I'm glad the Chief Alen sorted out his pump problem but I thought I could add a bit of info to the thread…

If it is a single A/C system installed (not more than one compressor or control) there will be no need for a pump relay.

If it is a older Cruisair 3 knob type cabin control…Yes the pump will run with the fan and not cycle with the compressor unless it has been changed or is a newer digital control.

The thought back in those days was that when wired to the fan circuit…The owner could verify flow over the side before switching the system to run…They have since re-thought, and have changed that practice/wiring…

Yes it can also be wired to the compressor circuit so that it will cycle with the compressor…
It can be a simple change, but first you need to find where the pump wire runs to…Some run to the unit, and others are run to the terminal strip that should be within 3 feet of the cabin control….You then just remove the black pump wire from under the red terminal (Cruisair) and reconnect it along with the purple compressor wire…The pump should then cycle with the t-stat/compressor.

The other thing to watch for with the March Mag drive pumps is if they are run dry for very long…They will heat up enough to melt the plastic on the back side of the impeller around the ceramic shaft, and also the impeller mating surface of the plastic head itself…
The impeller may appear to rotate fine when the cover is removed but it won't when the pressure of the cover is against the impeller with the cover installed…If this is the case…The housing & impeller can be replaced, or you can also get the complete head without having to replace the whole pump.

Steve~

• Then another gentleman posted…

“I guess I must have the ideal system. If the water flow is disrupted on mine…the breaker trips.”

• My reply…

That is a good thing…But not actually a function of design of the A/C system or control.

I'll try to explain…

Most Marine A/C units with manual cabin controls have what is called a “Line Voltage” automatic reset high pressure switch (meaning it is actually carrying the compressor current) that trips around 425 psi, but then resets itself at around 325 psi…

Mermaid was one exception…They had/have a Manual reset Hi PS Switch with a push to reset button…Anyway…

Normal system operating high side pressure is somewhere between 200-250 psi depending on water flow & the temp of that seawater…

Also…In a Normal startup (system has been idle for a few minutes) the low & high side pressures will be equal (or very close)
This pressure will depend on ambient air temp but it will not ever be much over 196 psi (that's assuming a 100 degree cabin)and usually much lower.

The compressor has a fairly easy time getting rolling against that kind of pressure….But at where the pressure switch resets (325 psi) it has a much tougher time and thus draws much more amperage (than normal) in trying to re-start against that High pressure…Therefore tripping the breaker…

Units that didn't come with a start relay & capacitor have a harder time starting than ones that did come so equipped…

The ones that came equipped usually will start on lower dock power, and also may re-start at that 325 psi if the breaker is not closely matched to the size system it is powering…

Breakers are most often spec'd for wire size & distance to any given piece of equipment (under normal load conditions) for that equipment…The wire is also sized to be able to carry that projected load.

So you might see…There are trade offs…

Tripping that breaker weakens it a bit every time…Yeah I know…It shouldn't happen that often…And it does protect my A/C & Pump…

But having a unit that did not come with start components can mean the difference of being cool, or not being cool with marginal dock power (Like at a lot of backwoods marinas, and behind many homes) It can also mean a given unit may or may not start on a smaller gen set or inverter with other essentials running…

Tanqueray…I'm not saying your unit does or does not have start components…I'd have to know what you have to know that, but I can also say to everyone that a start kit can be added very easily to most all systems that don't have a relay & capacitor already installed if your situation warrants it…

The reason most manufacturers didn't use them was cost…I can also add that up until a few years ago Cruisair had them on all their units.

Note: The digital controls of Cruisair/Marine Air handle Hi/Ps much differently than manual controls….But that's another post…

Edit Note: Since the time of this original post, Dometic Cruisair/Marine Air has come out with a patented SmartStart that far out performs a standard hard start kit, or even a real start capacitor/relay…I should also add that I have personally installed them and they really do live up to the claims.  

We offer them in our store…Just click on the link below to see it, and it’s info…There is even a video…

http://marine-ac.com/2010/11/0…..-assist-2/

Steve~

In boats with multiple systems running on one seawater pump there is most likely a Pump Relay to receive a signal from each system to tell the pump it needs to run…That is unless it has been bypassed and the pump runs 24/7 from the breaker ( I see this done fairly often)

Lets start with the breaker in the main panel…Most are labeled A/C pump or similar…Some boats may have the pump breaker tied in with one of the A/C system breakers (Ocean Yachts for one used to connect the pump to the Main Salon A/C Breaker) but most should have a stand alone breaker…

The breaker feeds the pump relay and carries the load of the pump only…this power can be either 115 or 230 volts depending on the pump installed…

The individual systems aboard will then have their own breaker that powers/protects each A/C system & it’s wiring…These can also be 115 or 230v supplied…The reason I describe this is that some boats have a mixture of system voltages…Hatteras for example, always supplied one of the smaller systems as 115v and the rest as 230v…They would then install a 115v pump so that if all that is available on the dock is 115v, you can still have at least one cabin of A/C for crew/kids while the owners/parents go to the resort for the night without starting the generator (Well this has always been my assumption and nowadays most docks will have 230v anyway)

There have been many types of pump relays over the years…Some with mechanical relays (in previous times) but most for many years have been “ignition protected” solid state designs…Here are the most current & popular versions that I see, and a few of the most common failures…

Some were for built for only 2 systems and could not be expanded for more…Marine Air still makes a 2 unit relay (2PRP) but they also offer one that can handle up to 6 systems (6PRP) on one pump…you don’t have to utilise all if you have say 3 or 4 systems, and it actually gives you spare circuits to re-connect a system to if the one it’s connected to were to fail (that is if you have the 6PRP)…These relays are just a metal box with a single circuit board and all needed components on that board…Other than complete board replacement, there are no repair parts available/needed.

Cruisair went about it a bit differently…Years ago they offered one pump relay that could handle up to 8 systems on one pump (PR8X) and it came with individualised components, all of which could be replaced…The “Circuit Board” & separate” Triac” (solid state relay that switches & carries the pump current) came with the box…You could then add “Triggers” for as many units as needed, and you could also select trigger voltage (115 or 230) to match the system voltage feeding it….These triggers of course were/are replaceable with two screws & two plug on wires in the event of failure…

The only problem with this Pump Relay was the box was a bit large for smaller areas in boats that may only need 2 or three triggers…So they built a smaller box that can house all the same basic components (circuit board & triac) but only has room for up to 3 triggers…It can also be purchased with the triggers pre-installed for either 2 or 3 systems (PR3X-?) The question mark would be the number of triggers pre-installed (either 2 or 3)…They still offer the PR8X, and it can also now be purchased with triggers pre-installed but either box is not hard to add/install trigger/s in…

The main purpose of these relays is to run the pump when a system is calling for it…That means any system that is running a compressor (Some with just fan, IE: older Three knob cabin controlled systems)…But if a trigger circuit in either type box fails (Marine Air, Cruisair, or any manufacturer) the pump may run for one or more systems…But not for the one that has the failed trigger circuit…

This sometimes causes confusion because if any one of the good triggers starts the pump…All systems that are connected to that pump will get seawater, and thus run normally….

However if the system/s with the good trigger/s cycle off, and the one with the bad trigger either continues to run, or starts up with the others off…It may/should shut down randomly with a fault code on the cabin display.

The easiest thing to do to verify if you have a problem is to test run each system” individually” to verify if you are pumping water with just that system running…If you find a system that won’t start the pump, the trigger circuit (in the pump relay) for that system has likely failed.

Also if the pump won’t run at all…These Pump Relays all have fuses installed either inside the box (Newer automotive type pull out fuse) or in a typical twist type fuse holder mounted thru the box ( It’s how Ocean Yachts gets away with running the pump on the salon breaker)…This should be one of the first things to check….If the fuse is blown, and then blows again after replacement, it’s time to look at the pump & associated wiring for shorts or bad/wet connections.

These Pumps & Relays are pretty easy to troubleshoot with a volt meter and a bit of common sense, but going into that here would be a bit long for all models…If you want help with your individual installation, I’ll be more that happy to answer in the forum troubleshooting section http://marine-ac.com/forum/troubleshooting/

Just let me know what you have and we can solve it there together…If you have another brand of equipment or model of pump relay, it’s likely that I have seen a few…We can figure those out together too…

Steve~