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~

The Advantages, and Disadvantages of a Hold Over system are similar to a block of ice because that is essentially what is done…

The hold over plate/s in the box is/are approx 3″ thick, and have copper tubing run through them which is then surrounded by a liquid that freezes (and re-freezes) when the refrigeration system is run… a Block of Ice really…The idea is to not to have to run the system but once or twice a day to maintain a box’s design temp.

The the solution/liquid is generally a glycol solution (or brine water in older plates) that is mixed for a lower freezing point than water…A refrigerator plate solution can be mixed to around 26 degrees F…A freezer plate can be 0 degrees or even lower depending on the temp you want the box maintained, and how much insulation is surrounding the box (more on that later)

Other than what I have stated above…There really aren’t that many advantages to a hold over plate system…Yes they can work well for the intended purpose, and have for many years for that purpose…Thing is, they really are nothing like what most folks are used to at home, and that in itself  can be one big disadvantage…

If you have, or are considering a hold over system I will try to list some of the lower points when compared to a traditional refrigeration system…

• Expense…Custom Hold Over Systems & Boxes are expensive to design & build…Proper insulation & door gaskets are key factors too…Insulation should be around 4″ thick everywhere (even against the hull) with no air pockets.
• Given Space…The insulation requirements and the space the plates take up, really tend to limit internal box size in the space allocated for a box.
• Weight…A plate full of liquid is quite heavy.
• Power…Big plates full of liquid require a large refrigeration unit to pull them down to temp, and Initial pull down time is also much longer to get the box temp to stabilize.
• Recovery…Holding Plates don’t recover temp very fast after opening the box…No more leaving the door open at all…In fact you really should only open them once a day…But what about my beers ?
• Defrost…Any ice build up acts like a insulation blanket over the plate, and thus raises box temp…By design, hold over plates can’t utilize auto defrost cycles effectively, so you end up scraping the ice off all the time to maintain temp, and the more you open the door…The more the ice/frost forms…Break out that windshield scraper you came south to get away from. (but at least up there you could put the beer outside)
• Safety…Last but not least is the possibility of a Glycol leak into your food without knowing it…Yes it’s rare but does happen…Glycol is like antifreeze in your car and is Very toxic.

I have found one of the biggest misunderstandings about Holding Plate systems is…Hold Over Period Box Temp/s…I’ll try to explain…

Hold Over Period is the time it takes the plates to thaw from a solid back to a liquid…Hold Over Period Temp is the box temp during that period…This is where the box/system was designed/intended to operate….

Any additional run time below the freezing point of the solution is pretty much wasted energy because the plates will rise in temp very rapidly back up to the change over (thawing ) point anyway…Then the box temp will again rise rapidly when the solution has thawed (if the compressor is not run)

This is where insulation, door gaskets, drains etc… Have the most effect…Ideally with all things at their best, you would like to maintain a box temp that is 15 degrees above the plate temp when the plate is frozen…That means with a 26 degree refrigerator plate you might expect a 41 degree hold over period, and with a 0 degree freezer plate you might expect +15 degree hold over temp  (not good for ice cream)….Yes you can go lower with the solution temps (mixture to attain lower freezing point) but to effectively do that you will need to use a system that is set up for a lower temp refrigerant than the old standard R-12 or it’s replacement R134A systems that we have seen for years…Grunert offers a R-404A system and plates mixed to do it…

Different than a conventional system (non hold over) that senses ambient air box temp, and cycles the compressor (if power is available) to maintain that temp…A good Hold Over system only senses Plate temp (to keep the plate frozen) in order to save energy, and thus the box temp…Is what it is…Depending on all the factors I have mentioned…

Thing is…Boxes, doors, gaskets & insulation are almost never perfect…Especially as they age…So as you might see…Maintaining these marginal temps for food storage is very critical, and a chore for the owner of such systems…Most never utilise the real design of the system as a hold over system so to speak…They generally end up adjusting the thermostats to get the box temp they want and leave it at that…Thus running it like a conventional system…Which it is not as good at as one designed to operate that way.

Knowing all the advantages & disadvantages is key…Yes Hold Over systems can still provide a long distance cruising option, but with things like today’s high efficiency DC to AC power inverters, or the little DC powered flat plate systems…A more standard system can offer the refrigeration you may be more accustomed to at less expense.

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~

First & Foremost are Filters…and not just the Sea Strainer I talked about in my last article….But the ones in the Return Air path too…

Direct Expansion A/C (as opposed to Chilled Water) systems rely heavily on good air flow to keep from icing up in the cool mode (Low pressure) or shutting off due to High pressure in the heating mode…

Generally speaking…Most boats may have two Return Air Filters per system or control…Some have more if there are more than one air handler on that system (compressor)

If you have a small cabin adjacent to another that the fan operates off the control in the other cabin (or just has a speed control installed in that small cabin) this may be one instance….

The trick to finding air handlers is to first look for the return air grill…

Once you have located the Return air Grill (also a likely spot for the first filter) it should be removed…Check if a filter is installed on the back side of the grill…If so remove it and set it aside…

Now with a flashlight…Look into that hole to see the path the air would take…At the end of that path should be the air handlers location…You may have to open another cabinet, or locker to gain access to that location, and boat builders can be very inventive in placing them in out of the way places…Ones that sometimes seem impossible to get to, but in most cases a panel can be removed for access…Here is where the second filter is located…On the aluminum fins of the evaporator coil…

All major manufacturers of marine A/C have shipped their units with a filter installed at the factory for some time now…If there is not one there…It has likely been removed after the system was installed…

I have serviced boats that the owners never knew of this factory installed filter…Or the air handler’s location for that matter…Thus when I removed the filter it looked like a piece of carpet…We all know Air does not flow thru carpet too well…

The manufactures install them there not knowing if a filter is going to be utilized in the grill at system installation…and If it is a clean path to the air handler (one that is not likely to get dust from another path) this filter can sometimes be discarded if it is tough to access, but it is better to have both in most cases…

In boats with multiple controls (more than one system) follow the above procedure till you have all the filters in hand…Take them out to the dock and have fun with the water hose…That is unless someone has installed the throw away fiberglass/paper filters, in which case I would recommend you replace them with some foam filter material that you might find at your marine A/C dealer…Or what Home Depot/Lowes etc…offers for home window A/C units.

Now either wait for them to dry…Or shake them out and re-install…A little water on them will not hurt a thing…

If you can get a good look at the aluminum fins…Check to see that they are not also clogged…If they are…That’s another story, and it’s probably time to call you’re A/C guy to clean them because he has access to cleaning solutions that work real well for this…and also likely has the equipment to do the job right…With less mess…

If you want to clean them yourself with a squirt bottle & garden hose…He may even sell you some cleaner, or you can try Home Depot…However what they offer never seems to work as well as what he gets from the wholesale A/C supply house…

Before you close the cabinet back up you should also check the condition/operation of the condensate drain…Especially after washing the coil…All that stuff that got washed off is now in the condensate pan and could have clogged the drain fitting/s or hose…

Best way to test this is with a quart or two of tap water…You can add some bleach/Lysol/whatever to kill bacteria…But first I would run some plain water into the drain pan to find out if the drain is clogged…This way you won’t have to suck out the bleach water to avoid it dripping onto surfaces when you remove the hose to blow it out…

Please Note: If you have to blow out the hose or fittings, and the only way you have to do so is by mouth…Try to find a clean piece of garden hose or the like to attach to the existing fitting or hose…These hoses are where bacteria like the Legionaires Virus live…You don’t want the old hose, or the condensate water to touch or enter your mouth!!!

Once you are done…We are ready to start up the system/s…

This is a good time for you to check the seawater pump & its pump relay operation (for boats with more than one A/C system supplied seawater by a single pump)

A seawater pump relay is a box connected to the pump that receives a signal from each separate unit…and then tells the pump to start pumping seawater when each, or any unit calls for it (starts to run)

They are easy to verify if they are working properly just by starting one system…Waiting for the compressor to come on…and then either looking over the side of the boat for water discharge, or if the pump actually runs by hearing it start…

Once you verify that that system started the pump…Switch the control off for that system…Wait for the pump to stop…Then follow the same procedure for each system by itself…You don’t want to run more than one system at a time for this test as it may be one of the other systems that brings the pump on…

If one of the systems does not bring the pump on…There is a problem with the pump relay box…Over the years there have been many kinds/models of pump relays, and to say how to troubleshoot each here would be kinda lengthy…However if you feel you have a problem, I will be happy to discuss your model/issue in the blog if you like…Otherwise you probably need to call Ole Faithful A/C Guy…

Now that we have verified Good seawater flow overboard and pump relay operation…We can run all systems…Do so for about half an hour….

Now we need to locate each compressor…

There are basically two kinds of direct expansion marine A/C systems.

• Self Contained units…sometimes called Stowaways or Cabin Mates…These are all in one units where the compressor, condenser, evaporator & blower are all on the same base…They are usually located behind the return air grille, and under a settee or in a closet…They are more like a window unit than central A/C…As the compressor is almost in the living space and thus you can get a bit more noise…Though now days the newer rotary compressors are pretty quiet.
• Split Systems…Sometimes called Remote…These are more like the central systems you may have at home…In that the compressor & condenser are “Remotely” located and connected to the Air handler/evaporator section by copper tubing to carry the refrigerant…These systems are quieter, and have more flexibility in that they can have more than one air handler/evaporator running off one compressor…and thus allow for installation in smaller cabinets, with smaller units, to reduce air noise even more…The total of air handler BTU just must pretty closely match the condensing units capacity….

Now that we have located the Compressor/s and they have run for a half hour…Be certain they are still running in cool mode…If not…Lower the temp setting of your control (t-stat)

Now locate the seawater condenser…It’s the tubes that horizontally (most are horizontal but some are vertical) wrap around the top of the compressor (condensing) unit…there should be at least two water hoses connected to it (some have 4) and with the palm of your hand…Grab the top coil….It should be Warm…but not Hot…

If it is Hot in more than one place…The unit is not performing as it should and is probably drawing more amperage than designed…

A couple of likely reasons…

• Seawater flow is not up to spec…Look for restrictions
• Seawater condenser is dirty…Needs to be flushed with an acid solution…Similar to the radiator in your car…At any rate…Not enough heat is being removed from the refrigerant…
• Not likely…But an overcharge of refrigerant…Maybe that last A/C guy didn’t do so good?…Or was it a buddy that said he knew what to do?
• Even less likely…Someone replaced the seawater pump with one that doesn’t have enough GPH pumping capacity for the system…Maybe somebody’s buddy was trying to save him a buck or two…You may laugh, but I’ve seen that happen too…Hey…He meant well!
• Most of the above reasons are probably best to call the A/C guy…Not the buddy…However you can & I will walk you thru flushing a coil to remove the scale that acts like a insulating blanket preventing proper heat transfer…Just at another time…

This next test is mostly for owners of Split/Remote systems…

Most of the newer Self Contained systems use what is called a Rotary compressor…Some of the older ones are still around that use the older reciprocating (piston) type…and to determine which you have is hard to explain if you have never seen either…But the Rotary type is usually slightly taller, and a lot skinnier than the Piston type…Have you got a Fat compressor matey?

Anyway…A piston type compressor that is too hot to touch on top…Is likely low on Freon…Rotary’s run Hot but Piston’s do not…

If either type is Cold or sweating…That’s another set of possibilities…

• Lack of air flow due to…Dirty filters or coils…Hey I thought we did that?
• Overcharge of refrigerant…That buddy again?
• Ductwork that is too small (or even crushed due to the space it was run thru) to properly handle the flow of air that the system BTU requires…We could blame…But let’s not…It won’t do any good anyway, and it’s gonna get too in depth here…Again we can discuss it in the blog for each individual circumstance…
• Last but not least…In a split system…Multiple Air handlers that don’t add up to compressor/condensing unit BTU…We won’t go into that here either….

This is getting a bit long…So in closing…I hope I have helped some…I look forward to answering any questions that I might…

Think of you’re A/C guy this New Years…I’m sure he thinks of you too…I know I do…

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~