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Marine A/C Cabin Controls
March 23, 2009
8:35 am
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Steve Pooler
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Forum Posts: 1127
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November 12, 2008
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Since they are not like home A/C controls...Sometimes marine A/C controls are misunderstood...Hope this can clear up some misconceptions...

They do provide the same function as in home systems as they are designed to keep the temperature in the cabin/room at a selected temp within a few degrees...

But different than Home...They can (and most do nowadays) offer more/different features than we use at home depending on the type of marine control that is installed...We can cover some of those features here but first we need to define which type of control is installed...Although there have been and are many types and models of controls with several variations...

Lets first describe the most popular two main types...

Manual Controls...

The old standby that has been around for many years is the three knob manual control (non digital) it has/had 3 control knobs each with a different function & was/is available in either a vertical or horizontal, rectangular form factor...

The typical function of each knob was as follows...

  • The first knob was a function knob with Off, Start (sometimes labeled Fan) and Run positions...

The functions of each position were pretty much self explanatory except that the Start/Fan position ran the fan but also started the seawater pump so that you could look over the side of the boat to verify it was pumping water before selecting Run.

The Run position sent power thru the thermostat which then would decide whether to start the compressor in the cool, or heat mode depending on cabin temp, and the position of the thermostat knob...(more on that in a minute)

  • The second knob was a Variable Fan Speed control (not usually available at home)

It was useful at night for lowering blower noise to watch TV etc...(how low was low was adjustable but the control needed to be removed to adjust this parameter with a small screwdriver)

  • The third knob was the thermostat...

This was the most misunderstood function of this control because it had no temp numbers (It was also the hardest to explain to folks)  Most would just crank it to it's coldest setting thinking that the further they turned it...The colder the air would come out...That's true in a way, but it's not a function of the control, and A/C does not work that way...I will try to explain...

Regardless of control setting or type...All R-22 A/C systems work on a 15 to 18 degree temp exchange design...Meaning that the air coming out of the discharge grill should be 15 to 18 degrees colder than what is going into the return air grill (if it's more, or less than that, there is likely a problem with the system or installation)

So if it's 90 degrees in the space being cooled...don't expect much colder than 75-72 degree air coming out the discharge with any R-22 A/C system...But as the room/cabin cools, that exchange is maintained, and the air continues to come out colder by 15 to 18 degrees of what goes in....Lower room temp=lower discharge air temp.

The thermostat tells the compressor when to run or not run, and energizes the reversing valve when reverse cycle heat is needed to maintain cabin temp of the setting on the dial (position of the arrow)...Generally speaking...A properly calibrated t-stat set with the arrow pointed vertical should maintain around 70-74 degrees cabin temp...Anything to the right is a colder cabin, and to the left of vertical is warmer...The T-Stat will allow the compressor to run until the cabin gets to the selected temp (on the knob) at which point it cuts power to the compressor until the cabin either warms or cools a few degrees, at which point it then again sends power back to the compressor/reversing valve as needed to bring the cabin back to selected temp.

These controls offered very little in system protection and thus it was not advisable to run the A/C system unattended for very long.

Digital Controls...

Started hitting the market in the mid 80's...They at first like any new product, had a few weaknesses and thus some early problems, but as they matured they have now become the standard, and are very reliable...

They also allow the marine A/C manufacturers to offer extended warranty periods when installed with new equipment because of the system fault protection/shutdown features they provide, along with the many other benefits/features...I can't count the compressors & pumps they have saved for my customers...Many of them don't realize it either, but they do help immensely...

The features & protections offered by these newer microprocessor based cabin/system controls are likely too many to mention here but I will list some of the more important below...

Protection:

  • High Pressure Shutdown...Loss of Seawater flow...This can save a compressor and or seawater pump failure, from running without water. (Did you check that strainer & thru hull ?)
  • Low Pressure Shutdown...Loss of Freon or a dirty air filter...Can also save a compressor....
  • Low AC power Shutdown...One of the worst things for a compressor is low voltage.
  • Seawater Pump shuts off with compressor (on single system boats) so that pump can be saved from no water burnout of pump head (this will also protect pump if only one system is run while away on multi system boats)
  • Programmable Time Delays...So that multiple systems don't all start at once when shifting from Shore to Gen or vise verse causing tripped breakers and/or heavy surges.

Features:

  • Digital Display of Setpoint (Temp you want the cabin to be) & Actual Temp of that cabin.
  • Much more accurate temp control. (when compared to the 3 knob control or others)
  • Automatic Fan Speed control...Fan speed can be controlled by how far away you are from setpoint...Manual speed is also an option so that if you don't want it to change it won't.
  • Ability to program Continuous or Intermittent Fan (Like the Auto fan feature at home)
  • Humidity Control program...System can be activated to remove humidity only while away..This greatly reduces run time while keeping mold out (you probably won't find this feature in most home controls)
  • Some have IR remote control availability/operation.
  • Some have networking ability...So that they can all be linked and controlled from a central (keypad) location, or from any individual control location aboard.

These are some of the most important features, but the controls are fully programmable for many more user adjustable functions (depending on model)

If you have one or more of these digital controls...The not so funny thing I find here in West Central Fl...Is that about 95% of the boats I go on have never had the digital controls setup for the boat/installation...

The A/C manufacturer has a set of Default settings they ship the controls programmed with...These are just that...Defaults that the manufacturer feels that all systems should operate safely with once installed...It's generally thought that it's the installers responsibility to perform the calibration...Unfortunately it's likely that the boat builder or original installer did not do this for many reasons...One of which might be they don't know about them ?...

Anyway as I said...Most of the boats I go on still have the factory defaults set in the controls, and a lot of these boats are many years old...They are by no means the best settings for each installation or system, and again if you have these controls, you may find that you will enjoy your system/s much more if you get your trusty A/C guy out to to go thru them...Or get the manual out yourself to take a look....

Many also ask if the newer digital controls can be retrofit to take the place of others...Yes they can...I have installed them on just about every application of marine A/C made in the last 40 years...Yeah...That's longer than I have been working on marine A/C but I do/have seen some old stuff still in operation...and Usually after we install one on a boat and calibrate it...The owner almost always wants another...

Thanks for reading, and Comments are always welcome in the forum...

Steve~