I Recently received an email from a gentleman named Tyler…

Tyler was inquiring about a DIY job installing Marine A/C on a 30 foot sailboat, and then running the Marine Air Conditioner on an inverter when he was away from shore power.

His question asked about wattage draw of a 8,000 BTU Cruisair self contained Turbo AC unit & if I thought the 8,000 BTU marine ac unit would be large enough for his 30′ Irwin.

Since this “Air Conditioning on Inverter” question is being asked more & more often now days, I thought I would post my answer here for everyone’s review…

Hi Tyler…Thanks for asking~

It’s not really as simple an answer as you might expect…But I’ll try to keep it somewhat simple…So here goes…

• The 8K Turbo unit is rated to draw 5.5 amps which using the conversion formula of 5.5 amps X 115 volts = 632.5 watts in cool mode (slightly more in heat)
• The 10K Turbo unit draws 6.7 amps X 115 volts = 770.5 watts in cool (slightly more in heat)
• Depending on your location (seawater & air temps) 10K is likely better suited for a 30′ sailboat…At least down here in central Florida…
  

Cruisair Stowaway Turbo Unit (SMXII) W/R410A 10,000 BTU Marine AC

OR

Marine Airrr Vector Turbo Self Contained A/C 10,000 Btu W/410A

Yes they can be run on an inverter, but starting amps can be many times the run load, and that can depend on several factors such as the inverter’s quality & the voltage it maintains during that surge…

The inverter’s Surge rating is as important as it’s continuous wattage rating…And as with anything electronic… Over sizing is usually best for the life & performance of the product…Don’t plan on just the minimum needed.

Also…A Dometic “SmartStart” can be added to any A/C unit to reduce it’s starting amperage draw/surge by as much as 65%…In my experience they really do live up to the claims…

Once started…How long your battery bank can run the A/C depends on a few things too…

• Size of the bank in amp-hours…Meaning size of each battery & how many batteries connected in parallel (Two 100 amp hour 12 volt batteries connected together in parallel provide for a 200 amp hour bank…4 would be 400 amp hours etc..)
• Length Of, and Wire Size, run…DC Voltage to the inverter tends to drop pretty quickly (length of run is measured as a total of both to & from batteries)
• The Longer the DC Voltage Run…The Larger the wire needs to be to limit voltage drop & overheating…(It’s why Tesla won out with his AC voltage we now use over Einstein’s DC voltage back in the day)
• Duty Cycle of the A/C (actual compressor run time to pull down to & maintain a given set point temp)
• Duty Cycle is usually figured as a compressor run percentage of overall time on…
• Then also figure the small draw of the fan & pump…Which may, or may not cycle with the compressor (Now days with the digital t-stats most pumps cycle with the compressor but the fan may not, or is selectable)

Most battery manufactures recommend not running batteries below 50% charge state before re-charging (so figure battery amp hour rating 50% less than actual rating)

As a general rule of thumb…It’s a 10 x conversion for DC to AC amperage…10 amps of AC amp draw (uninterrupted) per hour is around 100 amp hours of DC removed from the batteries…

So taking the 6.7 amp figure from the 10K unit above…Figure 67-70 DC amps being removed from the batteries (actually inverters are never 100% efficient and differ in ratings so YMMV but 10x is easy to figure)

Basically, It takes a pretty large battery bank to get a full nights sleep…Then you must figure recharge time to replace that amperage used & the charging system…Alternator or Charger output rating determines that factor…But other questions are…

• How long do you have to run the engine to replace that energy ?
• Or how long do you typically run the engine a day, and is it enough ?

To run that 10 K Dometic Turbo unit 9 hours (one hour pull down, 8 hrs sleeping) and I don’t think you can find a similar sized unit that draws less…figure say…

• 7.7 amps running (6.7 unit amps plus 1 amp for the pump) 7.7 x 10 (basic DC to AC conversion factor) 77 amps x 9 hours = 693 amp hours used…
• 693 x .80 (80% duty cycle) equals 554 amp hours without loss thru wire, inverter efficiency, etc…
• Then double that for the 50% discharge factor the battery manufacturers recommend & you would need a battery bank capacity of 1,108.8 amp hours…
• That’s 11 or more 100 amp hour 12v batteries…
• Or you could use six pairs (12 batteries @ 225 Ah per pair) of 6 volt golf cart batteries for a capacity of 1350 amp hours…This should keep you above the 50% discharge for a full night of sleep in slightly less space than twelve 100 Ah 12 volt batteries that would only total 1200 Ah…

If you want to keep the cabin cool 24/7…

• Does the alternator have enough output to keep up with the draw without the battery bank loosing percentage of charge & are you taxing that alternator doing so ? (at maximum output all the time)
• Or can the alternator “Catch Up” After some engine off time while still running A/C underway ?
• Meaning do I need to consider a larger alternator to do what I want without it overheating causing shorter life ?
• Typically the standard alternators on small diesels or sailboat auxiliary engines range from 35 to 65 amp output…

There are trade offs…Yes it can be done, and is…

You can play with the figures for differing usage, but knowing all this info going in provides for fairly realistic operational & pocketbook expense expectations…

Hope that helps…Any questions please ask…

For folks reading this in Marketplace…We are linking this post to the “Installation” section of the Forum here on Marine-AC.com so that it can be discussed further if needed…

Steve~

One compressor & 5 air handlers in this case…This system is in a Berger Yacht & cools the lower deck cabins…

The Cruisair units in the second picture were added after the Pilot House/Aft Deck were enclosed…

Sorry for the not so good pictures but my iPhone has no flash (waiting for upgrade time)

Steve~

• 2 PRP circuit board
• Includes thermal compound packet for retrofit

Call for details and shipping options 727-365-6123

• Two Unit Solid State Relay
• 115V/230V 1phase
• Pump activation for two units supplied by one seawater pump

Call for details and shipping options 727-365-6123

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

Any questions…Please Ask.

Steve~

FEATURES BENEFITS Elite or Passport I/O Display Versatility in choice of displays w/Passport I/O technologyHigh Velocity Blower Increased air flow with no motor overhangRust-free Molded Composite Drain Pan Never rusts and minimal standing condensate waterBlower Rotatable to Left or Right Hand Reduced installation time and SKU #’sBuilt-in Isolation Mounting System Minimizes vibration to the mounting deckState-of-the-Art Patented Technology Increased capacity & reduced Amperage with compact size.

Call Or E-Mail For Discounted Pricing.

Note: Picture may show “Optional” compressor sound shield which is sold separately.

Watch the Dometic Turbo U-Tube Video

FEATURES BENEFITS Elite or Passport I/O Display Versatility in choice of displays w/Passport I/O technologyHigh Velocity Blower Increased air flow with no motor overhangRust-free Molded Composite Drain Pan Never rusts and minimal standing condensate waterBlower Rotatable to Left or Right Hand Reduced installation time and SKU #’sBuilt-in Isolation Mounting System Minimizes vibration to the mounting deckState-of-the-Art Patented Technology Increased capacity & reduced Amperage with compact size.

Call or E-Mail For Discounted Pricing.

Note: Picture may show “Optional” compressor sound shield which is sold separately.

Watch the Dometic Turbo U-Tube Video http://www.youtube.com/watch?v=7nVRFaniPQo&feature=player_embedded#!

FEATURES BENEFITS Elite or Passport I/O Display Versatility in choice of displays w/Passport I/O technologyHigh Velocity Blower Increased air flow with no motor overhangRust-free Molded Composite Drain Pan Never rusts and minimal standing condensate waterBlower Rotatable to Left or Right Hand Reduced installation time and SKU #’sBuilt-in Isolation Mounting System Minimizes vibration to the mounting deckState-of-the-Art Patented Technology Increased capacity & reduced Amperagewith compact size

Call or E-Mail for Discounted Pricing.

Note: Picture may show “Optional” compressor sound shield which is sold separately.

Watch the Dometic Turbo U-Tube Video http://www.youtube.com/watch?v=7nVRFaniPQo&feature=player_embedded#!

FEATURES BENEFITS Elite or Passport I/O Display Versatility in choice of displays w/Passport I/O technologyHigh Velocity Blower Increased air flow with no motor overhangRust-free Molded Composite Drain Pan Never rusts and minimal standing condensate waterBlower Rotatable to Left or Right Hand Reduced installation time and SKU #’sBuilt-in Isolation Mounting System Minimizes vibration to the mounting deckState-of-the-Art Patented Technology

Call or E-Mail for Discounted Pricing.

Note: Picture may show “Optional” compressor sound shield which is sold separately.

Watch the Dometic Turbo U-Tube Video

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~