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Old 05-14-2013, 08:09 PM   #1
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AC or DC Fridge?

Hi folks,

Figuring this out a little bit at a time. I have a few questions. First, my AC 10 year old fridge is making it's last very loud gasps.. I have a 1000 watt inverter, a westerbeke 7.5kw genset, and twin ford lehman 120's. I would like very much to minimize generator sounds while at anchor and I'm wondering about a dc fridge.. how much more is the draw on the batteries going through the inverter to an AC fridge? I know I've got a seperate starting battery for the genset and that makes sense to me, and I'm guessing I've got 2 8d batteries but I'm not sure. I saw one but I'm electrically challenged and this wiring is very strange to me. Don't people usually have two 8d's on a boat this size, 42'? I'm also trying figure out how long it would take to charge the batteries with the generator. With the engines? Shore power? What's an average size battery charger?

Thanks again for your time and consideration!
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Old 05-14-2013, 09:59 PM   #2
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There's no free lunch. A modern, store-bought, fridge, AC or DC is going to use essentially the same amount of power per cubic foot. That said, you can get a lot more fridge for the dollar in a home-style AC vs. boat DC ones.

If you have a GOOD inverter with low overhead (check the amperage draw when no AC items are running) I'd recommend a good, high tech AC unit. The money you save over a DC one can go to more AHs in your battery bank.

If you have 72" of height you can find a good, full size such as Sanyo, LG or Hitachi. What's in common with these brands? All Asian. Some models have high end features, ice makers, drawers, glass shelves, polycarbonate bins. And they tend to be energy efficient. You can read the yellow annual usage tag to figure out daily watt usage. Divide by 12 for 12 volt amps - add about 10% for inverter overhead. Times 24 for daily battery amp hours used.

Many boat DC boat fridges were engineered decades ago and look it.
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Old 05-15-2013, 05:53 AM   #3
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"There's no free lunch. A modern, store-bought, fridge, AC or DC is going to use essentially the same amount of power per cubic foot. That said, you can get a lot more fridge for the dollar in a home-style AC vs. boat DC ones."


Actually there are huge differences.

A modern (expensive) DC fridge is built to lower energy consumption in a number of ways.

The insulation will be better grade and far thicker (see Sun Frost) and the compressor will be variable speed , so it only works as hard as it needs to.
In some the compressor is on top ,getting rid of the heat above the box , on others the heated air is ducted out into the cabin, not dumped behind the reefer.

House stuff has very thin insulation to maximize the interior volume.

The insulation is so thin most units require daily use of heat strips to dehumidify the insulation. This extra energy is not in the EPA cost for a year figures.

The power consumption can easily be 50% less for a good DC unit, weather it pays for you ????

For 100% electric free the Servelle and other propane units offer the best results.
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Old 05-15-2013, 07:25 AM   #4
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Fred is absolutely correct about the efficiency of the higher cost DC units. In addition to his comments, DC compressors actually have an internal inverter which producest AC at several hundred Herz (which is variable with load). That higher frequency is inherently more efficient for small motors.

Back to the OP's question. Buy a bigger shore power charger to minimize genset running time. With two 8Ds (not the best choice- either AGMs or golf cart batteries would be better) he can use as much as a 100 amp charger and drastically reduce genset running time.

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Old 05-15-2013, 08:58 AM   #5
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From the questions you ask I think you would profit from hiring a good marine electrician. Let him crawl over your boat alone and then have him explain the systems to you. Not knowing where everything is and what it does is dangerous! You state that you do not know how many batteries you have. These require regular service and missing one could be an expensive mistake.
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Old 05-15-2013, 09:46 AM   #6
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Ditto Tingum times 10. It is essential that you know your vessel's systems. No point in trying to deal with issues piecemeal until you do. Some people falsely believe they can ease into boating, learning along the way. That's possible, but if you're like me, you'll likely make mistakes that will cost you much more than if you'd hired a professional to teach you what you need to know. You may find that will be the wisest investment you could possibly make! Best of luck!
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Old 05-15-2013, 10:09 AM   #7
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Quote:
Originally Posted by FF View Post
"There's no free lunch. A modern, store-bought, fridge, AC or DC is going to use essentially the same amount of power per cubic foot. That said, you can get a lot more fridge for the dollar in a home-style AC vs. boat DC ones."


Actually there are huge differences.

A modern (expensive) DC fridge is built to lower energy consumption in a number of ways.

The insulation will be better grade and far thicker (see Sun Frost) and the compressor will be variable speed , so it only works as hard as it needs to.
In some the compressor is on top ,getting rid of the heat above the box , on others the heated air is ducted out into the cabin, not dumped behind the reefer.

House stuff has very thin insulation to maximize the interior volume.

The insulation is so thin most units require daily use of heat strips to dehumidify the insulation. This extra energy is not in the EPA cost for a year figures.

The power consumption can easily be 50% less for a good DC unit, weather it pays for you ????

For 100% electric free the Servelle and other propane units offer the best results.
I stand corrected - the Sun Frost line is extremely efficient. It is difficult to figure out capacity from their website, however. From altestore.com, I found out that the Sun Frost RF16 is 14.3 cu.ft./406 liters and uses .79 kW hours per 24-hour day. That works out to an average of 33 watts/hour. I was thinking of an Hitachi R-S42BMJL which is 415 liters (a bit larger) that uses .99 KW hours/day - 41 watts/hours - 25% more - but still less than most of the other "store-bought" fridges out there of this size. The Hitachi would use 92 AH of 12v per 24 hours, including the 10% inverting overhead.

The reason I prefer the Hitachi over the Sun Frost and essentially ALL other Marine refrigerators is the form-factor and high-end execution. It will fit in a 24" wide x 26" deep x 72" high space - has one door and 4 drawers - built-in ice maker with its own water reservoir (no plumbing and you can easily use bottle or filtered water) plus pull-out bins inside the two big drawers. Yes it uses 25% more battery - but it uses half what most other DC fridges do.

Below are photos of the Sun Frost 19 (16.1 cu.ft.) and an older Toshiba of 400 liters plus (didn't have as good a photo of the current Hitachi). You can see what I mean by different approaches to the interior and exterior spaces. It is my guess that shown side by side, most chefs would prefer the Japanese fridge?

SunFrost RF16 - $2,300
Hitachi RS42BMJL - $39,700 Taiwan Dollars/USD 1,324 plus freight and duty. BTW, Taiwan uses 120VAC/60Hz, just like the USA.
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Old 05-15-2013, 10:55 AM   #8
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I made the switch to an apartment size fridge years ago when my DC compressor blew. It was only approximately 8 cu ft and fit in my center window opening by a mere 3/4 inch. I spent 235.00 for the unit at Best Buy at the time. It only drew 155 watts. I timed it on an 80 degree day and it ran 20 minutes, then off for 60 minutes. That's 4 hours a day.

It was very manageable with my house bank and Prosine inverter.
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Old 05-15-2013, 11:39 AM   #9
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Personally, I'd focus on size first. How big a fridge do you want/need? Are you looking for a fridge only, or combined fridge/freezer? Depending on the size, you may not have much of a choice re AC vs DC. As the fridge gets larger, AC will dominate the market. In the little fridge category, there will be plenty of DC and AC models.

Once you get into a larger unit, the only DC models are likely to be something like Sunfrost which, although very efficient, are also wildly expensive.

The point about insulation is a good one, and a lot of improvement can be gained from any fridge by adding insulation around the outside. In a boat, this is often much easier said than done, but it's worth considering if the specifics of your boat allow it.

If you end up with an AC unit, take a look at the EPA energy star web site. It has very good listings of typical annual energy consumption for most models, and I've found them to be very good predictors of actual consumption. You can see if thee is any significant difference between models that you are considering.
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Old 05-15-2013, 04:47 PM   #10
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No one has mentioned building a "sailboat" type box with remote comprressor and perhaps holdover plates? It's not costly if you're handy to glue up some foam insulation as thick as you have space for - make a liner of FRP, sheet plastic or aluminum - and mount the condenser plate. This would take the least power to keep cold.

But, since you have a sizable inverter, you have endless choices for both under counter and apartment sized fridges. Some ridiculously inexpensive - and some, very efficient. Spend the money you save on adding to your AGM house battery - 4 8Ds are much better than 2. 6 is even better. The larger your battery, the faster it will take a charge from either your engine alternators or your shore charger, which iI assume is part of your inverter?

Unless you're planning on many days at a time on the hook, you'll do just fine.

Are you cooking with LPG or electric? If electric, you need to run your genset a couple times a day anyway - so you'll always be ahead of the drain from the fridge.
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Old 05-15-2013, 07:41 PM   #11
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BigFish---- If you don't want to build something with components here and there, and you don't have room for a full-size refrigerator/freezer you can have a very workable setup with a basic marine AC/DC refrigerator-freezer, particularly if your boat is currently configured for an under-counter refrigerator/freezer.

Our boat was built with a teak-doored, under-the-galley-counter icebox. I don't know when this box was removed and an under-counter refrigerator put in its place but the year before we bought the boat the previous owner installed a new AC/DC Norcold refrigerator/freezer in this space.

While the Norcold has been operating 24-7-365 for the last 15 years almost flawlessly I would not recommend a Norcold. It has a funky compressor/power system that is virtually unrepairable, it is a power-hog, and the repair shops we talked to, including one that was a Norcold dealer, all advised us not to buy one when we thought we had to replace ours.

BUT.... we think for coastal cruising the AC/DC setup is terrific. You don't need an inverter even though we have one, and it doesn't draw on the batteries when there is shoreside AC available. Operation is dirt simple--- when we're out we run it on DC. When we're in the slip or at a marina dock we run it on AC.

When we thought we had to replace the Norcold a few years ago we did a LOT of research on what was available. Our requirements were:

1. Absolute minimum hassle in fitting a replacement. No building up some sort of remote compressor system, etc. We wanted a drop-in, turn-key AC/DC replacement.
2. Had to fit the same space as the Norcold and connect to the same AC and DC wiring.
3. Had to have a Danfoss compressor.
4. Had to be as efficient as this type of unit can be.
5. Had to have a user-friendly interior layout (the Norcold we have now is outstanding in this regard).
6. Had to have a proven record of reliability and longevity.

We talked to a lot of refrigerator dealers and repair shops. This right away took several contenders off the list of candidates. Norcold, of course, and NovaCool had the worst reputations at the independent and factory-authorized repair shops we talked to.

There were several survivors but in the end we decided on Isotherm. They had a unit that fit every one of our requirements to a T.

We were about to order one when whatever was ailing the Norcold went away and the unit has been running flawlessly again ever since. But when the day comes that it dies--- and it will eventually--- we will go with Isotherm unless we find there is something even better by then.

Other people have had good experiences with other brands so I'm not saying that it's Isotherm or nothing. You'll have to do your own research and determine which one best suits your requirements.

But my point is that a basic, undercounter, AC/DC refrigerator/freezer can be an excellent choice if it fits the kind of boating you do. You can certainly build a very effective, very efficient system like some of the ones that have been described earlier. I don't mean to say that's a bad road to go down.

Only that if you don't want to go down it for whatever reason, the off-the-shelf marine undercounter (or stand-alone) AC/DC units offer a solution that can be every bit as effective for you.
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Old 05-15-2013, 10:18 PM   #12
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Since you said you want to reduce run time, energy use is probably the single most important factor.

If you go to EnergyStar.gov Refrigerators : ENERGY STAR you can click on the Excel icon in the upper right hand corner and download a file with energy consumption data for 2184 different refrigerator/freezer models. You can then sort by which ever parameter is most important to you. For example you can sort by "% Better than Federal Standard" but that seams to favor smaller units due to reduced surface area for heat transfer. I thought it was energy consumed per unit volume, so I created a column to calculate that and did a sort. In this sort the Sun Frost R-19 came out on top but the other models weren't far behind. I think to get a good comparison you need to first sort by Adjusted Volume and then sort the Adjusted Volume range that suits your needs based on Annual Energy Use (kWh/year). Happy hunting.
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Old 05-15-2013, 11:10 PM   #13
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Quote:
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.... I'm guessing I've got 2 8d batteries but I'm not sure. I saw one but I'm electrically challenged and this wiring is very strange to me. Don't people usually have two 8d's on a boat this size, 42'?
Two 8Ds on a boat like yours is kind of under-batteried these days given the typical electrical loads of today's cruisers. Our boat's battery system consisted of a pair of 8Ds and that was what was used back in 1973 when electrical loads were pretty minimal. One battery was selected for house power and the other was selected for engine starting. Actually our boat's starting setup is more complicated than that but that's a typical two-battery configuration.

We, like you, also have a separate battery (4D) just for the generator.

If you do, in fact have just two 8Ds, one thing you can do without changing much of anything to increase your battery capacity is change out the two 12vdc 8Ds for six, 6vdc golf cart batteries. An 8D battery box will hold three golf cart batteries. The golf cart batteries are then wired together in pairs so you end up with three 12vdc "sets." Typically, two pairs are connected together to provide house power and the third pair is dedicated to engine starting.

One pair of golf cart batteries delivers about the same amp hours as a single 8D. So by having two pairs of golf cart batteries as the house bank, you will have doubled your house capacity without physically adding any more battery footprint to the boat.

Another advantage is that a golf cart battery is a hell of a lot easier to get in and out of the boat than an 8D.

We did this on our boat a few years ago and it made a big difference to us in terms of house power capacity, particularly when we have guests on board.

The nice thing about this change is that it requires no change to the boat's wiring system, breakers, switches, etc. The leads that had connected to the "house" 8D were simply connected to the new golf cart battery house bank. And the leads that had connected to the "start" 8D were connected to the third pair of golf cart batteries. The only wiring work involved was the fabrication and installation of the short cables that paired up and connected the golf cart batteries themselves.
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Old 05-16-2013, 05:54 AM   #14
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"he can use as much as a 100 amp charger and drastically reduce genset running time."

That depends on two ratings . The charger and the noisemaker.

Many chargers are "rated" at what they can pump into an almost dead battery bank, 100A at 12v output into a 10.5v set is a far different charger from 100A at 14.4 V into a mostly charged batt set.

Until the noisemaker is about 10KW many do bot provide enough power (area under the sine wave)to run a good charger at full power.

Observe the charger amps output with a down bank when starting the noisemaker , and observe the output returning to a dock with the batts in a similar state.

For most there will be a big TIME difference to full charged.

The best way to reduce noisemaker run tine is usually to belt a 135A truck alt with 2 belts and a smart 3-4 stage charge V regulator.
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