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3-Way Refrigerators

 
 
 3-Way Gas, 240V AC & 12V DC Refrigerators

 
Just a brief discussion on the subject of those 3-way absorption refrigerators.

I am referring here to the Electrolux/Dometic product but would apply to other makes.
These are usually a good,
*reliable product and for these units to work effectively, the makers installation and operating instructions must be strictly adhered to.   There are several areas to pay attention to.
* See note at bottom of page

Having said that, once ambient temperature exceeds 40 deg C, absorption fridges are no match for compressor types.
Compressor fridges will work effectively past 50 deg C and at all temperatures will pull the fridge temperature down more quickly. 

However for most people the absorption fridge is the more suitable option for long term operation (that is, more than just a few consecutive days) in the warmer summer months unless you have at least 250 watts of solar panels or always have mains or generator power available when you stop. 
(Don't be talked into believing that a 12V compressor fridge can be satisfactorily operated from one solar panel and battery for more than a few of days at a time without being properly recharged unless it is a small chest type, to attempt to do so will cause the battery to cycle too deeply before it can be properly and fully charged with a significant reduction in battery life - see solar page for more details.)


 Vent position & sealing

 
Pay particular attention to the position of the external air vents in relation to the unit as shown in the illustration.

The performance of the unit will be greatly impaired if the convection air flow is restricted around the condenser and absorber in this rear area behind the refrigerator.

The bottom of the lower vent must be level with the bottom of the unit and the bottom of the top vent level with the top of the unit.

It is also very important that this rear area behind the fridge is completely sealed from the area around the sides and top of the unit.

If, during operation you can feel the bench top above the refrigerator is warmer than ambient by more than a few degrees, then you can be reasonably sure that the rear sealing is less than perfect.

It may be worthwhile to fit a small 12V DC fan in this rear area to assist air movement during the warmer days particularly if the manufacturers installation instructions have not been strictly adhered to.   There are thermostatically controlled fan kits available from caravan parts suppliers for this purpose.

A small 12V Pentium computer fan inside the food area blowing on the cooling fins circulating the air gives a worthwhile improvement also. 
 
 
 Voltage drop and leakage

 
The operation of these absorption fridges relies on a correct heat source.   This heat source is derived from a gas flame, a 240 VAC element and a 12 VDC element selected, as appropriate, from the front of the refrigerator.

The gas flame is rarely a problem if serviced annually by a qualified gas fitter but there can be problems with the electrical elements.


12V element

The usual problem here is voltage drop in the cable feeding the element due to too small wire gauge.

The (nominal) 12V element is around 150 Watts which works out to 11 Amps @ 13.8V.   If, as is often the case, the wire size is too small, 3-4V can easily be dropped across the supply cable leaving only 10-11V for the element.

If, for example, 3V is dropped across the supply cable, the power dissipated in the element is only 93 Watts, less than two thirds that intended by the manufacturers.

No wonder the performance on 12V can be disappointing.

To remedy this, the wire size from your vehicle alternator / battery, not just your house battery, must be taken into account and upgraded.

The easiest way to accurately measure the voltage drop is to take two measurements of the voltage at the refrigerator with a multi-meter with the engine running such that the vehicle battery reads 13.8V, first with the fridge selector off and second with the selector in the 12V position.    The difference between the two readings is the voltage drop or loss across the cable.    I would not like to see more than 0.5V dropped.

(This voltage drop problem associated with inadequate wire size is a problem with electric brakes also, but that's another story.) 

Make sure the AC feed to your RV is unplugged prior to doing this as the AC and DC terminal strips can be in close proximity to each other depending on the model.
If you have any problem following these steps it is probably a good indication that you should enlist the help of a suitably qualified person.


( I don't use the 12V facility at all, I use the 240V position even when driving, it works much better.    See my Batteries & 3-Way Fridge Problems page. )  


240V element

Voltage drop is not a problem with 240 VAC side.    The current with a 150W element is only 0.625 Amp and the minuscule voltage drop is an extremely small percentage of the supply.   (In marked contrast to the 12V side where 3V is a large percentage of the supply)
However there can be another problem.

The heating element consists of a resistance wire inside, and insulated from, an outer metal tube casing.   So long as there is no electrical connection between the resistance wire and the outer casing, all is well.

A situation can arise, and probable will in time, where a partial circuit will form between the resistance wire and the outer casing which is earthed.    This can be caused by partial breakdown of the insulation between the two or an ingress of moisture.

If this happens it will cause RCDs (safety switches) to trip out if a leakage current of greater than 30 mA flows.    (See page describing Safety Switches)
Even if you don't have an RCD in your RV (and you should), many caravan parks now do and this will result in your 240V power tripping off every time you turn you fridge to 240V.

If the leakage is due to a breakdown inside of the element, you will have to have it replaced.    This is a simple job that can be accessed by removing the lower exterior vent.

If it is due to moisture in the element, you can try drying it out by running on gas for a while.   However, prevention is a better option.   If you run your fridge on 240V on a frequent regular basis, it will prevent the moisture build up.

Remember, all work involving 240V and gas must be done by a suitability qualified person.


 Level

 
Absorption refrigerators must be levelled to work efficiently and can be damaged if operated out of level for more than a short time.  Mobile operation out of level is OK since it will be level on average.  This is extremely important.

Circular 'bulls-eye' spirit levels are ideal.   I have one on the drawbar and one on the bench above the fridge.

Fore and aft levelling is done with the jockey wheel and sideways levelling with a nifty tough plastic ramp made for the job by Fiamma.    There are two ramps in a set each measuring 500x200x90mm.   Also, make sure you buy the optional matching chocks that lock in to the ramp.


 Other problems

 
Whilst absorption fridges do not have any moving mechanical parts, they are not without their problems.

The cooling system relies on a complex ammonia/water/hydrogen cycle.
Faults can occur due to pipe corrosion, blockage, leakage and air locks.

In the event of cooling failure, where the heat source is not at fault, the entire cooling system will need to be replaced. The cost of a replacement system is in the vicinity of 60% of a new fridge.   Reconditioned cooling units are available but are still approximately 50% of the new fridge price and usually with a shorter or no warranty.

If you are very lucky, the problem may be an air lock.   In this event, try inverting the fridge for several hours and giving it a bit of a shake and when right way up again, try operating on 240V.   This often works and if it does you will have saved $600-700.

  Update


Jan 07

Based on recent personal experience and several emails, I am not so sure the statement on reliability is accurate.

We have been told (or we assumed) that no moving mechanical parts equals good reliability and long life.
I have personally just recently been associated with two Dometic 3-way fridges that failed, one at 8 years and the other 4 years.
The former was used continuously for 8 weeks per year and monthly for several days at a time the rest of the year.   The latter was run continuously for 4 years.
Both suffered the same fate, rusted pipes causing loss of coolant.
The cost of repairs (new cooling unit needed at 60-70% of new fridge cost depending on repairer) and in some instances, lack of worthwhile warranty of the repair meant both were scrapped.

To my surprise, authorised repairers have told me 10 years is a good life expectancy.
Obviously this is planned obsolescence since a metal not subject to corrosion could be used. 

Pretty disappointing for a $1300 fridge (depending on state, Qld was cheapest, SA dearest, in fact when I enquired, it worked out more than $100 cheaper to buy in Qld an freight to SA than buy in SA).

If my new one fails anytime soon, I can't see myself getting another one.

My recent experiences and several email criticizing my comment on their good reliability have prompted this update.


 
Animated graphic of the absorption refrigerator cycle.
  



Reproduced by kind permission of RV Air 'n' Fridges Co. 
http://www.rvairnfridges.com/



 Also have a look at my page on portable fridge/freezers

 

 

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