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CHOOSING AND INSTALLING FRIDGES

People have been refrigerating things for a long time: scientist Charles Boyle was cooling his alcohol that way in 1691. He wrote "that part of the beer or the wine that was next to the sides of the bottle could be refrigerated.” That's more than many an unhappy RV owner can claim for their fridges three hundred years later!

In reality there are very few bad fridges. Most simply do not work remotely as well as they can because (a) the fridge is not designed for the climate in which it's being used; (b) it is connected by inadequate wiring and/or has inadequate ventilation; (c) users, in particular those who go seriously fishing, have unrealistic expectations of the amount of energy needed to freeze multiple kilos of non- pre-cooled stuff.

Fridges do not generate 'cold'. They work by pumping unwanted heat to somewhere it does not matter - and particularly to somewhere where it cannot heat up the fridge. Installing so that heat is shifted effectively is not hard, but how to do it it is not as obvious as it might initially appear. This is covered toward the end of this article.

Buying the Right Fridge

Large fridges use more energy than small fridges - but not in direct proportion. If everything else is equal, a fridge twice the size is likely to use about two-thirds more energy, not twice as much.

The amount of energy a fridge uses will vary from brand to brand but primarily by thickness and effectiveness of its heat insulation. If marketed as being suitable for caravan or motor home use, most are designed specifically for that use, but not usually the very big ones. Most big 12/24-volt fridges are originally 230-volt domestic units retrofitted with a 12 volt Danfoss compressor. They use less energy after they are modified, but may lack adequate insulation for truly efficient operation. There are a few truly efficient domestic 230 volt units (e.g. Liebherr, Mielle) that can be run via an inverter, but cost $2500 up to about $5000.

Because cold air falls it escapes every time the door is opened, but as air carries little heat, a door-opening fridge uses only a little more energy than a top-opening fridge. But whilst top-opening fridges are more efficient, anything you want to get out will always be underneath everything else. (This is due to Murphy's Law of Selective Migration). Also, door opening fridges use a great deal of energy unless their door seals work 100%.

A further major difference relates to whether the unit runs only on electricity or, as in the often called 'three-way' units, run from gas whilst camping, from 12-volts from the vehicle supply whilst driving, and from 230 volts when available. The two types are substantially different and which you choose is best influenced by where you mostly stay at night.

Electric-only fridges are effective and convenient but those over 220 or so litres are only really practicable if you have access to mains power most nights, or have a great deal of roof area for six or more 130 watt solar modules and ample battery capacity. Running one from solar is however technically feasible.

A 70-100 litre top opening fridge will run from two 100 watt solar modules, and a 250 amp hour battery will keep one going for two to three days with no solar input.  A 120-litre door-opening electric fridge needs three 120 watt solar modules. A 180-litre upward fridge needs at least six, and a 300-litre unit need eight. With any electric fridge it's advisable to have a back-up generator, or be prepared to waste food from time to time.

Fridges can alternatively be run from a generator (ideally by charging a battery bank), but is costly. Alternator charging assists but only if driving a few hours each day.

Most small to medium-sized electric fridges intended for RV use are designed specifically for that. They use about one-third of the electrical energy of domestic units of similar capacity.

Three-way Fridges

Long time RV owners are likely to have had bad experiences with earlier versions of these fridges, but current  such fridges work very well indeed.

These fridges run from gas whilst camping (or from 230-volts when available). Whilst driving they run off the vehicle's 12 volt system (never on gas). They draw a lot of power whilst doing, and whilst this is within the capacity of the vehicle's alternator, they draw too much (from 12-25 amps) to run them from solar. A 180-litre unit typically uses a third of a kilo of gas each day.

These fridges are designed to work over limited and defined ranges of ambient temperature. Only those specifically designated as having a 'Climate Class T' rating can be relied upon to work satisfactorily in the very high ambient temperatures experienced in the north and northwest of Australia.

Climate Class Ratings

Many fridges sold in Australia are marketed as 'tropicalised'. Whilst this is a reasonable description of their design and construction, the term 'tropicalised' (originated by Dometic in the early 1990s, following an update to all local models) can unintentionally mislead. This is because the increasingly accepted European Union (CEN) fridge performance standard includes so-called 'Climate Classes' in which the most stringent is Climate Class T. And that 'T' stands for Tropical. A 'tropicalised' fridge is thus not the same thing as Climate Class T.

There are four (CEN Standard) Climate Class ratings. Fridges rated 'SN', and 'N' (which stands for Sub Normal, and Normal respectively) are designed and rated to work up to 32 degrees C.

Fridges marked 'ST', (which stands for Sub Tropical) are designed and rated to work up to 36 degrees C. Those marked 'T' (which stands for Tropical) are designed and rated to work up to 43 degrees C. They lose some cooling in ambients below 18 degrees C. This latter effect only related to the depth of freezing.

A correctly installed fridge Climate Class rated fridge can be relied upon to work satisfactorily up to the highest ambient temperature for which it is rated. But once past that temperature, cooling performance is likely to drop off. If, for example, you have a Climate Class SN or N fridge (i.e. designed for 32 degrees maximum) if it's 42 degrees outside, freezing may be affected. Further, gas fridges (surprisingly perhaps) will lose some performance at low ambient temperatures. The 'SN' unit (for example is designed to operate at ambient temperatures below 14 degrees C, the others, below 18 degrees C. It affects only the depth of freezing. Few users are even aware of it.

If planning to be very hot places, use a 'T-rated' unit. I have yet to encounter anyone using a correctly installed 'T-rated' fridge who was not satisfied with its tropical heat performance.

The European Union Climate Class ratings are not obligatory in Australia. Dometic and Chescold market 'Climate Class' rated fridges (the actual rating is noted on the second-last line on their compliance plate).  Dometic's 'T-rated units are (in 2012) the  RM 2453, RM 2553, and RM 4601. The company claims only that its other fridges are 'tropicalised'.

Outside the EU Standards, it is close to impossible to obtain any reliable guide to probable fridge performance.

There are vague de facto 'standards', but are really only of value for comparative purposes. Individual fridge performance is affected by so many variables, particularly installation, but also of usage, that it's all but impossible to give typical data. A seller who seems evasive about this is probably being more honest than those who make specific claims!

Approximate Consumption

As a very rough guide, 40-110 litre chest opening electric units such as Engel and Waeco etc draw three to four amps. Small door-opening units such as Waeco, Vitrifrigo, Frostbite) draw three and a half to six amps. Big purpose-made fridges such as Vitrifrigo and Frostbite draw from six to eight amps. Big converted domestic fridges (BP, Fisher Paykel etc) draw seven to ten amps.

But knowing the steady state draw does not assist a great deal. This because most fridges maintain temperature by cycling on and off. Their daily draw is thus related also to the ratio of their on/off time. It is far from uncommon to find one that draws more current when on - but is on less often (due perhaps to better insulation) and thus uses less energy per day. The only meaningful data is thus the daily energy draw.

There is also a trend to variable speed compressors. These run constantly, adjusting speed to compensate for internal and external temperature changes.

Three-way fridges draw a great deal more energy when on their electrical mode of use. Small-medium sized three-way fridges draw twelve to fifteen amps at 12 volts. Those over 170 litres or so draw fifteen to twenty amps at 12 volts. The 300-litre plus units may draw up to twenty-five amps at 12 volts. That's a lot!

Unrealistic Expectations

Most people know pretty much what to expect of a fridge, but some seriously complain their fridge cannot bring a warm carton of beer down to Australian drinking temperature in an hour or two! No normal domestic or RV can do this either. When you buy beer always obtain a cold carton and put it straight into the fridge. Likewise no normal fridge has a chance of quickly freezing an ever-growing stack of newly caught fish. If you attempt this the fridge/freezer will draw power continuously - instead of cycling on and off - and energy consumption will double or even triple. Yet the fridge will still not freeze quickly.

A fridge that is correctly installed and reasonably used will perform as its maker intended. But if that design  intention was for an ambient not exceeding 32 degrees, it is unreasonable to complain if it does not cool adequately when beyond that temperature. Marketing concentrates on what things can do, not what they cannot do, nevertheless a fridge's designed working temperature range is usually disclosed in vendors' technical data, but not necessarily in their promotional literature.

Installation

Many fridges do not perform as well as had been hoped but almost always because they are not installed as their makers' specified. This is sadly true of many so-called professional installations. Some are dreadful. Because of this, almost all RV fridges can be made to work better. Some dramatically so. The changes required may cost next to nothing - and you can usually do it yourself. But in some extreme cases, a total re-installation may be the only possible solution. I quote an example or two at the end of this article.

Even if you feel your fridge is cooling things well, it's still worth seeing if you can improve the installation as this reduces energy consumption. In one example a couple, who had a fridge that appeared to use more energy than reasonable, had planned to increase solar capacity and add a third battery. But most of the existing power was wasted in heat losses in a grossly too thin feed cable. Instead of the several hundred dollars and a fair bit if work, all that was needed was ten dollars spent on heavier cable. That not only resulted in the fridge working better, but they then had power to spare.

The Essentials

The fridge must be out of direct sunlight. This may seem blindingly obvious, but I met one user who had his fridge outside his caravan in full Broome summer sun. He bitterly complained to anyone who'd listen: "my b..y mongrel Electrolux won't keep my %#@^& beer cold." And he would listen to nobody who tried to explain why.

The fridge must be adequately ventilated. This is absolutely vital for three-way fridges and for electric fridges running on solar. There must be a cool air entry at or around floor level. There must be a hot air exit, preferably at roof level. Baffles are necessary to direct the incoming cool air upward and through the fridge's cooling fins - rather than around them. It is equally necessary to channel the rising air so no pockets of hot air can be trapped.

Figures 1, 2 and 3 show the main requirements. The bottom (cool air) vent can enter via the side of the vehicle, or through the floor (but not if the entry is above or to the rear of the vehicle's exhaust pipe) and it must allow the cool air to enter below the fridge's lowest cooling fin. It is necessary to make some provision for blanking off this air intake to prevent the ingress of dust and sand whilst travelling.

Ideally, the rising warm air should be vented through the roof. Dometic markets an excellent vent made for this purpose. Where a roof vent is not possible, a side vent must be located well above the top of the fridge's cooling fins (Figure 3).

As mentioned above, the baffles direct the incoming cool air over the cooling fins and only over the cooling fins. These baffles can be made out of rigid foam (possibly covered in aluminium foil) aluminium sheet, plywood or whatever. They must come to within a few millimetres of the fridges cooling fins.

Figures 4 and 5 show various combinations of how NOT to do it. Three-way fridges need to be level. Some fridges can tolerate six degrees tilt, others only three degrees. Electric fridges are less sensitive (and Engel fridges in particular) but few actually welcome working on a tilt.

Figs 1-3:

Here's how to install a fridge correctly.

Cold air enters from below the cooling fins and is directed by baffles that must extend to a few millimetres of the fridge's cooling fins, Hot air must be channelled to the exit vent.

In all cases an extractor fan will further enhance performance.

 

Fig 4:

Here the lack of baffles causes the incoming air to bypass the cooling fins. The side (exit) vent is far too low. Worsening this, a pocket of hot air will be trapped above it.

 


Fig 5:

The baffles are too short to be effective (or are non-existent). Rising warm air is trapped in the dead space above the fridge itself. This air must be channelled (Fig 2) or the dead space blanked off (Fig 1).

 

Problems on Twelve Volts

Most electric fridge problems are due to inadequate sized 12 volt wiring. An absolute give-away is if the fridge works well on 230-volts but not on 12 volts. This fault can also be caused or worsened by a faulty fuse holder and particularly by cigarette lighter plugs. Never use the latter.

You absolutely must have an adequate voltage across that fridge and this calls for surprisingly heavy cable. The minimum is spelled out in Table 1. Ideally use at least one size heavier.

The only way of knowing if the voltage is correct is to measure it, or have an electrician measure it. Make sure the fridge is switched on and put something warm inside to make sure it is also cycled 'on', Measure the voltage directly across the battery that drives it, then check the voltage directly across the fridge - as close to its terminals as possible.

There should be no more than about 0.30 of a volt difference: ideally only 0.15 - 0.2 volt. Many fridges have close to 1.0 volt drop! Fixing this makes an extraordinary difference.  Replace the cable using one at least as heavy as in Table 1, or run a second cable in parallel.

Table 1: Cable sizes for 12-volt fridge and other circuits.

The sizes, in square millimetres, (and, in brackets, AWG/B&S) cause less than 3% voltage drop at the maximum distance in each range. In the above Table, 4 mm auto cable can replace 1.5 sq. mm, and 6.0 mm auto cable can replace any size up to 4.0 sq. mm.

Auto Cable Trap

You also (especially) need to know about a huge trap that even catches out professionals.

Most non-US appliance makers specify cable by the cross-sectional area of the copper conductor (that's the bit that carries the current). They quote this in square millimetres. Incredibly, auto cable makers rate their product by cable diameter - and not the area of copper bit that carries the current - but across the insulation as well. And that varies considerably from maker to maker.

Fridge makers specify the size of the copper conductor (usually in sq. mm) they want you to use. But auto cable makers tell you only the size hole their cable will pass through! This is a very serious and widespread problem because the actual 'numbers' can be identical.

Most fridge makers assume the fridge will be within three metres of the battery and specify 4.00 sq. mm or 6.00 sq. mm cable accordingly. The most commonly sold auto cable is also 4.0 and 6.0 mm. But that 4.0 mm auto cable is typically only 1.8 sq. mm (some is 2.0 sq. mm). It may even be as small as 1.25 sq. mm. Six mm auto cable is not as bad - but that's still typically less than 4.6 sq. mm.

As a result countless thousands of such fridges are connected via cable that is less than half the specified and necessary size. Most fridges that disappoint on 12-volts do so for this cause. No electric fridge wired this way can or will ever work properly - particularly in hot climates.

A further trap are cable 'ratings' such as '30-amp', '50-amp' etc. These indicate only the current it can carry before it melts. It gives some indication of the fuse or circuit breaker needed to protect it but tells you nothing about voltage drop over distance.

So - how to tell the 'true size' (that is area in square millimetres) of auto cable? It's useless asking the vendor - few are even aware it's an issue. But some makers include it in small print on the side of the drum.

Table 1 shows the minimum size cable (in sq. mm and also the often available AWG sizes) that needs to be used. It's not feasible to supply this data in auto cable sizes because they vary hugely from maker to maker. The only generally applicable rule is that 6.0 mm auto cable (typically 4.59 sq. mm) can be safely substituted for 4.0 mm auto cable. And 8 mm auto cable is between 7-8 sq. mm.

Table 1 assumes you are using twin cable (that is one positive and one negative lead). It shows the cable sizes you need for different current draw and lengths of cable.

Measuring the distance is easy - work out the shortest practicable route. Run a length of string along that route and measure it. Determining current draw is a bit harder. It is usually shown on the appliance rating plate, in the technical manual, or in the promotional literature. You will see something like (say) 4.5 amps. This is often abbreviated to (say) 4.5 A.

(A better and virtually exact way of obtaining cable size is shown in the Article headed Cabling on this for any length, cable size and current flow).

As a very rough guide, 40-110 litre chest opening electric units such as Engel and Waeco etc draw three to four amps. Small door-opening units such as Waeco, Vitrifrigo, Frostbite) draw three and a half to six amps. Big purpose-made fridges such as Vitrifrigo and Frostbite draw from six to eight amps. Big converted domestic fridges (BP, Fisher Paykel etc) draw seven to ten amps.

Three-way fridges draw a great deal more. Small-medium sized three-way fridges draw twelve to fifteen amps. Those over 170 litres or so draw fifteen to twenty amps. The 300-litre plus units may draw up to twenty five amps. That's a lot! If you cannot determine the exact draw, use the highest of the relevant numbers I've quoted above.

Always use cable that is too heavy rather than too light. If it's heavier than needed you've spent a few dollars more than really needed but the fridge will work even better for it. If the cable is too light it can never work correctly.
For caravan-located electric fridges consider locating the auxiliary battery as close as possible to the fridge and install a dc-dc alternator charger close to that fridge. See the Article - Dc-dc Charging that will be posted on this site on or around 9th September 2012.

Other Problems

Door seals tend to leak cold air after five or so years. You can buy replacements from some hardware stores and also from Clarke Rubber.

Most fridges benefit from extra heat insulation, particularly at the top. But check before doing this because some fridges (such as the Autofridge) are intended to dissipate heat from their sides. So don't add insulation to these - except to the lid or door!

Whilst less common a problem, a three-way fridge that works adequately on 230 volts but not on gas may have the incorrect jet size - or need adjusting. This can also because a 'grey-imported' fridge may be set up for a different type of gas from that used in Australia. Here, you will need expert advice as some may not be legally usable here because of excess emission of carbon monoxide other than by just incorrect jetting.

Energy Consumption

Amongst other factors, a fridge's energy consumption depends on the amount of heat that must be pumped out. Things that affect this include:

The external temperature - the hotter the day, the more energy the fridge will use. That is typically 5% more for every degree C above 25 degrees C. 

The internal temperature setting. The colder you set the thermostat, the more energy the fridge will use. The optimum is about 4 degrees C and most fridges use about 5% more energy for every degree below that. In particular, don't set the freezer temperature lower than the -18 degrees C really needed (but some settle for - 14 degrees C.
Don't put anything hot into the fridge - let it cool down first. Conserve energy by dithering frozen food in the fridge section. Buy goods that are as cold as possible, and put them into the fridge before they begin to warm up. If you have to buy warm beer, let it cool outside overnight.

Open the fridge as little as possible.

Keep the fridge full. This reduces the amount of cold air that falls out when the fridge door is opened. Fill empty spaces with rectangular plastic vessels full of water. (This can make a surprising difference). Why rectangular? They pack more closely, so there's less free air space.

It is often a big job, but a really good way to assess fridge performance is to remove it from the RV and check its performance whilst it is free standing in a cool garage.

Worst Case Examples

The very worst was a $500,000 motor home that had a 450 litre fridge located in a totally unventilated thick timber enclosure only 25 mm larger all round and top than the fridge. No heat whatever could escape. The unfortunate fridge cycled on 24/7 and could barely cool - let alone freeze. It had been designed to dissipate heat from its sides and rear - and ran literally too hot to touch. This unit had a similarly installed 120 litre chest freezer in an underneath locker. Fixing it involved major carpentry in a virtually new unit. The maker (no longer in business) refused point-blank, back then, to accept any responsibility.

Another was a 110 volt electric fridge in a caravan, powered by a battery under the bonnet of the towing vehicle and connected via some 12 metres of 1.5 sq. mm house lighting cable. The DIY owner thought and maintained he had done a really good job ("it's housing cable - just has to be good") despite only some 10.5 volts across that fridge. He had gone from one fridge repairer to another - totally convinced the fridge was faulty and rejecting all advice that the fault was that grossly inadequate cable.

Summary

You really need to have the right fridge, particularly if travelling up north. There, even winter days can exceed 30 degrees C, and summer days well over 40 degrees C. It also stays hot at night.

 But even if you don't have the ideal fridge, you can almost always improve the cooling and reduce the energy consumption by making the changes I've recommended.

Think hard before running a large electric fridge from solar modules. It's technically feasible but you'll need a lot of solar modules, a lot of roof space to house them, a heap of batteries and, for peace of mind, a back-up generator.
Forget everything you've heard or experienced about deficiencies of such and such a fridge. As long as it comes from any of the major established makers and, if it's a three-way unit has the appropriate 'Climate Class' for its proposed use, any cooling problems are almost certainly due to faulty installation. Remember that a fridge described as 'tropicalised' does not necessarily meet the requirements of Climate Class T.

Further information on both three-way and electric-only fridges is contained in Caravan & Motorhome Electrics (to be published around November 2012). That book will even show how to build your own fridge that will leave the commercial units for dead in terms of cooling performance and economy. There's also a lot of information about running them from solar in my now 3rd Edition Solar That Really Works.

 
Collyn's books are available from the suppliers listed on the Where to Buy section of this site or directly from the publisher (Caravan & Motorhome Books).
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