Making fridges work

In the second part of this two-part feature, Collyn Rivers shows how to install fridges correctly, and how to fix those that disappoint.

Whilst many fridges do not perform as well as had been hoped this is rarely the fault of the fridge. In nearly always because they are not installed as their makers' had intended. Because of this, most fridges can be made to work better. Some can be made to work dramatically better. The changes required may cost next to nothing - and you can usually do the fixing yourself. At worst you are up for a roof vent and/or a few metres of proper cable.

Even if your fridge is cooling things well, it's still often worth seeing if you can improve the installation as this reduces energy consumption. I recently advised a couple who had a fridge that appeared to use more energy than reasonable: they were planning to increase solar capacity and add a third battery. But most of the existing power was wasted in heating up a much too thin fridge cable. Instead of the better part of $1000, all that was needed was ten dollars spent on heavier cable. Then, not only did the fridge work fine; they 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, including myself, who tried to tell him 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 directly above the exhaust pipe) and it must allow the cool air to enter below the fridge's lowest cooling fin. It is desirable 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

What we'll look at now is how to make three-way and electric-only fridges work properly on 12 volts. The flip answer is simply do what the maker requires - but this is not always sufficiently spelled out. And, as you will see, even if it is, installers can go horribly wrong.

Many fridge problems are built in by whoever fitted the fridge in the first place. This is particularly true of electric fridges. An absolute give-away is if the fridge works well on 240-volts but not on 12 volts. This fault is almost always due to inadequate wiring and/or faulty connectors.

You absolutely must have an adequate voltage across that fridge and this calls for surprisingly heavy cable. Just how heavy is spelled out in Table 1.

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 0.2 volt. Many fridges have close to 1.0 volt drop! Fixing this makes an extraordinary difference. To do this, replace the cable using one at least as heavy as in Table 1, or run a second similar-size cable in parallel.

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

The sizes, in square millimetres, (and, in brackets, AWG) 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

You also (especially) need to know about a huge trap that event 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 however, auto cable makers rate their product by cable diameter - and not just across the copper bit that carries the current - but across the insulation as well. And that varies considerably from maker to maker.

In other words fridge makers specify the size of the copper conductor they want you to use. But auto cable makers tell you only what size hole their cable will pass through! This is a very serious and widespread problem because the actual 'numbers' that both use 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 cable is typically 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 only 4.59 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 diddly-squat 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. 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.

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.

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.

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 some branches of Clarke Rubber.

Most fridges benefit from extra heat insulation, particularly at the top. But check before doing planning 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 240-volts but not on gas may have the incorrect jet size - or need adjusting. I have heard but cannot confirm that this can be 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 attention.

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.

• 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 really needed.

• Don't put anything hot into the fridge - let it cool down first. Conserve energy by dethawing 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.

Summary

You really need to have the right fridge, particularly if travelling up north. There, even winter days can exceed 30 degrees C. 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 Motorhome Electrics. That book even shows you 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 new book 'Solar That Really Works - Motorhome Edition (there's also a Caravan Edition so you need to spell out which you need).