Words and Images by: Collyn Rivers N8054

This and my next few Tech Notes attempts to answer the most common queries that I receive from CMCA members, and from readers of my articles and books. They cover a wide range of issues. This one relates to fridges. I have précised the queries to save editorial space. Feedback and (general) questions are welcomed but I cannot discuss any named products or services.

Q: My own electric fridge does not work nearly as well I had hoped for – having read its maker’s claims. I was talking about this at a CMCA happy hour recently. Many there claimed their own were much the same. How do fridge makers get away with it?

A: Over a third of the queries I get from readers of my books and articles concern fridge cooling and their energy use. In many such cases the fridges are good products that have been installed in ways that show almost cynical disregard for their makers’ usually detailed instructions. Most such problems so introduced can usually be fixed. In some instances it will require carpentry and/or a hole or two cut in the side of the RV to enable vents to allow cool air to enter and hot air to escape. It almost always requires the 12-volt supply cable to be replaced by one of adequate size.

Q: Why does a fridge need ventilation? Cannot it just ‘make more cold’ in much the same way you turn a fire up when you need more warmth?

A: Many people assume a fridge is a sort of electric freezer in reverse. It is not. A fridge is a pump that shifts heat from where you don’t want it – to where it does not matter. In this case that must be to outside the RV. To do so it needs a cool air inlet low down, and a warm air exit that is least 100 mm above the fridge’s highest cooling fin.

Most installation problems are due to this not being understood. I have seen a plus $500,000 motorhome that had the main (600-litre) fridge totally boxed in, plus twin 275-litre chest fridge freezers in air sealed non-insulated lockers exposed to the sun. None would cool their contents below 10° C (let alone remotely freeze them) except on ultra-cold days.

Q: Is there any quick, easy and reliable way to see whether there really is an installation problem rather than a fridge fault?

A: This can readily be done with a compact fridge. Take it out, place it somewhere (such as a cool garage floor) that has an ambient temperature of 20-25° C. Connect it to a fully-charged battery by a metre or two of 4.0 square mm (or 10 B&S or AWG) cable, and monitor it over 24 hours. It’s odds on it will work just fine. If not, the fridge is faulty.

This is less feasible with larger fridges as most are over 600 mm wide despite RVs having less than 600 mm wide doors. They are built in during construction and may need the door frame to be removed to allow the clearance required if the fridge ever needs to be replaced!

Q: Can I improve cooling by increasing cable size alone?

A: If the fridge is physically installed so as to provide ventilation, as per the two bottom examples shown in the accompanying sketches (above) increasing cable size will assist. Where the previous cable is seriously too small this can make a dramatic difference.

If the fridge is not ventilated, however, it is likely to cool to lower temperatures, but energy usage will soar because the heat being pumped out has nowhere to escape  to, thus increasing the temperature around the fridge. This in turn necessitates it working even harder – and increasing that ambient temperature – and so on. This is known technically as a ‘positive feedback loop’.

Q: A dealer has told me that European fridge specifications show they draw less power than those made in Australia. Is this really true?

A: Like is not being compared with like. The local AS/NZS 4474.1 specification requires fridges to be tested at an ambient temperature of 32° C. Europe uses the ISO standard of 25° C for all fridges – except those ‘T’ (Tropical) rated. These are tested at 32° C.

Q: How many amps are drawn by a typical 12-volt RV electric fridge each day?

A: Amperage is a measure of the instantaneous current flowing. That which you need to know is the amount over time, in this case ‘amp hours drawn each day’. This depends very much on the ambient temperature, fridge volume, quality and when made; and also by how well or otherwise it is installed.

Assuming competent installation, in temperate climates (pre-2013) 12-volt fridges up to 120 litres typically draw about  1.0 amp hour per litre a day. Above that 120-litre volume the draw per litre reduces progressively to about 0.7 amp hour per litre a day. A correctly installed 120-litre fridge is thus likely to draw 70-100 amp hours a day (840-1200 watt hours a day). Following an increasing need for reducing energy usage generally, some top-quality post-2013 fridges have constantly running variable speed compressors and are likely to draw 25 percent or so less.

Incompetently installed fridges may draw more than the above. Fridge-freezers may be unable to attain the desired low temperature.

All fridges draw about five percent more energy for every 1° C increase in ambient temperature (above their rated temperature – see answer above). They do likewise for each degree C below that to which they are set. Do not cool below 4° C, nor freeze below -18° C.

Q: I’d like to have two fridges – each of about 200 litres. A friend recollects your suggesting this is not a good idea – but forgets why. Is this really so?

A: It is not a good idea because surface to area ratio decreases with increasing  volume – hence the heat losses become proportionally smaller. Assuming the 200-litre fridges your friend has in mind – a pair (of typical shape) fridges will have a surface area of about six square metres. A single 400-litre fridge, however, will have a surface area of about 3.7 square metres and thus incur far less heat loss. This is why larger fridges draw proportionately far less.

Q: I’m considering buying a threeway (230/12-volt LP gas) fridge but hear totally conflicting reports. Are they worth considering?

A: Their major benefit is that they slash electrical energy usage. They will meet their specifications – but not necessarily your expectations unless you buy one that is of the ambient temperature Climate Class required. I recommend only ST (18° C to 36° C) or T (18° C to 43° C). It is essential that they are installed fully and only as their makers spell out. This is typically as the lower pics on the previous page.

Q: Is any type of battery particularly suitable for driving an electric fridge?

A: Theoretically, LiFePO4 batteries are the logical choice. This is because, while  driving light loads (and a fridge in this context is a tiny load), their output voltage is likely to remain within 13.1-13.0 volts. This voltage, that is totally safe, is higher than that of fully-charged lead acid, AGM or gel cells (that may then drop to well under 12 volts).

I need to qualify the above as specialised knowledge is required to charge LiFePO4 batteries safely in RVs. Either wait until ‘drop in’ direct replacements are more widely available (a few now are) or only if you know people (or a company) that know how to do it.

This subject, including that of Climate Class, is covered in depth at: http://caravanandmotorhomebooks.com/makingcaravan-fridges-work-as-claimed/ and even more so in my published books. These are directly available from CMCA NHQ.

Category: Technology
Written: Tue 01 Dec 2015
Printed: December, 2015
Published By: