THREE WAY TO COMPRESSOR FRIDGE CONVERSION – PART 2

I would suggest reading Part 1 first (if you have not done so) as it explains why this madness commenced!

First off, this post is not a set of instructions, it's a write up of how I achieved what I set out to achieve. The process involves work on both 240 (mains) voltage and 12 volts. It also involves LPG gas connections which NEED to be tested by a registered engineer for safety. DO NOT mess with any of this unless you know what you are doing. It is perfectly feasible that you may injure/maim/kill yourself, your family or anyone within the blast radius. It's no joke so take the warning seriously.

Secondly, I started the work on the fridge with the full intention of photographing every stage. That never happened so all the photos are from after the installation was completed. Sorry!

With a box of bits and a rough plan in my head, it was time for work to commence! After disconnecting the gas, mains power, isolating the solar panel and disconnecting the battery, it was time to take out the fridge. As with everything someone had already produced a YouTube video of the process on a similar fridge/'van combo which did prove to be useful.

The first step is to remove both external vent grills from the outside of the caravan. This exposes the rear of the fridge.

In the lower panel, you disconnect both the 12v (1) and 240v (2) lines and also undo the gas coupling (3).

Thetford 3 way fridge rear connections

Back inside the caravan, I then removed the fixing screws from inside the fridge. On this model, there are 2 on each side accessed from inside the fridge and hidden under trim caps. Our fridge had been in and out so many times that they were bloody difficult to get out, purely because they were not unscrewing, just spinning around. After a bit of swearing and the use of a highly magnetised screwdriver, they did eventually come out.

With everything undone, the fridge then eased straight outwards, a lot easier than I had expected! I was hoping to get the fridge out of the caravan and onto a workbench but it was pretty obvious that it was not going to go through the door! This meant that all work on the fridge had to be completed within the 'van. It was tight, but 'doable'.

With the freezer compartment door removed (2 screws), it was time to make a hole for the evaporator pipework to exit the back of the freezer compartment. I had poked, prodded and tapped the back of that compartment numerous times and was convinced it was plastic. But, no such luck dear friends! Top, bottom and sides are plastic, but the back is a sheet of metal. Deep joy! With the limited access available, it took the best part of 2 hours to cut a 30mm(ish) square hole in the back with one of those multi-cutter tools. I got there, but it was a PITA!

Next. Mount the evaporator.

The installation guide (which I did actually read) indicates that all surfaces should be at least 10mm from any wall. This is all well and good, but the 'kit' doesn't contain any spacers to achieve this. I did actually suss this out beforehand so had drawn some up in Autodesk Fusion 360 and printed them on my 3D printer beforehand. I could have probably got away with some of those plastic cabinet blocks, but I was able to produce rear blocks that allowed the evaporator to clip in, only requiring screws at the front.

For those who are interested or perhaps looking for a way of mounting the same evaporator, you can download the 3D .stl files from Thingiverse.

Evaporator clips
Evaporator clips

The printed blocks were bonded to the roof of the freezer compartment with a gel type super glue. This literally 'welds' the plastics together making a very strong bond. I allowed the glue to 'go off' for a couple of hours before mounting the evaporator plate. All the pipework needed to be carefully threaded through the hole I made in the back, together with the capillary pipe for the thermostat sensor. I didn't think about that pipe until I had actually mounted the plate which caused a fair bit of swearing!

Dometic VD-07 evaporator plate mounted inside a Thetford N112 fridge.
Dometic VD-07 evaporator plate mounted inside a Thetford N112 fridge.

With the evaporator mounted, it was time to get the thermostat control in place. I was slightly disappointed with the supplied thermostat. It's about twice the size it needs to be, has a stupidly large bright blue control knob and in my opinion looks like a cheap piece of tat. It can be mounted within the fridge or remotely (up to the length of the capillary sensor pipe). Once I had removed the fridge, it became apparent that I could mount the 'workings' of the supplied thermostat on the existing fridge control panel. In fact, because the same panel is used on multiple fridge models, I was able to use a pre-drilled hole that was covered by a stick on facia. The thermostat components also include an indicator LED which flashes error codes (should they occur), so I did have to drill one hole. To finish things off, I also found a more suitable knob which didn't look like it would be more at home on a child's toy.

The thermostat supplied with the Dometic Cold Machine
The thermostat supplied with the Dometic Cold Machine
The same thermostat mounted in the fridge control panel (right of pic).
The same thermostat mounted in the fridge control panel (right of pic).

With these bits in place, cables routed to the back and an access hole cut through to the compressor location I was able to get the fridge back in place. Incidentally, I did also fix the gas ignition problem beforehand. When our friendly caravan workshop replaced the burner, they had nicked the ignitor cable, so it was sparking in completely the wrong place. A short length of heat-shrink sleeving sorted this. Bonus!

Manoeuvring the fridge back in place was a two-person job with myself man-handling the fridge and Sal, my wife, feeding pipework and cables through the vent apertures on the outside of the caravan.

Next up, mounting the compressor.

This has to be reasonably close to the fridge. The pipework is about 2 metres long but you cannot bend it through tight corners. The obvious place was under the front seating/bed right next to the fridge. This would provide enough airflow and give me a decent amount of working room. There's also a fair bit of other 'gubbins' under there (such as the solar charge controller, motor-mover) so it's not really used for storage. An added bonus is the battery compartment also enters the 'van in the same location so accessing 12 volts for the compressor supply was simple.

Dometic CU55 Compressor mounted under caravan bed/seating.
Dometic CU55 Compressor mounted under caravan bed/seating.

The next step was to connect up the self-sealing connectors between the evaporator and compressor. There are all sorts of warnings about this. You don't want to cause the gas to escape! I just took it slowly and followed the instructions to the tee. With the pipework done, both the 12 and 240v was connected up and the thermostat/warning indicator/compressor connections made. These are just spade connectors so simple enough.

At this point, with nothing permanently fixed, I connected the 12 volts up and gave the system a test. Whilst I was pretty confident everything had been connected correctly, there was that inevitable spark of relief when everything powered up and started working! The evaporator was cold in a matter of minutes and (thankfully) the 'noise' produced by the compressor was pretty minimal. Certainly not loud enough to disturb sleep.

Everything was bolted/screwed/cable tied and the hole cut in the back of the fridge filled with some low-expansion fire-retardant foam. Fire retardant was chosen because of the proximity of the hole to the gas burner flue. I am sure that we will never need it again, but it's one of those 'just in case' scenarios.

Expanding foam in 'the hole'. It's not pretty, but it's not exactly on show!
Expanding foam in 'the hole'. It's not pretty, but it's not exactly on show!

I used a combination of gaffa (duct) tape and a plastic plate around the entry point inside the fridge to make sure as little foam as possible found it's way inside. Once again, not that pretty but it's hardly visible behind the evaporator.

That covers the basic installation using the components supplied with the Cold Machine. This was completed a week before we were setting off for a 10 day holiday in Cornwall at the Marazion Caravan Club site. We'd booked one of the 2 available non-electric pitches so were reliant on the 'new' fridge operating correctly. As it happened, our stay coincided with one of the hottest weeks of the year (hottest on record for May) so it was one hell of a test!

On the whole, everything worked as expected but we did experience a few teething issues.

  1. The wide top door shelf could not be used as it was preventing cold air from the evaporator from circulating into the fridge. Not a great issue as we had a spare 'thin' shelf to use in its place.
  2. The freezer door cannot be left in place. We were 'sort of' expecting this, but there was a vain hope that the slab of metal that conducts cooling from the fridge to freezer when operating conventionally may have worked in reverse.
  3. From midday onwards, the side of the caravan where the rear fridge vents are situated was bathed in sunlight. This appeared to have an adverse effect on the accuracy of the thermostat. I am guessing that the small length of capillary pipe exiting the fridge was getting hot, causing the compressor to run almost continuously even though the evaporator was at around -18 degrees!
  4. A combination of this and the overall ambient temperature caused the compressor to get very hot. More airflow was needed.
  5. Overall, we were just not happy with the thermostat. You just didn't know where the hell it needed from one day to the next, it was like stirring a stick in sh*t or playing darts blindfolded.

Having said all that, we never needed our 'emergency' spare leisure battery. Food and drinkies were suitably cold for the whole period. Just a little bit of 'massaging' required to the initial install.

The wayward thermostat came first. This has now been replaced with a digital heat/cold controller. These can be had for less than a tenner from Amazon so are an absolute bargain. I chose to install it in the control panel area of the fridge which did mean pulling it out again but only by about 6 inches so the evaporator pipework didn't need to be disconnected. I left the supplied thermostat in place just in case we need to go back to the original equipment at some point.

Digital heat/cool controller installed next to the original thermostat.
Digital heat/cool controller installed next to the original thermostat.

It did take a little experimentation to find the sweet spot for the controller but at least we were playing on a level and predictable playing field! The temperature sensor is mounted on the evaporator plate with a small bulldog clip (you can see this in the evaporator photo towards the top of the page). There is a school of thought that says it should be insulated and in 'free air' to minimize rapid fluctuations but we found having it in contact with the evaporator (which is the method used for domestic fridges) more predictable. We've currently got the target temperature set to -12 with a slew of 10 degrees. This means the compressor will run until the evaporator reaches -12 at which point the controller turns off the compressor. The evaporator will then start to warm. When it reaches -2 the controller starts up the compressor. Rinse and repeat.

These temperatures will freeze water and maintain ice-cubes within the evaporator 'O'. The fridge temperature is also maintained between 3 and 5 degrees which is within the ideal range. There is some variance depending on ambient temperatures but it is minimal.

I've also had to add an isolator switch to the circuit, allowing the unit to be turned off easily (the previous thermostat also acted as an on-off switch).

To lower the effects of sun raising the temperature of the back of the fridge, I've installed reflective insulation on 5mm ply in the vent apertures. This is held in nicely by the vents themselves and actually fits better than the original fly screens which used to rattle in the wind! All original parts have been retained in case we need to return to 'old style' cooling.

In addition to this, I've also installed a 'hit and miss' vent at each end of the door on the seating/bed plinth, also adding a ply baffle at the side of the compressor to encourage 'in/out' airflow.

Hit and Miss vent installed in seating plinth
Hit and Miss vent installed in seating plinth
Reflective insulation under external vent (I know, it needs a clean!)
Reflective insulation under external vent (I know, it needs a clean!)
Full installation under the seat (key below)
Full installation under the seat (key below)
  1. Dometic CU55 Compressor.
  2. Auto switching mains/battery power supply.
  3. Isolator switch for compressor and heat controller (also contains appropriate supply fusing).
  4. Baffle to encourage in/out airflow.
  5. NASA Marine battery monitor shunt on the rear of the leisure battery box.
  6. Solar charge controller.
  7. Motor-mover control box.
  8. Motor-mover isolator switch.
  9. Picnic blanket (not required to convert a fridge).

This has been an extremely satisfying and successful project. We now only need to use gas for cooking/heating and don't need mains electricity at all. We've effectively removed the reliability issue of the fridge running on gas and are also using more of the 'free' solar generation capacity.

I am sure that many will not see the point in doing this but being able to enjoy locations that do not have mains power vastly increases the value to be had from caravanning.

There is also the environmental aspect. Any reduction in our reliance on mains power and fossil fuel has to be a good thing, no matter how small. Perhaps caravan manufacturers should be working more towards increasing the ‘off-grid’ capabilities of their new models instead of continually increasing overall power demands and installing under-sized solar panels?

It would also be nice to see organisations such as The Caravan and Motorhome Club providing more choice. Instead of having one or two (if any) non-electric pitches, give members the choice of whether they use (and pay for) power on all pitches.

In the future, I may well look at fabricating some form of door for the freezer compartment that doesn't inhibit downward airflow but, at the moment, it's a minor inconvenience.

Leave a Reply