So you’ve made the decision to turn that old icebox into true boat refrigeration… or have decided that your current ‘ice tray’ bin-style freezer is just a little too small.
Can a spillover system work for you? Well… it depends…
Here are some things to consider to help you make your decision.
First… what is a Spillover system?
A ‘spillover’ system is a refrigeration compartment box that is divided into two separate sections with a partition dividing the box into a freezer section and a refrigeration section. Typically the box is designed to provide a ratio of 1/3rd freezer space to 2/3 refrigerator space separated by a well insulated partition.
The evaporator plate for the system is installed in the freezer section, and the system is designed to have the compressor operate based on freezer temperature requirements .
How does it work?
The system is cooled by air spilt over from the freezer section into the refrigeration section. It is important that the spillover divider is insulated – typically to R10 (that’s the equivalent of 2 layers of ‘blue board’ extruded polystyrene foam board (XPS), or about 2”/4cm of thickness). The divider should be moisture penetration proof, and sealed with fiberglass, plastic or formica.
Practically, some spillover systems just have air passages/holes in the bottom of the divider between the fridge and freezer. Often the air passages are made from PVC tubing, caulked into place. This design depends on natural convection to keep air flowing. How many holes should the divider have? It depends. A good test is determining the flow needed to keep the ice cream hard when the fridge reaches 32-40° F/0-4°C.
Depending on the box shape and more importantly how the box is loaded with food and filled, some boxes just can't get good convection air movement and need a fan to enable air flow for higher efficiency and temperature uniformity. The fans are thermostatically ‘triggered’ when the refrigeration box demands it.
The fan should be small and located midway or higher up on the divider. With a fan, the air passage hole should be about equal size to the fan and be located at the top of the divider to provide circulation between sections.
If I use fans, which way should they blow?
Intuitively, since the evaporator plate is located in the freezer section, it makes sense to use the fan to force that colder air into the refrigeration section to cool it. However, if the air blows from the freezer to the fridge, ice may build up at the exchange hole side where the air should flow from the fridge to the freezer. The fan(s) may suffer freeze up problems and stop working or work slowly (but this isn't always a problem).
Many systems recommend keeping the fan on the refrigeration side to prevent fan freeze up because the fan never encounters temperatures below freezing– although this positioning may seem counter-intuitive.
The advantages and disadvantages of a Spillover
The biggest advantage of a spillover is that you can conquer both your refrigeration and freezer needs in one compartment…. and importantly from one compressor. This can make it more economical to install and a great option for smaller boats.
As for disadvantages? When compared to separate refrigeration and freezer compartments, temperatures in a spillover may be more erratic making the system less energy efficient.Generally freezer temperature performance will range from 10-20° F/-6-12°C (not a true deep freeze).
How much fridge and freezer space do you need?
While often dictated by refrigeration compartment configuration size, food storage needs truly depend on personal preferences and habits.
Do you spend weekends or weeks aboard?Are fresh foods essential, or are handy dry good staples good for you?Are you at dock or close to provisioners? Or are you remote and needing your food stores to keep for weeks? You will have to consider these factors.
A cruising couple on a 40-foot boat probably would want a 5- or 6-cubic-foot refrigerator and a 3-cubic-foot freezer.
Consider Spillover maintenance needs
Spillover systems can cause faster freezer plate frost up than ‘freezer only’ compartments.
Why? The spillover fan blows the air between the refrigeration and freezer compartments. This air circulation introduces warm air from the refrigeration compartment onto the ‘ice cold’ evaporator plate causing it to frost up. A stand alone freezer is opened less frequently than a 'combined' spillover fridge/freezer, and therefore does not require defrosting as often.
How to avoid counter top condensation
Spillover boxes that have top-opening hatches can be susceptible to loss of cold air and subsequent inflow of replacement air if there are weak lid seals (gaskets) accelerated by a forced-air fan circulation system.
Make certain that the spillover fan produces no more than a gentle breeze to ensure that pressure is not built up, forcing air out of a weak seal and drawing cabin air in to replace it through a different location.
If there is any weak seal on a front-opening door, or an unplugged drain or access hole, then the heavier cold air will escape and condensation can result on the counter.
Seven tips for best Spillover performance
1. Insulate for optimal energy efficiency! We did tests on 2 popular insulating materials and here's the Cruiser's Forum link for more information http://www.cruisersforum.com/forums/f115/polyiso-vs-blueboad-xps-test-193232.html
2. Don’t forget to ensure the compartment lid is well insulated. See this video for tips https://www.youtube.com/watch?v=F8SAku1HjXY
3. Keep your fan size small. A little air movement is good, but too much can be bad causing your system to run excessively, cause condensation and even freeze food stuffs that should not be frozen.
4. Check to ensure your fan is operating. Small fans have a limited lifespan and will need replacement. (carry spares)
5. Keep air ducts in the separation panel unblocked by food – air needs to circulate even when the freezer or refrigeration is full.
6. Cut a piece of plastic, create a lip and mount/hang it on the evaporator plate across from the spillover wall holes to protect it and prevent ice blockage that can prevent door sealing.
7. Don’t let frost build to more than 1/4 inch/1cm on the plate – defrost regularly to prevent efficient loss of heat moving into the plate and being removed from the box.