Bear in mind that the degree of warmth and the period of warmth is critical with food. +40degC for 4 days might be enough to make a cheese sandwich walk out of the enclosure on its own.....

The legendary British Rail Sandwich used to <snip>......

Keep in mind that putting condenser coils in an enclosed chamber reduces the efficiency of the cooling system.

Is it cheaper to supply more air conditioning for the room where the waste heat is dumped, is it cheaper to run the refrigeration unit longer at lower efficiency, or is it cheaper to heat food the old fashioned way? Given the cost of making a hot box (which doesn't make people suspicious of its sanitation), I would guess "the old fashioned way" will win out.

Hope you like warm soda and spoiled food. If you contain the condenser in the box the heat build up will lower the efficiency so your refrigeration will no longer cool your food or beverages. A secondary condenser system is needed to maintain the cooling efficiency outside the hot box. May need to switch from one to the other the condensers can be quit hot. If you only want to warm the food and not cook it.

Good subject - I have a concept to use this waste heat to recycle and purify water but do not have the facilities to build it - (needs a plumber) I can see many uses. Any body got a suitable place and the gear to build it - essentially pipework?

I think what is needed is a method to remove the heat from the coils and upgrade it, otherwise the fridge will stop working correctly. This will help no one.

The best way I feel would be a heat pump.....eg a tiny fridge system, to take the heat from the coils (thus cooling them down), upgrade it and place the heat in the hot box, which should be well insulated.....

An easy way to make contact between both systems I feel would be to construct a water cooling box for the fridge coils and for the tiny system to have its cooling element placed at the top of the box to capture the heat and cool the water.....

If the box is not insulated and also made of metal, it should allow the fridge to work normally even when the hot box heating is not required.....

Absolutely to the point (GA). The efficiency of a heat pump is approximately inversely proportional to the temperature differential, so increasing the temperature of the cooling coils will degrade the efficiency of the refrigeration - possibly to the point that the system no longer meets its requirements.

Only if the cooling coil is already at the temperature you require can you sensibly use its output - and you will still need to be careful that any physical modification you make does not cause a temperature increase.

GA from me for a perfect answer/comment.

Waste heat recovery has been used for many years in the grocery business, as they use lots of refrigeration. The hot refrigerant is diverted to a indoor coil and air handler to provide free heating when required, or sent to the outside condenser. You will need a three way valve and a thermostatic control. You can check the "Sporlan" web site for more information on heat reclaim valves.

If you're air conditioning the shop in summer, you would probably save more energy by venting the heat produced by the cooler condensers outside. In winter (at least here in the NE US) the heat from the coolers helps to heat the shop.