There is a similar previous thread referring to refrigerators. Cooling air conditioners dump the heat outside of buildings, adding more heat to the outside environment. The outside grill is a heat exchanger that heats the surrounding air, but perhaps it could be built to exchange heat with water pipes entering the hot water system, i.e. preheating the building's hot water and reducing the energy cost to generate hot water both for commercial and home use. It would have to be disabled for reverse cycle conditioners during the heating cycle to avoid water pipes freezing. I would not be surprised if some industrial refrigerators already use this energy but I haven't seen the idea used in home use.
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I like this idea, but it might have some problems. You can't put the heat exchanger in the waterheater itself, so you would basically have to have a separate waterheater for it. The heat will build up in it to the point that the water will no longer be an effective heat-sink unless you use the hot water regularly throughout the day. That would probably make this more viable in a commercial building that uses a lot of hot water. It might be possible to overcome this problem by circulating the water, but I'm not sure about that. You could also solve this problem by using a really big tank, but that would be very expensive.
Thanks for your feedback, Dwane. It caused me to go and crunch some data.Lets assume a 10,000 btu/hr or approx 3kW A/C unit delivers 3kW of heat to the outside sink. Thats 3kJ in one hour.It takes approx 4kJ to heat 1 litre of water one degree C from room temp.I have no idea of the efficiency of heat exchangers but even if it 100% could be achieved my arithmetic says 1 litre of water would have a temp increase of just over 1 deg C. Please feel free to correct my arithmetic. In my original idea I was thinking of a preheating tank to main H/W tank and recirculating via siphon or auxiliary pump. I think it might be more a problem of not getting enough heat to justify the cost of redesigning the A/C.
There is an error in your math. 1 watt hour = 3600 joules. So 3kWH = 10,800 kJ. If 4kJ heats 1 liter 1 degree C, then 10,800 kJ will heat 1 liter 2700 degrees C, or 100 liters 27 degrees C. Bear in mind, that's per hour. In 10 hours it could raise 1000 liters 27 degrees C.
Still, given those numbers, this would work if your preheater tank held over 1000 liters, which is not impossible. Also, your A/C may not run continuously, so you might generate a lot less heat than that per day. On extra hot days, you could vent excess heat outside.
Thinking about this again, I think this could be made to work pretty well. You could use a small tank (about the same as a regular water heater) and a regular outside condenser. The refrigerant runs first through the water tank, then on through the regular condenser. When the water is cold, the water tank will absorb all the heat and then the cooled refrigerant will go through the condenser, where nothing will happen. As the water tank gets warmer, the refrigerant coming out will be warmer, but then will simply be cooled in the condenser like normal.
This would be very easy to design and install. You simply connect the tank into the system between the evaporator and the original condenser, and between the regular water heater and the incoming cold water main. You'll need to add more refrigerant to fill the system.
It would be very safe, as the refrigerant never gets hot enough to boil water. The device has no moving parts and requires no electricity.